Wednesday, January 23, 2013

January 29 Severe Weather Event

The Storm Prediction Center has outlined a risk of severe thunderstorms for January 29, 2013.

Before we get into this, it is worth noting that outlining severe weather risk areas this far out is very rare. The Storm Prediction Center will only do such an outline if confidence is high that a severe weather event (possibly significant) could occur. Some of the worst severe weather outbreaks have been predicted at this same forecast period of 6 days away (although it days Day 7 (D7), it is actually 6 days away because Day 1 is referred to as the present day). I'm not saying this will be an incredible weather event, but such a long range forecast certainly says something.

We'll start off with the general synopsis. A strong, positively tilted storm system will be pushing east after coming into contact with the Southwest. The phrase 'positive tilt' refers to the highest vorticity values being positioned towards the southwest. This is good news, as negatively tilted storms (highest vorticity pointing towards the southeast) are known to be more volatile as far as severe weather goes. Nevertheless, this storm system will be pushing east towards Texas. This image is valid for the evening of January 29, which is when we expect this severe weather event to be ongoing. Seeing as how high pressure is in place over the Southeast, I would not be surprised to see the storm system take on a neutral (vorticity maximums pointing to the south) or even negative tilt as it rolls up the hill of high pressure. This could cause more severe weather chances for states in the Southeast in the future, but that remains to be determined. One bone I have to pick with this forecast is I am not comfortable with the storm system being so far from the predicted severe weather zone. I can see a band of vorticity readings in southeast Oklahoma that would attempt to provoke severe weather, but in order to see this risk verify, I would like to see the system further east, closer to the severe weather zone.

Upper level winds show great strain being inflicted upon the jet stream for the same forecast timeframe of the late hours of January 29th. The storm system is clearly shown by the blade of quiet winds in the Northern Plains into the Rockies. Again, you can see the positioning of this 'blade' depicts a positively-tilted storm system. Winds reaching above 120 knots are forecasted across much of Texas and northern Mexico in response to this great strain, not unlike a rubber band being stretched. As the system moves east, the jet stream should be provoked to tilt this system to a more neutral angle as high pressure tries to push in on the West Coast. Anyhow, strong upper level winds exist in Arkansas at this time, the predicted zone of severe weather. However, again, the strongest winds are centered further west in Texas, making me revisit the question of whether this risk should be moved further west in accordance with more favorable upper level winds in that area.

But all that doubt in the placement of the severe weather risk area subsides when you take a look at the 700 millibar forecast map for the same timeframe as the above two maps. This forecast shows wind speeds in the lower levels of the atmosphere, in the domain of the nocturnal lower level jet stream (LLJ). The LLJ is a well-known instigator of severe weather and even tornadoes overnight. So if you're wondering how the atmosphere can sustain that energy when the sun has gone down, there's your answer. There are poised to be strong winds in the lower level jet stream over eastern Kansas and parts of Oklahoma and Arkansas. This does ease many fears about the Storm Prediction Center being wrong in their placement of the severe weather risk, but I'm persuaded to believe that the risk should be pushed a tad west, more in line with west AR, east OK and east KS for the maximum risk areas.

Finally, we take a look at deep layer shear, or DLS values for the same evening of January 29th timeframe. The higher shearing values indicate a more favorable environment for tornadoes in the spring, summer, and any time the climate is prepped for severe weather. If this forecast comes to fruition, I see no reason why a substantial tornado threat would not exist in the aforementioned areas in Kansas, Oklahoma and Arkansas. We see very elevated valies over these same areas, even extending into Illinois and Wisconsin.

Here's my map and discussion for this event, subject to potentially major changes:

I expect the system to be too far west to comply with the Storm Prediction Center's outlook, and positioned my highest risk area over the regions where highest shearing would occur. I made sure to include where more favorable upper level winds would be positioned to aid in severe thunderstorm development, and lower level winds were also factored into this map. While the dewpoint gradient may be a bit further east than this highest risk I am projecting, I believe this is an accurate depiction of my thoughts at the moment, which are fluid as forecasts continue to come in.


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