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000
FXUS63 KLSX 212043
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
343 PM CDT TUE MAY 21 2013

.SHORT TERM...(THROUGH LATE TONIGHT)
ISSUED AT 314 PM CDT TUE MAY 21 2013

THE OVERALL SEVERE WEATHER THREAT HAS DIMINISHED SIGNIFICANTLY
FROM EARLIER THINKING OWING TO A SERIES OF MCS`S TRACKING TO OUR
SOUTH FROM OKLAHOMA TO ARKANSAS INTO THE TN VALLEY. THESE SYSTEMS
HAVE PRODUCED A GOOD DEAL OF CLOUD COVER WHICH HAS LIMITED HEATING
AND THAT COMBINED WITH MUCH WEAKER LAPSE RATES HAS LIMITED THE
INSTABILITY. THE HIGHEST VALUES OF SBCAPE PRESENTLY ARE FROM
1000-2000 J/KG EXTENDING FROM METRO ST LOUIS INTO EAST CENTRAL
IL...THIS HALF OR LESS THE AMOUNT OF THE PREVIOUS FEW DAYS. ALL
THIS SAID THE PRECIPITATION CHANCES STILL LOOK PRETTY HIGH TONIGHT
ACROSS EASTERN MO INTO SRN IL. WE MIGHT SEE SOME ISOLATED STORMS
DEVELOP IN THIS HIGHER CAPE AIR THRU SUNSET AND WE WILL ALSO SEE
THE NORTHERN PORTION OF THE STRATIFORM RAIN REGION OF THE AR MCS
IMPACT SE MO INTO SRN IL. THE MAIN THREAT OF PRECIPITATION HOWEVER
APPEARS TO BE LATER THIS EVENING INTO THE OVERNIGHT HOURS. WATER
VAPOR IMAGERY AND VWP DATA IS SHOWING A POWERFUL SHORT WAVE ACROSS
EXTREME WRN OK AND THIS WILL EJECT ENEWD TONIGHT AND BE CENTERED
THRU ERN MO AT DAYBREAK ON WED. DEEP LARGE SCALE ASCENT ASSOCIATED
WITH THIS SHORT WAVE SHOULD LEAD TO AN UPSWING IN SHOWERS AND
THUNDERSTORMS LATER THIS EVENING. I CANT RULE OUT AN ISOLATED
SEVERE THREAT BUT AGAIN THE MARGINAL INSTABILITY WOULD SUGGEST THE
CHANCE IS LOW. RAINFALL AMOUNTS WILL BE SUBSTANTIALLY LESS THAN
LAST NIGHT...LESS THAN AN INCH.

GLASS

.LONG TERM...(WEDNESDAY THROUGH NEXT TUESDAY)
ISSUED AT 314 PM CDT TUE MAY 21 2013

THE MAJORITY OF THE PRECIPITATION SHOULD BE EAST OF THE MS RIVER
IN ILLINOIS OR FURTHER EAST AT DAYBREAK AND CONTINUE TO MOVE
EASTWARD AS THE SHORTWAVE TROF AND ATTENDANT COLD FRONT MOVE EAST.
COOLER AND DRIER AIR WILL OVERSPREAD THE REGION IN THE WAKE OF THE
FRONT AND ANY ADDITIONAL PRECIPITATION THREAT AFTER MID-MORNING
SHOULD BE FROM ISOLATED DIURNAL SHOWERS. ANOTHER SECONDARY COLD
FRONT WILL PUSH THROUGH THE AREA WED NIGHT USHERING HIGH PRESSURE
AND EVEN COOLER AIR INTO THE AREA. THIS LARGE HIGH WILL DOMINATE
THE AREA THURSDAY INTO FRIDAY.

NOT MUCH HAS CHANGED IN THE EXTENDED RANGES OF THE FORECAST. AN
UPPER RIDGE WILL DOMINATE THE CENTRAL U.S. WITH THE RIDGE AXIS
REMAINING JUST WEST OF MISSOURI SAT-MON. THIS WILL PLACE US JUST
DOWNSTREAM OF THE RIDGE CREST AND SUBJECT TO WEAK SHORT WAVES
TOPPING THE RIDGE. THESE DISTURBANCES COMBINED WITH RETURN FLOW
AND LOW LEVEL WAA ON THE BACKSIDE OF THE RETREATING LOW LEVEL HIGH
PRESSURE SYSTEM WILL BRING A PROLONGED THREAT OF SHOWERS AND
THUNDERSTORMS TO PRIMARILY THE NORTHERN HALF OF THE CWA FROM SAT
THROUGH MON NIGHT. THEREAFTER THE UPPER RIDGE AXIS MOVES TO OUR
EAST...PRECIPITATION CHANCES DIMINISH AND TEMPERATURES WARM.

GLASS

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY AFTERNOON)
ISSUED AT 1243 PM CDT TUE MAY 21 2013

DIURNAL CU AND CIRRUS SHIELD TO PERSIST OVER TAF SITES TODAY AHEAD
OF COLD FRONT. TIMING AND COVERAGE OF STORM DEVELOPMENT IS STILL
HARD TO PIN DOWN FOR LATE THIS AFTERNOON AND TONIGHT AS ATMOSPHERE
IS STRUGGLING TO DESTABILIZE. FEEL THAT KCOU SHOULD REMAIN DRY.
OTHERWISE...SOME STORMS POSSIBLE OVER METRO AREA AFTER 22Z AND
LINGER OVER AREA THROUGH DAYBREAK ON WEDNESDAY. AS FOR KUIN...THEY
WILL BE ON NORTHERN FRINGES OF ACTIVITY AFTER 05Z...TAPERING OFF
BY 10Z. OTHERWISE...SOUTHWEST WINDS TO DIMINISH TOWARDS
SUNSET...VEERING TO THE WEST BEHIND THE FRONTAL BOUNDARY EARLY
WEDNESDAY. THE WINDS WILL PICKUP TO NEAR 15KTS BY MID MORNING ON WEDNESDAY.

SPECIFICS FOR KSTL...DIURNAL CU AND CIRRUS SHIELD TO PERSIST OVER
METRO SITES TODAY AHEAD OF COLD FRONT. TIMING AND COVERAGE OF STORM
DEVELOPMENT IS STILL HARD TO PIN DOWN FOR LATE THIS AFTERNOON AND
TONIGHT AS ATMOSPHERE IS STRUGGLING TO DESTABILIZE. SOME STORMS
POSSIBLE OVER METRO AREA AFTER 22Z AND LINGER OVER AREA THROUGH
11Z WEDNESDAY. OTHERWISE...SOUTHWEST WINDS TO DIMINISH TOWARDS
SUNSET...VEERING TO THE WEST BEHIND THE FRONTAL BOUNDARY BY 11Z
WEDNESDAY. THE WINDS WILL PICKUP TO NEAR 15KTS BY 17Z WEDNESDAY.

BYRD

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...NONE.
IL...NONE.
&&

$$

WFO LSX