Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212333
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
633 PM CDT TUE APR 21 2015

.UPDATE...
ISSUED AT 552 PM CDT TUE APR 21 2015

WILL EXPIRE THE WIND ADVISORY AT 6 PM. A FEW GUSTS WILL LINGER AND
WITH SHOWERS OVER SOUTHERN IOWA AND DRY AIR AT THE SFC...SOME
CONVECTIVE GUSTS MAY OCCUR WITH VIRGA. OTHERWISE...GRADIENT WINDS
AND GUSTS WILL RELAX AFTER 6 PM. /REV

&&

.SHORT TERM /TONIGHT/...
ISSUED AT 214 PM CDT TUE APR 21 2015

A MID-LEVEL SHORTWAVE IMPULSE ROUNDING THE CYCLONIC FLOW ALOFT IS
PROGRESSING FROM SOUTH DAKOTA AND NEBRASKA TOWARD IOWA THIS
AFTERNOON. AT THE SURFACE THIS IS BEING REFLECTED IN A SUBTLE
FRONT/WIND SURGE...AND THE FEATURE IS ALSO GENERATING A BAND OF
CLOUDS AND LIGHT RADAR ECHOES STRETCHING FROM AROUND YANKTON TO
ATLANTIC. THUS FAR IT APPEARS LITTLE...IF ANY...OF THIS
PRECIPITATION HAS BEEN REACHING THE GROUND. HOWEVER...THE SHORTWAVE
IS JUST MATURING NOW AND IN THE NEXT FEW HOURS MOST HIGH RESOLUTION
MODELS INDICATE A SLIGHT INTENSIFICATION OF THE PRECIPITATION
RESULTING IN A FEW LIGHT SHOWERS LIKELY REACHING THE GROUND ACROSS
SOUTHERN IOWA. HAVE THUS MAINTAINED THE LOW POPS IN THIS AREA
THROUGH SUNSET...BUT ANY RAINFALL WILL BE EXTREMELY LIGHT AND OF NO
CONSEQUENCE. A WIND ADVISORY REMAINS IN EFFECT THIS AFTERNOON BUT IS
SLATED TO EXPIRE AT 6 PM...AND GIVEN THE EXPECTED DECREASE IN MIXING
BY SUNSET AND THE FACT THAT WINDS HAVE ALREADY BEGUN TO TAPER OFF
JUST A LITTLE BIT...EXPECT THAT THIS TIMELINE WILL HOLD UP NICELY.
THE OTHER FORECAST ISSUE FOR TONIGHT IS TEMPERATURES GIVEN THE
COOLER AIR BEING PUMPED IN FROM THE NORTHWEST AND GRADUALLY CLEARING
SKIES EXPECTED AFTER DARK. THIS SHOULD ALLOW TEMPERATURES TO FALL TO
NEAR OR BELOW FREEZING ACROSS THE FORECAST AREA. HOWEVER...WITH
WINDS STAYING IN THE 5 TO 10 MPH RANGE AND DAMP GROUND A FROST SEEMS
FAR FROM CERTAIN AND GIVEN THAT THE AGRICULTURAL AND GARDENING
GROWING SEASONS HAVE NOT REALLY BEGUN YET NO HEADLINES ARE BEING
ISSUED AT THIS TIME.

.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
ISSUED AT 214 PM CDT TUE APR 21 2015

THE LARGE LOW PRESSURE SYSTEM THAT HAS BROUGHT WITH IT CHILLY
TEMPERATURES WILL CONTINUE ITS MOVE EASTWARD OVER SOUTHEAST CANADA
AND THE NORTHERN GREAT LAKES. AS IT SLIDES EAST A SURFACE HIGH
PRESSURE RIDGE WILL SLIDE IN BEHIND IT AND BE THE MAIN DRIVER FOR
THE NEXT COUPLE OF DAYS BEFORE MAKING WAY FOR A LOW PRESSURE
SYSTEM MOVES OFF THE ROCKIES THAT WILL MOVE INTO THE REGION BY
THE END OF THE WORK WEEK.

WEDNESDAY WILL BE HIGHLIGHTED BY SURFACE HIGH PRESSURE THAT WILL
KEEP US IN NORTHWEST FLOW...THOUGH MUCH LIGHTER THAN EARLY IN THE
WEEK. SKIES WILL CLEAR OUT AS A RESULT AS WELL AND BRING ABOUT NEAR
AND SUB-FREEZING LOW TEMPERATURES ACROSS THE REGION THANKS TO THE
ADDITIONAL RADIATIONAL COOLING THAT WILL BE SEEN OVERNIGHT. THE
COLDEST TEMPERATURES WILL LIKELY BE REALIZED WEDNESDAY
NIGHT/THURSDAY MORNING WHERE SUB-FREEZING TEMPERATURES AND FROST MAY
BE AS FAR SOUTH AS INTERSTATE-80. THURSDAY WILL BEGIN SIMILAR TO
WEDNESDAY WITH THE PRIMARY DRIVER STILL THE SURFACE HIGH
PRESSURE RIDGE...BUT ITS INFLUENCE WILL DIMINISH THROUGHOUT THE
DAY AS IT SLIDES EASTWARD AND A LOW PRESSURE SYSTEM DEVELOPS AND
SLIDES OFF OF THE ROCKIES AND INTO WESTERN KANSAS. AS THAT
HAPPENS...THE PERSISTENT NORTHWEST FLOW OF LATE WILL DIMINISH TO
NEAR CALM AND ALLOW SOME RETURN FLOW FROM THE SOUTH LATER THURSDAY
AND INTO FRIDAY.

THE MODELS HAVE COME INTO BETTER AGREEMENT OVER THE PROGRESSION OF
THE SYSTEM AND HAVE IT SETTING UP AS A RELATIVELY WEAK CUT OFF
LOW AND SLIDES IT JUST SOUTH OF THE STATE FRIDAY AND SATURDAY.
ALONG WITH A SHORTWAVE MOVING THROUGH DURING THE SAME TIME
PERIOD...SHOWERS AND POSSIBLE ISOLATED THUNDER WILL AFFECT MUCH OF
THE STATE. THE GREATEST POTENTIAL COMES FRIDAY EVENING AND INTO
EARLY SATURDAY. WITH THE LOW POSITIONED SOUTH OF THE STATE AND
LIFT ON THE WEAK SIDE...THE CHANCES FOR SEVERE WEATHER WILL BE
MINIMAL. BEYOND FRIDAY AND SATURDAY...TEMPERATURES WILL MODERATE
SOME BY NEXT WEEK AND A WEAK TROUGH MAY BRING WITH IT CHANCES FOR
PRECIP NEXT MONDAY AND INTO TUESDAY.

&&

.AVIATION...22/00Z
ISSUED AT 633 PM CDT TUE APR 21 2015

BAND OF SHOWERS MOVING THROUGH SOUTHERN IA WILL EXIT THE AREA BY
04Z...ONLY AFFECTING KDSM AND KOTM FOR THROUGH 02Z. WIND GUSTS TO
RELAX AFT 02Z WITH GRADIENT WINDS CONTINUING...ESPECIALLY NORTH
THROUGH OVERNIGHT AROUND 12 TO 15KTS. GENERALLY VFR CONDITIONS
EXPECTED. BY 13 TO 15Z WINDS AGAIN BEGIN TO MIX WITH GUSTS NEARING
30 TO 35 KTS ACROSS MOST OF THE AREA. AREAS OF FEW TO SCT060-080
EXPECTED WITH DRY AIRMASS WED AFT 16Z. /REV

&&

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

$$

UPDATE...REV
SHORT TERM...LEE
LONG TERM...CURTIS
AVIATION...REV



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