Severe Thunderstorm Warning 173, NWS Des Moines, IA

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE DES MOINES IA
620 AM CDT SUN AUG 9 2009

...UPDATED 12Z AVIATION DISCUSSION...

.SHORT TERM...TODAY
COLD FRONT CURRENTLY SETTLING SOUTH THROUGH NORTH CENTRAL AND
NORTHWEST IOWA. THE BOUNDARY WILL STALL OVER CENTRAL IOWA THIS
MORNING AND EVENTUALLY WILL BECOME THE FOCUS FOR THUNDERSTORM
DEVELOPMENT TODAY. LATEST WATER VAPOR IMAGERY SHOWING TWO PLUMES OF
MOISTURE CONVERGING ON THE CENTRAL PLAINS AND MIDWEST WITH A
TROPICAL CONNECTION FEEDING RIGHT INTO IOWA FROM THE SOUTHWEST AND
GULF MOISTURE LIFTING NORTHWARD AS WELL. TIMING FOR CONVECTIVE
INITIATION IS PROBLEMATIC TODAY. THE LATEST NAM AND 4 KM WRF RUNS
ALONG WITH LOCAL WRF RUNS SUGGEST INITIATION WITH FIRST ROUND THIS
MORNING OVER KS AND SOUTHEAST NEBRASKA THEN A NORTHEAST STORM
MOVEMENT INTO CNTRL IA...PARALLEL TO THE STALLED BOUNDARY. THIS
SCENARIO DOES SEEM PLAUSIBLE WITH NEW WAVE OF MOISTURE TRANSPORT NOW
LIFTING NORTHEAST THROUGH CNTRL KS AND WILL BE PRIMED TO LIFT OVER
THE BOUNDARY. MID LEVEL LAPSE RATES ALSO FAVORABLE AT AROUND 7.5
C/KM.

WOULD EXPECT AT LEAST A FEW ELEVATED STORMS TO DEVELOP THIS MORNING
ABOVE THE CAPPED LAYER...THEN CHANCES INCREASING DURING THE
AFTERNOON. HOURS. POTENTIAL FOR HEAVY RAINFALL COULD BECOME REALITY
BY LATE THIS AFTERNOON WITH STORM MOTIONS PARALLEL TO THE STALLED
BOUNDARY AND PWATS EXCEEDING 2 INCHES FLOW INTO THE REGION. ANYTHING
THAT DOES DEVELOP BY THIS AFTERNOON WILL BE EFFICIENT RAIN
PRODUCERS. THE SEVERE POTENTIAL MAY BE CONDITIONAL BASED ON HOW MUCH
CLOUD COVER DEVELOPS BEFORE PEAK HEATING. IF GOOD INSTABILITY IS
OBTAINED...THEN SEVERE STORMS WILL BE POSSIBLE WITH POTENTIAL FOR
LARGE HAIL EARLY EVENTUALLY EVOLVING INTO BOWING SEGMENTS LATE IN
THE PERIOD AND BEYOND WITH GOOD POTENTIAL FOR COLD POOL DEVELOPMENT.

.LONG TERM...TONIGHT THROUGH SATURDAY
SOME DISCREPANCIES OVER FRONTAL PLACEMENT INTO THE BEGINNING OF THE
PERIOD TONIGHT...WITH THE NAM SLIGHTLY FURTHER NORTH THAN THE GFS
BUT ALL MODELS IN GENERAL AGREEMENT THAT FRONTAL BOUNDARY WILL BE
DRAPED ACROSS THE CWA PROVIDING FOCUS FOR SCATTERED THUNDERSTORM
DEVELOPMENT TONIGHT. STORMS MAY BE ONGOING ALREADY BY THIS
EVENING...BUT MID LEVEL SHORTWAVE CURRENTLY OVER WY/CO TO MOVE
ACROSS THE AREA TONIGHT. MUCAPE VALUES LARGEST LATE THIS AFTERNOON
INTO THE EVENING HOURS...WITH ATMOSPHERE BEGINNING TO STABILIZE AS
THE FRONT MOVES SOUTHEAST THROUGH THE CWA BY LATE TONIGHT. MOISTURE
TRANSPORT INCREASING THROUGH THE EVENING HOURS...AND ORIENTED INTO
CENTRAL/NE IOWA. PWATS CLIMB TO AROUND 2 BY TONIGHT. MID LEVEL FLOW
PARALLEL TO THE FRONTAL BOUNDARY...SO COULD END UP SEEING SOME
TRAINING OF ECHOES ALONG THE FRONT...AND POTENTIAL FOR LOCALIZED
HEAVY RAINS. INCREASED POPS INTO THE LIKELY CATEGORY ACROSS CENTRAL
IOWA THIS EVENING...AND TRANSLATED THAT TO THE SOUTHEASTERN CWA BY
LATE TONIGHT AS THE FRONTAL BOUNDARY PROGRESSES THROUGH THE CWA.
UNIDIRECTIONAL SHEAR PROFILE...WITH DECENT SPEED SHEAR WILL ALLOW
THE THREAT OF SEVERE WEATHER TO CONTINUE FOR THE TONIGHT
PERIOD...WITH HAIL AND HIGH WINDS THE MAIN SEVERE THREATS.

FRONTAL BOUNDARY STALLS OUT ACROSS THE SOUTHERN CWA MONDAY INTO
MONDAY NIGHT...WITH ANOTHER SHORTWAVE DROPPING SOUTHEASTWARD THROUGH
THE CWA. LOOKS LIKE MAJORITY OF THE PRECIP WILL BE TO THE SOUTH OF
THE CWA. OTHERWISE COOLER AIR TO MOVE IN BEHIND THE SYSTEM FOR MID
TO LATE WEEK WITH H85 TEMPS HOVERING IN THE MID TEENS. THEREFORE
EXPECT A SLIGHT COOLING TREND AS TEMPERATURES EXPECTED TO BE NEAR
THE SEASONAL AVERAGES THROUGH THE END OF THE WEEK. AS UPPER RIDGE
BUILDS IN TO THE AREA...DRY CONDITIONS WILL PERSIST TUESDAY THROUGH
THE END OF THE WEEK. UPPER RIDGE LOOKS TO BREAK DOWN TOWARD THE END
OF THE WEEK INTO NEXT WEEKEND...SO COULD SEE THE CHANCE FOR
PRECIPITATION RETURN TO THE FORECAST JUST BEYOND THE CURRENT
FORECAST PERIOD.

&&

.AVIATION...09/12Z
COLD FRONT WILL BE STALLED OVER CENTRAL IOWA MUCH OF TODAY AND INTO
THIS EVENING. THE BOUNDARY WILL BE THE FOCUS FOR THUNDERSTORM
DEVELOPMENT DURING THE PERIOD WITH KFOD/KALO/KDSM UNDER A PROLONGED
THREAT AS THUNDERSTORMS REPEATEDLY ROLL THROUGH THE CENTRAL PORTION
OF THE STATE. THE BOUNDARY WILL BEGIN TO SHIFT SOUTHEAST LATE IN THE
PERIOD HOWEVER WILL HAVE NOT SHIFTED FAR YET BY 12Z. IFR STRATUS TO
LOCAL LIFR CONDITIONS WILL BE IN PLACE NORTH OF THE BOUNDARY EARLY
TODAY BUT SHOULD EVENTUALLY ERODE AWAY LATER THIS MORNING.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

SHORT TERM...DONAVON
AVIATION...DONAVON
LONG TERM...ALBRECHT

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
904 AM CDT SUN AUG 9 2009

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086-
092>097-101100-
EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO-
POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN-
WEBSTER-HAMILTON-HARDIN-GRUNDY-BLACK HAWK-CRAWFORD-CARROLL-GREENE-
BOONE-STORY-MARSHALL-TAMA-AUDUBON-GUTHRIE-DALLAS-POLK-JASPER-
POWESHIEK-CASS-ADAIR-MADISON-WARREN-MARION-MAHASKA-ADAMS-UNION-
CLARKE-LUCAS-MONROE-WAPELLO-TAYLOR-RINGGOLD-DECATUR-WAYNE-APPANOOSE-
DAVIS-
904 AM CDT SUN AUG 9 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA.

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS WILL INCREASE IN COVERAGE AND INTENSITY TODAY
THROUGH THIS AFTERNOON AND EVENING AND INTO TONIGHT. SOME STORMS
WILL BE SEVERE WITH LARGE HAIL OF GOLF BALL SIZE OR
GREATER...STRONG DAMAGING WINDS...AND LOCALLY HEAVY RAIN THE
PRIMARY THREATS. AN ISOLATED TORNADO IS POSSIBLE AS WELL.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

A FEW THUNDERSTORMS WILL STILL BE POSSIBLE ACROSS SOUTHERN IOWA
MONDAY INTO MONDAY NIGHT...WITH A SLIGHT RISK OF SEVERE WEATHER.
LARGE HAIL AND DAMAGING WINDS WILL BE THE PRIMARY SEVERE THREATS.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL BE NEEDED TODAY INTO TONIGHT. AN
EMERGENCY MANAGEMENT CONFERENCE CALL WILL BE HELD AT 930 AM THIS
MORNING.

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