Saturday, December 26, 2015

December 26-29 Potentially Catastrophic Flooding

A potentially catastrophic flooding situation is unfolding for parts of the South Plains in the next couple of days.

The Weather Prediction Center has outlooked portions of eastern Oklahoma and extreme northwest Arkansas in a High risk of rainfall exceeding flash flood guidance to the right of a line. In essence, this is the risk of very heavy rainfall in a given area. There is a Moderate risk extending from northern Texas, portions of eastern Oklahoma, northwest Arkansas, central and southern Missouri, and extreme west-central Illinois. Those in the Moderate risk should make preparations for very heavy rainfall, while those in the High risk area should make preparations for potentially-widespread flash flooding.

The Weather Prediction Center places an 11.87" rainfall maximum over eastern Oklahoma for this storm system over the next 72 hours. Given that Oklahoma went through a record-breaking rainfall event in May of this year, this impending rain event will only add stress to an environment that has already dealt with a very intense rainfall event this year. Amounts on the order of 4-7" are projected for northeast Texas, while central and southern Missouri into northwest Arkansas and portions of east Oklahoma are outlooked for 7-10"+ of rain. Again, this is not something to take lightly. Rainfall on this magnitude, especially in areas that received extreme rainfall several months ago, is expected to cause flash flooding, possibly on a severe scale.

To Summarize:

- Heavy rain is expected to cause flash flooding over the next few days.
- This rain could be as high as 12", with amounts over 6" possible in portions of eastern Oklahoma, southern Missouri, and western Arkansas.
- Preparations should begin now for potentially severe flash flooding, especially in the areas mentioned above.


December 26-29 Significant Ice Storm and Snowstorm

It appears that a significant snowstorm, as well as a significant ice storm, will unfold in the December 26-29 period.

VALID: 12/27 at 4 AM CST
All further images with this layout are also from the CMC
The event begins as the storm system exits the Southwest. We will see a swath of moderate to heavy rainfall extending from the Great Lakes into Texas, but the real story will be the wintry precipitation. In this image, the forecasted precipitation type valid for December 27th at 4 AM CST, moderate snow will be falling in northern Texas into the Oklahoma Panhandle, as well as the western portion of Texas into New Mexico. Heavy mixed precipitation will be falling in southwest Oklahoma into northern Texas, draped across the Red River. Freezing rain is forecasted very close to Oklahoma City, while moderate rain falls across the remainder of the state. Mixing will also be a concern in northwest Missouri.

VALID: 12/27 at 5 PM CST
By the evening of December 27th, model guidance foresees snow continuing to fall in northern and western Texas, with continued moderate to heavy mixing in portions of Oklahoma. Oklahoma City now appears at risk for a freezing rain event in this timeframe, with heavy rain located in Arkansas and Missouri. Our storm system is also observed moving across the Southern Plains at this time.

VALID: 12/28 at 1 AM CST
In the early morning hours of December 28th, the system will begin to spread its influence to the north. Moderate snow will be ongoing on the back end of the system in Oklahoma, and now also falling in southern and eastern Kansas into far northwest Missouri. Mixed precipitation will be falling at this time in portions of Oklahoma and Texas, and falling at a heavier rate in northern Missouri into extreme western Illinois. Heavy rain will continue falling in Missouri and Arkansas, setting up a pronounced flooding risk which we will analyze in a follow-up post, which will be published shortly.

VALID: 12/28 at 7 AM CST
As the sun begins to rise, the situation becomes more dicey for those in the Midwest and Great Lakes. Model guidance now foresees moderate snow falling on the back end of the system in central Oklahoma, with a tight gradient from snow to mix to rain in eastern Oklahoma. Moderate snow will be falling in much of Kansas into southeast Nebraska, with heavy snow impacting southern Iowa and northwest Missouri. A large swath of mixed precipitation will be falling from central and northern Missouri into much of northern Illinois and Indiana, including much of metropolitan Chicago, IL. A narrow, but moderate band of freezing rain looks to hit central Illinois into northern Indiana at this time period, setting up a potentially catastrophic rush hour situation for urban areas.

VALID: 12/28 at 2 PM CST
By the early afternoon hours of December 28th, the storm system is now mature, with a defined dry slot shooting into southern Illinois, and a frontal band producing moderate to heavy rain from Alabama and Georgia to Tennessee and Kentucky. Light to moderate snow is projected to be falling from east Kansas into east Nebraska, as well as much of Iowa and northwest Missouri. Heavy snow is outlooked in southern Wisconsin and central Iowa. A heavy band of mixed precipitation is forecasted in central Missouri, eastern Iowa, northern Illinois, extreme southern Wisconsin, southern Michigan, and even portions of Pennsylvania. A narrow but intense band of freezing rain looks to set up immediately south of downtown Chicago, extending east into the heart of Gary, IN, and just making its way into southern Michigan. This would likely set up a treacherous commute home for any workers able to make it in during the morning mess.

Now that we've analyzed the hour-by-hour, let's check out the total accumulations.

Meteocentre - Snow accumulations
Accumulations for this image and the next two accumulation images are in MILLIMETERS.
The Canadian model projects a general swath of 20mm to 40mm of liquid-equivalent snow in the Texas/New Mexico/Oklahoma region. Putting this into a conversion chart tells us this is roughly 0.8" to 1.6" of liquid-equivalent snow. The average snow-to-liquid ratio is 10:1, meaning one inch of liquid would produce 10" of snow in that environment. However, given the lack of a preceding cold air mass, it would not surprise me to see totals lower than that, as the ratio goes down and snow becomes heavier. Thus, using a general ratio of 8:1, we could expect about 6-12" of snow for this region.

A band from northern Missouri to southern Wisconsin shows about the same liquid equivalent values, and again, a lower snow ratio is expected, so 6-12" of snow could be expected if this forecast were to verify as is. These heavier accumulations would just miss downtown Chicago, but would greatly impact Madison, WI and Milwaukee, WI.

Meteocentre - Ice Pellet accumulations
For the projected ice pellet accumulations, a swath of 10-25 millimeters exists in portions of Oklahoma, which is converted to about 0.4" to 1.0" of liquid-equivalent ice pellets. The amounts only increase, with a higher band clocking Chicago and into Detroit. Amounts on the order of 25mm to 40mm of ice pellets, converted to 1.0" to 1.6" of liquid-equivalent ice pellets, would be expected in this band. Additional, albeit lesser amounts are then outlooked for the Northeast.

Meteocentre - Freezing Rain accumulations
Lastly, we examine the projected freezing rain accumulations from this system. The main band appears to be placed immediately south of downtown Chicago, if not slightly inside the main metropolitan area. The swath contains values on the order of 7.5 millimeters to about 15 millimeters, with the highest amounts in Illinois. Using our handy conversions, we can identify that this storm drops 0.3" to 0.6" of freezing rain, a potentially catastrophic scenario with the projected wind fields.

To Summarize:

- A significant snowstorm and ice storm looks to unfold across the country over the December 26-29 period.
- Freezing rain on the order of up to 0.75" could impact portions of Illinois, Indiana, Missouri, and Michigan.
- Snowfall on the order of 6-12" could impact portions of Texas, Oklahoma, Missouri, Iowa, Wisconsin, and Michigan.
- Preparations should begin for those outlooked in snow, mix, ice, and heavy rain areas.

Additional posts on this storm are forthcoming.


Thursday, December 24, 2015

December 26-29 Significant Winter Storm

There is growing concern that a potentially-historic snowstorm may impact the Southern Plains, with lesser, but still-substantial amounts to the north and east in the December 26-29 timeframe.

Tropical Tidbits
The system of interest is currently in the Gulf of Alaska, pushing towards the Pacific Northwest as of posting time. We see the higher vorticity values in the circled region, indicating the presence of the trough. Looking over the map right now, the pattern downstream (to the east) of this energy is rather zonal (not wavy; flow is west-to-east), which would keep this storm from going too far north. However, the flow immediately upstream (to the west) of this piece of energy is rather meridional (wavy; flow can appear to move in a north-south direction at times), which will likely play a role in the storm track later on. For now though, we can only confirm the presence of our storm system in the Gulf of Alaska.

Tropical Tidbits
The first snowfall map we'll analyze is the 00z GFS model, the most recent as of the time I am writing this post (it's currently 1:11 AM on Thursday, sleep is not my friend right now). The latest GFS shows amounts in the 6-14" range across northern, western and central Texas, with scattered swaths of 14-20", particularly in far western Texas into New Mexico, and into north-central Texas. A narrow, but intense band of amounts exceeding 24" is projected onto the southwest portion of Oklahoma into north-central Texas. Amounts in the range of 12-18" are then outlooked in much of central and northern Oklahoma, before amounts taper off to the 5-10" range from southeast Kansas through Chicago. The energy then appears to transfer east along the East Coast, dropping significant amounts in the Northeast, with New York, Massachusetts, Vermont, and New Hampshire among the states projected to receive up to 18".
This model has been quite variable with snow amounts, but has maintained consistency with the storm actually occurring, and confidence is increasing in the storm occurring in at least some format.

Tropical Tidbits
Next, we'll take a look at the Canadian GEM model forecast for this system. This model paints a different scenario, expressing the variability that still exists with this event. This model drops a general 6-18" swath across northern, western and central Texas, with the highest amounts in the far northern and western regions. Oklahoma also experiences significant snows, with a general 6-12" swath for much of the state, and even a 24" bullseye on the Oklahoma/Texas border just southeast of the Oklahoma Panhandle. Amounts then diminish to a 6-12" range in northern Missouri to northern Illinois and Indiana, southern Michigan, and boosting amounts slightly as the energy hits the Northeast.

While I'm not buying into any particular snowfall solution just yet, I am advocating a particular track. Next, I'll explain why I believe we will see the heavy snow axis fall where it is currently projected, or even further south and east than the above maps show.

Both images from CPC
The first two items to discuss are the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). The AO measures the strength of the tropospheric polar vortex- a positive AO indicates a stronger than normal polar vortex, while a negative AO indicates a weaker polar vortex. The positive phase of the AO tends to promote a more zonal flow, generally warmer than normal and less stormy. The positive phase of the NAO works similarly, promoting warmer weather in the eastern 2/3rds of the country, and zonal flow. Both of these indices are forecasted to maintain positive strength when this storm comes around, telling me that the storm will be hard-pressed to push northward like earlier model runs had suggested.

We're also looking at a positive Pacific-North American (PNA) index for when this storm is forecasted to hit. The positive phase of the PNA promotes high pressure in the west, and low pressure in the east. It's typically conducive for storms in the Midwest and Ohio Valley for this reason, and can also lead to these transfer events, which give the Northeast the threat for stormy weather as well. A typical positive PNA set up is shown below, and we are expecting this positive PNA set-up for the storm's occurrence.

But the real kicker is a piece of energy that, in a way, is both the least and most important part of this storm.

Tropical Tidbits
The above image shows the 500mb vorticity map, forecasted by the 00z GFS for 84 hours out. We see our storm now well developed and centered in far northern Mexico, beginning to push eastward into the southern Plains. However, the point of discussion here is that area of higher vorticity in the northern Plains and Midwest, and it is this item that tells me we could see the storm track stay to the south.

That band of higher vorticity values is an area of confluence aloft. Confluence is the act of air essentially rushing into a given space- similarly, diffluence is the act of air evacuating a given space. In the atmosphere, confluence aloft can result in high pressure at the surface, which acts to force storm systems away (in this case, further to the south). It is this that completes my belief that the storm will take this more eastern track and give cities like Oklahoma City, perhaps Dallas, Chicago, Detroit, and much of the Northeast a solid snow after Christmas.

To summarize:

- A significant winter storm is forecasted to hit a large portion of the country in the December 26-29 timeframe.
- Snowfall amounts in excess of 12" cannot be ruled out for the Southern Plains, including Texas and Oklahoma.
- Snowfall amounts in excess of 6" cannot be ruled out for portions of the Midwest and Great Lakes, including Chicago and Detroit.
- Snowfall amounts in excess of 12" cannot be ruled out for the Northeast.
- High uncertainty still exists with this system.


Tuesday, December 22, 2015

December 26-29 Potentially Significant Winter Storm

Model guidance is beginning to pick up on the idea of a strong winter storm impacting the Plains.

The above image shows today's 06z GFS snowfall forecast from the time of posting to midnight on December 29th, 2015. We can see the large swath of snowfall extending from northern Texas through Kansas, Nebraska, and into Minnesota. The heaviest-hit region in the deep south appears to be extreme northern Texas, with amounts in the 12" to 24" range, particularly along the border with Oklahoma, as well as in a spot near New Mexico.
Portions of the Oklahoma Panhandle, as well as southwest Kansas and eastern Colorado, may be in line for up to 36" of snow, according to this forecast model. These are very extreme amounts, and although they have been showing up multiple times on previous forecast runs, extreme caution should be used with these maps, especially given the timeframe.
Amounts in excess of 24" are found in spotty areas to the north and east, particularly eastern Nebraska into southeast South Dakota, with lesser (but still significant) amounts in Minnesota.

The above image shows the 12z GFS interpretation of the coming storm, with total snowfall from posting time to 6 AM December 29th shown. Snowfall amounts are still incredibly high in northern Texas, the Oklahoma Panhandle, and much of Kansas into southeast Nebraska, but trail off from there. Amounts in the 12-16" range can still be found in Iowa and Minnesota, but these amounts are substantially lower than those in the 06z forecast. This variability, both in amounts and track (as you'll notice the heavy snow axis has shifted east) expresses the extreme uncertainty still associated with this system.

Tropical Tidbits
Lastly, we'll analyze today's 12z Canadian model, the GEM model. Amounts on the order of 20-24" are found in north and west Texas into the Oklahoma Panhandle, even in south-central Kansas, but overall snowfall totals are drastically less than the two GFS runs we analyzed earlier. This is a red flag that may indicate one model is either over-doing snowfall (GFS) or one model is under-doing snow (GEM). There's also a plausible chance that they're both incorrect- the point is, there's a lot of uncertainty here.

But just where is this system now?

Tropical Tidbits
Our system is currently traversing the Aleutian Islands, and will drop into the west coast of North America to produce a very strong trough, as per latest model guidance. It remains to be seen if this is model error, as medium and long-range guidance has a tendency to over-amplify storm systems, but for now, it's merely a system to monitor.

To Summarize:

- There is the threat of a winter storm in the Plains over the December 26-29 period.
- Model guidance is expressing high snow totals in the South Plains, but high uncertainty indicates that these models should not be taken at face value right now.


Upper Stratospheric Polar Vortex Warming in Short Term

(Note: This post discusses the stratospheric polar vortex in the short term (out to Day 10). The other stratospheric post discusses expectations next month.)

The upper stratospheric polar vortex is forecasted to experience some intensive warming, in the atmosphere's attempt to get this winter started.

The image above shows a graph of the forecasted Wave-1 temperature outlook from the ECMWF model, 10 days from today. While daunting at first glance, it can be explained.
The different levels of the atmosphere are listed on the left-hand legend. We can see the graph covers the atmosphere from the 1000-millibar level up to the 1-millibar level. Note the warmest colors centered right around the 3-millibar line. The bottom legend shows lines of longitude.

Let's depart this forecast for a moment and discuss different modes of stratospheric vortex disruption. There are two primary modes of disruption: Wave-1 and Wave-2. In a Wave-1 stratospheric polar vortex disruption event, a single ridge / body of warm temperatures forms aloft and singlehandedly tries to displace the polar vortex. This singular body of warmth / ridging is why it is called a Wave-1 event- Wave-1 events are also the strongest type of disruption event. Similarly, Wave-2 events involve two bodies of ridging / warmth trying to squeeze into the Arctic Circle, usually to split the polar vortex into two vortexes. This is weaker than a Wave-1 event, but still wields significant power in the atmosphere.

This temperature chart above shows a temperature spike at the 3-millibar level. Remembering this chart specifically identifies Wave-1 events, we can then deduce that the ECMWF model is expecting a Wave-1 disruption attempt at the 3-millibar level of the atmosphere. We can look at this on a 3-millibar forecast map below, valid for the same timeframe as the graph above:

Note how we see a single body of warm temperatures on the right-hand side of the hemisphere (if you look closely, the warmth is centered over Eurasia), visually showing that Wave-1 pattern I discussed earlier.

Why is this important? It means that the atmosphere is applying pressure to the stratosphere to try and make the pattern more conducive for wintry weather here in the troposphere, something that's been lacking this December. Now, vortex disruption at the 3-millibar level won't do much down here at the surface, but if it can expand to lower levels of the stratosphere (ideally 30, 50, and/or 100-millibar levels), the influence becomes greater on us here at the surface.

To summarize:

- The upper portion of the stratospheric polar vortex looks to experience warming in the next 10 days and beyond.
- While insignificant at that height of the stratosphere (3-millibar level), it could indicate a pattern more conducive for wintry weather may set up down the road (i.e. into next month).


Sunday, December 20, 2015

Early Signals For Pattern Change in Early January

There are emerging signals for a change to a colder and snowier pattern in the early days of January 2016.

The image above shows the GFS Ensemble mean 500mb height anomaly forecast for hour 384- the infamous fantasy land, end of the run panel. The ensembles are expecting ridging to build up along the west coast of North America, with troughing taking over in the Bering Sea and Arctic Circle. This kind of pattern would likely promote an overall-cooler set-up than the one we're currently in, which will feature well above normal temperatures on Christmas Day.

You all know I wouldn't post a 384-hour graphic without good reason, so let's dive in.

The ensemble forecasts out of the Climate Prediction Center for the Pacific North American (PNA) index show a dip to negative values as we head to Christmas, but then rising to neutral, and even positive values to kick off 2016. The majority of members are in positive territory by the start of the new year. The relative consensus of most ensemble members on a neutral or positive value of the PNA is encouraging for winter weather fans. Positive PNA values typically result in colder and snowier weather for the Central and East US, while negative PNA values result in warmer weather.
Some of you may ask why it's been so warm even though the PNA was positive in December- that can be attributed to the El Nino, producing enhanced convection in a part of the Equatorial Pacific conducive for warm weather here in the U.S., however that's a topic for another post.

A grand overview of four key teleconnections reveals this improving pattern (improving, at least in the eyes of winter weather fans). We see a continued positive EPO through the forecast timeframe- if you're more knowledgeable, you'll recall that the positive phase of the East Pacific Oscillation (EPO) encourages colder weather in the eastern 2/3rds of the country, while the negative phase encourages warmth. The WPO (West Pacific Oscillation) closely follows those guidelines as well.
As noted earlier, we see the forecasted PNA rising to at least neutral values in the long range near the start of 2016.

If all of these forecasted indices and teleconnections verify on an as-is basis (which, I may add, is rather unlikely), we could expect a cooler pattern in the Central and East US. This would be mitigated by the positive phases of the Arctic Oscillation (shown in the first image by negative height anomalies across the Arctic Circle) and North Atlantic Oscillation (shown in the first image by weak troughing over Greenland), both of which tend to discourage persistence of colder weather.

To summarize:

- There are indications that a pattern change may be on the way for the start of 2016.
- Model guidance will change drastically, and this is nowhere near set in stone. However, if this does verify to a certain degree, a cooler pattern may be on the horizon.


Stratospheric Polar Vortex Expected to Weaken in January

The stratospheric polar vortex is expected to exhibit weakening in the near future.

Garfinkel/Hartmann Publication
(Image obtained from Eric Webb)
The above image, from the Garfinkel / Hartmann publication, 'Tropospheric Precursors of Anomalous Northern Hemisphere Stratospheric Polar Vortices', shows precursors in the tropospheric mid-levels to weakening of the stratospheric polar vortex. In layman's terms, the images above (particularly the left panel) give us a glimpse at what the weather pattern should look like a short period of time before the stratospheric polar vortex weakens.

In that left panel, showing geopotential height anomalies at the 500-millibar level, we see strong negative anomalies (troughs / storm systems) in the Bering Sea extending into the Arctic Circle, even a bit into Greenland, and positive anomalies (ridges / high pressure) over Canada, as well as in the north-central Pacific. We also see ridging over Europe. These are indicators that the stratospheric polar vortex could be weakening down the road.

Let's compare these indicators to the long-range forecast from the GFS Ensembles.

Tropical Tidbits
Attached is the 500-millibar height anomaly prognostication for the 11-15 day forecast period (Dec. 30 to Jan. 4). Here, we see negative anomalies across the Bering Sea into Siberia, all the way across the Arctic Circle into Greenland. We also see weak ridging in Canada and the East US, and ridging in the north-central Pacific. Most notable is a strong ridge over Europe.

If you compare this forecast graphic and that left panel earlier in this post, you'll find that they're incredibly similar, almost identical. In sum, long range model guidance is telling us that we should see the stratospheric polar vortex weaken in January. This is good news for snow and cold fans who have been suffering through this first month of 'winter' (more like an extended fall with these warm temperatures)- even though a weakened polar vortex does not necessarily mean cold and snow, it does raise the chances of more cold air intrusions and snow events for North America.

To summarize:

- Long range model guidance is indicating the stratospheric polar vortex will weaken in January.
- Consequentially, the chances of cold and snow may be on the rise for next month.


Tuesday, December 1, 2015

Upper Stratosphere Experiencing Minor Warming

The upper stratosphere appears to be undergoing some minor warming, although it is unlikely to set up a polar vortex disruption event.

The above animation shows temperature anomalies at the 10-hPa level of the atmosphere, in the upper levels of the stratosphere, over the last month. Beginning around November 20th, we started to see increasing temperatures just east of Japan, which really amplified in strength around November 27th. They aren't really moving in a particular direction, although a slight northward push has been noted over the past couple weeks.

This warming is likely thanks in part to a substantial increase in north poleward eddy heat flux values since about the last two weeks of November, right at the time when we started to see that warming commence in the 10-hPa layer. Typically, increased eddy heat flux values reflect an increased movement of warm air from the lower latitudes to the poles. Higher values can precede or coincide with stratospheric warming events, which is likely why we're seeing warming at the 10-hPa and 30-hPa levels.

The 30-hPa level, just a bit higher than the middle of the stratosphere, is showing some warming, albeit deflected a bit further east than the 10-hPa warming. In addition, this 30-hPa warming is showing an eastward push, rather than a poleward push. The latter would be more favorable for a polar vortex disruption event / stratospheric warming, so winter weather fans may not be too encouraged to see this.

To summarize:

- Minor warming is being observed at the 10-hPa and 30-hPa levels of the stratosphere.
- This warming is not strengthening quickly and is not making a strong poleward movement, so a stratospheric warming event is not expected over the next several days.