What constitutes a High Risk from the SPC?

[Jerry Prsha]

I'm reading the convective outlook for today (6/26/2010) and see this:

...NERN NEB...
OBJECTIVE ANALYSES INDICATED THE AIR MASS ALONG THE MID MO VALLEY
HAS BECOME VERY UNSTABLE...THUS FAR THIS AFTERNOON...WITH MLCAPE
VALUES EXCEEDING 4000 J/KG. GIVEN THAT STRONGER INSTABILITY EXTENDS
SWD INTO NEB AND EFFECTIVE BULK SHEAR WILL INCREASE SWD ACROSS PARTS
OF NERN NEB/NWRN IA INTO THE EVENING WITH APPROACH OF UPSTREAM
TROUGH...MODERATE RISK/SEVERE PROBABILITIES HAVE BEEN EXPANDED SWD.
VERY LARGE HAIL AND EVENTUAL TORNADO DEVELOPMENT... SOME STRONG TO
SIGNIFICANT...REMAIN LIKELY ACROSS THE MODERATE RISK AREA INTO THE
EVENING.

From the language, it sounds like the area should be in High Risk. Obviously, I'm not sure what criteria they base this on and would love to understand.

Anyone?

 

[Chris Hayes]

According to the SPC a high risk verification for an area the size of Oklahoma without the panhandle would would be 20 or more tornadoes, with at least 3 (I think 3, might be 2) of them being EF3 or greater. Now, as you cut the area down in size you just cut the numbers down with it. Cut it in half, you need 10, with 1 or more for verification. I forget the criteria for wind.

 

[Robert Dewey]

I think they used to be based on hard quantities (i.e. expected report numbers), but they have since moved to a probability system: http://www.spc.noaa.gov/faq/#3.3

...again, that's just a guess.

 

[Chris Foltz]

This link will provide the specific breakdown of what constitutes a slight, moderate or high risk...

http://www.spc.noaa.gov/misc/SPC_Prob_Conv_Otlk_Change_20060214.html

 

[Jerry Prsha]

Great! Sorry I did not find that on my own.

The word "Significant" is what my eyes keyed on. ..Which makes me thing that conditions are favorable for large and long lived tornadoes... At least the way *my* mind works.