Stormy Weather Next Week

As the latest model guidance has come in, there are a lot more questions than answers about the storm system that will move into the Tennessee Valley next week. Over the past 24 hours, the American-run GFS model has shown some significant speed with this system; accelerating it through here on Wednesday instead of Thursday. Here’s a comparison of the midday GFS “operational run” and the midday “ensemble run.” The operational is based on actual observed initial conditions with no additional tweaking; the ensemble is an average of multiple “runs” of the model with some small adjustments to each run. Ensembles help us remove some of the biases (errors) in the operational models. What you’re seeing here is barometric pressure and precipitation in the operational; it is barometric pressure and height contours representing the jet stream’s flow on the ensemble:

On the other hand, the European-based ECMWF model maintains a dynamic but very slow-moving system that arrives Wednesday night and lasts through Friday.

This look is very different from the GFS guidance; the images below are showing the same features as the GFS data above, but it is definitely in disagreement with the GFS:


*Note: The ECMWF ensemble does not have a graphic available for 1 PM Wednesday, so 7 AM was substituted. It’s close enough for comparison’s sake.

Because of the shift in model guidance toward a faster solution (even the Canadian GEMS model is doing that), we’re making a big adjustment to the forecast tonight placing the emphasis for the strong/severe storm potential in a Wednesday daytime time frame instead of a Wednesday night/Thursday period.

There is still room for the timing to change; we’ve been emphasizing all week that there’s not a lot of accuracy in timing (and specifics on the individual storms) in the long-range guidance. As it draws closer, we’ll be able to refine it even more. At any rate, be alert for a severe weather threat on Wednesday and possibly Thursday of next week.

What kind of threat are we talking about?

For the first time with this particular event, we are able to see some analogs to previous severe weather events. Analogs help us compare conditions, patterns, and the end results of systems that have looked and behaved the way this one is forecast. Here are some initial conclusions:

From the Cooperative Institute for Precipitation Systems · Saint Louis University

From the Cooperative Institute for Precipitation Systems · Saint Louis University

The above image is the analog Wednesday’s forecast; the “hot spot” looks like Middle Tennessee, West Tennessee, North Mississippi and North Alabama right now. This may change over time, but it’s something we’re keeping a close watch on for future trends.

There is still no specific information about whether a particular place is in any more danger than another; it’s just clear enough to resolve that kind of picture.

All you need to really know right now is that severe weather is possible next week; spend this weekend getting prepared. Know what you need to do if you are in a Severe Thunderstorm or Tornado Warning, and make sure you’ve got a way to get that warning through NOAA Weather Radio or any of the applications offered and/or recommended by WHNT News 19, as well Baron Saf-T-Net (free in Alabama).

Above all, even though this looks scary, don’t get too worried just yet. This is only an “outlook.” These things can change, and we can hope it will just end up as a good rain event. If it stays on track, though, you’ll want to be ready for whatever may come our way.

-Jason
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