Flash Flooding and Damaging Winds in Iowa, August 13, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
624 AM CDT FRI AUG 13 2010

...UPDATED AVIATION DISCUSSION...

.SHORT TERM /TODAY/...
MAIN FORECAST TODAY IS WITH TIMING OF CONVECTION DEVELOPING AHEAD OF
COLD FRONT THIS AFTERNOON.  MODELS ARE IN DECENT AGREEMENT WITH
TIMING OF FROPA AND STRENGTH OF SHORTWAVE CUTTING ACROSS THE STATE
THIS AFTERNOON.  WENT WITH A BLEND OF NAM/SREF/GFS THROUGH 00Z
SATURDAY.

POTENT SHORTWAVE AND SHARP COLD FRONT LOCATED ACROSS THE NORTHERN
AND CENTRAL PLAINS WILL CONTINUE TO PRESS EASTWARD THROUGHOUT THE
DAY TODAY. STRONG MOISTURE TRANSPORT AND WAA AHEAD OF THE FRONT
CONTINUES TO FEED THE AIRMASS THROUGH THE AFTERNOON AND PW VALUES 2
TO 2.25 INCHES. INSTABILITY PARAMETERS ARE EXTREMELY HIGH...WITH ML
CAPES RANGING FROM 2500 TO 4000 J/KG BY 18Z AND MUCAPES NEAR 7000
J/KG WITH NOT A VERY STRONG CAP OR ANY AT ALL WITH 700MB TEMPS
10-12C. PLENTY OF QG FORCING OUT ALONG AND AHEAD OF THE FRONT...AND
NAM HAS CAP ERODING B/T 18-21Z. 0-1 KM SHEAR AVERAGES AROUND 25KTS
WITH 0-1KM HELICITY RANGING ANYWHERE FROM 50 TO 200M2/S2 BY
18Z...AND THEN ALONG THE FRONT GENERALLY 100 OR GREATER BY 00Z
SATURDAY. POTENTIAL FOR SUPERCELL DEVELOPMENT WITH THE INITIAL
CONVECTION THEN TRANSITIONING TO LINEAR MCS BY LATE THIS AFTERNOON.
HAVE MENTION OF HEAVY RAIN AND SEVERE WX PAST 21Z ACROSS THE CENTRAL
AND SOUTHEAST. POTENTIAL IS THERE FOR SPOTS ACROSS CENTRAL IOWA TO
RECEIVE HEAVY RAIN BY THIS AFTERNOON AGGRAVATING THE FLOODING
SITUATION.

HIGH TEMPS LOOK TO QUICKLY RISE INTO THE LOWER TO MIDDLE 90S WITH
THE STRONG WAA AHEAD OF FRONT. THIS COMBINED WITH DEW POINTS IN THE
UPPER 70S...KEPT WITH THE HEAT HEADLINES OVER THE SOUTH. ESPECIALLY
SINCE TEMPS HAVE NOT FALLEN OFF TOO MUCH OVER NIGHT SOUTH OF I-80.

.LONG TERM /TONIGHT THROUGH THURSDAY/...
PRIMARY CONCERN WILL BE EVOLUTION AND TRENDS OF CONVECTION WHICH
WILL LIKELY BE ONGOING AT THE START OF THE PERIOD.  NEARLY ALL
MODELS PLACE SLOW MOVING COLD FRONT INTO WRN IA BY 00Z WITH
CONVECTION ALONG AND AHEAD OF IT DURING THE EARLY EVE.  CURRENT CO
SHORT WAVE SHOULD REACH SIOUXLAND AREA 18Z-00Z WITH ADEQUATE
FORCING WHICH SHOULD HAVE NO TROUBLE BLOSSOMING SIGNIFICANT MCS
ACROSS IA CONSIDERING AIRMASS IN PLACE.  THIS FORCING AND
INSTABILITY DOES NOT COMBINE TO OFTEN IN AUGUST.

HAVE INCLUDED SVR AND HEAVY RAIN MENTION IN THE FORECAST...WITH
HEAVY RAIN AREA VERY SIMILAR TO HPC EXCESSIVE RAIN OUTLOOK.  MLCAPES
ARE ALREADY 3K J/KG IN WRN IA SO INSTABILITY PRESENT BY EARLY
EVENING SHOULD BE MORE THAN ENOUGH TO COMBINE WITH MODERATE DEEP
SHEAR TO ORGANIZE CONVECTION INTO SUPERCELLS...AT LEAST INITIALLY.
LARGE HAIL...DAMAGING WINDS AND TORNADOES ALL APPEAR POSSIBLE WITH
TORNADIC TREAT IN THIS PERIOD DEPENDENT ON INITIATION TIME.  HEAVY
RAIN WISE...THE LACK OF MORNING CONVECTION MAY HAVE SPARED NRN IA
BUT CHANGED THE FOCUS INTO AREAS FARTHER SOUTH THAT CERTAINLY CANNOT
TAKE MUCH MORE RAINFALL.  ALL MODELS SUGGEST LITTLE CHANGE IN THE
SW-NE ORIENTED MEAN WIND THROUGH THE LATE AFTERNOON AND EARLY
EVENING HOURS SUGGESTING STORMS HAVE THE POTENTIAL TO TRAIN OVER
SIMILAR AREAS FOR SEVERAL HOURS.  THIS ALL OCCURRING IN ENVIRONMENT
QUITE CONDUCIVE FOR PRODUCING EFFICIENT RAINS WITH HIGH PWS...WCDS
AND K INDICES WITH PROLONGED STRONG MOISTURE TRANSPORT.  MOISTURE
TRANSPORT VECTORS IN LOWEST 100MB ARE JUST AS STEADY STATE AS MEAN
WIND...AND OPPOSITE...WHICH MAY ANCHOR RAIN AXIS SIMILAR TO A MADDOX
SYNOPTIC HEAVY RAIN EVENT.  H85 MOISTURE TRANSPORT ANOMALIES ARE UP
TO 3 SDS IN A FEW MODELS.  WITH THIS IN MIND HAVE EXTENDED FLASH
FLOOD WATCH THROUGH 06Z FOR ALL BUT WRN SECTIONS.

FRONT AND ASSOCIATED FORCING SHOULD EXIT OVERNIGHT.  HOWEVER YET
ANOTHER SHORT WAVE MAY BRING LIGHT PRECIP TO REGION SAT NIGHT.
FORCING IS CERTAINLY NOT STRONG...BUT BITTERROOTS SHORT WAVE WILL
DROP INTO THE MO RIVER VALLEY AND MAY DEVELOP SCT SHOWERS/TSTMS SO
HAVE ADDED LOW POPS.  THERE ARE A FEW OTHER POTENTIAL PRECIP WINDOWS
INTO EARLY NEXT WEEK...BUT AMOUNT OF UVM AND CONFIDENCE IN THOSE
FEATURES IS NOT GREAT ENOUGH TO INCLUDE MENTION AT THIS TIME.

STAYED CLOSE TO GUIDANCE BLEND FOR TEMP FORECAST.  HOWEVER CONTINUED
TO NUDGE HIGHS UP A TAD SAT AS COOLER/LESS HUMID AIRMASS DOES NOT
REALLY APPEAR TO TAKE HOLD UNTIL SUN PER FORECAST BUFR SOUNDINGS.

&&

.AVIATION...13/12Z
WAA AHEAD OF THE STRONG SHORTWAVE AND COLD FRONT WILL PROVIDE SOME
BREEZY WINDS THIS MORNING. TIMING OF CONVECTION DEVELOPMENT LOOKS TO
BE AROUND MIDDAY AND INTO THE AFTERNOON...BUT KEPT WITH VFR
CONDITIONS THROUGH THE MAJORITY OF THE TAF FORECAST.  WAS MORE
CONFIDENT WITH MENTION OF MVFR VIS FOR TSRA AT ALO AND DSM...BUT
COULD VERY EASILY GET DOWN TO IFR CONDITIONS DURING THE AFTERNOON.
LOW CONFIDENCE WITH TIMING SO THIS WILL BE BETTER MONITORED BY NEXT
TAF ISSUANCE.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
EXCESSIVE HEAT WARNING ALONG AND SOUTH OF I80 TIL 00Z
FLASH FLOOD WATCH ALL BUT SW SECTIONS TIL 00Z
FLASH FLOOD WATCH ALL BUT W CENTRAL AND NW SECTIONS TIL 06Z

&&

SHORT TERM...KLP
LONG TERM...BSS
AVIATION...KLP

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
606 AM CDT FRI AUG 13 2010

IAZ004>007-015>017-023>028-033>039-044>050-141100-
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-
606 AM CDT FRI AUG 13 2010

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

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS ARE EXPECTED TO EXPAND IN AREAL COVERAGE AND
INTENSITY ACROSS THE AREA LATER TODAY AND POSSIBLY INTO THE
EVENING HOURS. THESE STORMS MAY BE SEVERE WITH HEAVY
RAINS...POSSIBLY TWO TO FOUR INCHES LOCALLY. WITH THE GROUND
SATURATED IN SOME AREAS...FLASH FLOODING MAY DEVELOP. LARGE
HAIL...DAMAGING WINDS AND ISOLATED TORNADOES ARE ALSO POSSIBLE.

FLASH FLOOD WATCHES AND A FEW RIVER FLOOD WARNINGS ARE IN EFFECT
FOR THE AREA. PLEASE REFER TO THE NATIONAL WEATHER SERVICE WEBSITE
AT WEATHER.GOV/DESMOINES FOR FURTHER DETAILS.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

THUNDERSTORM CHANCES RETURN TO THE FORECAST AROUND TUESDAY. THE
SEVERE WEATHER AND HEAVY RAIN POTENTIAL APPEARS TO BE LOW AT THIS
TIME.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL LIKELY BE NEEDED FOR A PERIOD LATER TODAY
OR THIS EVENING.

IAZ057>062-070>075-081>086-092>097-141100-
AUDUBON-GUTHRIE-DALLAS-POLK-JASPER-POWESHIEK-CASS-ADAIR-MADISON-
WARREN-MARION-MAHASKA-ADAMS-UNION-CLARKE-LUCAS-MONROE-WAPELLO-TAYLOR-
RINGGOLD-DECATUR-WAYNE-APPANOOSE-DAVIS-
606 AM CDT FRI AUG 13 2010

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

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS ARE EXPECTED TO EXPAND IN AREAL COVERAGE AND
INTENSITY ACROSS THE AREA LATER TODAY AND INTO THE EVENING HOURS.
THESE STORMS MAY BE SEVERE WITH HEAVY RAINS...POSSIBLY TWO TO
FOUR INCHES LOCALLY. WITH THE GROUND SATURATED IN SOME
AREAS...FLASH FLOODING MAY DEVELOP. LARGE HAIL...DAMAGING WINDS
AND ISOLATED TORNADOES ARE ALSO POSSIBLE.

IT WILL BE ANOTHER HOT AND HUMID DAY ACROSS THE AREA AS WELL. WITH
HIGHS IN THE LOWER TO MIDDLE 90S AND DEWPOINTS WELL INTO THE
70S...HEAT INDICES WILL AGAIN REACH 105 TO 112 DEGREES DURING THE
PEAK HEATING OF THE AFTERNOON.

FLASH FLOOD WATCHES...AN EXCESSIVE HEAT WARNING...AND A FEW RIVER
FLOOD WARNINGS ARE IN EFFECT FOR THE AREA. PLEASE REFER TO THE
NATIONAL WEATHER SERVICE WEBSITE AT WEATHER.GOV/DESMOINES FOR
FURTHER DETAILS.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

THUNDERSTORM CHANCES RETURN TO THE FORECAST SATURDAY NIGHT AND
AGAIN AROUND TUESDAY. THE SEVERE WEATHER AND HEAVY RAIN POTENTIAL
APPEARS TO BE LOW AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL LIKELY BE NEEDED LATER TODAY AND THIS
EVENING.

SMALL

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
1110 AM CDT FRI AUG 13 2010

IAZ039-131645-
BLACK HAWK-
1110 AM CDT FRI AUG 13 2010

...THUNDERSTORM WITH SMALL HAIL...

AT 1108 AM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
STRONG THUNDERSTORM OVER NORTHEASTERN BLACK HAWK COUNTY...MOVING
NORTHEAST AT 30 MPH.

PENNY SIZE HAIL...WINDS GREATER THAN 40 MPH...BRIEF HEAVY
DOWNPOURS...ARE POSSIBLE WITH THIS STORM.

* THE THUNDERSTORM WILL BE NEAR...
  RURAL BLACK HAWK COUNTY AT 1120 AM CDT

GUSTY WINDS MAY CAUSE SMALL OBJECTS SUCH AS TRASH BINS TO BLOW
AROUND. SEEK SHELTER IN A STURDY STRUCTURE UNTIL THIS STORM HAS
PASSED.

BEERENDS