Flash Flood Watch 9, NWS Omaha, NE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
251 PM CDT THU AUG 23 2007

.DISCUSSION...
CONTINUED CHANCE FOR THUNDERSTORMS REMAINS THE PRIMARY FORECAST
CONCERN.

VERY HEAVY RAIN FELL OVERNIGHT...WITH THE HIGHEST AMOUNTS OVER
NORTHEAST NEBRASKA...MOST NOTABLY FROM STANTON TO WAYNE WHERE 4 TO
5 INCH AMOUNTS WERE COMMON. FLASH FLOOD WATCH WAS CANCELLED AROUND
NOON AS PRECIP HAS ENDED THERE...BUT COUNTY OR RIVER FLOOD
WARNINGS REMAIN IN EFFECT FOR SOME AREAS. LOOKS LIKE FOCUS FOR
POTENTIALLY HEAVY RAIN HAS SHIFTED SOUTH...SO NO ADDITIONAL
FLOODING IS EXPECTED WHERE WATER IS ALREADY RUNNING HIGH.

SOUTHEAST NEBRASKA AND SOUTHWEST IOWA ARE THE PRIMARY AREAS OF
CONCERN TONIGHT AND FRIDAY. COLD FRONT HAS SET UP THERE THIS
AFTERNOON WITH SOME CONVECTION ALREADY POPPING NEAR THIS BOUNDARY.
FRONT EXTENDED FROM CENTRAL IOWA INTO NORTHWEST KANSAS AT EARLY
AFTERNOON...WITH SOUTH TO SOUTHWEST WINDS PUMPING LOW TO MID 70S
DEW POINTS INTO SOUTHEAST NEBRASKA AND SOUTHWEST IOWA. INCREASE IN
CLOUD COVER HAS HELD INSTABILITY DOWN SOMEWHAT...BUT STILL LAPS
CAPE VALUES WERE INCREASING TO OVER 2000 J/KG WITH CAP ERODING.
ALSO OF NOTE IN VISIBLE SATELLITE LOOPS WAS VORT CENTER FORMING
OVER NORTHWEST KANSAS...AND BEGINNING TO DRIFT NORTHEAST ALONG
FRONTAL BOUNDARY. AS THIS LOW PROGRESSES NORTHEAST THIS
EVENING...INCREASING CONVERGENCE AND THETA-E ADVECTION AHEAD OF IT
WILL TRIGGER CONVECTION FROM NORTHEAST KANSAS INTO OUR SOUTHERN
CWA. HEAVY RAIN IS LIKELY AS PRECIPITABLE WATER VALUES AROUND 2
INCHES REMAIN POOLED IN THIS AREA. 3- AND 6-HOUR FLASH FLOOD
GUIDANCE VALUES ARE BETWEEN 1.5 AND 2.5 INCHES ACROSS SOUTHEAST
NEBRASKA AND SOUTHWEST IOWA...SO WILL LIKELY HAVE SOME WATER
PROBLEMS IF STORMS FIRE AS EXPECTED TONIGHT. SO HAVE OPTED TO
ISSUE FLASH FLOOD WATCH FOR SOUTHERN CWA TONIGHT.

CONVECTION WILL EXIT TO OUR EAST FRIDAY MORNING...DRAGGING FRONTAL
BOUNDARY SOUTH IN ITS WAKE. THIS SHOULD TAKE MOST OF THE
PRECIPITATION OUT OF OUR CWA...BUT SOME OVERRUNNING
SHOWERS/THUNDER POSSIBLE IN THE SOUTHEAST THROUGH THE AFTERNOON OR
EARLY EVENING. FARTHER TO THE NORTH...NAM AND GFS ARE SHOWING A
SECONDARY VORT MAX DROPPING INTO SOUTH DAKOTA DURING THE
AFTERNOON...AND SPREADING LIGHT QPF INTO NORTHERN NEBRASKA BY LATE
IN THE DAY. GFS IS MORE BEARISH ON PRECIP IN NORTHEAST NEBRASKA
AND WAS GENERALLY FOLLOWED. DID ADD A SLIGHT CHANCE FOR RAIN
THERE...BUT LACK OF MOISTURE IS A NEGATIVE FOR WIDESPREAD PRECIP EVENT.

COOLER AND DRIER CONDITIONS EXPECTED BEHIND FRONTAL SYSTEM THIS
WEEKEND. MODELS ARE SHOWING DEW POINTS IN THE 50S OR LOWER
60S...WHICH WOULD ALLOW MORNING LOWS TO DIP BELOW 60 IN THE NORTH.
RELATIVELY PLEASANT DAY ON SATURDAY WITH HIGHS IN THE LOWER 80S
AND LIGHT WINDS. RETURN FLOW ON SUNDAY WILL BEGIN TO DRAW MOISTURE
BACK INTO THE REGION...PROVIDING A KEY INGREDIENT FOR
PRECIPITATION FOR THE NEXT FRONTAL SYSTEM ON TUESDAY. WARMER TEMPS
EXPECTED AHEAD OF THAT FRONT ON MONDAY AND TUESDAY WITH HIGHS BACK
INTO THE LOWER 90S.

&&

.AVIATION...
SURFACE FRONT HAS PUSHED INTO SOUTHEAST NEBRASKA THIS AFTN WITH A
COUPLE OF BANDS OF SHOWERS POPPING UP IN ADVANCE OVER SERN NEB AND
SWRN IA. HAVE KEPT SOME CHANCE OF TRW IN TAFS FOR KOMA AND KLNK FOR
THE LATE AFTN AND EVENING DEPENDENT ON HOW EXTENSIVE CONVECTION
DEVELOPS AHEAD OF NEXT SHORT WAVE OVER WESTERN KS LATE THIS
AFTERNOON OR THIS EVENING. OTHERWISE PROBLEM FFURTHER NORTH WILL BE
DEALING WITH LOW CEILINGS AND SHOWERS IN THE COOLER AIR.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...FLASH FLOOD WATCH FROM 7 PM CDT THIS EVENING THROUGH FRIDAY
     MORNING FOR NEZ066>068-078-088>093.

IA...FLASH FLOOD WATCH FROM 7 PM CDT THIS EVENING THROUGH FRIDAY
     MORNING FOR IAZ079-080-090-091.

&&

DERGAN/FOBERT

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