Forecast Update for Wednesday May 2nd Tornado Threat

I was just glancing over model data this morning at work before I head out of town and the trend with the GFS over the last 24 hours is to improve directional shear for Wednesday, making the threat of tornadoes a much bigger concern. I need to keep this brief since I need to get a few things done at work before I leave town, but since I largely skipped over the setup for Wednesday in last night’s post due to the poor directional component to shear profiles in the 850-500 layer, I wanted to update on it┬ábecause if the current trends hold true, it will have a significant impact on any tornado threat Wednesday IMO.

Having good directional shear is a must for a solid tornado threat. The trough coming through is going to have a positive tilt to it, so poor directional shear as the trough fully ejects into the plains is kind of the MO with that style of setup. Since the 12Z run yesterday though, the GFS has backed 850mb winds more and more with each run, to the point that now with last nights 00Z and this mornings 6Z runs, you actually have good turning in the 850-500 layer over the Kansas portion of the threat area. Below are 3 screen grabs which clear show this trend in the supercell composite maps (SP composite shows 850-500mb winds overlayed if you aren’t familiar with it).

Yesterday’s 12Z run

Yesterday’s 18Z run

Last night’s 00Z run

As you can see by scrolling down the 3 model runs from yesterday, it’s not hard to see the trend of 850’s backing. Shear profiles as a whole improve across those three runs, but the backing 850’s is the most notable improvement. Instability has also improved, which is why the composite indices have spiked. I’m tight on time so I can’t get into all the details, but this is bringing Wednesday into more of a serious tornado threat type setup. Below is 850mb winds from yesterday’s 12Z run and then yesterday’s 00Z run. Again it is not hard to see the change/trend.

yesterday’s 12Z run

Yesterday’s 00Z run

Now, we are a long ways out so again a lot can and likely will change. This is a positive trend if you’re rooting for tornadoes though. I try not to give much credit to specifics with model data this far out. I try to just focus on the bigger picture, like moisture quality and trends with larger scale features like the tilt/evolution of the trough. Generally when you have a positive tilt you get more veering of 850’s. 850’s are damn near straight out of the south in the latest runs of the GFS, which isn’t really what you’d expect with a positive tilt. Based on the 30 seconds of research I’ve done, the other big change that is creating this with the GFS is the strength and location of the upper level jet (which is causing your pressure falls and the strength/direction of the low level winds). The tilt of the upper level trough has also been moving more towards neutral, which is helping to back low level winds. The images below of 250mb winds/upper level jet will show why.

The first map is the 12Z runs depiction of the 250mb winds and the second image is the 00Z runs depiction of the 250mb winds from the GFS. Big difference there. If you look at the continental US view it’s very clear the trough has moved to more of a neutral tilt with the last few runs as well. The tilt of the trough is critical for tornadoes. You can get big severe days with a positively tilted trough, but you’ll be hard pressed to find very many examples of big tornado days with a positively tilted trough. Almost always, large scale tornado outbreaks are associated with a neutral or negatively tilted trough.

Anyway, who knows what to believe with the GFS. This far out you can’t take too much seriously, but if the latest trends are accurate it will have a big impact on the type of severe weather (tornadoes or more of just hail and wind type threat) we see and the extent of the threat on Wednesday. There is a bit of a veer back veer profile to hodographs over Kansas in the latest run, but the better instability and greatly improved shear profiles with current GFS runs vs. the runs from 24 hours ago is a major shift and if current runs are accurate, strong tornadoes could also be a real concern on Wednesday. South central Kansas, including Wichita, are right in the wheelhouse for both Tuesday and Wednesday so pay close attention to the forecast as we get closer. Again I am out turkey hunting the rest of today and tomorrow morning, so don’t expect another update until tomorrow afternoon at the earliest. If you’re interested, check back then.

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