Severe Thunderstorm Warning 43, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
326 PM CDT FRI MAY 8 2009

.SHORT TERM.../TONIGHT THOUGH MON/
A VIGOROUS SHORTWAVE IS PUSHING ACROSS CENTRAL IOWA AT THIS TIME.
AT THE SURFACE...A COMPLEX PATTERN HAS SET UP OVER CENTRAL IOWA.  A
LOW PRESSURE SYSTEM HAS DEVELOPED OVER SOUTHWEST IOWA AND A WEAK
FRONTAL BOUNDARY THAT WAS A QUASI STATIONARY COLD FRONT HAS MOVED
BACK NORTH AS A WARM FRONT.  NEAR THE TRIPLE POINT THERE IS ENOUGH
SHEAR TO CAUSE SOME OF THE STORMS TO ROTATE AND PRODUCE A COUPLE
LANDSPOUTS.  AS THE EVENING HOURS APPROACH AND STORMS PUSH FURTHER
AWAY FROM THE TRIPLE POINT...THEY WILL LOSE THEIR ROTATION AND
BECOME MORE OF A HAIL THREAT.  HAVING SAID THAT...ALTHOUGH
CONVERGENCE ALONG THE FRONTAL BOUNDARY HAS IMPROVED...MARGINAL CAPE
AND WEAK SHEAR WILL FIGHT AGAINST STEEP MID LEVEL LAPSE RATES AND
SOME DECENT STRETCHING POTENTIAL.  FOR THIS REASON I HAVE HIT
THUNDERSTORMS HARD IN THE GRIDS AND THE HWO.  IF THE STORMS CAN
BECOME MORE ORGANIZED AND SUSTAIN THEMSELVES THEY SHOULD BE
CAPABLE OF PRODUCING INCH OR SO HAIL WITH SOME STRONGER WIND GUSTS.
STORMS THAT DEVELOP RIGHT ON THE FRONT WILL HAVE AT LEAST A
POTENTIAL FOR LANDSPOUTS.  I MADE LITTLE CHANGE TO TEMPS ALTHOUGH I
DID BUMP MOST LOWS UP A DEGREE OR SO.

CURRENT SYSTEMS SHOULD BE EAST OF FORECAST AREA BY SAT MORNING WITH
FAIR WEATHER FOR START OF WEEKEND AS SFC HIGH APPROACHES MO RIVER
VALLEY.  SUBTLE SHORT WAVE ENTERING CA COAST LATE THIS AFTERNOON
THEN REACHES THE MIDWEST BY SUNDAY.  PV AND QG FORCING IS RATHER
WEAK...BUT COMBINED WITH STEEP LAPSE RATES I AM NOT COMFORTABLE
LEAVING FORECAST DRY SO HAVE INSERTED SLIGHT CHANCES SUN.  THIS IS
SUPPORT BY NAM...GFS AND SREF.  ANOTHER WEAK SHORT WAVE TRAVERSES
THE FLOW SUN NIGHT.  THIS SIGNAL IS SOMEWHAT LESS IMPRESSIVE IN
DETERMINISTIC MODELS...HOWEVER SREF PRECIP PROBS ARE STILL IN THE
LIKELY CATEGORY SO HAVE OPTED TO LEAVE SLIGHT CHANCES GOING THROUGH
THE NIGHT CENTRAL AND SOUTH.  SIMILAR THINKING TO SUN FOR MON...WITH
SUBTLE SHORT WAVES AND STEEP LAPSE RATES SO CONTINUED SLIGHT
CHANCES.  NO BIG CHANGES TO TEMPS WITH FORECAST SOUNDINGS SUPPORTING
GUIDANCE BLEND AND PERSISTENCE.

.LONG TERM.../MON NIGHT THROUGH FRI/
SCENARIO HAS CHANGED LITTLE SINCE YESTERDAY AND CANNOT ARGUE MUCH
WITH HPC SOLUTION WHICH IS HEAVILY WEIGHTED TOWARD 00Z ECMWF
ENSEMBLE.  THIS KEEPS CLOSED UPPER LOW IN PLACE TO OUR NORTH FOR THE
MIDDLE TO LATTER PORTIONS OF NEXT WEEK...BUT TO A LESSER EXTENT THAN
DEPICTED BY THE OPERATIONAL 00Z ECMWF.  CANNOT REALLY FIND MUCH
SUPPORT FOR ANY PRECIP AHEAD OF MATURING SYSTEM SO HAVE REMOVED ANY
TOKEN SLIGHT CHANCE WORDING MON AND MON NIGHT.  ISENT LIFT
DRAMATICALLY INCREASES LATE TUE...AND ESPECIALLY TUE
NIGHT...FOLLOWED BY KINEMATIC FORCING AND FROPA WED MORNING.  THIS
FAVORED SCENARIO HAS BEEN FAIRLY CONSISTENT SO HAVE RAISED POPS
FURTHER THROUGH CHANCE CATEGORY DURING PEAK PERIODS TUE NIGHT/WED.
WITH SEVERAL WEAK SHORT WAVES DIVING BEHIND CLOSE LOW...HAVE ALSO
INCLUDED INTERMITTENT SLIGHT CHANCES FOLLOWING SYSTEM DAYS 6 AND 7.
NO BIG AIRMASS CHANGE DURING THIS PERIOD...EVEN WITH FROPA.
SEASONAL TEMPS WITH FORECAST SOUNDINGS HINTING AT HIGHER END OF
ENSEMBLE MOS MEMBERS.

&&

.AVIATION...08/18Z
A SHORT WAVE ALOFT COUPLED WITH A COLD FRONT ALONG/NEAR THE HIGHWAY
20 CORRIDOR WILL PRODUCE THUNDERSTORMS OVER KFOD...KALO AND KMCW
THROUGH 21Z.  AS THE FRONT SAGS SOUTH AND THE UPPER LEVEL FEATURE
PUSHES EAST...THUNDERSTORMS WILL MOVE SOUTH ACROSS THE TAF LOCATIONS
THROUGH 06Z.  MVFR COND ARE EXPECTED IN THE STORMS.  SOME STORMS
WILL BE CAPABLE OF STRONG DOWNBURST WINDS...INCH TO INCH AND A HALF
HAIL AND AN ISOLATED TORNADO ALONG THE BOUNDARY.

&&

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

&&

FAB/BSS

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1200 PM CDT FRI MAY 8 2009

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-091700-
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-
1200 PM CDT FRI MAY 8 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA.

.DAY ONE...THIS AFTERNOON AND TONIGHT

SCATTERED THUNDERSTORMS ARE EXPECTED ACROSS CENTRAL IOWA THIS
AFTERNOON INTO THE EARLY EVENING. SEVERE STORMS ARE POSSIBLE WITH
LARGE HAIL AND DAMAGING WIND THE MAIN THREAT. HAIL SIZE BETWEEN A
QUARTER AND A HALF DOLLAR IS POSSIBLE WITH THE STRONGER STORMS
ALONG WITH 60 MPH WIND GUSTS. ISOLATED TORNADOES  A FEW
THUNDERSTORMS ARE POSSIBLE ACROSS THE AREA TODAY. A FEW TORNADOES
ARE ALSO POSSIBLE MAINLY THIS AFTERNOON.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

ADDITIONAL THUNDERSTORM CHANCES ARE FORECAST FOR TUESDAY THROUGH WEDNESDAY
NIGHT. THE MOST WIDESPREAD THUNDERSTORM POTENTIAL ARRIVES TUESDAY NIGHT.
THE POTENTIAL FOR SEVERE WEATHER DURING THIS PERIOD WILL BE RESOLVED
WITH FUTURE FORECASTS.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NEEDED THIS AFTERNOON AND EARLY EVENING.

FAB

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
459 PM CDT FRI MAY 8 2009

IAZ036-047-048-082130-
BOONE-HAMILTON-STORY-
459 PM CDT FRI MAY 8 2009

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN BOONE...NORTHWESTERN
STORY AND SOUTHERN HAMILTON COUNTIES UNTIL 430 PM CDT...

AT 359 PM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR WAS TRACKING A
STRONG THUNDERSTORM 6 MILES NORTHWEST OF BOONE...OR 29 MILES
SOUTHEAST OF FORT DODGE...MOVING EAST AT 45 MPH.

WINDS OF 45 TO 55 MPH ARE EXPECTED WITH THIS STORM.

THIS STORM WILL BE NEAR...
  BOONE AIRPORT BY 405 PM CDT.
  GILBERT BY 420 PM CDT.
  RANDALL...STORY CITY AND JEWELL BY 425 PM CDT.

KINNEY

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