re: General Tornado Thread

URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 108
NWS STORM PREDICTION CENTER NORMAN OK
1240 PM CDT MON APR 28 2014

THE NWS STORM PREDICTION CENTER HAS ISSUED A

* TORNADO WATCH FOR PORTIONS OF
NORTH AND NORTHWEST ALABAMA
NORTHEAST LOUISIANA
CENTRAL AND NORTHEAST MISSISSIPPI
SOUTHERN MIDDLE TENNESSEE

* EFFECTIVE THIS MONDAY AFTERNOON AND EVENING FROM 1240 PM UNTIL
900 PM CDT.

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

* PRIMARY THREATS INCLUDE...
NUMEROUS TORNADOES EXPECTED WITH A FEW INTENSE TORNADOES LIKELY
WIDESPREAD LARGE HAIL EXPECTED WITH ISOLATED VERY LARGE HAIL
EVENTS TO 3 INCHES IN DIAMETER LIKELY
SCATTERED DAMAGING WIND GUSTS TO 70 MPH LIKELY

THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 75 STATUTE
MILES EAST AND WEST OF A LINE FROM 65 MILES NORTH OF HUNTSVILLE
ALABAMA TO 10 MILES SOUTH OF NATCHEZ MISSISSIPPI. FOR A COMPLETE
DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE
(WOUS64 KWNS WOU8).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
AND POSSIBLE WARNINGS.

&&

DISCUSSION...SURFACE-BASED STORMS ARE BEGINNING TO DEVELOP FROM NE
LA INTO W CENTRAL MS...WITH ADDITIONAL DEVELOPMENT EXPECTED NEWD
TOWARD NE MS/NW AL AS CLOUDS CLEAR AND THE LOW LEVELS DESTABILIZE.
A COMBINATION OF MODERATE INSTABILITY /MLCAPE NEAR 2000 J PER KG/
AND STRONG DEEP-LAYER VERTICAL SHEAR WILL SUPPORT A BROKEN BAND OF
SUPERCELLS...WHILE SOME INCREASE IN LOW-LEVEL SHEAR IS EXPECTED THIS
AFTERNOON FROM CENTRAL MS INTO NE MS/NW AL...UP TO A RESIDUAL
OUTFLOW BOUNDARY IN SRN MIDDLE TN. SOME OF THE SUPERCELLS WILL BE
LONG-LIVED AND CAPABLE OF PRODUCING STRONG/LONG-TRACK TORNADOES...AS
WELL AS VERY LARGE HAIL.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
SURFACE AND ALOFT TO 3 INCHES. EXTREME TURBULENCE AND SURFACE
WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
550. MEAN STORM MOTION VECTOR 24035.






PDS watches are quite uncommon; less than 3% of watches issued by the SPC from 1996 to 2005 were PDS watches, or an average of 24 each year.[2] When a PDS watch is issued, there are often more PDS watches issued for the same weather system, even on the same day during major outbreaks, so the number of days per year that a PDS watch is issued is significantly lower.

PDS tornado watches are issued when there is a higher than normal risk of multiple strong to violent tornadoes – especially those that are predicted to be long-track in nature, with path lengths of more than 20 miles – in the watch area (usually amounting to damage consistent with EF4 or EF5 tornadoes), in addition to including significant wind and hail damage. While there are no set criteria for a PDS watch to be issued, they are usually issued when the potential exists for a major tornado outbreak. These types of tornado watches represent about 90% of PDS watches issued by the Storm Prediction Center.[2]