Wednesday, April 23, 2014

**Significant Severe Weather Outbreak Expected This Weekend**

A significant severe weather outbreak, possibly including strong to violent tornadoes, is expected Saturday, Sunday and Monday.

The Storm Prediction Center has outlined three days of potential severe weather in their long range outlook. In these outlooks, the SPC will mark regions it believes are at risk for enhanced severe weather. Typically, one or two days may be outlined at a time, but it is uncommon (though not unheard of) to see three different areas outlined, especially considering how much area they cover. By SPC estimates, over 47 million people may be affected by this potential severe weather outbreak.

The wording by the Storm Prediction Center is very strong, as this excerpt below shows:

Jet Stream Forecast for Saturday
The whole situation looks to unfold as a strong trough digs into the Southwest United States, developing a negative tilt in the process. A negative tilt, commonly seen how the isobars seem to push towards the southeast like you see above over Arizona, indicates the maturity of the trough in question, and thus means weather of any kind is generally stronger than it would be if the storm were not fully mature yet, or if it was beginning to occlude and weaken. In this forecast of the jet stream, valid Saturday night, we see a strong Pacific jet stream powering the trough as it digs east, and the divergence over the South Central Plains, shown as how the two jet streams (subtropical jet stream, seen along the bottom of the image, and the Pacific jet stream) separate just west of Texas. Divergence in this sense means the air is rising in monumental fashion, also a big signal for severe weather potential.

Lifted Index and Dewpoints over the Southern Plains, valid Saturday
The Lifted Index (LI), a measurement of how unstable the atmosphere is, looks to be plummeting as low as -11 or -12, indicating extreme amounts of instability. Worse yet, the sharp gradient in dewpoints (shown by the shaded colors) indicates the presence of a dryline, literally meaning a line where the air is dry to the west and very humid to the east. These drylines almost always originate and stick around in the Plains, and are a big reason why we see major severe weather down there. I have little doubt based on the projected Lifted Index that if/when a storm does break the capping inversion predicted to be in place (a capping inversion hinders thunderstorm development, and is the opposite of instability), we may very well see violent thunderstorms capable of strong tornadoes and potentially extreme severe weather. The rhetoric here is elevated, but there is quite a bit of concern here on how this will evolve.

By Sunday, the trough looks to continue to mature and eventually close off, as the full-circle contours in the 500mb wind speed image above show. That negative tilt is also evident, as we see the contours tilting towards the south-east direction. In response to this strong trough, as well as the intense Pacific jet stream we discussed above which is also evident in the image above, the mid-level jet stream will be absolutely howling over the Plains, right over the area projected to be hit on Sunday. We could see mid-level winds of over 80 knots, an astounding strength for this event.

Projected Energy-Helicity Index (EHI) values look to be heading towards double-digit territory on Saturday, a major concern for tornadic activity that evening. The EHI combines instability and helicity (spinning) to make an index that tries to predict situations where tornadoes are likely to occur. Values as elevated as the ones shown above confirm my concern for this event, which won't be just one day, but possibly three days of continuous severe weather problems.

To summarize:
• A potentially significant severe weather event is expected Saturday, Sunday and Monday.
• Potentially strong tornadoes are anticipated Saturday evening.
• Risks beyond Saturday look to focus more on other modes of severe weather.
• Those in the risk areas outlined by the SPC should review severe weather guidelines and preparations.



An old friend :) said...

A few constructive criticisms:
1. Model disagreement is tremendous at the moment, and the run-to-run continuity is particularly awful. Though it is pretty much expected that an event will happen, I'd include that little bit as a sort of calming effect in the post.
2. I suggest starting to back away from indices (such as EHI) and beginning to look more at what's known as "organic" forecasting. While indices are helpful at summing everything up in one number, they may overlook some smaller details that may enhance/decrease the threat. Try reading Tim Vasquez's books; they really are tremendous at helping a person start out. They certainly helped me :)
3. This is more nit-picky than anything, but the SPC's outlooks overlap, so the number of people they have listed on each day also overlap. This affects the D5/D6 outlooks especially. There isn't quite 47 million people in that area.
Thanks for reading this if you got this far. Good luck Andrew :)

Andrew said...

Definitely some good tips & ideas here, thanks for sharing. Yeah, tried to work around the population conundrum, but was in a bit of a rush trying to get the post to a publishable level that I never went back to correct it.
Agreed on the model disagreement, I see the SPC is favoring the ECMWF at this point, will need some time to refine the details and such.
I've tried moving away from the EHI, but I'm trying to see if a general summation, despite its operational shortfalls, gets the message of severe weather to the public easier than the long, drawn-out explanations. It's all being refined, hopefully for a compromise between the two.