Sunday, June 29, 2014

Potentially Dangerous Tornado & High Wind Discussion for Monday, June 30

This discussion will focus on the anticipated tornado and high-wind threat on Monday, June 30.

SPC
The Storm Prediction Center has issued a Slight Risk of severe weather on Monday in the 15% shading, which extends from Oklahoma into Kansas, Missouri, Iowa, Illinois, Wisconsin, and into Michigan. An enhanced risk of severe weather exists from eastern Kansas into southeast Iowa, northern Missouri, much of Illinois and portions of Indiana and Wisconsin. This enhanced area is where we are watching for potentially significant severe weather on Monday, June 30, which may include tornadoes and a swath of damaging winds.

WPC
Short range analysis from the Weather Prediction Center, valid for Monday morning, shows multiple features relevant to tomorrow's expected severe weather outbreak. We see a decaying mesoscale convective system (MCS) progressing across Illinois, moving eastward as it does so. This complex of thunderstorms will have formed today (Sunday) out to the west, and will have produced additional severe weather in those areas (see Sunday's severe weather discussion here). We also see a stationary front draped across Nebraska, Iowa, and Wisconsin, which is the focus of some early-day convection in northwest Iowa per this forecast graphic. However, the main concern here is if the morning MCS over Illinois will limit convection later in the day due to the MCS-associated cloud cover.

COD
If we look at the forecasted relative humidity values at the 500mb level of the atmosphere from the NAM, valid in the early hours of Monday, we see only limited moisture across Illinois and Indiana, despite the expected presence of the decaying MCS. This tells me that while we can expect cloud cover in the aforementioned regions due to the thunderstorm complex moving east, the cloud deck shouldn't be thick or extensive enough to severely limit convection later in the day, and this is what makes me believe we are looking at a potentially more explosive environment on Monday afternoon.

COD
By Monday afternoon, we see extensive instability has developed over the Midwest, focused in on the areas projected to receive severe weather. We see a swath of 4000 j/kg CAPE (convective available potential energy) stretching from Oklahoma to Illinois, with a pocket of 5000 j/kg CAPE existing in south-central Iowa and northern Missouri, likely the initialization point of Monday's convection.

There is ample concern that Monday's event could produce tornadoes. This stems from a very dynamic wind field in the upper, middle, and lower levels of the atmosphere, thanks in part to the dominant upper level low in southern Canada. These dynamic wind fields then create ample wind shear, a necessary ingredient for tornadoes. With the aforementioned extreme instability, tornadoes are very possible with the initial cells that form Monday afternoon out in Iowa and northwest Illinois, prior to coagulation into a cluster of thunderstorms. From there, a potentially dangerous damaging wind threat may be expected to evolve. You can stay tuned to our Facebook Page and Twitter account for more frequent updates on the situation.

Andrew

Severe Weather Outbreak Discussion for Sunday, June 29

A severe weather outbreak is expected today, June 29.

SPC
The Storm Prediction Center has posted a Moderate Risk of severe weather for southeast Nebraska, northeast Kansas, northern Missouri, and the southern half of Iowa in advance of severe weather expected today. Current discussions indicate ongoing convection over Iowa will be clearing later on in the day, paving the way for diurnal heating to develop and destabilize the environment to provide a breeding ground for severe thunderstorms.

COD
The NAM model indicates deep instability will develop over Iowa and Nebraska later on this evening, where convective available potential energy (CAPE) values look to exceed 3000 j/kg in spots. The development of this instability depends severely on the morning convection over the same areas right now. If cloud cover tends to linger over the area, we could see a reduced severe weather risk, as destabilization will be limited. However, if cloud cover can clear quickly enough, such CAPE values will be easy to come by.

TwisterData
When the storms do form, it looks like they will have decent potential to be originally tornadic. The NAM model places over 50 knots of shear near Iowa, increasing the likelihood that these initial cells that form may have a tornadic component to them. With such deep instability and shear, it is no wonder the Storm Prediction Center elected to go for a higher-level tornado threat for this Moderate Risk area. As the night progresses, the initial cells are expected to come together to form a mesoscale convective system (MCS), also known as a body of severe thunderstorms, that will be moving to the east.

To summarize, those in the Moderate Risk area today should prepare for:
• Damaging winds in eastern Iowa
• Tornadoes and very large hail in central Iowa into Missouri with the initial cells
• General poor weather conditions across the Plains and Midwest

Andrew

Friday, June 27, 2014

Severe Weather Discussion for Monday, June 30

This is a discussion for potentially significant weather on Monday, June 30.

HOOT OU
Model guidance indicates a shortwave trough will extend southward across the Plains and Midwest from a parent upper level low centered in south-central Canada. This shortwave trough is expected to be the firing mechanism for the anticipated severe weather, as it shifts eastward amid strong mid-level winds and a steadfast zonal flow. This chart shows the aforementioned mid-level winds at speeds of over 50 knots in a wide swath of land across the north Plains, supporting the zonal flow regime due to a slightly suppressed ridge along the Gulf Coast. It is this zonal flow that makes me concerned for a severe damaging wind event, possibly to the extent of a derecho.

COD
A zoomed-in view of upper level winds over the Plains and Midwest shows a band of intense winds rounding the base of the upper level low in Canada, and these winds look to push down as far south as southern Minnesota and Wisconsin, per the NAM model. I am using the NAM model primarily due to observed convective feedback issues in the GFS model immediately prior to Monday's event. These convective feedback issues have been slowly eroded today, but due to their continued presence, I feel more comfortable sticking with the NAM. The presence of these strong upper level winds tells me that we are likely looking at a continued supportive environment for a severe wind event.

Let's now analyze the typical characteristics of a damaging wind environment.

This image is from an academic journal, and shows a screenshot of a tornado environment setup on the top panel, with a "typical" damaging wind environment displayed on the bottom. The area where storms are favored to form is outlined in the hatched region. For the bottom panel, we see that the average damaging wind environment has the polar jet (the primary jet stream for all intents and purposes) shooting northeast, mainly over the northern portion of where storms are anticipated to form, eastward of the cold front. We also see the lower level jet stream shooting more north-northeast into the southern portion of the storm formation region, eventually coming together later on down the road.

If we compare the aforementioned factors with the ones we've already gone over, we can see the jet stream winds slanting northeast across the Midwest, north of where storms are expected to form on Monday (they are expected to form in Iowa/Illinois/Wisconsin). A glance at the frontal boundaries on Monday shows a cold front in a similar stance to the west of IA/WI/IL (shown below) as the bottom panel above.

HPC
Forecasted frontal boundaries on Monday morning
Let's now move on to an analysis of the projected instability values for Monday evening to try and identify any other boundaries that may ignite storms.

Twisterdata
The NAM's forecasted convective available potential energy (CAPE) is quite impressive for this event. We see the band of elevated CAPE ahead of the cold front in the north Plains, but the real story is the swath of 3000+ j/kg of instability from Oklahoma to Ohio, and it is this area of instability we will focus on. When taking into account that storms are expected to form in Iowa, Wisconsin and Illinois, we look that way and find what looks to be a boundary of sorts over northern Illinois. This appears to be defined by a steep drop-off in CAPE values from the tip of Lake Michigan to the WI/IL border. This makes me wonder if storms could fire along that potential boundary there, though the long range format of this forecast tells me to wait and watch for this particular feature. Nonetheless, it is apparent that ample instability will be present for this event.

SPC
The final item to look over is the idea of analog forecasting. This method involves analyzing past conditions, and choosing dates with conditions that best match up with forecasted conditions. In this case, the SLU CIPS program was utilized to see what dates had the most similar conditions as those forecasted to be present on Monday. The top analog that came up was June 18, 2010, and the storm reports for that day are shown above. It is quite apparent that this top analog greatly favors a damaging wind environment, as we've already evaluated for ourselves with the two-panel graphic earlier in this post. In this top analog, the damaging wind swath extended from Illinois and Wisconsin, as well as Iowa, into Indiana and Michigan. This is actually quite similar to what I think may happen if current forecasted predictions are to verify, and this must be watched closely.

To summarize, a potentially significant severe weather event is projected for Monday, with damaging winds being a primary threat. The areas affected look to be Iowa, Illinois and Wisconsin. More information will be delivered as it becomes available.

Andrew

Severe Weather Discussion for Sunday, June 29

Sunday is being monitored for potentially significant severe weather.

SPC
The Storm Prediction Center has highlighted portions of the Plains and Midwest for severe weather on Sunday. The Slight Risk covers eastern Nebraska, northern Kansas, northwestern Missouri, Iowa, southeast South Dakota, much of Missouri, Wisconsin, and the Upper Peninsula of Michigan. The most elevated risk inside this overall outlook area extends from eastern Nebraska to west-central Wisconsin.

Instant Weather Maps
Projected instability values off of the NAM model show figures in excess of 4000 j/kg situated from Kansas to Iowa, which should be more than enough to fire off these storms as a disturbance propagates eastward across the northern Plains. Shear from the surface to the 500mb layer looks to grow to upwards of 60 knots across the northern Plains and Upper Midwest, with spots of 70 knots also being found. It does appear that the initial storms that fire look to have some substantial tornado potential, with the aforementioned instability and shear present. As the night wears on, the expectation is for these cells to coagulate into a mesoscale convective system, which will then likely shift off to the east overnight. A strengthening nocturnal lower level jet stream should enable this eastward progression to continue.

Andrew

Thursday, June 26, 2014

Long Range Guidance Supporting Snowy East Coast Winter

In a marked change from previous forecasts, long range guidance is now indicating that the East Coast is in line for a snowier than normal winter.

The image above shows the average precipitation anomaly forecast for January 2015 from multiple forecast guidance models. In this image, we see a textbook El Nino precipitation pattern. This pattern is complete with a drier than normal Pacific Northwest region, leading into a wetter than normal Southwest. This wetter than normal Southwest area would likely arise from the enhanced subtropical jet stream, a result of the El Nino phenomenon, and bring quite a beatdown to the drought ravaging the West US. We see these wetter than normal anomalies transpiring throughout the Southern US, and along the Gulf Coast, before cutting up the coast. As I have previously discussed, sea surface temperatures close to Greenland are becoming increasingly supportive of a pattern which could bend the jet stream and induce additional opportunities for coastal storms in the East. These long range models would seem to confirm this theory, and will have to be monitored in the near future.

Andrew

Wednesday, June 25, 2014

Update on Winter 2014-2015 Forthcoming

An update to the 2014-2015 winter season will be released tomorrow at 2:00 PM Central Time. 

Andrew 

Saturday, June 21, 2014

Quiet Period Expected for Severe Weather

A quiet period in the realm of severe weather is looming on the horizon.

CFS Long Range Severe Weather Forecast
The image above shows The dates on the right display the date the forecast was made on, and the dates on the bottom legend indicate when this forecast is valid for. Warmer colors indicate the higher potential for severe weather. For instance, if I want to know the severe weather potential for July 20th on a forecast made on June 5th, I go up the legend on the left to find June 5th, then scroll on the corresponding horizontal line to find the July 20th box. We have been going through an active period of severe weather, which was predicted earlier by the CFS model, as those red and yellow boxes on the left side of this chart show. However, long range forecasts in the top right corner show a distinct drop-off in colored boxes, indicating a quieter weather pattern.

The image above shows 500mb height anomalies across the Northern Hemisphere, valid on Monday, June 23. In this image, we see below normal height anomalies across the Japanese/East Asian region, located in the top-left corner of this graphic. These negative height anomalies indicate a cooler and stormier regime, something that will carry over in the United States 6-10 days later using the Typhoon Rule. This particular ensemble set, referred to as the PSD Ensemble guidance system, continues the negative height anomalies for multiple days ahead, meaning that the correlation period is extended and the cooler pattern should persist in the US. At this rate, the CFS forecast and PSD Ensembles do match up in the general quieting down of the weather pattern; the CFS model shows the supercell composite values across the nation dwindling, going from colorful boxes to dark blue boxes, and the PSD ensembles keep cold & stormy conditions over Japan for multiple days, meaning cold and stormy conditions can be expected in the US 6-10 days later, and then for a longer time period.

Andrew

Thursday, June 19, 2014

Global Sea Surface Temperatures Supporting Cold Winter Ahead

Global sea surface temperature anomalies continue to point towards an increased likelihood for a cold winter ahead.

ESRL
There are four specific items I want to review that are pointing towards this cold winter idea.

1. El Nino
It's been discussed countless times already by countless weather agencies and enthusiasts, but we'll discuss it again here. The anomalously warm waters appearing west of Ecuador, nicknamed the El Nino phenomenon, look to be a crucial piece to this puzzle indicating a chilly winter ahead. In typical El Nino situations, we tend to see warm weather confined to the North and West US regions, while cool anomalies prevail in the South and East. Considering the El Nino retains the highest confidence for still being present in the coming winter, confidence in a cooler than normal winter for the aforementioned regions rises in response.

2. Cooler than Normal West Pacific 
This factor doesn't constitute an oscillation or index, per se, but will likely play into what we see happen this winter. The past cold season, I based the majority of my long range posts off of storms I was seeing hit Japan, based on the Typhoon Rule popularized by Joe Renken. It only takes a simple understanding of the relationship between high/low pressure patterns and sea surface anomalies to recognize what this means. The presence of below normal water temperatures extending eastward from Japan could very well mean that we see a stormy pattern over Japan this winter. Translating that through the Typhoon Rule, it means that the prospects of a stormy (and consequentially cooler) winter are enhanced.

3. Positive Water Temperature Anomalies in the Gulf of Alaska
In addition to the waters off Ecuador, the Gulf of Alaska is also experiencing above normal water temperatures. However, the placement of this particular body of warm water is key. Last winter, well above normal water temperature anomalies in the Northeast Pacific resulted in consistent high pressure along the West Coast, which provoked the polar vortex and incredibly cold air to push south, affecting much of the United States in the process. Barring any significant changes this summer and fall, a similar situation could unfold this next cold season.

4. AMO
The Atlantic Multidecadal Oscillation, abbreviated as AMO, is the wild card this winter. In recent months, it has reversed from its positive phase, which appears as warm waters along the coast of Greenland and into the Arctic Circle, into its negative phase, which features cold water temperatures in those same areas. The two conflicting masses of water are outlined clearly on the image above, and right now, it looks like the warm water is winning the fight. If it does hold out, we can expect increased probabilities of high pressure being sustained over Greenland, which would buckle the jet stream to the south and allow for cold air to penetrate deep into the Eastern US. Again, however, this is the wild card, and we likely won't know if this will happen for another handful of months.

Andrew

Monday, June 16, 2014

Updated Severe Weather Discussion for Thursday, June 19

This is an updated severe weather discussion for Thursday, June 19.

CIPS/SLU
This discussion will utilize only analog guidance, as there are some things that need to be clarified with the method as a whole (those caveats will be discussed as we move along). The image above shows tornado, damaging wind and hail reports from the top 15 dates whose conditions match up the best with those forecasted on Thursday. When atmospheric conditions similar to those on Thursday set up, the result is typically a severe weather event in the Midwest, as the image above shows. The hardest hit areas include Minnesota, Iowa, Wisconsin, Illinois, Indiana and Missouri. Phrased better, these states should be prepared to be affected by potentially significant weather conditions this Thursday if analog guidance verifies.

CIPS/SLU
If we look at a probability map from the top eight most severe analog dates, we see that the analog guidance suggests that there may be a 60% or greater chance of severe weather in Illinois, Wisconsin, Iowa, and extreme southeast Minnesota. These percentages are definitely threatening, although considering most recent model guidance, these percentages may actually be too high, not only due to recent developments, but the natural tendency of the CIPS system to seemingly overestimate these events.

CIPS/SLU
Use caution when utilizing this graph. This image shows the percentage of the top 15 analog dates that had a long-track tornado occur, where a long-tracked tornado is defined as a track over 30 kilometers. As you can see, this risk is anomalously high over north central Iowa, where nearly half of the chosen analogs had long tracked tornadoes occur. This may all seem frightening, but the caveats we discussed with the last image stand here as well- I don't think things will be this bad.

The threat for severe weather on Thursday will be examined in the next discussion.

Andrew

Updated Severe Weather Discussion for Wednesday, June 18

This is the latest severe weather discussion for Wednesday, June 18

SPC
The Storm Prediction Center has outlined a large Slight Risk of severe weather for the North Plains all the way into the Northeast region. States affected include North Dakota, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Wisconsin, Illinois, Michigan, Indiana, Ohio, Pennsylvania, and New York. The Plains states that look to be affected by this event are included in a hatched outlook, indicating that the risk of significant severe weather (i.e. strong tornadoes, extremely large hail, and extreme damaging winds) is elevated in those regions.

CIPS/SLU
We can also use analog dates to predict the future. This method of forecasting takes decades of weather observations and matches a handful of select dates up that are closest to the forecasted conditions. This image shows compiled severe weather reports from the top 15 analog dates, basically the 15 days that were the closest to projected conditions on Wednesday evening. When we do take a look at these compiled storm reports, the prognosis for Wednesday is shocking. We see an epidemic of significant storm reports over the northern Plains, including significant and long tracked tornadoes. Conditions similar to the ones expected Wednesday resulted in an outbreak of severe weather primarily over southern Minnesota, western Iowa, much of the Dakotas, and eastern Nebraska.

CIPS/SLU
The graphic above shows a mean probability of severe weather being reported in any given area, based on the top eight analogs. As the image shows, probabilities exceed 60% in northeast Nebraska, southeast South Dakota, Minnesota, and northwest Iowa. A still-threatening 45% risk extends across a large chunk of the north Plains, but it is that 60% risk area that has me worried. Right now, the SPC is retaining a Slight Risk for this area with the significant risk hatching, and I feel like that is the right move. While I'm nowhere near the level of expertise of an SPC forecaster, retaining a Slight Risk into the event, with the chance (NOT a guarantee) of an upgrade to a Moderate Risk on the table for Wednesday would be what I were to do if I were an SPC forecaster. For right now, though, this event does look to produce severe weather across the northern Plains. The question is, how severe will things get? That question will be answered in tomorrow's discussion.

Andrew

Saturday, June 14, 2014

Potentially Significant Severe Weather Discussion for Thursday, June 19

(Click here for the discussion concerning Wednesday's severe weather threat)

Thursday's severe weather event is looking far more threatening than initially thought.

WPC
In the Weather Prediction Center's surface analysis forecast for Thursday, we see the storm system centered in southwestern Minnesota slowly shifting eastward. A warm front is draped across Wisconsin and Michigan, heralding the boundary between the seasonal air mass and the hot, humid and unstable air mass to the frontal boundary's south. A cold front is then projected to meander eastward, as it is attached to the low pressure system from Minnesota down to Texas. Based on the location of the frontal boundaries, and anticipated wind fields due to the upper level low, I'm watching Iowa, Illinois, Wisconsin, Missouri, Kansas and Indiana for severe weather threats on this particular day.

CIPS/SLU
We can also use analog dates to predict the future. This method of forecasting takes decades of weather observations and matches a handful of select dates up that are closest to the forecasted conditions. This image shows compiled severe weather reports from the top 15 analog dates, basically the 15 days that were the closest to projected conditions on Thursday evening. If we take a glance at all of these severe weather reports, it is clear that environments similar to what we may see on Thursday tend to produce significant severe weather episodes. In this image, we see significant tornado, damaging wind, and hail reports from North Dakota to Oklahoma and Texas, and from Ohio to Michigan. The states of Illinois, Iowa, Minnesota, Wisconsin, Nebraska and Kansas look to be hit awfully hard in these types of set-ups. While these analog projections cannot exactly predict where severe weather may strike, and with what vigor, it's clear by the number and frequency of severe weather reports that residents in the aforementioned states should keep both eyes to the skies on Thursday, if current forecasts hold.

CIPS/SLU
The folks at the institute that compiles all of these analog dates also makes an image that essentially takes the frequency and intensity of all of the above severe weather reports, and puts them into a probabilistic scale of receiving severe weather in the form of a tornado, hail, or damaging winds, based on the top eight analogs. According to this image, if current forecasts hold, residents of Illinois and Iowa could have over a 60% chance of seeing some type of severe weather. Needless to say, that is an awfully high risk, and one that must be paid close attention to. Residents of Wisconsin, Missouri, Kansas and Oklahoma would still have a very-respectable 45% to 59% chance of severe weather based on all of these analogs. This is all concerning to me, and warrants the issuance of a new severe weather discussion.

Andrew

Potentially Significant Severe Weather Discussion for Wednesday, June 18

This is a special severe weather discussion for Wednesday, June 18th, as new data suggests this may be a multi-day severe weather episode.

WPC
The projected surface analysis forecast for Wednesday by the Weather Prediction shows a storm system located in the Plains, with a warm front extended nearly due east through Minnesota and Wisconsin, making available a large swath of hot, humid air to the Central United States in the process. We see a cold front snaking down from the Dakotas into Nebraska and looping around the Rockies. A special point of interest is the dryline feature that is projected to extend from Kansas into Texas and New Mexico.

According to this graphic, we may be expecting two areas of severe weather on Wednesday. The first area would likely be near the dryline feature in the Southern Plains. As that cold front meanders east through the day on Wednesday, some thunderstorms should be able to initiate and sustain themselves in the very unstable environment, which we will discuss more about later. The second region of potentially active weather should be located in the Northern Plains, in the vicinity of the low pressure system. This upper level low feature looks to create favorable lower and mid-level wind fields that should be conducive for thunderstorm formation. The proximity of the warm front to the low pressure center means those in South Dakota, eastern Nebraska, and Minnesota may have to watch for some of the strongest storms, at least initially.

COD
The forecast for convective available potential energy (CAPE) on Wednesday evening is shown above from the most recent GFS model forecast. In this image, we see that mass of warm and unstable air that has been pulled north by the aforementioned warm front in the North Plains. CAPE values look to exceed 6000 j/kg in eastern Minnesota, an incredible feat when one considers that 2000 j/kg of CAPE is typically sufficient for severe thunderstorm formation. This high instability combined with a favorable wind field makes the North Plains region more susceptible to the potential severe weather than the dryline feature in the South Plains.

CIPS/SLU
We can also use analog dates to predict the future. This method of forecasting takes decades of weather observations and matches a handful of select dates up that are closest to the forecasted conditions. This image shows compiled severe weather reports from the top 15 analog dates, basically the 15 days that were the closest to projected conditions on Wednesday evening. When we look at the compiled severe weather reports, it is confirmed that the North Plains does look to be the hotspot for severe weather on Wednesday. We see numerous damaging wind and hail reports across the Dakotas, Michigan, Nebraska and Colorado, with multiple tornado reports intertwined. While this by no means confirms that this will be how the event transpires, it does give us a key glance into what this event has the potential to become.

Related: Potentially Significant Severe Weather Discussion for Thursday, June 19: CLICK HERE

Andrew

Preliminary 2014-2015 Winter Forecast

"Another Cold Winter May Be Brewing..."

Hello everyone, and thanks for tuning in to our Preliminary 2014-2015 Winter Forecast. This forecast will focus on factors that will be influencing this winter, with an analysis of analog years and long range guidance for this upcoming cold season.

The first thing we will take a good look at is the latest weekly analysis of sea surface temperatures around the world.



ESRL
We'll be referring to this image quite a bit in this post, so let's get to know it. This image shows averaged sea surface temperature anomalies from June 1st to June 7th across the world. Below normal temperature anomalies are displayed in blue, while above normal anomalies can be observed in yellows and reds. We're going to begin this outlook by analyzing the band of above normal waters along the Equator in the Pacific Ocean.

The waters in the central Pacific may seem like an awfully desolate place, but for weather enthusiasts, it's one of the most (in)famous areas to look at when creating seasonal forecasts. The reason is due to a phenomenon called the El Nino-Southern Oscillation index, or ENSO for short. This ENSO phenomenon involves water temperature anomalies and enhanced or suppressed convection along the Equator, which then leads to climate-altering weather patterns for a prolonged period of time. We have recently emerged from a period when this ENSO phenomenon was at its weakest, as there was little or no designated positive or negative water temperature anomaly in the Central and East Pacific. However, in recent months, the waters along the Equator have begun warming, a sign that things are changing.


This warming of the Equatorial waters is one of two phases of the ENSO phenomenon, this one called the El Nino or warm phase. The El NiƱo is officially defined as water temperatures in the Central and East Pacific being at or above 0.5 degrees Celsius above normal. According to this chart, we're already there. According to many respectable meteorologists, we have been there for a while now, but the atmosphere is failing to recognize the El Nino (however, that's a different story for a different time). When this El Nino does become well defined in the atmosphere, we can be assured that it will have consequences on our winter season this year. Let's take a gander at some effects of the El Nino.



CPC

In an El Nino winter, we tend to see below average snowfall in the Midwest and Great Lakes, due to an unfavorable precipitation pattern. A wide swath of above normal snowfall is then observed in the Mid-Atlantic, and well into the Northeast. This comes as the result of an enhanced subtropical jet stream, the mechanism that allows storm systems to dip south and eventually become Nor'easters and coastal storms in general. Ski resorts out west tend to suffer in many areas, but some portions of the Rockies do fare well with above normal snowfall in a typical El Nino winter. Temperatures in an El Nino winter are opposite of what you may expect. Instead of mild winters in the South, El Ninos tend to bring chilly winters. The warmth is then displaced into the North and West US. Precipitation composites echo the snowfall composite, with wet conditions in the East, dry conditions in the Ohio Valley, and a little of both in the West US. While the ENSO phenomenon is just one factor in the huge machine we call the atmosphere, it has some real power when it comes to large-scale weather patterns, and for this reason, we need to monitor how this progresses into the summer.



IRI
Model guidance from multiple global climate models indicates the above normal water temperatures will continue to strengthen as we head into the fall months, shown as ASO (August-September-October) and SON (September-October-November). By winter, the El Nino slackens, but already-iffy climate model accuracy is greatly degraded by the end of the forecast window. The agreed consensus is that this El Nino will continue to strengthen, and is very likely to be with us for the winter of 2014-2015.


Image re-published for ease of access
Continuing on with our sea surface temperature analysis, we will now take a look at the above normal water temperatures in the northeast Pacific.

The ENSO phenomenon is a significant climate-influencer, but it is nowhere near the only factor that is able to alter long-term weather patterns. That body of above normal water temperatures in the Gulf of Alaska depicts the positive phase of the Pacific Decadal Oscillation, or PDO. The PDO has two phases, just like the ENSO phenomenon- a warm phase, and a cold phase. During the warm phase/+PDO, above normal water temperatures are seen right against the coastline of British Columbia and Alaska, with a swath of colder than normal water temperatures in the open Northeast Pacific waters. The cold phase/-PDO is exactly the opposite. While this isn't exactly a clear-cut positive PDO regime, data from the University of Washington indicates that is what we are experiencing. 


Typically, the phases of the PDO and ENSO are opposite each other. For example, an El Nino / warm phase ENSO is typically accompanied by a warm phase / +PDO regime. Likewise, a La Nina / cold phase ENSO is commonly observed in conjunction with a cold phase / -PDO regime. In this case, though, it looks like we are seeing a warm phase PDO and warm phase ENSO situation. While not "rare", these events aren't exactly common, either. During a positive PDO, temperature trends see warm weather in the North and West, with cold weather in the South and East during the winter seasons. Basically, the positive PDO and positive ENSO phases could very well combine to create an even colder environment for the South and East, and a warmer one for the North and West this winter. Since we're still several months away, however, confidence is naturally lower than if winter were instead several weeks away.


But there's more than just correlations and stats saying that this +PDO regime may allow for another cold winter. For more, we look to last winter.



ESRL
The above image shows a glimpse of mid-level height anomalies last winter, averaged out from December 2013 to February 2014. In this image, warm colors show positive height anomalies, resulting in quiet and warm weather. Similarly, cold colors indicate cold and stormy weather. If we look over at the United States, we see our friend, the polar vortex, in the position that brought us the horrendous cold in January and February. But the vortex pushing so far south wasn't a random act of God. It was greatly influenced by that strip of positive height anomalies over the Northeast Pacific at the center of the image above. And how did that body of positive height anomalies appear? That's right, it came from the warmer than normal water temperatures in the Gulf of Alaska. If you put the pieces together, the climate pattern that made the winter so cold last year may very well return again this winter. This is because the warm waters in the Northeast Pacific are still here today, and if they remain there this winter, the chances of ridging appearing in that area are high, which could lead to another very cold winter.


Image re-published for ease of access
The final sea temperature-related topic we have to discuss is the Atlantic Multidecadal Oscillation, or AMO. The AMO, much like the indices already discussed, has two phases: a positive/warm phase, and a negative/cool phase. In the positive AMO regime, waters near Greenland are warmer than normal. This allows for high pressure to build in over Greenland and the Arctic Circle, which then can disrupt the polar vortex and send cold air down to lower latitude regions like North America. This ridging over Greenland can also alter the jet stream and persuade more coastal storms to affect the Northeast. In the negative AMO phase, temperatures near Greenland are cooler than normal, resulting in a stormier than normal pattern in the area. The polar vortex is strengthened by this, but ends up moving further south, which can also increase the potential for a cold winter in North America.

Recently, we have observed a belt of below normal sea surface temperatures developing south of Greenland, which is still visible today on the chart above. This prompted the AMO to flip from its positive phase to negative phase, something that sent meteorologists and weather enthusiasts into a frenzy. This negative AMO persisted for a handful of months before the latest May AMO update declared the AMO back in its positive phase. This flip back to the positive phase is likely attributed to the snakelike swath of positive water temperature anomalies immediately south and east of Greenland, encompassing Iceland. In case you can't already tell, the AMO is clearly in a pretty chaotic state.

Because the AMO has been erratically flipping, and isn't in any clear phase right now, it's harder to use this index in our seasonal forecast. This will be updated in our Official 2014-2015 Winter Forecast later this fall, but for now, I find it best not to look to this particular oscillation for advice on our winter right now.

Now that we've exhausted the topic of water temperatures, let's go into another out-of-this-world discussion: the Sun.


NOAA/SWPC
We are in an interesting time with respect to the Sun. The image above shows observed sunspot values, with a trend line superimposed, since January of 2000. You can see how we were sustained in triple-digit sunspot values for the first few years of the new decade, as Solar Cycle 23 peaked and double-peaked before plummeting down in 2005 through 2009 to begin Solar Cycle 24, which we are currently in. But things aren't as similar as our last cycle. Notice how our first peak was only around 60 sunspots, instead of 125 sunspots in 2000. Now, as we finish our double-peak of this solar cycle, the average number of sunspots will begin to dramatically fall off, just as it did in 2005-2009. However, this time, it won't recover as well. The abnormally low sunspot numbers will  likely have some sort of effect on the Earth. It may not be this year, though we could see increased chances of a chilly winter if sunspots plummet around wintertime. No, it will take a number of years before we see a string of potentially noticeably-cold winters due to this new "hibernation" of the sun. This can be elaborated on much more on another day in another post, but the thing to take away here is that we are more prone to a chilly winter with sunspots projected to drop than a warm one.

Next, let's move on to a subject that has origins dozens of miles above the surface, but has implications across the troposphere and stratosphere.



The Quasi-Biennial Oscillation (QBO) is a wind-driven oscillation located around the 30 millibar mark high up in the atmosphere (consider 1000mb is the surface, and planes fly at around 200 millibars). The QBO has two phases- a positive phase, and a negative phase. In the positive QBO phase, winds in the stratosphere are westerly, or going eastward. When one considers that low pressure in the northern Hemisphere also has westerly winds, one can match up the two and come to the conclusion that the stratospheric polar vortex, which controls much of the cold air reservoirs in the Arctic, is strengthened by this +QBO. As a result, the cold air is locked in up north, and winters in the lower latitudes (like the US) tend to see warmer winters.During negative QBO years, stratospheric winds are easterly, or going westward. These winds are going in the opposite direction as the polar vortex, which means the vortex is weakened. Cold air then has a better chance at flowing south into the lower latitudes.

The image above shows the history of the Quasi-Biennial Oscillation, with the years on the bottom legend and height in millibars on the left legend. Measures in km are on the right legend. Dark gray shadings indicate the presence of a positive QBO wind shift, while white contours and shadings show a negative QBO. If you look at the bottom of this graph, you can see how we are now beginning to emerge from a positive QBO regime this past winter, and how we're heading for a negative QBO this winter. Although the positive QBO this past winter was supposed to allow for a warmer winter, it was that high pressure in the Northeast Pacific we discussed earlier that didn't allow it. Now, this upcoming winter, it looks like that high pressure in the Northeast might be back again, and this time, the QBO ought to be more supportive for the polar vortex to be torn apart, and could very well allow for a cold winter again this upcoming season.


CPC
One other item I want to look at is the current soil moisture anomaly across the United States over this past May. This shows us how wet (green shading) or dry (red shading) the soil in a given area is, and helps us analyze not only precipitation patterns we have observed, but precipitation patterns we will observe. Looking over this chart, we see that the Pacific Northwest was inundated with rain in the month of May, resulting in western Montana seeing over 160% of its average soil moisture. The Southwest fared much worse, seeing extreme negative soil moisture anomalies, likely due in large part to the feedback loop the drought is creating (we will discuss this later). We see this trend continuing in the Plains, and even into the Midwest, before the Great Lakes observes above normal soil moisture. The East Coast and Southeast are also in this above normal precipitation trend for the month of May.

We brought up the idea of a feedback loop in the last paragraph, and that's something I want to discuss more in-depth right now. To describe the feedback loop concept, we can use a real-life example in California. California is experiencing a severe drought right now, leading to absolutely parched soil, and thus extreme wildfires. This dry soil greatly limits the process of evapotranspiration, where moisture in the soil is evaporated into water vapor and lifted into the air to create clouds. Due to the lack of soil moisture, this evapotranspiration does not happen. Consequentially, no clouds are created, meaning the chances for rain to fall from these clouds are severely lowered. When storm systems do stray over California, the dry environment actually ends up cutting down on any helpful rain, as the storm cannot complete the evapotranspiration process, thus cutting off the whole water cycle, and crippling the storm. As a result of the weakened storm, no rain falls, and the drought worsens. This is the feedback loop. A graphical image of this feedback loop is shown below.


The Weather Centre


We can use this feedback loop concept to our advantage in long range forecasting. It is plausible to suggest that this drought continues into the winter. It's not impossible, but at this rate, it's more likely than not. Just going solely off of that soil moisture guide, one might expect a stormy winter in the Northwest, and a dry one in the Southwest. Some drier conditions may be anticipated in the Plains, while a snowy winter might be anticipated along the East Coast. We're still half a year away from winter, so these precipitation trends are bound to change. However, it can be helpful to see what hints we can take away about the long range outlook by analyzing soil moisture content.

To discuss potential analog years, below is the content from the Preliminary 2014-2015 Winter Analogs post, first published in mid-May. It is republished here, as the years are still valid.


The image above shows 500mb height anomalies across the northern hemisphere, where cool colors signify stormy and cool weather. Warm colors define the presence of warm and quiet weather. In this image, which shows 500mb height anomalies averaged out across these 9 analog winters, we see significant negative height anomalies across the western coast of North America, defining a textbook negative-Pacific North American index (PNA) pattern. In negative PNA patterns, low pressure dominates the West US, leading to warm weather in the Central and East US. We see ridging in southern Canada, spilling over into the northern United States. There is no clear blocking pattern over the Arctic Circle, meaning cold weather may be even more difficult to come by.

The temperature composite for these same analog years confirms this assumption. We see warm weather dominating the northern United States, maximized in the northern Plains. This sort of temperature pattern is typically associated with an El Nino, where we see warmer than normal weather in the Northern US, with cooler than normal weather in the South or West US.

The precipitation anomaly for these same years confirms the textbook El Nino set-up, as we see below normal precipitation affixed over the southern Ohio Valley down towards the Gulf Coast, and the Southwest US along the Gulf Coast experiencing above normal precipitation. This belt of above normal precipitation looks to be due to the abnormally active subtropical jet stream, another characteristic of an El Nino. We also see a stripe of wetter than normal conditions along the East Coast, which can be attributed to that El Nino-induced subtropical jet stream.

Let's now go over my preferred analogs, which match three of the five chosen parameters that fit the upcoming winter.

Looking at the 500mb height anomaly composite for the four years that match three of my five parameters, we see a much different story than we saw with the less-preferred analog set we just went over. We now see a well-defined blocking situation over the Arctic Circle, with strong positive height anomalies in Greenland and into Canada. This exemplifies the negative phases of the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO). The negative phases of these indexes both allow cold air to push into the United States, and the negative NAO permits the storm track to push north and threaten the East Coast with more coastal snowstorms.

The temperature composite for these four choice analog years is also a change from the other analog set. Rather than most of the United States being warmer than normal, the negative AO and NAO seem to influence temperatures to be well below normal across much of the nation, maximized in the southern Plains. We still see warmer than normal conditions in the upper Midwest, but these warm anomalies are more restricted than they were in the first analog set. Unfortunately, this could mean the second straight colder than normal winter, as I place more trust in this analog set than the one we first analyzed.


The precipitation composite for these four analog years shows a very stormy East Coast, with anomalies anywhere from 4 to 6 inches above normal, most enhanced in the Southeast. This precipitation pattern is still somewhat aligned with a typical El Nino pattern, though the dryness in the Southeast argues against this somewhat. I'm not as ready to accept this precipitation composite, considering how dominating the El Nino may be this winter, but as always, the analog set will be refined in coming months. 



The Forecast
What Can We Expect This Winter?

It's too early to make maps for this upcoming winter, but we have an abundance of hints we can use for some early forecasting. Predictions for temperature and precipitation will be given. Anomalies in confidence will be listed; if no confidence level is listed, confidence is average.

For the Pacific Northwest: A warmer than normal winter with around average precipitation is currently favored, due to the state of the Pacific Ocean and choice analog year. Snowfall is projected to be slightly above normal.

For the Southwest: A warmer than normal winter with above average precipitation is currently favored, due to expected high pressure along the West Coast and an enhanced subtropical jet stream. Snowfall is projected to be around average.

For the North Plains: A cooler than normal winter with average precipitation is currently favored, due to the expected Pacific set-up and choice analog years. Snowfall is projected to be around average.

For the South Plains: A cooler than normal winter with slightly below average precipitation is currently favored. Snowfall is projected to be slightly above average.

For the Midwest and Great Lakes: A slightly cooler than normal winter with around average precipitation is currently favored. Low confidence. Snowfall is projected to be below normal.

For the Ohio Valley: A slightly cooler than normal winter with slightly below average precipitation is currently favored. Snowfall is projected to be slightly below normal.

For the Southeast: A cooler than normal winter with wetter than normal precipitation is currently favored. Higher than normal confidence. Snowfall is projected to be above normal.

For the Mid-Atlantic: A cooler than normal winter with above average precipitation is currently favored. Snowfall is projected to be above normal.

For the Northeast: A cooler than normal winter with above average precipitation is currently favored. Snowfall is projected to be above normal.


Thank you for reading the Preliminary 2014-2015 Winter Forecast. Please bear in mind this is not a full-fledged forecast, but an overview of expected conditions with some early estimates of what may be to come this winter season. Make sure to stick with The Weather Centre as we continue to head towards the winter season. If you have any questions or comments, don't hesitate to post them in the comment box below.

**Please do not ask what your location will be like this winter. It's far too early for such questions, and such questions will not be answered!**

Don't forget to share the forecast below! 

Andrew

Friday, June 13, 2014

Multi-Day Severe Weather Discussion for June 17 - 20

This is a severe weather discussion concerning the potential multi-day severe weather threat from June 17 to 20.

COD
The severe weather potential kicks off on the evening of June 17th, when this instability image above is valid for. In this forecast, we see a reservoir of extremely unstable air extending from the Gulf Coast to southern Canada. At the heart of all of this instability, we see convective available potential energy, or CAPE values exceeding 6000 j/kg, an absolutely massive amount of fuel for thunderstorms. This kind of extreme instability finds itself from Nebraska to Minnesota, down through Illinois and Missouri. All of these areas may be in line for the first of potentially multiple rounds of severe weather on the back end of this upcoming workweek.

COD
Fast-forwarding to Thursday evening, we see lower-level wind fields are suddenly much more supportive for severe weather than they had been on Tuesday and Wednesday. This comes as a low pressure system slowly meanders across the Plains gradually attaining a negative tilt in the process. A negative tilt means the contour lines appear to be digging to the southeast direction, which then enhances the risk of severe weather. In this case, we see low-level wind speeds exceeding 40 knots in the Midwest, and this is where I believe we may see some organized severe weather on Thursday evening, as continued extreme instability and now-supportive wind fields combine to create for what could be an impressive environment.

CIPS/SLU
We can also use analog dates to predict the future. This method of forecasting takes decades of weather observations and matches a handful of select dates up that are closest to the forecasted conditions. This image shows compiled severe weather reports from the top 15 analog dates, basically the 15 days that were the closest to projected conditions on Wednesday evening. On this image, we can see that the majority of severe weather events that happened under similar conditions as this coming Wednesday occurred in the Plains regions. We see an anomalous swath of hail, wind and tornado reports throughout the Dakotas, Nebraska, Kansas and Colorado. We then see an area where strong tornadoes hit under similar conditions in central Oklahoma into Texas. While this coming Wednesday won't exactly follow this set-up and put tornadoes/hail/damaging winds down in these exact areas, it does give us a good glimpse at some regions that might be affected by potentially tornadic weather. While I don't have images for Thursday's threat and onward, it can be anticipated that the threat would shift east.

Andrew