Flash Flood Watch 6, NWS Des Moines, IA

Adair County, IA, Adams County, IA, Audubon County, IA, Black Hawk County, IA, Boone County, IA, Bremer County, IA, Butler County, IA, Calhoun County, IA, Carroll County, IA, Cass County, IA, Cerro Gordo County, IA, Clarke County, IA, Crawford County, IA, Dallas County, IA, Emmet County, IA, Franklin County, IA, Greene County, IA, Grundy County, IA, Guthrie County, IA, Hamilton County, IA, Hancock County, IA, Hardin County, IA, Humboldt County, IA, Jasper County, IA, Kossuth County, IA, Lucas County, IA, Madison County, IA, Mahaska County, IA, Marion County, IA, Marshall County, IA, Palo Alto County, IA, Pocahontas County, IA, Polk County, IA, Poweshiek County, IA, Ringgold County, IA, Sac County, IA, Story County, IA, Tama County, IA, Taylor County, IA, Union County, IA, Warren County, IA, Webster County, IA, Winnebago County, IA, Worth County, IA, Wright County, IA

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE DES MOINES IA
1153 PM CDT WED MAY 28 2008

...UPDATED AVIATION DISCUSSION...

.SHORT TERM...TONIGHT THROUGH FRI NIGHT
SFC-850 MB RIDGE TO THE EAST OF THE UPPER MISSISSIPPI RIVER VALLEY
HAS KEPT A COOLER AND DRIER THAN NORMAL AIRMASS ACROSS MUCH OF THE
CWA. THE SFC RIDGE WILL SLOWLY GIVE WAY EASTWARD WITH TIME...WHILE A
SHORTWAVE OVER THE CENTRAL ROCKIES IS SPREADING RAPIDLY INTO THE
CENTRAL PLAINS TONIGHT. PLENTY OF SIMILARITIES IN THE RUC/NAM/GFS
BRINGING INTENSE 850 MOISTURE TRANSPORT AND THETA-E ADVECTION ACROSS
CENTRAL/EASTERN NEBRASKA OVERNIGHT...AND HAVE GONE WITH CATEGORICAL
HIGH POPS IN THE NORTHWEST AFTER 06Z. TEMPS WILL BE HARD WITH ON
GOING T-ADVECTION. POTENTIAL FOR HEAVY RAIN RATES IN THIS ZONE...
HOWEVER LIMITED INITIAL SEVERE THREAT AS SHOULD BE MAINLY ELEVATED
WITH LACK OF SIGNIFICANT CAPE.

DOMESTIC MODELS ARE PAINTING SIMILAR PICTURE FOR CONVECTIVE TRENDS
THU INTO FRI AND COULD VERY WELL TURN INTO ANOTHER SIGNIFICANT
EVENT.  EXPECT OVERNIGHT WARM ADVECTION MCS TO LINGER INTO FRI OVER
AT LEAST NRN HALF OF IA...PUSHING INTO NERN SECTIONS LATE IN THE
DAY.  ANY BREAK WILL LIKELY NOT LAST TOO LONG HOWEVER.  NAM/GFS
MLCAPE/CIN TRENDS BOTH INDICATE DEVELOPMENT ALONG NE FRONTAL
BOUNDARY NOSING INTO SIOUXLAND AREA.  SEE NO REASON WHY CIN WILL NOT
ERODE CONSIDERING ACTIVE SWLY FLOW AND PROXIMITY TO JET WITH COMPLEX
FIRING TO OUR WEST IN 2000-3500 J/KG MLCAPE AXIS THEN MOVING QUICKLY
INTO IA AT 30-40KTS...MAYBE EVEN TOWARD 50KTS.  THIS INSTABILITY
COMBINED WITH PLENTY OF SHALLOW AND DEEP SHEAR MAKES ALL STORM MODES
POSSIBLE /LARGE HAIL...DAMAGING WIND...AND TORNADOES/.  ALTHOUGH
INSTABILITY DIMINISHES SOMEWHAT OVERNIGHT...CIN STILL RELATIVELY LOW
WITH PERSISTENT AFOREMENTIONED SHALLOW AND DEEP SHEAR SO WOULD NOT
BE SURPRISED TO HAVE SVR AND EVEN TORNADO THREAT CONTINUE LONGER
THAN TYPICAL DIURNAL PEAK.  HEAVY RAINS AND FLOODING POSSIBLE AS
WELL WITH DETAILS NOTED IN HYDROLOGY SECTION BELOW.

MCS SHOULD LINGER INTO FRI BUT DISSIPATE TOWARD MIDDAY AS LOW LEVEL
JET VEERS.  FRONTAL BOUNDARY WILL STILL LAG HOWEVER AND BE ORIENTED
SW-NE ACROSS HEART OF IA DURING PEAK HEATING.  NAM/GFS AGAIN
DIMINISH CIN AND REALLY BOOST MLCAPE BY AFTERNOON INTO 3000-4000
J/KG RANGE...WHICH MAKES SENSE CONSIDERING MOISTURE HAS ANOTHER DAY
TO ESTABLISH ITSELF IN THE WARM SECTOR WITH MOIST SOILS IN PLACE.
ANOTHER ROUND OF CONVECTION WILL LIKELY DEVELOP SERN HALF AND MOVE
OFF TO THE EAST.  ALTHOUGH KINEMATIC PARAMETERS ARE NOT AS
STRONG...HEIGHTENED INSTABILITY MAY OFFSET THAT SOMEWHAT WITH MORE
SVR WX POSSIBLE.  CONVECTIVE CHANCES SHOULD THEN FINALLY DIMINISH
INTO THE NIGHT.

PCPN TOMORROW MAY RESULT A DECENT TEMP GRADIENT SO HAVE COOLED
THINGS OFF NE AND WARMED THEM SW...BELOW AND ABOVE RELATIVELY
GUIDANCE EXTREMES.  NOT AS MUCH OF A CHANGE FRI BUT STILL TRIMMED
HIGHS NE DUE TO PCPN.

.LONG TERM...SAT THROUGH WED
CONVECTIVE TRENDS WILL AGAIN BE THE PREDOMINATE WX CONCERN THIS
PERIOD...BUT OVERALL LESS ACTIVE THAN THE END OF THIS WORK WEEK.
FAIR WX SHOULD BE IN PLACE FOR THE WEEKEND AS WEAK ROCKIES RIDGING
SLOWLY ADVANCES TOWARD THE MO VALLEY AFTER PASSAGE OF LATE WEEK
FRONT.  GENERALLY FOLLOWED BLEND OF GEFS/ECMWF INTO NEXT WEEK WITH
INFLUENCES FROM HPC AND ECMWF MEAN.  RIDGE SHOULD MOVE FAR ENOUGH
EAST TO GET IA INTO THE WARM ADVECTION ZONE BY LATE SUN NIGHT INTO
MONDAY.  THIS SHOULD BE FOLLOWED BY A PROLONGED PERIOD OF IL-DEFINED
CHANCES WITH MODEL CONCENSUS SMOOTHING OUT ANY DETAILS.  THIS
SCENARIO WOULD SEEM TO KEEP CONVECTIVE POTENTIAL ELEVATED WITH
RELATIVELY WEAK LOW LEVEL FLOW IN PLACE AND SFC BOUNDARYS REMAINING
TO OUR SOUTH AND WEST.  ECMWF DOES DEPICT A VERY STRONG SYSTEM
AROUND WED...BUT THIS WILL BE IGNORED FOR THE TIME BEING CONSIDERING
IT IS DAY 7 WITH LITTLE ADDITIONAL SUPPORT AT THE MOMENT.  NO
SIGNIFICANT CHANGES TO GOING TEMPS WITH RELATIVELY FLAT SEASONAL
READINGS ANTICIPATED.

&&

.HYDROLOGY...
MANY HEAVY RAIN AND FLOODING PARAMETERS ARE IN PLACE FOR THU NIGHT.
310K/H85 MOISTURE TRANSPORT AND CONVERGENCE IS VERY STRONG INTO
CENTRAL IA WITH PWS APPROACHING 2 INCHES...VERY ANOMALOUS FOR LATE
MAY...AND EVEN EXCEEDING THAT IN ISOLATED SPOTS.  12-13KFT WCDS WILL
MAKE FOR VERY EFFICIENT RAINERS.  ONLY DRAWBACK WILL BE FAST MEAN
WIND AND SYSTEM MOTION.  ALTHOUGH PATTERN DOES FIT CONCEPTUAL UPPER
AND LOW LEVEL JET MODELS SOMEWHAT...ULJ MAY BE TOO CLOSE AND PUSH
THE MCS OUT QUICK ENOUGH.  STRENGTH OF MEAN WIND AND LLJ ALSO
RESULTS IN RELATIVELY TRANSIENT CORFIDI VECTORS.  BECAUSE THIS IS
3RD PERIOD AND ABOVE CONCERNS...WILL NOT GO WITH FLASH FLOOD WATCH
YET BUT CONTINUE ESF.  FFA COULD CERTAINLY BE NEEDED BY MID SHIFT OR
TOMORROW HOWEVER WITH STORM TOTAL PCPN 1-3.5 INCHES IN GRIDS AT THE
MOMENT.

&&

.AVIATION...29/06Z
THUNDERSTORMS HAVE FIRED OVER NORTHEAST AND CENTRAL NEBRASKA.
PRIMARY CONCERN FOR TAF SITES WILL BE THE CENTRAL NEBRASKA
CONVECTION WITH STORM MOTION VECTORS BRINGING THIS INTO NORTHERN TAF
SITES LATE THROUGH EARLY FRIDAY MORNING. ADDITIONAL CONVECTION
EXPECTED NEAR DSM/FOD/MCW LATE THURSDAY AFTERNOON WITH POTENTIAL
CONTINUING THURSDAY EVENING. VFR CONDTIONS EXPECTED TO MAINTAIN AT
OTM THROUGH THE PERIOD AND AT DSM FOR MUCH OF THE PERIOD.

&&

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

&&

LONG TERM/SHORT TERM...SMALL/RUSSELL
AVIATION...DONAVON

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