Wednesday, April 27, 2011

...THIS IS A PARTICULARLY DANGEROUS SITUATION...

The SPC has issued a tornado watch that covers Rowan County, effective until 2 AM. 


What the pros out in Oklahoma are thinking (edited):
URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 241
NWS STORM PREDICTION CENTER NORMAN OK
650 PM EDT WED APR 27 2011
EFFECTIVE THIS WEDNESDAY NIGHT AND THURSDAY MORNING FROM 650 PM UNTIL 200 AM EDT.
...THIS IS A PARTICULARLY DANGEROUS SITUATION...
DESTRUCTIVE TORNADOES...LARGE HAIL TO 2 INCHES IN DIAMETER...
THUNDERSTORM WIND GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE
POSSIBLE IN THESE AREAS.

TORNADOES OVER PORTIONS OF ERN TN AND AL ARE EXPECTED TO RAPIDLY MOVE EWD INTO THE WW AREA THIS EVENING. EWD MIGRATION OF 50-60+ KT LLJ AXIS WILL SUPPORT CONTINUED AIR MASSD ESTABILIZATION WITH MLCAPE VALUES REMAINING IN THE 1000-2000 J/KG RANGE. WHEN COUPLED WITH 50-70 KT EFFECTIVE BULK SHEAR AND STEADILY STRENGTHENING LOW-LEVEL SHEAR...SETUP WILL REMAIN QUITE SUPPORTIVE OF SIGNIFICANT TORNADOES...SOME OF WHICH COULD BE LONG-TRACKED
DISCUSSION...LONG-LIVED SUPERCELLS WITH A HISTORY OF DESTRUCTIVE
A particularly dangerous situation means higher than normal probabilities of tornadoes in a watch area and/or long lived (stronger) tornadoes, or if you want a very vague definition, the SPC wording:

"
The "particularly dangerous situation" wording is used in rare situations when long-lived, strong and violent tornadoes are possible. This enhanced wording may also accompany severe thunderstorm watches for exceptionally intense and well organized convective wind storms. PDS watches are issued, when in the opinion of the forecaster, the likelihood of significant events is boosted by very volatile atmospheric conditions. Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective. There is no hard threshold or criteria. In high risk outlooks PDS watches are issued most often."

For our watch area (which is a big chunk of real estate) the probabilities are as follows:

WT 0241 PDS
PROBABILITY TABLE:
PROB OF 2 OR MORE TORNADOES                        :  80%
PROB OF 1 OR MORE STRONG /F2-F5/ TORNADOES         :  70%
PROB OF 10 OR MORE SEVERE WIND EVENTS              :  50%
PROB OF 1 OR MORE WIND EVENTS >= 65 KNOTS          :  60%
PROB OF 10 OR MORE SEVERE HAIL EVENTS              :  60%
PROB OF 1 OR MORE HAIL EVENTS >= 2 INCHES          :  50%
PROB OF 6 OR MORE COMBINED SEVERE HAIL/WIND EVENTS : >95%
This begs the question, I think, what is going to happen in Salisbury tonight? If that's what you're reading this for, stop being ridiculous. No one can answer that with any certainty. We will have thunderstorms late tonight, the atmosphere is "ripe" for severe weather; and that is about as exact as one can get until the storms approach and then "now-casting" takes over. Then NWS issues tornado warnings - based on radar indicated rotation or spotter observations. 

After the front passes and the low pressure system responsible for this severe weather outbreak (see maps below) moves East, a windy day is in store as skies become partly cloudy, then clear by sunset as high pressure builds in. Biggest difference will be the drop in humidity and overnight low temps dropping back to seasonal values. 

Current watches and today's storm reports - this is system is bringing severe weather to much of the nation: