Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182052
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
352 PM CDT FRI APR 18 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 305 PM CDT FRI APR 18 2014

AREA OF LOW PRESSURE DEVELOPING ACROSS THE HIGH PLAINS
TODAY...AND WILL PUSH EAST INTO THE EASTERN DAKOTAS INTO SW NE BY
EARLY SATURDAY. SOUTH TO SOUTHWESTERLY FLOW WILL INCREASE IN THE
LOW LEVELS WITH WAA OVERNIGHT. SFC PRES GRADIENT WILL CONTINUE TO
TIGHTEN OVERNIGHT AS THE SFC LOW TRANSLATES EASTWARD...THUS EXPECT
WINDS TO STAY UP SOME OVERNIGHT. EXCEPTION WILL BE ACROSS THE
EASTERN CWA TOWARD THE EARLY EVENING HOURS WHERE WINDS MAY BECOME
LIGHT...UNTIL THE GRADIENT BEGINS TO TIGHTEN TOWARD LATE EVENING
INTO OVERNIGHT HOURS. WITH THE WINDS STAYING UP SOME AND THE
WAA...HAVE KEPT LOWS MILDER IN THE 40S ACROSS THE CWA.

.LONG TERM /SATURDAY THROUGH FRIDAY/...
ISSUED AT 305 PM CDT FRI APR 18 2014

SPLIT UPPER ZONAL FLOW WILL PREVAIL INTO EARLY NEXT WEEK. AFTER
THAT...GFS/ECMWF ARE CONSISTENT IN AMPLIFYING THE NORTHERN STREAM
BY THE MIDDLE OF NEXT WEEK MOVING A STRONGER UPPER TROUGH THROUGH
THE CENTRAL UNITED STATES.

SURFACE RIDGE FROM OVER THE UPPER MIDWEST TODAY WILL MOVE EAST
TONIGHT AND DECENT SOUTHERLY FLOW WILL DEVELOP BY SATURDAY
MORNING. WARMER TEMPERATURES AND BREEZY CONDITIONS ARE EXPECTED DUE
TO WARM ADVECTION AND GOOD MIXING ON SATURDAY. SINCE IT IS A WARM
ADVECTION SITUATION...WE DO NOT EXPECT THE FULL MIXED LAYER WINDS
TO BE MAXIMIZED AT THE SURFACE. GIVEN THE EXPECTED DEWPOINTS IN
THE UPPER 40S...MINIMUM RELATIVE HUMIDITY VALUES SHOULD FALL TO
NEAR 30 PERCENT. AT THIS POINT...NEITHER THE WIND OR THE MINIMUM
RELATIVE HUMIDITY REACH RED FLAG WARNING CRITERIA...SO NO ACTION
IS NEEDED IN THAT ARENA. THERE IS CONCERN THAT THE DEWPOINTS MAY
BE TOO HIGH ON SATURDAY SINCE RECENTLY THE MODELS HAVE BEEN TO
MOIST DURING RETURN MOISTURE SITUATIONS.

A COLD FRONT WILL STALL SOMEWHERE OVER NORTHERN IOWA OR SOUTHERN
MINNESOTA SATURDAY NIGHT AND SUNDAY. THE FRONT SHOULD STALL AS A
NORTHERN STREAM SHORTWAVE LIFTS NORTHEAST INTO SOUTHEAST CANADA.
THE FRONT WILL REMAIN QUASI-STATIONARY UNTIL THE NEXT SHORTWAVE
AMPLIFIES ENOUGH TO DRIVE THE FRONT THROUGH IOWA SUNDAY NIGHT AND
MONDAY. PROLONGED MOISTURE ADVECTION AND TRANSPORT SATURDAY AND
SUNDAY EVENTUALLY RESULTS IN FAIRLY HIGH PRECIPITABLE WATER VALUES
FOR LATE APRIL. GIVEN AMPLE MOISTURE...MODEST INSTABILITY AND A
RATHER SLOW MOVING FRONT...EXPECT SHOWERS AND THUNDERSTORMS NEAR
THE BOUNDARY SATURDAY NIGHT AND EARLY SUNDAY IN THE NORTH. AS THE
BOUNDARY MOVES SOUTHEAST SUNDAY NIGHT AND MONDAY...SHOWERS AND
THUNDERSTORMS SHOULD OCCUR ACROSS MUCH OF CENTRAL IOWA. RAINFALL
AMOUNTS SHOULD BE MODERATE WITH ISOLATED HEAVIER AMOUNTS DUE TO
AMPLE MOISTURE SUPPLY.

FOR THE EXTENDED...THE 12Z ECMWF AND GFS BOTH BRING A STRONG SHORTWAVE
THROUGH THE CENTRAL UNITED STATES DURING THE MIDDLE PART OF NEXT
WEEK. THIS SYSTEM WILL BE PRIMARY WEATHER MAKER IN THE EXTENDED. GIVEN
THE APPARENT DYNAMICS AND WIND FIELDS...THERE SHOULD BE A SEVERE
WEATHER THREAT AT SOME POINT IN CENTRAL IOWA. TIMING AND
INSTABILITY DETAILS ARE DIFFICULT TO PREDICT AT THIS POINT. WE
WILL FORECAST DECENT THUNDERSTORM POPS FOR NOW.

&&

.AVIATION...18/18Z
ISSUED AT 1240 PM CDT FRI APR 18 2014

VFR CONDITIONS EXPECTED THROUGHOUT THE TAF FORECAST PERIOD.


&&

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

$$

SHORT TERM...BEERENDS
LONG TERM...JOHNSON
AVIATION...BEERENDS



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