Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 291742
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1242 PM CDT FRI AUG 29 2014

.SHORT TERM /TODAY/...
ISSUED AT 329 AM CDT FRI AUG 29 2014

THE FORECAST IS BEGINNING TO SOUND LIKE A BROKEN RECORD BUT TODAY
THE UPPER TROUGH FINALLY BEGINS TO SHIFT EAST ACROSS THE UPPER
MIDWEST.  THE TROUGH AXIS IS PROGGED TO BE OVER THE MISSOURI RIVER
VALLEY AROUND 18Z AND SLOWLY PROGRESS ACROSS IOWA THROUGH TONIGHT.
A SURFACE LOW OVER THE SIOUX FALLS AREA WILL LIFT OFF TO THE
NORTHEAST TODAY WHICH WILL DRAG A COLD FRONT ACROSS CENTRAL IOWA
DURING PEAK HEATING.  UNFORTUNATELY THAT WILL MEAN MORE
THUNDERSTORM DEVELOPMENT THIS AFTERNOON INTO THE EVENING.

CAPES ARE FORECAST TO BE BETWEEN 1500 AND 2500 SO WE WILL BE QUITE
UNSTABLE TODAY WITH QUITE A BIT OF FORCING MOVING ACROSS THE
FORECAST AREA ASSOCIATED WITH THE FRONT.  WHILE SHEAR IS NOT GREAT
IT SHOULD BE ENOUGH FOR A FEW STRONG TO SEVERE STORMS TO DEVELOP.
HEAVY RAINFALL IS ONCE AGAIN LIKELY ESPECIALLY WITH THE STRONGEST
STORMS.  HOWEVER THE HEAVIEST RAINFALL SHOULD BE ACROSS NORTHERN
IOWA WHERE THEY CAN TAKE A BIT MORE RAINFALL. WE WILL HAVE THE ADDED
CONCERN THIS AFTERNOON FOR SOME STORMS CAPABLE OF PRODUCING LARGE
HAIL AND DAMAGING WIND IN ADDITION TO THE HEAVY RAINFALL POTENTIAL.

MORNING CONVECTION SHOULD LIFT OFF TO THE NORTHEAST AND DIMINISH
BEFORE RE-DEVELOPMENT THIS AFTERNOON.  THERE IS SOME CONCERN ABOUT
THE STRATUS THAT HAS BEEN PREVALENT ACROSS THE NORTH AND EAST BUT
ONCE THE SUN IS UP THE LOW CLOUDS SHOULD LIFT AND THERE SHOULD BE
ENOUGH BREAKS IN THE CLOUDS TO ALLOW SUFFICIENT HEATING BY THE
AFTERNOON TO DESTABILIZE THE AIRMASS. AS FOR TEMPS...THEY SHOULD BE
A FEW DEGREES WARMER ACROSS THE SOUTHERN TWO THIRDS OR SO OF THE
FORECAST AREA.  WE WILL BE IN WARM ADVECTION AHEAD OF THE FRONTAL
BOUNDARY AND SHOULD BREAK OUT OF THE CLOUDS A LITTLE EARLIER TODAY
ACROSS THAT AREA.  NORTHERN IOWA WILL STILL BE SOMEWHAT COOLER WITH
HIGHS IN THE MID TO UPPER 70S.

.LONG TERM /TONIGHT THROUGH THURSDAY/...
ISSUED AT 329 AM CDT FRI AUG 29 2014

PERSISTENT UPPER LEVEL LOW WILL CONTINUE TO PULL EASTWARD AWAY
FROM IOWA THROUGH THE BEGINNING OF THE PERIOD...HOWEVER...ONE LAST
ROUND OF PRECIPITATION WILL BE POSSIBLE WITH THIS SYSTEM TONIGHT
BEFORE IT FINALLY PUSHES EAST. AS FRONTAL BOUNDARY PUSHES EAST
ACROSS IOWA OVERNIGHT TONIGHT...PRECIPITATION CHANCES WILL REMAIN
POSSIBLE...AND ANOTHER ROUND OF HEAVY RAINFALL WILL ALSO BE
POSSIBLE. FORCING IS SLIGHTLY LESS WITH THIS SYSTEM...THOUGH GIVEN
BOUNDARY PUSHING THROUGH AND MOISTURE ENVIRONMENT...EXPECT
PRECIPITATION...ESPECIALLY ACROSS THE EAST WHERE FORCING IS BEST.
BEHIND SYSTEM THERE WILL BE A BRIEF DRY PERIOD SATURDAY AND MOST
OF SUNDAY. HOWEVER...THIS WILL BE SHORT LIVED AS ANOTHER LOW
PRESSURE SYSTEM PUSHES EAST ACROSS SOUTHERN CANADA/NORTHERN US.
THIS WILL DRAG A FRONTAL BOUNDARY INTO IOWA BEGINNING LATE SUNDAY.
AHEAD OF THE SYSTEM...SOUTHERLY FLOW WILL HELP TO KEEP MOISTURE IN
PLACE...AND THUNDERSTORMS MAY AGAIN BRING HEAVY RAINFALL SUNDAY
AND SUNDAY NIGHT...WITH PWAT VALUES AGAIN OVER 1.5
INCHES...ALMOST 2 INCHES IN PLACES. IN ADDITION...INSTABILITY WILL
BE HIGH AHEAD OF BOUNDARY...WITH CAPE VALUES OF 2000 J/KG OR
HIGHER...WITH HIGH ENOUGH SHEAR TO SUPPORT THUNDERSTORM
DEVELOPMENT. THIS MAY ALLOW FOR THUNDERSTORMS WHICH DEVELOP TO
BECOME SEVERE. HOWEVER...MUCH WILL DEPEND ON THE TIMING OF THE
SYSTEM...WITH MODELS CONTINUING A SLIGHTLY SLOWER TREND...WHICH
MAY ALLOW FOR MOST ROBUST CONVECTION TO BE WEST OF AREA.
HOWEVER...GIVEN DIURNAL HEATING...FEEL CONFIDENT IN KEEPING
THUNDERSTORM CHANCES IN THE NORTHWEST LATE SUNDAY. SYSTEM WILL
CONTINUE TO PUSH EAST OVERNIGHT AND MAY LINGER INTO MONDAY
MORNING.

HIGH PRESSURE WILL PUSH IN BEHIND SYSTEM...HOWEVER MODELS ATTEMPT
TO HANG BOUNDARY UP ACROSS NORTHERN MO WHICH MAY ALLOW FOR
PRECIPITATION CHANCES TO LINGER INTO SOUTHERN IOWA THROUGH THE
EARLY WEEK. OVERALL...CONDITIONS WILL BE MOSTLY DRY UNTIL ANOTHER
LOW PRESSURE SYSTEM PUSHES EAST ACROSS THE NORTHERN US LATE IN THE
PERIOD. MODELS HAVE VERY POOR AGREEMENT ON TIMING AND PLACEMENT OF
THIS SYSTEM...AND THEREFORE HAVE ONLY KEPT SLIGHT AND CHANCE POPS
FOR THURSDAY.

TEMPERATURES WILL BE WARMER SUNDAY WITH SOUTHERLY FLOW AND WAA
AHEAD OF BOUNDARY...HOWEVER TEMPERATURES WILL COOL BEHIND
BOUNDARY. TEMPS WILL WARM AGAIN LATE IN PERIOD.

&&

.AVIATION...29/18Z
ISSUED AT 1241 PM CDT FRI AUG 29 2014

EXPECT MAINLY VFR CONDITIONS THIS AFTERNOON...DETERIORATING LATE
AFTERNOON/EVENING AS SHRA/TSRA DEVELOP ACROSS THE AREA WITH
LOCALIZED MVFR OR LOWER CONDITIONS...THEN DETERIORATING FURTHER
OVERNIGHT EVEN AS PRECIP ENDS...WITH REDEVELOPMENT OF LOW
CIGS/VSBYS IN BR ONCE AGAIN. HAVE TRENDED TAFS THIS WAY AND
ATTEMPTED TO TIME MOST LIKELY PERIOD FOR TSRA AT
DSM/ALO/OTM...WITH AMENDMENTS ANTICIPATED AS THE STORMS MOVE
ACROSS THE AREA LATER TODAY.

&&

.HYDROLOGY...
ISSUED AT 315 PM CDT THU AUG 28 2014

SEVERAL ADDITIONAL EPISODES OF RAIN ARE EXPECTED THROUGH SATURDAY
MORNING AND AGAIN ANOTHER SUNDAY AFTERNOON THROUGH MONDAY MORNING.
AT THIS TIME EXPECTED RAINFALL AMOUNTS THROUGH SATURDAY MORNING WITH
EACH EVENT SHOULD BE LIGHT TO MODERATE. THIS WILL LIMIT THE LIKELIHOOD
OF FLASH FLOODING BUT STILL HAVE SOME EFFECT ON RISES ON SMALL STREAM
AND AREA RIVERS. THERE IS A GREATER RISK OF MORE SUBSTANTIAL RAINFALL
OVER PORTIONS OF THE FORECAST AREA SUNDAY EVENING. THIS TIME PERIOD
WILL BE MONITORED FOR BOTH SHORT TERM FLOODING AND RIVER FLOODING
OVER THE UPCOMING HOLIDAY WEEKEND.

&&

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

&&

$$

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



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