Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 140504
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1204 AM CDT SUN SEP 14 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 330 PM CDT SAT SEP 13 2014

RIDGE OF HIGH PRESSURE CONTINUES TO DRIFT EAST OF THE AREA THIS
AFTERNOON AND WILL BE IN THE OHIO RIVER VALLEY BY TONIGHT.
MEANWHILE LOW PRESSURE OVER NORTH DAKOTA AND SOUTHERN CANADA IS
EXPECTED TO MOVE EAST...PULLING A WARM FRONT AND MILDER SW FLOW
BACK INTO THE AREA FOR THE REMAINDER OF THE AFTERNOON AND TONIGHT.
THIS WILL ALLOW FOR A MODERATION IN OVERNIGHT
TEMPERATURES...ESPECIALLY ACROSS CENTRAL AND WESTERN IOWA. WHERE
THE SFC PRESSURE GRADIENT CONTINUES ON THE LIGHT SIDE OVER EASTERN
IOWA TONIGHT...EXPECT THAT TEMPS NEAR THE CEDAR RIVER VALLEY WILL
DROP A BIT MORE OVERNIGHT BUT NOT AS FAR ANYWHERE AS COMPARED TO
LAST NIGHT AS H850 TEMPS AND THICKNESS WILL INCREASE DURING THE
OVERNIGHT HOURS. AS THE LOW OVER NORTHERN MN TRACKS EAST AND THE
TROUGH PASSES INTO IOWA OVERNIGHT...SHOULD SEE AN INCREASE IN MID
LEVEL CLOUDS AFT 06Z...THOUGH NO PRECIPITATION EXPECTED.
OTHERWISE...QUIET WEATHER WILL BE WITH US THROUGH 12Z.

.LONG TERM /SUNDAY THROUGH SATURDAY/...
ISSUED AT 330 PM CDT SAT SEP 13 2014

TWO MAIN SYSTEMS OF NOTE AT THE BEGINNING AND END OF THE EXTENDED
PERIOD WITH PLEASANT EARLY FALL WEATHER IN BETWEEN. CLOUDS WILL
GRADUALLY INCREASE DURING THE DAY ON SUNDAY AS THE FIRST OF TWO
300MB SHORTWAVES ROTATES AROUND THE MEAN LONGWAVE TROUGH CENTERED
OVER MANITOBA. THE BEST FORCING REMAINS WELL NORTH OF IOWA AND IS
REFLECTED WELL IN THE HIGH RES MODELS...BUT THE WAVE WILL DRAG A
WEAK SURFACE BOUNDARY INTO NORTHERN IOWA BY 06Z MONDAY. A SECOND
UPPER LEVEL SHORTWAVE WILL DIVE FURTHER SOUTH AND CAUSE A SURFACE
LOW IN THE LEE OF THE FRONT RANGE TO DRIFT EASTWARD...WITH AN
ATTENDANT WARM FRONT LIFTING AS FAR NORTH AS NORTHERN KANSAS BY 12Z
MONDAY. OVERRUNNING MOISTURE ABOVE THE WARM FRONT COMBINED WITH AN
INCREASE IN MID LEVEL FRONTOGENETICAL FORCING SHOULD RESULT IN THE
DEVELOPMENT OF SHOWERS ACROSS THE CWA. A FEW THUNDERSTORMS ARE
POSSIBLE ACROSS THE SOUTH FROM 12-18Z WHERE A WEDGE OF ELEVATED
INSTABILITY EXISTS AND MUCAPES ARE AROUND 300 J/KG. TWO BANDS OF
PRECIP MAY INITIALLY DEVELOP ON MONDAY MORNING...ONE WITH THE FG
BAND IN NORTHERN IOWA AND A SECOND WITH THE 850-925MB THETA-E
ADVECTION NORTH OF THE WARM FRONT.

LATEST SHORT TO MEDIUM RANGE MODEL GUIDANCE SLOWED THE TRANSIT OF
THE PRECIP BY 3-6 HOURS AND THIS TREND IS REFLECTED IN THIS
FORECAST UPDATE. THIS WILL KEEP CENTRAL AND EASTERN IOWA IN THE
CLOUDS ALL DAY AND HAVE SUBSEQUENTLY LOWERED HIGHS BY 3-4 DEGREES
FOR MONDAY. SOUNDING PROFILES SHOW DRY AIR PUNCHING IN BEHIND THIS
SYSTEM...WHICH WILL QUICKLY PUSH THE PRECIP OUT OF THE CWA BY
MONDAY EVENING. AN UPPER LEVEL RIDGE CURRENTLY OVER THE PACIFIC
NORTHWEST WILL MIGRATE OVER THE ROCKIES BY TUESDAY AND OVER IOWA
BY THE LATTER PART OF THE WEEK. THIS WILL PLACE IOWA IN BROAD
NORTHWEST FLOW WITH SURFACE HIGH PRESSURE IN CONTROL FOR TUESDAY
AND WEDNESDAY. A WEAK 500MB WAVE PROPAGATING OVER THE RIDGE MAY
BRING AN INCREASE IN CLOUDS FOR WEDNESDAY BUT PRECIPITATION IS NOT
EXPECTED. TEMPERATURES WILL GRADUALLY RISE CLOSER TO NORMAL
THROUGH THE WEEK AS THE SURFACE HIGH SHIFTS EASTWARD AND SOUTHERLY
FLOW RETURNS TO IOWA.

THE SURFACE RIDGE STRENGTHENS AND BEGINS TO SET UP A BLOCKING
PATTERN AS IT MOVES TOWARDS THE EAST COAST FOR NEXT WEEKEND. WHILE
IOWA REMAINS IN WARM SOUTHERLY FLOW...A TROUGH OF LOW PRESSURE
DEVELOPS EAST OF THE ROCKIES AND WILL BRING A MORE ACTIVE PATTERN TO
THE REGION FOR FRIDAY AND NEXT WEEKEND. MODEL SOLUTIONS DIFFER ON
THE TIMING AND PLACEMENT OF THE TROUGH WITH THE EC BEING MORE
PROGRESSIVE THAN THE GFS.

&&

.AVIATION...14/06Z
ISSUED AT 1158 PM CDT SAT SEP 13 2014

SLY FLOW OVER IA AHEAD OF COLD FRONT EXTENDING FROM NWRN MN INTO
ERN CO. COLD FRONT WILL MOVE INTO IA SUN EVENING AND PASS
SOUTHEAST OF THE STATE MONDAY AFTERNOON AND NIGHT. PATCHY MIFG IS
POSSIBLE TONIGHT...BUT DROPPED OUT OF TAFS THIS SET. ONLY MVFR
CIGS EXPECTED THIS SET.

OUTLOOK...CIGS WILL LOWER WITH SCATTERED SHOWERS LATER SUNDAY
NIGHT AND MONDAY WITH SOME MVFR CONDITIONS DEVELOPING. HIGH
PRESSURE WILL BUILD INTO THE AREA WITH VFR CONDITIONS MONDAY
NIGHT INTO TUE.

&&

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

$$

SHORT TERM...REV
LONG TERM...SKOW
AVIATION...MS SEP 14



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