Monday, March 31, 2014

Thursday Severe Weather Threat Could be Season's First Outbreak

I'm looking at Thursday for the nation's first chance at a severe weather outbreak this season.

Note: There will be some technical terminology in this post. For explanations of a specific topic, rather than copying and pasting the information in a time-consuming manner, I will paste the link to an explanation alongside the topic discussed.

We look to see an upper level low emerge over the Central Plains by Thursday afternoon, taking on an apparent negative tilt by the time we reach Thursday evening. The presence of a negative tilt, depicted in the 500mb vorticity image above as the maxima vorticity values pushing to the southeast, will add to the intensity of these storms, as it is known that negatively-tilted storms define the mature stage of a storm system, priming the atmosphere for a major severe weather event. (More on negatively-tilted storms in middle of this post). As the storm digs into the Plains, we will see ridge formation in the Central and East US, indicating the open flow of moisture and instability across the East, concentrated particularly in the Oklahoma, Arkansas, Missouri, and Louisiana/Texas areas.

A look at the jet stream for Thursday evening confirms the potential severity of the event, as we see a split jet flow across the United States. The jet stream splits over the same areas we mentioned above, as the subtropical jet stream is forced south along the Gulf Coast and the Pacific jet is pushed north into the Upper Midwest. This split indicates the presence of divergence over our severe weather area, highlighted below by the Storm Prediction Center. With divergence, we see air rising strongly into the middle and upper layers of the atmosphere, helping to form thunderstorms. (More on divergence on bottom of this post)

Storm Prediction Center outlook for Thursday.
Outlined area denotes enhances severe weather threat.
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Skew-T chart for northern Oklahoma
The latest run of the NAM model has come in quite a bit more tornadic than what other guidance suggests. This image is valid at 1 AM Central local time for early Thursday morning, not Thursday night, which we have been talking about up to this point. There is a severe weather risk on Wednesday night, and this skew-t chart shows it well. By 1 AM, we see just over 2000 j/kg of CAPE, a substantial amount of instability. We also observe the EHI, or Energy-Helicity Index to be at a whopping 7.4 for this time period. This would be a very dangerous situation, but it's not being talked about as much. The reason for that is because it looks like our capping inversion may hold through the night and kill off the risk. As the skew-t shows, we see the red temperature line not really curving to the left much from the surface to 700 millibars. This means temperatures want to stay warm, and that limits thunderstorm formation. It's possible this does end up being a substantial severe weather event, but I'm not seeing too much activity for Wednesday night. The big story is Thursday night.

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Back to Thursday night, lower level winds look to be roaring just southeast of the storm center, as this forecast 700mb wind speed chart shows. These elevated lower level winds extend all the way down to the 925 millibar level, which isn't too high above the surface (a couple thousand feet, roughly). These lower level winds should act to strengthen the lower level jet stream, shown well on that 700mb chart above, which will then act with the rest of the environment to provoke this severe weather event.

My thoughts follow the Storm Prediction Center's outlook above, with the primary threats being hail, damaging winds, and an isolated tornado.