Area Forecast Discussion
Issued by NWS Des Moines, IA

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913
FXUS63 KDMX 200848
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
348 AM CDT WED MAY 20 2015

.SHORT TERM /TODAY/...
ISSUED AT 348 AM CDT WED MAY 20 2015

INVERTED TROUGH WITH WEAK UPPER LEVEL SUPPORT CONTINUES TO MOVE
INTO WESTERN IOWA EARLY THIS MORNING. ISENTROPIC LIFT FROM
NORTHERN MISSOURI INTO SOUTHERN IOWA AIDING LIGHT RAIN DEVELOPMENT
AS NORTHERN PLAINS HIGH OVER NORTH DAKOTA AT 06Z CONTINUES TO
IMPACT AREA WITH COOL AND RELATIVELY DRY SURFACE CONDITIONS. WITH
ARRIVAL OF RAIN EARLY TODAY...TEMPERATURES WILL FALL THIS MORNING
INTO THE 40S THEN STRUGGLE TO MOVE UP WITH WIDESPREAD CLOUDS AND
NORTHEAST WINDS MOST OF THE DAY. THIS SETS THE STAGE FOR A RATHER
COOL DAY WITH HIGHS IN THE LOWER TO MID 50S ACROSS THE REGION. IN
FACT IN SOME CASES ACROSS CENTRAL AND SOUTHERN IOWA WE MAY BE
WITHIN A FEW DEGREES OF RECORD LOW MAX TEMPS FOR THE DAY. WITH
SUPPORT FOR LIGHT RAIN OVER THE SOUTH BETTER PRIOR TO NOON THE
AFTERNOON WILL LIKELY SEE LESSENING RAIN CHANCES AND CLOUDS AS
WAVE EVIDENT IN WATER VAPOR IMAGERY EARLY TODAY BEGINS TO PULL
EAST THROUGH THE AREA BY AFTERNOON. THERE MAY BE SOME LINGERING
SHOWERS OVER THE CENTRAL AND NORTH THIS AFTERNOON AS THE UPPER
LEVEL LOW MOVES EAST. OTHERWISE...THE COOL TEMPS AND CLOUDS
SHOULD LINGER THROUGH THE AFTERNOON.

.LONG TERM /TONIGHT THROUGH TUESDAY/...
ISSUED AT 348 AM CDT WED MAY 20 2015

MAIN FOCUS OF THE EXTENDED PERIOD WAS ON THE RAIN/THUNDERSTORM
CHANCES OVER THE HOLIDAY WEEKEND. MODELS ARE IN FAIRLY GOOD
AGREEMENT THROUGH THE END OF THE WORK WEEK AND EVEN INTO THE EARLY
PART OF SATURDAY...BUT BY SATURDAY NIGHT INTO EARLY NEXT WEEK
MODELS REMAIN IN DISAGREEMENT.

TONIGHT THROUGH FRIDAY...SURFACE HIGH PRESSURE DOMINATES THE
WEATHER PATTERN AND BRINGS CLEAR SKIES AND WARMER TEMPERATURES.
WEAK MIXING THURSDAY SHOULD PUT TEMPERATURES BACK INTO THE 70S
OVER MUCH OF THE FORECAST AREA EXCEPT THE SOUTHWEST WHERE THE
CENTER OF THE HIGH AND THUS COOLER AIR WILL BE LOCATED AND LESS
MIXING. RAISED MAX TEMPS ON FRIDAY PLENTY OF SUNSHINE AND A
WARMER START FRIDAY MORNING.

SATURDAY THROUGH TUESDAY...SOUTHWEST FLOW ALOFT RETURNS TO IOWA
OVER THE WEEKEND RESULTING IN WARMER TEMPERATURES BUT ALSO MEANS
SEVERAL SHORTWAVES EMBEDDED IN THE 500MB FLOW TO AFFECT THE STATE.
MODELS TYPICALLY HAVE TROUBLE DISCERNING THE TIMING AND LOCATION
OF THE WEAKER SHORTWAVES AND THUS RESULTING IN A LOWER CONFIDENCE
IN PRECIP CHANCES DAYS 5 TO 7. THAT BEING SAID...MUCH HIGHER
CONFIDENCE IN A DRIER FORECAST FRIDAY NIGHT INTO SATURDAY WITH
STRONG SURFACE HIGH HOLDING STEADY OVER THE GREAT LAKES/OHIO
VALLEY SLOWING DOWN THE NEXT LOW MOVING OUT OF THE CENTRAL
ROCKIES. LOWERED POPS ACROSS THE FORECAST AREA FRIDAY NIGHT INTO
SATURDAY...RESULTING IN GOING COMPLETELY DRY FOR MUCH OF THE AREA
FRIDAY NIGHT AND EAST OF INTERSTATE 35 SATURDAY MORNING.
PLUS...WITH THE DRIER FORECAST SATURDAY...RAISED MAX TEMPS UP A
COUPLE OF DEGREES AND LIKELY STILL TOO COLD AS STRONG MIXING
DEVELOPS THROUGH THE DAY.

THE ECMWF IS MUCH FURTHER SOUTH WITH THE RAIN CHANCES SATURDAY
NIGHT INTO SUNDAY AS IT WANTS TO SPLIT THE UPPER LEVEL VORT MAX
DURING THIS TIME. IT PLACES A PIECE OF ENERGY INTO THE DAKOTAS BY
SUNDAY MORNING AND ANOTHER ACROSS THE MISSOURI VALLEY. GFS HAS
BEEN FAIRLY CONSISTENT BRINGING A WEAK SURFACE LOW AND COLD FRONT
ACROSS THE STATE SATURDAY NIGHT INTO SUNDAY. THE FRONT LOOKS TO
STALL ACROSS IOWA INTO MONDAY WITH ANOTHER COUPLE OF WEAK
SHORTWAVES BRINGING ADDITIONAL THUNDERSTORMS CHANCES. NOTHING TO
REALLY HANG YOUR HAT ON FOR SEVERE WEATHER POTENTIAL OVER THE
WEEKEND. CERTAINLY CANNOT COMPLETELY RULE OUT THE ISOLATED SEVERE
THREAT...BUT LOW CONFIDENCE ATTM WITH ANY WIDESPREAD SEVERE
WEATHER.

LOWERED POPS MONDAY NIGHT INTO TUESDAY WITH THE GFS AND ECMWF
TRENDING DRY AS THEY ARE SOMEWHAT GOOD AGREEMENT. WARMER
TEMPERATURES IN STORE FOR TUESDAY AND WEDNESDAY OF NEXT WEEK.

&&

.AVIATION...20/06Z
ISSUED AT 1157 PM CDT TUE MAY 19 2015

LITTLE CHANGE FROM PREVIOUS THINKING. PRIMARY CONCERNS WILL BE
PRECIP TIMING AND LATITUDINAL EXTENT AND MAGNITUDE OF CIG
REDUCTIONS WITH NAM QUITE AGGRESSIVE AND HI RES MODELS LESS SO
/HRRR/ARW/NMM/. OPTED FOR A MIDDLE OF ROAD APPROACH WHICH KEEPS
KMCW VFR/DRY AND TRENDS TOWARD MVFR CIGS SOUTH AND AROUND A NINE
HOUR PERIOD OF LIGHT RAIN. SOME POTENTIAL TO DROP TO IFR...BUT
CONFIDENCE IS NOT THERE YET. INSTABILITY DOES NOT APPEAR
SUFFICIENT FOR THUNDER.


&&

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

$$

SHORT TERM...REV
LONG TERM...PODRAZIK
AVIATION...SMALL



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