Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 281720
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1220 PM CDT THU AUG 28 2014

.SHORT TERM /TODAY/...
ISSUED AT 305 AM CDT THU AUG 28 2014

THE UPPER LEVEL LONGWAVE TROUGH WILL SHIFT A LITTLE MORE TO THE EAST
TODAY PUSHING THE UPPER LEVEL RIDGE OF HIGH PRESSURE INTO ILLINOIS.
THE UPPER JET WILL BE RIGHT OVER IOWA TODAY WITH BROAD UPPER LEVEL
DIVERGENCE MOVING ACROSS THE CWA THROUGH 18Z-21Z.  COINCIDENT WITH
THIS WILL BE AN AREA OF QG FORCING ACROSS CENTRAL INTO NORTHERN
IOWA.  AT THE SURFACE...A BROAD LOW PRESSURE SYSTEM OVER THE
CENTRAL/SOUTHERN PLAINS WILL PULL SLOWLY TO THE NORTHEAST DRAGGING A
WARM FRONT UP ACROSS CENTRAL IOWA THROUGH THE AFTERNOON.

THE MODELS HAVE BEEN SLOWER IN BRINGING THE HEAVIER PRECIP INTO
CENTRAL IOWA BUT DO IN FACT BRING IT ACROSS US FROM MID TO LATE
MORNING INTO EARLY AFTERNOON.  THERE IS NO QUESTION THAT WE WILL SEE
THUNDERSTORMS MOVE INTO THE AREA BY DAYBREAK AND THEY WILL INCREASE
IN COVERAGE THROUGH MID MORNING BUT THE MODELS ARE FAIRLY
PROGRESSIVE WITH THE PRECIPITATION ACROSS THE SOUTHWEST WHERE FLASH
FLOOD GUIDANCE IS LOWEST. HOWEVER IT WILL NOT TAKE MUCH TO PRODUCE
FLASH FLOODING IN THE AREA GIVEN THE ANTECEDENT MOISTURE CONDITIONS.
PRECIP IS EXPECTED TO THEN MOVE INTO AND ACROSS NORTHERN
IOWA...BETWEEN HIGHWAY 30 AND THE MINNESOTA BORDER BY LATE MORNING
INTO EARLY AFTERNOON.  THERE WILL STILL BE A HEAVY RAIN POTENTIAL
BUT NORTHERN IOWA CAN TAKE MORE RAINFALL.

THE CURRENT FLASH FLOOD WATCH LOOKS GOOD ALTHOUGH IT MAY NEED TO BE
EXTENDED IF THE PRECIP DOES IN FACT MOVE OUT SLOWER THAN EXPECTED.
LOCAL RAINFALL AMOUNTS OF 1 TO 3 INCHES ARE LIKELY WITH STRONGER
STORMS PRODUCING LOCALLY HIGHER AMOUNTS.

.LONG TERM /TONIGHT THROUGH TUESDAY/...
ISSUED AT 305 AM CDT THU AUG 28 2014

UPPER LEVEL LOW PRESSURE SYSTEM WILL FINALLY BEGIN TO PUSH
EASTWARD THROUGH THE DAY TODAY...WHICH WILL HELP TO EVENTUALLY
BRING AN END TO PROLONGED PERIODS OF PRECIPITATION CHANCES.
HOWEVER...BEFORE SYSTEM PUSHES EAST FOR GOOD...A FEW MORE ROUNDS
OF PRECIPITATION CAN BE EXPECTED. FOR BEGINNING OF
PERIOD...ANTICIPATE DAYTIME CONVECTION TO CONTINUE TO PUSH TO THE
NORTHEAST NORTH OF IOWA...THOUGH MAY LINGER INTO NORTHERN CWA.
BEHIND THIS PUSH OF PRECIPITATION FORCING WILL AGAIN
INCREASE...LIFTING INTO IOWA BEHIND BOUNDARY. FORCING IS MUCH LESS
WELL DEFINED AND MODELS ARE NOT IN GOOD AGREEMENT WITH TIMING OR
PLACEMENT OF PRECIPITATION. GIVEN AIRMASS WILL GENERALLY BE
WORKED OVER FROM DAYTIME PRECIPITATION...THOUGH WILL REMAIN VERY
MOIST...HAVE CUT BACK ON POPS WITH MORE SCATTERED PRECIPITATION
EXPECTED. HOWEVER...STORMS WILL AGAIN BE SLOW MOVING AND PWAT
RATES REMAIN NEAR 1.5 INCHES WITH GOOD WARM CLOUD DEPTHS.
THEREFORE...ANY PRECIPITATION MAY AGAIN BE HEAVY. WILL HOLD OFF ON
FLASH FLOOD WATCH ATTM...THOUGH MAY BE NEEDED OVERNIGHT DEPENDING
ON EVOLUTION OF DAYTIME PRECIPITATION TRENDS ACROSS THE CWA.
ANOTHER ROUND OF PRECIPITATION WILL BE POSSIBLE FRIDAY...AS
SECONDARY BOUNDARY AND UPPER LEVEL TROUGH PUSH EAST ACROSS IOWA.
FORCING IS AGAIN SOMEWHAT LACKING DURING THE DAY...THOUGH
INCREASES ACROSS THE SOUTHEAST LATE IN THE DAY AND HAVE THEREFORE
KEPT BEST POPS THERE...LINGERING INTO FRIDAY NIGHT.

SYSTEM WILL FINALLY PUSH EAST SATURDAY WITH A BRIEF DRY PERIOD IN
STORE. HOWEVER A STRONG UPPER LEVEL LOW WILL PUSH EAST ACROSS
CANADA WITH ASSOCIATED FRONTAL BOUNDARY PUSHING INTO WESTERN IOWA
BEGINNING SUNDAY. THIS WILL BRING ADDITIONAL CHANCES FOR
PRECIPITATION TO THE AREA FOR SUNDAY AND MONDAY BEFORE THIS SYSTEM
ALSO PUSHES EAST. MODELS HAVE BEGUN TO SLOW ONSET OF THIS SECOND
SYSTEM SOMEWHAT...WITH BEST PRECIPITATION CHANCES ARRIVING SUNDAY
NIGHT. HOWEVER...INSTABILITY WILL BE SUFFICIENT AHEAD OF SYSTEM
THAT IF SYSTEM ARRIVES DURING BEST DIURNAL HEATING MAY SEE THREAT
FOR SEVERE THUNDERSTORMS...THOUGH NOT AS CONFIDENT GIVEN SLOWER
TIMING.

TEMPERATURES WILL REMAIN FAIRLY STEAD THROUGH THE WEEK...THOUGH
PRECIPITATION TRENDS AND CLOUDS MAY HELP KEEP TEMPERATURES COOLER
THAN GOING AT TIMES...ESPECIALLY IN THE OVERNIGHT HOURS.
CONDITIONS WILL BE SLIGHTLY COOLER BEHIND SECONDARY LOW FOR EARLY
IN THE WORK WEEK.

&&

.AVIATION...28/18Z
ISSUED AT 1220 PM CDT THU AUG 28 2014

MORNING SHOWERS AND THUNDERSTORMS ARE CLEARING THE AREA TO THE
NORTHEAST...BUT WILL LINGER ESPECIALLY AT MCW/OTM INTO THIS
AFTERNOON. BR AND IFR/LIFR CIGS ARE SIMILARLY TRYING TO CLEAR OUT
AND SHOULD EXIT FOD/DSM WITHIN THE NEXT HOUR OR TWO...BUT AGAIN AT
MCW/OTM MAY PERSIST LONGER AND IT IS DIFFICULT TO TELL WHETHER
THEY WILL LIFT AT ALL AT MCW TODAY. TONIGHT EXPECT MORE SHRA/TSRA
DEVELOPMENT ESPECIALLY AT FOD/MCW...ALONG WITH MORE LOWER CIGS AND
BR IN MOST AREAS AFTER MIDNIGHT. HAVE TAKEN A SHOT AT TRENDING
THIS WAY IN THE 18Z TAFS...BUT DETAILS WILL NEED TO BE REFINED
THIS EVENING/TONIGHT.

&&

.HYDROLOGY...
ISSUED AT 305 AM CDT THU AUG 28 2014

RAINFALL RATES OF 1 TO 2 INCHES PER HOUR WITH LOCALLY HIGHER
RATES ARE POSSIBLE OVER WEST-CENTRAL TO SOUTHWEST IOWA EARLY THIS
MORNING AND... WILL LIFT NORTH AND EAST THROUGH THE DAY TODAY.
ACROSS THE SOUTH AND WEST...1-HR FFG IS AROUND AN INCH AND 3-HR
FFG IS LESS THAN 2 INCHES IN MOST PLACES...SO FLASH FLOODING
REMAINS A CONCERN. WITH THE WIDESPREAD QPF AMOUNTS OF 1 TO 3
INCHES TODAY...THE RACCOON AND SMALLER RIVER BASINS MAY SEE RISES
BY FRIDAY AND ATTM MAINLY REACHING ACTION STAGE OR MINOR FLOOD
STAGE.

&&

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

&&

$$

SHORT TERM...FAB
LONG TERM...AWB
AVIATION...LEE
HYDROLOGY...PODRAZIK/REV/AWB



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