Saturday, April 4, 2015

April 8th, 2015 Severe Weather Outlook

There is an elevated risk for severe weather on Wednesday, April 8th.

Storm Prediction Center
(click all images to enlarge)
The Storm Prediction Center has posted a 15% risk for severe weather within 25 miles of any given point for Oklahoma, eastern Kansas, and much of Missouri. If you're familiar with the Storm Prediction Center's Day 1 through Day 3 outlooks, you might be wondering why you don't see the 'Slight' or 'Enhanced' demarcations on this chart. In the long range (beyond Day 3), confidence can be high enough to outline an area for 15% or 30% risk of severe weather, but because it is so long-range, that's about as confident as these outlooks can get.

Weather Prediction Center
The first thing we want to do is look at the surface map to identify what features will be inciting this severe weather event over the aforementioned states. On the morning of Wednesday, April 8th, we see an elongated stationary front, stretching from western Kansas across southern Missouri, meeting up with a low pressure system in southern Indiana. We also see another storm system in western Oklahoma with a small warm front and associated trough in Texas. If you were to click through different days to get a context for where these boundaries are moving, you would find that the stationary front will be drifting north to cover Kansas and Missouri by evening on Wednesday. This means that the stationary front should cause the severe weather over Kansas and Missouri, while that smaller system in western Oklahoma combines forces with another system dropping out of Colorado to produce a severe weather threat in Oklahoma.

Now that we know what we're looking at, let's start checking out model forecasts for our severe weather parameters.

The first thing to look at is the forecasted instability, since instability (the action of warm air rising as cool air surrounds it) is necessary for all thunderstorm formation. This chart above shows the Convective Available Potential Energy (CAPE, the meteorological definition of instability) forecast for the evening of April 8th. We notice a corridor of high instability on the order of 3000+ joules per kilogram extending across eastern Kansas and central Oklahoma. It is expected that a boundary, potentially a dry line, will be available to conjure up such instability. We see lower, but still notable instability values over Missouri. Here, the stationary front should be able to fire up thunderstorms as well, some of them potentially being severe.

Also helpful is looking at the wind pattern aloft; here, the 300 millibar jet-stream wind forecast is displayed for the same timeframe as our graphic above. For this chart, we see a large trough dipping in the Western US, and allowing a sharp jet streak to form around the base of this trough, in Arizona and New Mexico. Additionally, we see the subtropical jet stream poking in to Baja California, and even providing a jet streak (a band of stronger wind speeds than the surrounding air) in western Texas and Oklahoma. The jet streak nosing into Kansas and Oklahoma will aid in severe weather development, especially with the positioning of the jet streak just west of the axis of highest instability in those states. In some research I've done this past winter, tornado formation can be favored in the areas directly east of a jet streak; in this case, that favors the Oklahoma, Missouri, and Kansas regions.

To summarize:

- A severe weather threat is unfolding on Wednesday, April 8th for eastern Kansas, Oklahoma, and much of Missouri.
- As of now, model forecasts are showing a potentially tornadic environment.
- Very large hail could be a prominent threat in Kansas and Oklahoma.
- All of this is subject to change.



Anonymous said...

Wednesday and ESPECIALLY Thursday. Thursday is looking worse and much more widespread. Might not want to leave that out.

Andrew said...

A post was made immediately after the Wednesday post: