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URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 258

NWS STORM PREDICTION CENTER NORMAN OK

145 PM CDT SUN MAY 6 2012



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       SOUTHEAST IOWA

       NORTHERN AND CENTRAL ILLINOIS

       NORTHERN MISSOURI



EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 145 PM UNTIL 900

PM CDT.



HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70

MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.



THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 60

STATUTE MILES NORTH AND SOUTH OF A LINE FROM 45 MILES NORTHWEST

OF KNOB NOSTER MISSOURI TO 55 MILES NORTH OF DANVILLE ILLINOIS.

FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH

OUTLINE UPDATE (WOUS64 KWNS WOU8).



REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE

FAVORABLE FOR 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. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY

DO PRODUCE TORNADOES.



OTHER WATCH INFORMATION...CONTINUE...WW 256...WW 257...



DISCUSSION...TSTMS ARE GRADUALLY INTENSIFYING THIS AFTERNOON AT THE

INTERSECTION OF MULTIPLE BOUNDARIES OVER NRN IL AS WELL AS ALONG A

RESIDUAL OUTFLOW BOUNDARY ACROSS N-CNTRL MO.  AMBIENT INFLOW AIR

MASS HAS WARMED WELL INTO THE 80S...AND WHEN COUPLED WITH DEWPOINTS

IN THE UPPER 60S-LOWER 70S...ENVIRONMENT HAS BECOME STRONGLY

UNSTABLE WITH MLCAPE VALUES OF 3000-3500 J/KG.  THE STRONGEST LOW

AND DEEP-LAYER SHEAR WILL RESIDE ALONG WARM FRONTAL SEGMENT OVER NRN

IL WHERE SUPERCELLS CAPABLE OF LARGE HAIL...DAMAGING WINDS...AND

PERHAPS A TORNADO ARE POSSIBLE.  ELSEWHERE...VERTICAL SHEAR DROPS

OFF WITH SWD EXTENT ACROSS THE WATCH AREA WITH MULTICELLS AND

MARGINAL SUPERCELL STRUCTURES POSSIBLE WITH A RISK FOR LARGE HAIL

AND DAMAGING WINDS.



AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT

TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60

KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 550. MEAN STORM

MOTION VECTOR 27020.





...MEAD

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