Flooding in Iowa, October 14 to 18, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
249 PM CDT SUN OCT 14 2007

.SHORT TERM...TONIGHT THROUGH TUESDAY.

WATER VAPOR IMAGERY THIS AFTERNOON SHOWING STRONG UPPER TROF PUSHING
THROUGH THE ROCKIES WITH SIGNIFICANT 500MB HGT FALLS SPREADING ACROSS
THE CENTRAL PLAINS. AT THE SURFACE...A FRONTAL BOUNDARY STRETCHED
FROM THE TX PANHANDLE TO ABOUT NRN IA WITH STRONG CAA PUSHING
SOUTHWARD THRU WESTERN NEB AND WESTERN KS. AHEAD OF THE FRONT...STOUT
LOW LEVEL MOISTURE WAS IN PLACE.

INGREDIENTS FOR SIGNIFICANT PCPN OVER THE CWA TONIGHT LEADS TO
CONCERN FOR POTENTIAL FLASH FLOODING. AS DPVA INCREASES...UNORGANIZED
AREA OF LOW PRESSURE OVER THE CENTRAL/SOUTHERN PLAINS WILL EVENTUALLY
STRENGTHEN AND MOVE ACROSS THE CWA. FAVORABLE POSITIONING OF LFQ OF
UPPER JET AND OVERALL SLOW PROGRESSION OF THE SYSTEM WILL WORK TO
ENHANCE RAINFALL TOTALS BY MONDAY MORNING. WITH MODEL QPF EXCEEDING
FLASH FLOOD GUIDANCE...FEEL THAN FLASH FLOOD WATCH IS WARRANTED
TONIGHT FOR AREAS GENERALLY SOUTH OF A LINE FROM ABOUT MAPLETON
IOWA...UEHLING...AND LEIGH. ON MONDAY CORE OF UPPER LOW TREKS ACROSS
NEB AND SHOVES PRIMARY DYNAMICS/MOISTURE AXIS INTO THE MS VLY. SYSTEM
STILL WOUND UP TIGHT AND EXPECT WRAP AROUND PCPN WILL MOSTLY AFFECT
THE NORTHERN CWA THROUGH MONDAY NIGHT. BY TUESDAY UPPER SHORT WAVE
RIDGE BUILDS EASTWARD FROM THE ROCKIES ALLOWING FOR A SHORT DRY
PERIOD UNTIL TILL WEDNESDAY MORNING WHEN THE NEXT POTENT UPPER TROF
APPROACHES THE PLAINS. LLVL JET ONCE AGAIN CRANKS UP AND PULLS GULF
MOISTURE INTO THE REGION WHILE INCREASING DPVA EVENTUALLY LEADS TO
LEE SFC TROF EVOLVING TO BROAD CYCLONIC CIRCULATION. BOTTOM
LINE IS SIMILAR SCENARIO SETTING UP FOR SIGNIFICANT PCPN OVER
THE CWA WEDNESDAY WITH DEEP LAYER MOISTURE IN PLACE AND STOUT DYNAMIC
LIFT ENCOMPASSING THE PLAINS.

.LONG TERM...TUESDAY NIGHT THROUGH SATURDAY.

TIMING REMAINS A BIG PROBLEM FOR THIS FORECAST PERIOD AS A SERIES OF
FAST MOVING SHOT WAVES EMBEDDED IN STRONG PACIFIC JET CONTINUE TO
TRACK TOWARD THE CENTRAL AND NORTHERN PLAINS WITH VARYING DEGREES OF
AMPLIFICATION.  IT APPEARS THAT A NEG TILTED SHORT WAVE MOVING
ACROSS THE AREA WED NIGHT INTO THU WILL AMPLIFY AS IT MOVES INTO THE
NRN PLAINS...LIKELY CLOSING OFF TOWARD THE UPPER GRT LAKES.  BEHIND
IT...A MORE SRLY SHORT WAVE TENDS TO AMPLIFY SOMEWHAT AS IT PASSES
MOSTLY TO OUR SOUTH ON FRI.  12Z GFS IS PROBABLY OVERDONE WITH THIS
FEATURE...BUT MODELS CONTINUE TO SHOW ENORMOUS SPREAD IN TIMING AND
DEGREE OF AMPLIFICATION.  HPC PICKED A REASONABLE MIDDLE GROUND
RELYING ON ENSEMBLE MEAN...BUT THIS WILL TEND TO SERIOUSLY DOWNPLAY
SOME RATHER STRONG GRADIENTS IN THE VICINITY OF FEATURES THAT
SUCCEED IN AMPLIFYING.  FOR THIS FORECAST...KEPT A CHC OF TSTMS
ACROSS THE AREA WED NIGHT AND A LINGERING CHC EAST THU AS THE FIRST
WAVE EJECTS EWD.  FOR NOW KEPT ONLY A SLGT CHC OF SHWRS FROM MORE
SOUTHERN SYSTEM THU NGT AND FRI...BUT THIS WILL NEED TO BE BOOSTED
IF SYSTEM TURNS OUT TO HAVE ANYTHING LIKE THE INTENSITY OF THE 12Z
GFS.  ONE AREA OF DEPARTURE FROM HPC WAS COOLER TEMPS THU.  SOME
MODELS AT LEAST ARE SHOWING A LOT OF COOL ADVECTION DURING THE DAY
BEHIND INTENSIFYING NORTHERN PLAINS LOW.

FOLLOWING THE DEPARTURE OF THE SOUTHERN SYSTEM FRIDAY...WE WILL
LIKELY GET SEVERAL DAYS OF DRIER WEATHER UNDER SHORT WAVE
RIDGING...WITH SOMEWHAT WARMER DAYTIME TEMPS.  BOTH THE 12Z GFS AND
THE 00Z AND 12Z ECMWF BRING ANOTHER SHORT WAVE INTO MID CONTINENT
LATE NEXT WEEKEND WHICH WOULD GIVE US A FRONTAL PASSAGE NEXT SUN AND
A CHC OF RAIN AS EARLY AS SAT NGT.  FOR NOW...KEPT WITH HPC SOLUTION
FAVORING DRY WX AND A SLOWER WAVE.

.AVIATION...
FOR TAF SITES KOFK/KOMA/KLNK.

MIX OF FLIGHT CATEGORIES WL CONT OVERNIGHT WITH LIFR CIGS ACROSS
NERN NEB AND MVFR CONDITIONS ACROSS OMA AND LNK. SCT TSTMS WL CONT
THRU PD AS FRONTAL BOUNDARY MOVES THRU OVERNIGHT WITH LIKELY IFR TO
MVFR CONDITIONS IN THESE TSTMS. WINDS WL GO VARIABLE FOR A WHILE AND
BECOME MORE WEST OR NORTHWESTERLY BY MID MORNING BEHIND BOUNDARY
WITH MAINLY DRIZZLE AFTER SUNRISE.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...FLASH FLOOD WATCH THROUGH MONDAY MORNING FOR NEZ034-043>045-
     050>053-065>068-078-088>093.

IA...FLASH FLOOD WATCH THROUGH MONDAY MORNING FOR IAZ043-055-056-069-
     079-080-090-091.
&&

SHORT TERM...DEE
LONG TERM...POLLACK
AVIATION...BG

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