Outbreak forecast w/o high risk

[Andy Wehrle]

...AN OUTBREAK OF SEVERE THUNDERSTORMS WITH TORNADOES IS FORECAST THIS AFTERNOON AND TONIGHT FROM EAST TX...ACROSS PARTS OF LA...SERN AR...MS...AL...FL PANHANDLE AND SWRN GA...
STRONG DEEP LAYER AND LOW-LEVEL SHEAR COMBINED WITH MODERATE INSTABILITY WILL RESULT IN DISCRETE SUPERCELLS WITH LARGE HAIL...TORNADOES...AND DAMAGING WINDS LIKELY.

...OUTBREAK OF SEVERE THUNDERSTORMS POSSIBLE THIS AFTERNOON AND TONIGHT...

THE COMBINATION OF AN UNUSUALLY STRONG SPRING STORM SYSTEM AND A VERY UNSTABLE ATMOSPHERE WILL LIKELY LEAD TO AN OUTBREAK OF SEVERE THUNDERSTORMS ACROSS THE LOWER MISSISSIPPI VALLEY THIS AFTERNOON AND TONIGHT. WE CANNOT RULE OUT THE POSSIBILITY OF LONG TRACK STRONG TORNADOES.

Anybody else somewhat confused by this strong wording without an accompanying categorical high risk?

 

[Mike Peregrine]

High risks are not necessarily required for events with tornado outbreak situations. Risk level is more of an indication of the possible impact to human interests ... and large bow echoes or destructive storms with strong outflow or damaging hail have an impact on a greater area and therefore affect more people. There are often scenarios (every year in fact) where you'll have a forecast for an outbreak of tornadoes, but maintain a moderate risk in the event area. There are a lot of reasons why it works this way - and always remember that some of the very BEST chase days are on days with a slight risk - or even outside the risk area altogether! - Risk areas are designed as a potential threat identification sytem for the general public ...

 

[mikegeukes]

A SLGT risk implies well-organized severe thunderstorms are expected but in small numbers and/or low coverage. Within a slight risk area, 5-29 reports of 1 inch of larger hail, and/or 3-5 tornadoes, and/or 5-29 wind events are forecast.

MDT risks imply a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather. Within a moderate risk area, at least 30 reports of hail 1 inch or larger, or 6-19 tornadoes, or numerous wind events (30 that might be associated with a squall line, bow echo or derecho) are forecast.

The HIGH risk area almost always means a major severe weather outbreak is expected, with great coverage of severe weather and enhanced likelihood of extreme severe (i.e., violent tornadoes or extreme convective wind events over a large area). Within a high risk area, expect at least 20 tornadoes with at least 2 of them rated F3+, or an extreme derecho causing 50+ widespread wind events (50+) with numerous higher end wind (80+ mph) and structural damage reports.

Source: http://www.spc.noaa.gov/misc/about.html
http://www.spc.noaa.gov/misc/about.html#Le...els%20of%20Risk

Mike

 

[Chris Hayes]

I know of two perfect examples where there was a moderate risk out and a high risk would have been good for event: June 8, 1995 in the Texas Panhandle, (22 tornados including 3 F4s), and June 24, 2003 in South Dakota and western Minnesota (89 tornados including 2 F3s and 1 F4). There's always the possibility that SPC may upgrade to high risk in the 2000 or 0100 outlook. This looks to be a nighttime event, so it's possible. May 22, 2004 wasnt upgraded until the 0100 outlook.

 

[Gabe Garfield]

I suppose there is some doubt as to whether or not the most unstable air will be able to juxtapose with the best shear. I really don't think that will be too much of a problem, since the area north of the warm front is getting some insolation and should help the w/f to lift to then north. I would expect a high risk at some point (either at 20 UTC or 01 UTC).

By the way, I think any substantial risk of significant, long-track tornadoes in the southeast warrants a high risk.

Gabe

 

[Chris Hayes]

I suppose there is some doubt as to whether or not the most unstable air will be able to juxtapose with the best shear. I really don't think that will be too much of a problem, since the area north of the warm front is getting some insolation and should help the w/f to lift to then north. I would expect a high risk at some point (either at 20 UTC or 01 UTC).

By the way, I think any substantial risk of significant, long-track tornadoes in the southeast warrants a high risk.

Gabe

Still a moderate in the 2000 outlook. If isolated supercells start exploding and dropping tornados, they'll probably upgrade to high risk. Gabe is right, long track tornados in the southeast do warrant a high risk. Some of the most deadly outbreaks occur down there and tommorows the anniversary of the Palm Sunday II outbreak.

 

[Andy Wehrle]

By the way, I think any substantial risk of significant, long-track tornadoes in the southeast warrants a high risk.Gabe

This was what I was thinking of when I started this thread. Looking back on events like November 15, 1989, Palm Sunday 1994, April 8, 1998, April 16, 1998, etc; all those were high risks and not without warrant. Strong or violent tornadoes struck metropolitan areas on all of those days. Of course, some recent high risk days in that region have essentially busted (April 6 and May 5, 2003 come to mind) and perhaps SPC wants to be extra sure before blowing the whistle, since the most activity will probably occur this evening/night anyway.

 

[Robert Dewey]

Hmm, interesting...

I actually don't think today warrants a HIGH risk, at least at this time anyway. The strength of the storms doesn't dictate the level of risk... MDT means that SPC expects between 6-19 tornadoes... Some of which may be violent... But, unless you exceed the "19", a HIGH risk isn't needed...

 

[Tim Johnson]

Is anyone chasing this event today? Quite a few cells going up in the surrounding Gulf state areas.

 

[David Drummond]

I'm doing a little armchair chasing, giving the new wxworx a trial run here, see how it does in a potential "outbreak" situation.

 

[Mike Mezeul II]

im nowcasting for scott peake and ken fugate....i slept in this morning so i wouldnt be able to make it down there in time...ah...precious sleep! :wink:

...just think, its only march

 

[Gabe Garfield]

Since I can't post in the NOW thread, I thought I'd post here. :wink:

Looks like this event is beginning to really unfold...two supercells are establishing themselves in southern/central MS. Looks like this could be the beginning of the big event.

AT 822 PM CST...NATIONAL WEATHER SERVICE DOPPLER RADAR CONTINUED TO INDICATE A STRONG TORNADO. THIS TORNADO WAS LOCATED 8 MILES SOUTH OF OMA...OR ABOUT 16 MILES EAST OF BROOKHAVEN...MOVING EAST AT 45 MPH.

By the way, to any mods who might be paying attention, it'd be awful nice if we could post in the target area without having to get special permission. Pretty significant event underway, and it's pretty quiet in there...just a suggestion. :wink:

Gabe

 

[Glen Romine]

Gabe, the ST rules are in place to avoid these very issues. Just follow the instructions - request permission by PMing Tim that you have read and agree to the special rules and you'll be able to post in the TA forum. It's really not that big of an inconvenience, is it? Otherwise, your posts here are just going to be deleted by the mods. Alternatively, you could consider visiting the chat group and there are probably folks there who would be happy to talk about the unfolding weather events.

Glen

 

[Gabe Garfield]

It's really not that big of an inconvenience, is it?

No, not really...not that big of a deal. It's just that last week I didn't need special permission and this week I do. I just thought it was a little odd, that's all.

Gabe

 

[Aaron Kennedy]

We actually had a special list in the past... in the past few months TA has deteoriated so we're trying to return TA to its standards.

Aaron