Flash Flood and Flooding in Iowa, August 26 to 27, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
351 AM CDT WED AUG 26 2009

.SHORT TERM.../TODAY/
SURFACE FRONT HAS SLIPPED INTO SOUTHERN IOWA...CURRENTLY ROUGHLY
ALONG A LINE FROM OSKALOOSA TO MOUNT AYR...HOWEVER THE 850 MB FRONT
IS DRAPED ROUGHLY ALONG THE HIGHWAY 30 CORRIDOR THROUGH OUR AREA AND
IS SERVING AS THE PRIMARY FOCUS FOR CONVECTIVE DEVELOPMENT AT THIS
TIME. TRAINING CELLS OVER CENTRAL TAMA COUNTY A FEW HOURS AGO
PRODUCED ABOUT 3 INCHES OF RAIN IN A COUPLE OF HOURS AS EVIDENCED BY
RADAR DATA...AND THIS IS CONSISTENT WITH OBSERVATIONS FROM CEDAR
COUNTY IN EASTERN IOWA WHERE THE CELLS EXHIBITED SIMILAR BEHAVIOR.
THUS IT IS APPARENT THAT WITH WARM AND SATURATED LOWER ATMOSPHERIC
LEVELS VERY EFFICIENT RAIN PROCESSES ARE TAKING PLACE...AND HEAVY
RAIN WILL CONTINUE TO BE THE PRIMARY THREAT FROM CONVECTION THIS
MORNING.

EXPECT POPCORN DEVELOPMENT OF SHOWERS/STORMS TO CONTINUE INTO MID
MORNING ALONG AND SOUTH OF THE 850 MB FRONT...OR ALONG AND SOUTH OF
HIGHWAY 30...WHILE CONCURRENTLY A CLUSTER OF THUNDERSTORMS
ASSOCIATED WITH AN MCV FROM OLD CONVECTION OVER KS IS MOVING
NORTHEAST INTO TAYLOR COUNTY AT THIS HOUR...AND SHOULD SPREAD ACROSS
MUCH OF OUR SOUTHERN AREAS EARLY THIS MORNING. FURTHER UPSTREAM
LARGE AREAS OF SHOWERS AND THUNDERSTORMS ARE MOVING EASTWARD ALONG
THE NE/KS BORDER IN ASSOCIATION WITH A SHORTWAVE KICKING OUT OF THE
ROCKIES...AND WILL ALSO AFFECT OUR SOUTHERN AREAS THIS MORNING. HAVE
KEPT LIKELY POPS IN ROUGHLY ALONG AND SOUTH OF I-80 THIS
MORNING...WITH A PRE-FIRST PERIOD FOR EVEN HIGHER POPS IN THE
SOUTHWEST GIVEN CURRENT RADAR TRENDS...BUT HAVE BACKED OF POPS IN
MOST AREAS THIS AFTERNOON AS IT APPEARS THAT ONCE AGAIN DIURNAL
CYCLES WILL RESULT IN DIMINISHING PRECIP AFTER ABOUT MID-MORNING
MUCH AS IT DID YESTERDAY.

ONLY OTHER SIGNIFICANT CONCERN/CHANGES IN FORECAST FOR TODAY ARE
WITH MAX TEMPS. WHILE THICK CLOUD COVER WILL AFFECT ALL BUT OUR
NORTHERN FRINGES FOR MOST OF THE DAY...OVERNIGHT TEMPS HAVE HELD
CONSIDERABLY HIGHER THAN EXPECTED ALONG AND SOUTH OF THE 850 MB
FRONT/HIGHWAY 30 CORRIDOR DUE TO HIGH SURFACE DEWPOINTS AND
PERSISTENT IF LIGHT SURFACE WINDS. WHILE HEATING WILL BE LIMITED
TODAY...MOS FCST MAX TEMPS AS WELL AS INHERITED FORECAST WERE ONLY
ABOUT A DEGREE ABOVE CURRENT TEMPERATURES WHICH IS A BIT
PESSIMISTIC. HAVE THEREFORE RAISED MAX TEMPS BY SEVERAL DEGREES IN
THESE AREAS. OVERALL...EXPECT FORECAST UPDATES THROUGH THE DAY AS
CONVECTIVE TRENDS EVOLVE.

.LONG TERM.../TONIGHT THROUGH TUE/
CONVECTIVE TRENDS AND HEAVY RAIN POTENTIAL WILL BE THE PRIMARY FOCUS
TONIGHT INTO THU.  UNFORTUNATELY CONFIDENCE REMAINS QUITE LOW WITH
MOST MODELS ALL HANDLING DETAILS DIFFERENTLY.  INITIALLY FAVORED 00Z
GFS/12Z ECMWF SOLUTION WITH 00Z NAM BEING OUTLIER IN ITS LESS
PROGRESSIVE HANDLING OF KS LOW...HOWEVER 00Z ECMWF HAS NOW LEANED IN
THAT DIRECTION TOO.  THUS HAVE PRETTY MUCH BLENDED ALL INTO A
COMPROMISE SOLUTION PAYING LITTLE ATTENTION TO DETAILS AS CONVECTIVE
FEEDBACK HAS AFFECT MOST OUTCOMES AT LEAST SOMEWHAT AND RENDERED
QPF OUTPUT USELESS.

GENERAL FAVORED SCENARIO SUGGESTS ADDITIONAL DEVELOPMENT LATER
TONIGHT INTO THU AS LARGER SCALE FORCING AND MOISTURE CONVERGENCE
INCREASE AS ROCKIES TROUGH APPROACHES MO RIVER VALLEY.  EXPECT
AIRMASS TO BE MUCH LIKE IS CURRENTLY IN PLACE WITH PWS AND WCDS BOTH
QUITE FAVORABLE FOR HEAVY RAINS.  WILL CONTINUE TO HIGHLIGHT LOCALLY
HEAVY RAINS IN HWO.  ALTHOUGH DIRECTIONS ARE CERTAINLY
DIFFERENT...ALL MODELS EVENTUALLY MOVE THE MEANDERING UPPER SYSTEM
OFF TO THE S OR E LATER THU WITH POPS GRADUALLY DIMINISHING.  IT IS
INTERESTING TO NOTE...AND REINFORCES THE LOW CONFIDENCE IN THESE
PATTERNS...THAT GFS MOS POPS ARE AROUND 40 THU AT FOD/MCW EVEN
THOUGH THE MODEL UVM AND QPF SUGGESTS PRECIP SHOULD REMAIN OFF TO
THE S AND E.

MUCH LIKE PRECIP TRENDS...BIG TEMP DIFFERENCES ALSO NOTED THU WITH
GFS KEEPING THINGS SOCKED IN WITH CLOUDS AND PRECIP.  GFS BUFR
SOUNDINGS ARE 10-15F COOLER THAN THE NAM.  DO NOT WANT TO GET TOO
EXTREME...SO HAVE PRETTY MUCH PLAYED A COMPROMISE STAYING NEAR
GUIDANCE.  DID UNDERCUT GUIDANCE TOWARD THE WEEKEND HOWEVER AS
ANOMALOUS UPPER LEVEL TROUGH ENTERS GREAT LAKES THIS WEEKEND.  HAVE
ABOUT A CATEGORY BELOW GFS MOS IN THESE PERIODS.

&&

.AVIATION...26/06Z
ACTIVITY HAS DWINDLED EAST TONIGHT...BUT A FEW MORE SHOWERS CONTINUE
TO DEVELOP ALONG THE BOUNDARY OVER WEST CENTRAL IA. GFS CONTINUES TO
HIT THE AREA HARD WITH HEAVY RAIN WED THROUGH 12Z/28 WITH UPPER
LEVEL LOW AND CLOSED H850 LOW TRACKING SLOWLY NORTH.  4KM WRF DOES
NOT SUPPORT THE SAME SOLN...BUT MORE SCT TO LINE ACTIVITY OVER THE
FAR SOUTH DURING THE NEXT 24 HOURS. WITH LITTLE IN TERMS OF
CONVERGENCE ALOFT AND WEAK FLOW...HAVE LEANED MORE TOWARD A SOUTHERN
SOLN WITH REGARD TO RAIN/THUNDER DEVELOPING OVERNIGHT. BR WILL BE AN
ISSUE...ESPECIALLY NORTH OVERNIGHT AND LOWER VSBYS GENERALLY NORTH
OF I80 THROUGH EARLY MORNING. FOR THE REMAINDER OF THE DAY...BEST
CHANCES FOR IFR/MVFR CONDITIONS WILL REMAIN OVER THE SOUTH SITES
DURING THE DAY. FEEL WRF/NAM SOLNS BETTER HANDLING NORTHERN EXTENT
OF LOWER CIGS/VSBYS AND THUNDER CHANCES AT THIS TIME.

&&

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

&&

SHORT TERM...JWL
LONG TERM...BSS
AVIATION...REV

HAZARDOUS WEATHER OUTLOOK...CORRECTED
NATIONAL WEATHER SERVICE DES MOINES IA
618 AM CDT WED AUG 26 2009

IAZ036>039-044>050-057>062-070>075-081>086-092>097-271130-
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-
618 AM CDT WED AUG 26 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL AND
SOUTHERN IOWA.

.DAY ONE...TODAY AND TONIGHT

SCATTERED SHOWERS AND THUNDERSTORMS WILL CONTINUE TO AFFECT
CENTRAL IOWA THROUGH TONIGHT WITH MORE WIDESPREAD COVERAGE
POSSIBLE SOUTH. ALTHOUGH THE POTENTIAL FOR SEVERE WEATHER IS
RELATIVELY LOW...A GREATER THREAT MAY BE LOCALLY HEAVY RAINS AND
POTENTIALLY MINOR FLOODING.  PONDING WATER ON ROADS AND RAPID
RISES IN SMALL STREAMS AND CREEKS ARE POSSIBLE. RAINFALL AMOUNTS
MAY REACH ONE TO FOUR INCHES.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY

CHANCES FOR SHOWERS AND A FEW THUNDERSTORMS REMAIN IN THE FORECAST
TO THE SOUTH AND EAST OF DES MOINES THURSDAY AND THURSDAY NIGHT.
THE SEVERE WEATHER THREAT REMAINS RATHER LOW...BUT LOCALLY HEAVY
RAINS AND THE POTENTIAL FOR MINOR FLOODING WILL STILL EXIST.

.SPOTTER INFORMATION STATEMENT...

THE LIKELIHOOD OF SPOTTER ACTIVATION FOR SEVERE WEATHER IS
LOW...BUT REPORTS OF HEAVY RAINS OR FLOODING MAY BE NEEDED AND ARE
ALWAYS APPRECIATED.

IAZ004>007-015>017-023>028-033>035-271130-
EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO-
POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN-
WEBSTER-
618 AM CDT WED AUG 26 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTHERN IOWA.

.DAY ONE...TODAY AND TONIGHT

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL NOT BE NEEDED.

SMALL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
647 AM CDT WED AUG 26 2009

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0647 AM     HEAVY RAIN       5 S MANNING             41.84N 95.06W
08/26/2009  E4.50 INCH       AUDUBON            IA   BROADCAST MEDIA

            FROM KCCI TV VIEWER

&&

SMALL