Polar Vortex Collapses For Summer

Observed zonal winds on left
Hour 240 forecasted zonal winds on right

The polar vortex, the entity that ignited massive discussion earlier this winter, will be commencing its final collapse to enter its summer stage, and likely bringing an end to the unusually chilly spring in the long range.

Shown above is a combined image of observed zonal winds as of yesterday morning throughout the stratosphere (millibar levels shown on the left-hand side of the image), while the right image depicts zonal winds 10 days from now. Positive zonal wind values indicate the presence of a storm system (in this case, the polar vortex), while negative zonal wind values tell of an anticyclone (high pressure system) in the vicinity. The observed image of zonal wind values tells me that the polar vortex is weakly present from the 3 millibar level through the rest of the stratosphere. We see how the positive zonal wind anomalies extend well beyond the 100 millibar level that defines the border between the stratosphere and the troposphere. The forecast image shows that the positive zonal wind anomalies have been eradicated from the stratosphere, and it is this eradication that indicates the polar vortex has collapsed and entered its summer stage. When the polar vortex collapses in spring, high pressure takes over the stratosphere, ending the risk for sudden stratospheric warming events. In the fall, this high pressure system breaks down again and the polar vortex returns to re-ignite the risk of collapse in the winter, like we very nearly saw this past winter.

When the polar vortex takes its summer leave, we can anticipate the unusually chilly spring to end. The stratosphere was bubbling with anomalously warm areas of air in the upper stratosphere, meaning that the stratosphere was still in its winter stage. Because the stratosphere was in its winter stage, the lower atmosphere, including the troposphere was most likely still in a rather wintry stage. Because the stratosphere is getting into its summery phase, we can expect the surface weather to calm down as well.

Andrew

Late Season Snowstorm Targets Midwest

The GFS model is predicting a late season snowstorm will hit the Midwest and western Great Lakes regions over the next 5-10 days.

Shown above is the accumulated snowfall forecast from now until May 6th. We see a swath of accumulating snowfall stretching from the eastern half of Iowa into Wisconsin and the Upper Peninsula of Michigan. The amounts in this swath of snow are anywhere from 2 inches to nearly one foot, with the latter end of the spectrum centered over Iowa and the U.P. of Michigan. Accumulating snow even goes as far south as Missouri, with that state and Illinois getting in on amounts as high as 2 to as much as 4 inches in isolated spots.

I am incredibly skeptical of this forecast. The GFS model is known to have a cold bias when it comes to winter weather, and the impending pattern change does nothing to help forecast accuracy. In my opinion, if this scenario even works out, we would see a chilly night in IL/MO without any snow, and maybe some plowable snow into Iowa- certainly not at the level being predicted. I find it hard to believe that the GFS model has a viable case at this point in time.

Andrew

Nasty Squall Line Hits Texas on Wednesday

The Storm Prediction Center has outlined a small area of severe weather potential for Wednesday, May 1st. This outline encompasses south central Oklahoma into north central Texas.

The NAM model shows a rather small region of high instability over central Texas, extending into central Oklahoma. This image is valid for the afternoon of Wednesday. This stretch of high instability is slightly different from the SPC outlook in that the most able regions for thunderstorm development are just a hair east of the outlook proposed by the SPC, especially in Oklahoma. Thunderstorm development is expected to commence in the late afternoon hours after the layer of stability erodes. This image shows surface-based instability, which is essential for thunderstorms to get going. However, in order for thunderstorms to start forming, the layer of stability must be eroded. The stability is seen in the shaded regions. When you have high instability with no blue shading, thunderstorms are most readily able to form.

Projected radar reflectivity for Wednesday evening shows the development of a nasty squall line stretching from Oklahoma into Texas. The greatest threat of severe weather then appears to be damaging winds and large hail. The lack of strong helicity in the vicinity of the highest instability tells me that the tornado risk will be rather minimized. Regardless of the biggest threat, it's certainly looking like a severe weather event will unfold on Wednesday.

Andrew

Summerlike Warmth Just Days Away

The first blast of summerlike warmth appears to be just a couple days away, as Old Man Winter is beat back for the first time in this unusually cold spring.

Maximum Tuesday Temperatures

The short range NAM model is projecting temperatures to soar past the 80 degree mark as far north as Chicago, IL and the Quad Cities in Iowa. This model forecast has been rather consistent with extremely warm temperatures stretching as far north as those two cities, although in this forecast the warmth appears to have been slightly dialed back. Temperatures in western Texas could flirt with the 100 degree mark, and the thermometer will situate itself well into the 80s and 90s across multiple Plains states on Tuesday. Unusually warm temperatures will continue out to the east into the Ohio Valley and even poking into Canada.

Maximum Wednesday Temperatures

The cold front will show its full force on Wednesday afternoon, but the warmth ahead of this frontal boundary will continue to show its might. We see a large swath of 80s from Texas as far north as Michigan. The warm sector shoots well into eastern Canada, where 60s and 70s prevail. It looks like almost every state in the central and eastern US will be able to achieve temperatures as high as the 60s on the low end and 80s on the high end. Once again, western Texas will see temperatures close to 100, but not at the three-digit benchmark.

Andrew

Severe Weather Targets Plains This Evening

A slight risk of severe weather has been issued by the Storm Prediction Center for today, April 27th. This risk covers the states of Texas, Louisiana, Arkansas, Mississippi, Alabama and Tennessee.

Current frontal analysis shows the storm system of interest currently crossing over into western Arkansas. Warm front extends through the middle of Arkansas and cuts through central Mississippi, while a trailing cold front is draped across Texas. High precipitable water values shown in green indicate the warm front is doing its job in transporting energy and warmth up from the Gulf of Mexico, adding fuel to the fire that will start this evening in the states mentioned above. The storm system appears to be pushing northeast, and this should allow the warm front to pull the warmth and moisture north to encompass the rest of the states expected to be under a threat for severe weather tonight.

Short range NAM model predicts surface-based instability to be in excess of 3000 j/kg in some spots, with little to no capping inversion in the slight risk area. Thunderstorm formation is anticipated to be consistent with a damaging wind/hail risk. Elevated helicity values combined with this excessive instability should allow for an isolated tornado threat, especially in eastern Texas into southeast Arkansas. Relatively lower instability values will be present towards the easternmost portion of the slight risk area in Tennessee. I believe that we will see thunderstorms fire in that area, although the comparison of Tennessee's instability and the 3000 j/kg in Louisiana tells me the storms in TN will not be as significant as those in the Gulf Coast states.

At the present time... latest upper air soundings reveal a fairly substantial capping inversion stretching from the Gulf Coast into Louisiana and Mississippi. Not concerned with this development, as lower level stability is normal in the mornings. The sounds analyzed with that lower level layer of stability were mainly outside of the 3000 j/kg belt shown above, and the one sounding within that belt had a rather weak stability layer. Upper and mid level wind fields have a streak of slightly enhanced winds on the south flank of the storm system, and these winds should fall along the slight risk area later on this evening.

Storm Action Day status: A Storm Action Day will NOT be declared today.

Andrew