Tuesday, April 29, 2014

Analysis of the Tupelo Tornado on April 28, 2014

On April 28th, 2014, a supercell moving northeast across northern Mississippi became tornadic, and exhibited multiple signals of a large, violent tornado before it struck Tupelo, MS.

At approximately 2:44 PM Central Time/3:44 PM Eastern Time, radar indicated a supercell with a defined hook echo signature located in the immediate vicinity of Tupelo, as indicated above. This supercell was exhibiting a tornado debris signature (TDS) at the time it struck Tupelo, indicating that a tornado was already on the ground and doing damage. The already-dynamic environment, which later warranted a rare 'High Risk' of severe weather designation by the Storm Prediction Center, provided a sustaining mechanism for this tornadic supercell which eventually did strike Tupelo.

At approximately 2:54 PM Central Time, or 3:54 PM Eastern Time, the supercell had now moved past Tupelo. Radar imagery indicated a much more defined TDS, culminating into the appearance of a debris ball, in which a tornado is present and is throwing debris high up into the air, which then appears on radar. Note that the debris ball is significantly more defined at 3:54 PM in comparison to the previous radar scan at 3:44 PM. This development, combined with further analysis which detailed a more specific track, eventually led to the realization that this tornado had directly hit the city of Tupelo, Mississippi.

The correlation coefficient, a tool of dual-polarization radar, also indicated the presence of a debris ball. Using two methods of scanning from the radar tower, the radar is able to pick up the likelihood of a debris ball. When looking at a correlation coefficient (CC) chart above, one would want to observe values below 0.800 or so (seen as blues on this chart) to identify a potential debris ball. As this CC image from 3:54 PM ET shows, the debris ball is clearly evident in the white circle, seen as a deep blue dot. This nearly confirmed the possibility that this tornadic supercell had struck Tupelo, producing significant damage.

According to the governor of Mississippi, 131 homes and numerous businesses had been damaged in Tupelo as a result of this tornado. Dozens of injuries were reported as a result of this storm, but none were told as life-threatening.

Additional analyses of other tornadic supercells will be released in coming days.


Saturday, April 26, 2014

Significant Tornado Outbreak Expected Sunday, Monday

A significant tornado outbreak is expected on Sunday and Monday, with strong to violent tornadoes expected on these two days.

The Storm Prediction Center has issued a moderate risk of severe weather for northeast Texas, northern Louisiana, eastern Oklahoma, southern Missouri, and most of the state of Arkansas. This moderate risk comes as a strong upper level low brings moisture and other destabilizing factors up to the Plains, likely resulting in one of the more significant tornado events in recent memory. The Storm Prediction Center has indicated that long-tracked and strong tornadoes are possible if this event comes to fruition, and an upgrade to the agency's highest risk level, the High Risk, will come tomorrow if model guidance continues to hone in on this severe weather event.

The image above shows the East WRF-ARW model forecast for maximum updraft helicity over its entire 48 hour forecast period. The idea of highlighting the maximum updraft helicity values is to identify where potentially tornadic supercells may strike for this severe weather event. A look at this image, showing maximum updraft helicity for the Sunday event, gives a clear indication of what we could be facing with this situation. This particular model shows multiple long-tracked, rotating supercells moving northeast from Texas into Arkansas. Based on the high helicity values, it looks like some of the most intense cells may be able to put down some strong tornadoes, but it will take another round of model guidance to confirm or reject this idea.

A look at the Storm Prediction Center's SREF (short range ensemble model guidance) projection for the probability of strong tornadoes only worsens this threat. We see the chance of a significant tornado at or above 75% in Arkansas, but there's a very small speck in the middle of Arkansas, where it seems to increase another contour level. The next contour level above 75% is 90%, meaning that this model run indicates there is a 90% chance of a significant tornado in central Arkansas. Predicting tornadoes is a very difficult endeavor, but this graphic gives us an idea of where the worst tornado threat may be.

On Monday, we see another moderate risk area, now located just to the east over northeastern Louisiana, much of Mississippi, and western Tennessee. This moderate risk area also has the potential to be upgraded to a High Risk, as many of the same dynamics that look to produce a potentially deadly environment on Sunday will be in place again on Monday. If you or someone you know resides in the moderate risk area for either Sunday or Monday, please alert them to this severe weather. This outbreak may be one of the more significant ones of the last few years, and should be dealt with with extreme caution.

The SPC SREF projection for the probability of significant tornadoes is also startlingly high for the Monday evening event, as the graphic above shows. We see the highest probabilities placed in far northeastern Mississippi into northwest Alabama, located a bit east of the current Monday moderate risk area. While this forecast may see slight adjustments in the near future, the trend of a significant tornado event on Sunday and into Monday remains clear.

To summarize:
• A significant tornado event is expected Sunday and Monday.
• Long-tracked, violent tornadoes may occur on either of those days.
• Anyone in either of the moderate risk areas on Sunday or Monday should immediately prepare for potentially extreme weather, which may threaten lives and property.


Friday, April 25, 2014

April 26-30 Severe Weather Outbreak: Monday Outlook

This forecast is solely for the severe weather on Monday, April 28th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center has outlined a rather large area of potential severe weather to kick off the workweek on Monday, April 28th. This is a continuation of the severe weather events on Saturday and Sunday, as a strong upper level low continues to ravage the country. When the SPC issues these sorts of long range outlooks, they highlight areas with a 30% chance or more of observing severe weather within 25 miles of any given point. This means that any of the areas highlighted in Arkansas, Louisiana, Mississippi, Alabama, Tennessee, Kentucky, Indiana, Illinois and Missouri may see some formidable severe weather, with a lesser-but-still-present severe weather threat surrounding this circular shape.

A look at the projected 500mb isobars and wind speeds for late Monday evening continues to exemplify why this event will be a multi-day, potentially significant severe weather outbreak. We see a long strand of anomalously high wind speeds extending from Oregon, rounding the closed low and punching into the area we're watching for Monday's severe weather risk around Missouri and Illinois. Although the closed low does not have a negative tilt that could then intensity the situation further, it does look like this piece of the Pacific jet stream will be what keeps the severe weather going, even though the closed low won't be doing much, other than continuing to pull moisture up from the Gulf of Mexico.

A look at 700mb winds, just a few thousand feet off the ground for the same time as the 500mb image shows strong lower-level winds over the area we're watching for the severe weather threat Monday evening. Considering the streak of 40 to 60 knot winds in the Midwest and southern Plains will only continue to intensity during Monday evening, as it is the nocturnal Lower Level Jet stream (LLJ), the severe weather threat will likely continue as well, at least through Monday evening.

A look at projected dewpoints and the Lifted Index for Monday evening really emphasizes the moisture this system has to work with. We see the projected dewpoints in shaded colors, and the dark blues represent the highest dewpoints, and thus some pretty muggy air. This all bodes well for severe weather potential, which is confirmed by the very low Lifted Index values, some as low as -9. The Lifted Index, also called the LI, is a measure of buoyancy of the air- in other words, when the LI is negative, air tends to rise and thus create thunderstorms. When the LI is positive, it indicates a stable atmosphere, suppressing thunderstorms. Seeing numbers close to double-digit negatives tells me this closed low will be keeping the severe weather fireworks going well into the workweek.

To summarize:
• A potentially significant severe weather event is possible on Monday.
• Those in the outlined area may want to review severe weather preparations and watch for further updates on this situation.

Other posts pertaining to the April 26-30 Severe Weather Outbreak:
• Saturday Severe Weather Outlook: Click Here 
• Sunday Severe Weather Outlook: Click Here
• Tuesday Severe Weather Outlook: Click Here (Coming soon)


April 26-30 Severe Weather Outbreak: Sunday Outlook

This forecast is solely for the severe weather on Sunday, April 27th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center has issued a severe weather outlook for Sunday, April 27th, indicating the continuance of Saturday's severe weather. In the graphical representation of the severe weather risk above, we see there is a large 'slight risk' of severe weather covering Nebraska, Kansas, Oklahoma, Texas, Louisiana, Mississippi, Tennessee, Kentucky, Arkansas, Indiana, Iowa and Missouri. We also see a much more alarming 'Moderate risk' of severe weather, covering extreme northeast Texas, southwest Arkansas, northwest Louisiana and southeast Oklahoma. This moderate risk is particularly alarming, as it appears to be only the 9th time in Storm Prediction Center history that a moderate risk has been issued two days away from the predicted event. 

The image above shows the Storm Prediction Center's graphical representation of the percentage chance of severe weather within 25 miles of any given point. We see the 'slight risk' area as defined by the 15% chance swath, with an enhanced severe weather chance in the 30% red region. But it's the 'moderate risk' area that wins the contest, clocking in at a 45% chance of severe weather within 25 miles of any point. Putting that in simpler terms, you could flip a coin, and there's just about a 50/50 chance of severe weather within 25 miles of that purple area. The black hatched area signifies a 10% or greater chance of significant severe weather (i.e. very large hail, very strong winds or strong tornadoes).

The image above shows isobars (contour lines) and wind speeds (colored) at the 500mb level of the atmosphere on Sunday evening. Here, we see our strong trough that we went over in the Saturday Outlook post has now become a closed low, as seen by the closed isobars in Kansas and Nebraska. This indicates that the storm has passed maturity, and is now on the path to weakening. Despite this, a screaming mid-level Pacific jet stream, clocking in at nearly 100 knots, will be feeding this severe weather threat for days to come. We see the closed low wrapping those strong mid-level winds into the warm sector, enhanced right over that meeting point of Texas, Arkansas and Oklahoma, and right over the Moderate Risk outline. These strong winds indicate the enhanced risk of severe weather.

If we look at wind speeds at the 700mb level, just a few thousand feet off the ground, we see wind speeds of close to 60 knots enhanced over our severe weather area, and especially over that Moderate Risk outline of Texas, Arkansas and Oklahoma. Seeing as this 700mb wind component is also known as the lower level jet stream (LLJ), and is a nocturnal jet stream, we can expect these winds to sustain themselves as Sunday wears on, thus continuing the severe weather threat. These high wind speeds combined with the high mid-level winds we just went over make me more and more concerned for Sunday.

According to model guidance, there is every right to be concerned. The image above shows the forecasted supercell composite for Sunday night. In other words, the higher colors on the legend indicate the higher likelihood for a supercell, which is defined as a rotating thunderstorm. Seeing as the supercell composite is enhanced right over that moderate risk area again, it's clear that this focal point of Louisiana, Arkansas, Texas and Oklahoma will be the bullseye for Sunday's severe weather threat.

Interestingly enough, the forecasted significant tornado composite, which is an index that can help determine the likelihood of tornadoes in a given area, goes off the scale right over the Moderate Risk area, but in my opinion, it's best to wait another day or so before jumping on this train of violent tornadoes (though I will agree that strong tornadoes are possible here).

To summarize:
• A potentially significant severe weather event is expected to occur Sunday.
• A Moderate Risk has been issued for Oklahoma, Texas, Louisiana, and Arkansas.
• Those in the Moderate Risk area may want to prepare for potentially strong tornadoes on this day.

Other posts pertaining to the April 26-30 Severe Weather Outbreak:

• Saturday Severe Weather Outlook: Click Here
• Monday Severe Weather Outlook: Click Here
• Tuesday Severe Weather Outlook: Click Here (Coming soon)

Links labeled Coming Soon will be out in the next few hours.


April 26-30 Severe Weather Outbreak: Saturday Forecast

This forecast is solely for the severe weather on Saturday, April 26th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center is highlighting an enhanced severe weather risk over the Southern Plains on Saturday, April 26th. The image above shows a graphical representation of the SPC's thoughts for Saturday's severe threat, with the percentages showing the likelihood of severe weather within 25 miles of any given point. In this graphic, we see that the states of Nebraska, South Dakota, Iowa, Kansas, Oklahoma, Missouri and Texas are in the general 'slight risk' categorical severe weather outlook, indicated by the yellow 15% shading, while south-central Kansas, central Oklahoma and north Texas are in a 30% swath of enhanced severe weather chances. We also see the black hatched denotation, meaning areas within that have a 10% or greater chance of seeing significant severe weather within 25 miles of any point (think extremely large hail, very high winds, and strong tornadoes instead of "tamer" severe weather). Let's go over how this will evolve.

The projected surface analysis on Saturday evening from the Weather Prediction Center shows our strong storm located in northeast Colorado, with a warm front extending through the Central Plains and snaking south into Arkansas and Louisiana. We then identify our cold front way back west in Colorado and New Mexico, but we also see an orange boundary in western Texas, Oklahoma and Kansas. What is that boundary? That is a dryline. In the same sense that warm and cold frontal boundaries separate warm and cold air, a dryline boundary separates dry and moist air, sometimes creating a substantial temperature gradient in the process. Drylines are the source of numerous storms in the Plains, and look to be the source for Saturday's severe weather, as well.

On Saturday evening, we will begin seeing a strong trough (storm system) in the Southwest begin to attain a negative tilt. A negative tilt is visually seen as the isobars trying to 'push' towards the south-east direction, indicating the storm system has reached maturity, and we see that phenomenon occurring in the 500mb vorticity forecast map above. This negative tilt looks to be attained in the late evening hours of Saturday, which brings up some timing issues for this severe weather event on Saturday, which we will discuss more next. The general thing to take away here, though, is that this will be a very strong storm system surging eastward to kick off this multi-day significant severe weather outbreak.

There does look to be some timing issues on Saturday, not just with respect to how fast the storm can acquire a negative tilt, but also with the presence of a capping inversion Saturday evening. Shown above is the Saturday evening forecast for instability, marked in j/kg by contour lines, and stability, shown by the presence of blue shading; the darker the blue shading, the higher the stability. Note that instability means air can rise because the air at the surface is warmer than the air above the surface, and stability means air cannot rise, as the air above the surface is warmer than the air at the surface. If we recall that air can only rise if the surrounding air is colder than the surface, the presence of stability means thunderstorms cannot form. In this forecast image, we see that there is a lot of projected instability over the Southern Plains, over 3000 j/kg in some places, which is a very high amount of instability. In some spots, we see white, which means no capping inversion, but across the remainder of the SPC-outlined severe weather risk, we see a significant capping inversion in place, as the dark shading shows. I'm worried that this cap will be too strong to break through, and the trough will be too slow attaining that negative tilt and moving closer to the Southern Plains leading to a bust in the forecast.

Let's hypothesize for a moment that storms do form on Saturday. A weather model named the WRF-ARW can predict the maximum updraft helicity for the entire 48 hours it forecasts for. In this instance, it means we can see how strong the storms that form rotate and spin from the morning of April 25th to the morning of April 27th, encompassing Saturday's severe weather in the process. We see on this image that the WRF-ARW model projects multiple tornadic storms to develop across the Central and Southern Plains, with two notably strong supercells violently rotating from southwest Oklahoma to the central portion of that state, and another from western to eastern Kansas. So while the environment may not be too favorable right now, any storms that do form and can sustain themselves look to be rather significant.

To summarize:
• A potentially significant severe weather event may occur on Saturday.
• The Central and Southern Plains would be affected.
• Model guidance is forecasting an inhospitable environment for storms, which may greatly hamper the severe weather threat.
• Any storms that do form have the potential to be significant and potentially tornadic.

Other posts pertaining to the April 26-30 Severe Weather Outbreak:

• Sunday Severe Weather Outlook: Click Here
• Monday Severe Weather Outlook: Click Here
• Tuesday Severe Weather Outlook: Click Here (Coming soon)

Links labeled Coming Soon will be out in the next few hours.


Wednesday, April 23, 2014

Long Range Forecast Update for Late April, Early May

This is an update to a previous long range outlook concerning the expected weather for late April and into early May.

Tropical Tidbits
For the last days of April and in to early May, I'm still looking at above normal temperatures for a decent portion of the nation. Shown above is the 500mb height anomaly pressure forecast over the West Pacific for April 26th. In this image, we see a strong ridge of high pressure stationed over Japan and East Asia, permitting the flow of above normal temperatures into the area. This event has significant implications on our weather here in the US. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. So, if we use that rule, we can expect warmth over the United States around the May 1-5 period, if not for a longer time period than that. 

Tropical Tidbits
By the end of the first week of May, I anticipate that we will see another cool-down for a few days as a closed low looks to move into Japan. Since this 500mb anomaly image is valid for April 29th, we can expect this cool-down to start showing itself sometime around the May 4-8 time period, though the timeframe could probably be extended to May 4-10 as the closed low sticks around Japan for a day or two more. Despite how long that timeframe may seem, the actual cold weather should only be around for a few days, not for this whole 6-day timeframe.

Tropical Tidbits
By the time we get to the second week of May, we're looking at another bout of sustained warmth, as the GFS ensembles bring about a ridge of high pressure over Japan and the waters surrounding the island nation. Ensemble guidance looks to have rather low confidence in this event, though we should get a more accurate look at this portion of the forecast in the next few days. 

To summarize:
• I am still forecasting warmer than normal weather for the last few days of April into very early May.
• Cooler weather looks to take hold near the end of the first week of May.
• Warm weather may return for the second week of May.

One final note, I received a multitude of arrogant responses to the previous long range outlook I had posted. Remember that every comment is moderated before it's published, and arrogant comments like the ones I've been getting for the last few days will not be published.


**Significant Severe Weather Outbreak Expected This Weekend**

A significant severe weather outbreak, possibly including strong to violent tornadoes, is expected Saturday, Sunday and Monday.

The Storm Prediction Center has outlined three days of potential severe weather in their long range outlook. In these outlooks, the SPC will mark regions it believes are at risk for enhanced severe weather. Typically, one or two days may be outlined at a time, but it is uncommon (though not unheard of) to see three different areas outlined, especially considering how much area they cover. By SPC estimates, over 47 million people may be affected by this potential severe weather outbreak.

The wording by the Storm Prediction Center is very strong, as this excerpt below shows:

Jet Stream Forecast for Saturday
The whole situation looks to unfold as a strong trough digs into the Southwest United States, developing a negative tilt in the process. A negative tilt, commonly seen how the isobars seem to push towards the southeast like you see above over Arizona, indicates the maturity of the trough in question, and thus means weather of any kind is generally stronger than it would be if the storm were not fully mature yet, or if it was beginning to occlude and weaken. In this forecast of the jet stream, valid Saturday night, we see a strong Pacific jet stream powering the trough as it digs east, and the divergence over the South Central Plains, shown as how the two jet streams (subtropical jet stream, seen along the bottom of the image, and the Pacific jet stream) separate just west of Texas. Divergence in this sense means the air is rising in monumental fashion, also a big signal for severe weather potential.

Lifted Index and Dewpoints over the Southern Plains, valid Saturday
The Lifted Index (LI), a measurement of how unstable the atmosphere is, looks to be plummeting as low as -11 or -12, indicating extreme amounts of instability. Worse yet, the sharp gradient in dewpoints (shown by the shaded colors) indicates the presence of a dryline, literally meaning a line where the air is dry to the west and very humid to the east. These drylines almost always originate and stick around in the Plains, and are a big reason why we see major severe weather down there. I have little doubt based on the projected Lifted Index that if/when a storm does break the capping inversion predicted to be in place (a capping inversion hinders thunderstorm development, and is the opposite of instability), we may very well see violent thunderstorms capable of strong tornadoes and potentially extreme severe weather. The rhetoric here is elevated, but there is quite a bit of concern here on how this will evolve.

By Sunday, the trough looks to continue to mature and eventually close off, as the full-circle contours in the 500mb wind speed image above show. That negative tilt is also evident, as we see the contours tilting towards the south-east direction. In response to this strong trough, as well as the intense Pacific jet stream we discussed above which is also evident in the image above, the mid-level jet stream will be absolutely howling over the Plains, right over the area projected to be hit on Sunday. We could see mid-level winds of over 80 knots, an astounding strength for this event.

Projected Energy-Helicity Index (EHI) values look to be heading towards double-digit territory on Saturday, a major concern for tornadic activity that evening. The EHI combines instability and helicity (spinning) to make an index that tries to predict situations where tornadoes are likely to occur. Values as elevated as the ones shown above confirm my concern for this event, which won't be just one day, but possibly three days of continuous severe weather problems.

To summarize:
• A potentially significant severe weather event is expected Saturday, Sunday and Monday.
• Potentially strong tornadoes are anticipated Saturday evening.
• Risks beyond Saturday look to focus more on other modes of severe weather.
• Those in the risk areas outlined by the SPC should review severe weather guidelines and preparations.


Sunday, April 20, 2014

Spring, Summerlike Warmth Expected in Late April, May

I'm expecting the end of April and beginning of May to have seasonal to above-normal warmth across much of the country.

Tropical Tidbits
Shown above is the 500mb height anomaly pressure forecast over the West Pacific for April 26th. In this image, we see a strong ridge of high pressure stationed over Japan and East Asia, permitting the flow of above normal temperatures into the area. This event has significant implications on our weather here in the US. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. So, if we use that rule, we can expect warmth over the United States around the May 1-5 period, if not for a longer time period than that.

The factors for warmth don't stop there- it goes far deeper, literally and metaphorically.

The image above may look complicated, but isn't that complex when explained thoroughly. Shown above is a graphical representation of a phenomenon called the Walker Circulation, also known as the Walker Cell. The Walker Circulation is most prominent in a La Nina situation, and forms when you have cool waters towards the Eastern Pacific, and warm waters near Oceania and Australia. The trade winds across the Pacific go from east to west in the Walker Circulation on the surface, due to the presence of high pressure in the East Pacific and low pressure in the West Pacific (think of it like a ball rolling downhill from the top of the hill (high pressure in the east Pacific) to the bottom of the hill (low pressure in the West Pacific), where the ball signifies the trade winds). When the trade winds reach the low pressure area in the western Pacific, the winds converge and begin pushing up into the atmosphere, creating thunderstorms. The air from the thunderstorms shoots up to roughly the 250mb to 200mb level before the winds spread out, as the air cannot rise further. From there, the air at 200mb flows from west-to-east, opposite the direction of the surface winds. Then, the high pressure and cool/dry atmosphere in the East Pacific pulls that air down back to the surface to start the whole process over again. As I said, this typically appears in La Nina situations, as cool waters in the East Pacific that are required for the Walker Circulation typically arise in conjunction with the La Nina.

The image above shows a view of surface pressure and surface wind vectors across the tropical regions of the world. We can use this image to identify the presence, or lack thereof, of the Walker Circulation. Taking a glance at surface pressures and winds from April 6th to April 10th, we do see surface winds flowing from east-to-west, a trait typically observed with the Walker Cell, and the low pressure center in Oceania also shows a characteristic of that circulation. This is rather odd, as we have been expecting an El Nino to arise, but the Walker Cell appearance here tells me that the El Nino has not hit the atmosphere yet, though it may start appearing in the surface temperatures. In recent days, the surface winds have been showing signs of weakening, indicating that the Walker Circulation may be weakening, which would herald the arrival of an El Nino if the winds reverse to a west-to-east stature.

For all intents and purposes, let's say that the Walker Cell's presence in the Pacific still indicates the presence of a La Nina (which is true in respect to the Walker Cell actually appearing, even though sea surface temperatures may argue otherwise). We can correlate sea surface temperatures in the Pacific to surface temperatures here in the United States. In the image above, the warm colors indicate a positive correlation of SSTs to surface temperatures (for example, warm Pacific SSTs would then correlate to warm temperatures in the US, or cool Pacific SSTs would lead to cool temperatures in the US). Blues in the above image then depict a negative correlation, where warm Pacific SSTs would lead to cool surface temperatures, and vice versa. In this case, with negative correlation signals across the US above, we can determine that below normal SSTs in the Pacific (La Nina) can then lead to above normal surface temperatures in the US, and vice versa. Thus, even though the SSTs in the Pacific indicate an El Nino, the atmospheric pattern is more supportive of a La Nina, which would then support warmer than normal conditions in the April-May-June timeframe. Despite this, since the Walker Circulation may be fading, this portion may just be a moot point in just a few weeks time.

This image here actually is pretty complicated, but I'll do my best to explain it. This graphic shows the layout of the Global Wind Oscillation (GWO), and what factors indicate the presence of the GWO in one of a possible eight phases. For example, if the GWO is to be in Phase 1, we would look out for negative mountain torque (MT) values, as well as enhanced tropical convection in the Indian Ocean. An additional description of the GWO comes from the ESRL, as shown in the screenshot below.

I outlined the Stage 1 description, as not only does it fit in with the current conditions that we're experiencing, but it is indicative of a La Nina, like we talked about earlier in this post. How do we know for sure that we're in the Phases 1-3 of the GWO (which are traditionally La Nina-esque phases)? Let's take a look at the indicators that depict Phases 1 through 3 of the GWO.

Observed Mountain Torque

Observed Relative AAM
If we look at the first GWO image we discussed, as well as the screenshot description of the GWO, we can see what defines these key phases. We see that negative mountain torque and convection in the Indian Ocean is a key definition of these first three GWO phases. Looking at that observed MT chart shown above, we see that the net MT is now below average, as the black solid line shows. While the convection in the Pacific is more displaced in Oceania instead of the Indian Ocean, the description highlights that the relative AAM is negative, and that relative AAM image above shows a negative anomaly. Overall, this does indicate the La Nina phases, also known as GWO phases 1-3, strengthening that correlation image, as well as the Walker Circulation discussion. Additionally, in the same sense that positive East Asian mountain torque can lead to cooler than normal temperatures in the US, the current negative East Asian mountain torque anomaly in the graphic above (red line two images above) indicates warm weather is more likely.

Importantly, in that screenshot description image, we see that the La Nina-like stages indicate a trough in the Western US. Typically, when we see a trough/stormy weather in the West US, we get a consequential ridge of high pressure in the East US, which is a textbook negative PNA pattern.

The image above shows observed sunspots in accordance with the 30-day sunspot cycle. As I've mentioned on this blog before, there is a known inverse correlation between the 30-day sunspot cycle and the Pacific-North American index (PNA). When the 30 day sunspot number is anomalously high, we tend to see the PNA in its negative phase. In the same sense, when the 30 day sunspot number is anomalously low, the PNA tends to reside more in its positive phase. Composite images of both the positive PNA and negative PNA are shown above, courtesy of NCSU.

500mb height anomalies of a negative PNA on the left (warm colors= high pressure, cool colors = low pressure)
Surface temperature anomalies of a negative PNA on the right

500mb height anomalies of a positive PNA on the left
Surface temperature anomalies of a positive PNA on the right
If we look back at the sunspot image, we see that the 30 day sunspot cycle is just off the charts, indicating a very high number of sunspots, or at the very least, elevated solar activity. If we use our solar correlation with the PNA, we may be expecting the emergence of a negative PNA regime in the near future. If we look at the composite image above for a negative PNA, we see that warm weather tends to prevail in that sort of situation. This fits in well with the GWO description we went over earlier, the presence of the La Nina Walker Circulation, and the East Asian correlation tool.

The last piece to the puzzle is the presence of enhanced tropical convection over Oceania.

The image above gives a global view of rising air in conjunction with enhanced tropical convection (blue shading), also known as negative Outgoing Longwave Radiation (OLR), as well as sinking air, exemplified by the beige shadings, which show suppressed tropical convection. The green contours highlight divergence, or air rising and spreading out due to thunderstorms, while red contours depict sinking of air. We see that there is an anomalous swath of tropical convection over Oceania, shown by the blues/tropical convection, and the wind vectors at 200mb going away from that convection. The placement of this tropical convection fits into a certain phase of the Madden-Julian Oscillation (MJO).

If we consider that the current center of the tropical convection is around the 165 East longitude line or 170 East line, we can see which MJO phase is best represented here. It looks like the MJO is currently in Phase 6 or 7, as the line on the 165 East shows splitting through the two panels on the right that are circled.

What does a Phase 6 MJO mean in the month of April?

The Phase 6 MJO looks like a typical negative PNA regime, with low pressure anomalies along the West Coast, and strong high pressure/warm weather in the Central and East US. This supports the overall warm weather idea, only adding to the rest of the factors supporting a warm weather regime in the late April and early May timeframes.

Let's summarize all of this.

• Based on a number of factors, warmer than normal weather is expected for the latter part of April, potentially well into the beginning of May.


Saturday, April 19, 2014

April 23rd Potential Severe Weather Event

The Storm Prediction Center has outlined an area of enhanced severe weather potential on April 23rd, as the severe weather season finally begins to show signs of activity.

The SPC is discussing the presence of a corridor of instability aligned just east of a dryline set up along the western Central Plains, and it is this corridor of instability that is outlined in the graphic above. In these dryline situations, it is not uncommon to see isolated storm cells fire up, which then enhance the likelihood of tornadoes and overall elevated severe weather. If enough individualized storm cells form, the cells can congeal into a nasty squall line, or may just remain as individualized cells, which would maintain a raised tornado threat.

Model guidance shows this narrow section of instability, as the GFS model graphic above shows. However, under the roughly-15000 j/kg of instability (indicating the atmosphere is decently unstable), we see a hatched gray area. This indicates the presence of a capping inversion, which just means there is warm air above the surface that is hindering the development of thunderstorms. An analysis of a sounding chart confirms a layer of warm air in the first few thousand feet above the surface. The concern here is that this capping inversion may prevent storms from forming at all, which could just wipe out the severe threat. In this case, there is a severe weather threat if storms form; the question is just if these storms will form.


Friday, April 18, 2014

Long Range Forecast for Late April, Early May

Let's examine the long range forecast for late April into the beginning days of May.

Long range analog guidance from the ESRL/PSD division, a special physics-based meteorology branch of the government weather service, indicates we will see troughing setting up in Western North America in late April as a strong upper level low drops into the Southwest, provoking high pressure out ahead of it in the Central and East US. This ridge out ahead of the upper level low will likely make for anomalously warm weather across the aforementioned sections of the country, a real treat in the face of such a nasty winter.

Beyond the last days of April, it is expected that the weather will take on a slightly cooler tone. In the wake of a Kelvin Wave currently pushing across the Pacific, enhanced tropical convection is expected to develop near the 60E Longitude demarcation, a classic Phase 1 MJO signal. When we see enhanced tropical convection in this Phase 1 signal, it typically means we can anticipate cooler than normal weather here in the United States.

I am a bit skeptical of this cold weather forecast, due to the response we're looking to see in East Asia around April 26th. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. In this image above, we see projected tropopause pressures, vector winds, and wind speeds way up in the middle-upper regions of the troposphere into the stratosphere. If we look to this forecast image, valid April 26th, and find Japan in the top left corner of the image, we can make out a bulge of orange pushing towards the center of this image. That orange bulge signifies the presence of a Rossby Wave. In simple terms, this Rossby Wave will 'break' over Japan and initiate an intensive warming spell. This may continue for some time, but if it does happen in late April, we would likely see the cooling effects of the Phase 1 MJO hurt, as this East Asian development would likely overrule it.

To summarize:
• A warm end to April is expected.
• A cool start to May is possible, but there are hints that the late April warmth may just carry over into May. More time is needed to investigate this potential.
• A severe weather event is possible in the final 7 days of April, due to the upper level low in the West US.