Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210503
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1203 AM CDT SUN SEP 21 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 332 PM CDT SAT SEP 20 2014

FORECAST CONCERN IS WITH NEXT QUICK MOVING SHORTWAVE
TROUGH TO MOVE ACROSS MINNESOTA INTO WISCONSIN AND LOOKS TO BARELY
CLIP NORTHEAST IOWA EARLY THIS EVENING. A VERY SMALL WINDOW FOR
CONVECTION...23Z TO 03Z TONIGHT OVER THE FAR NORTHEAST SECTIONS OF
THE FORECAST AREA. LIMITED INSTABILITY BUT LOW LEVEL SHEAR IS DECENT
AND ENOUGH DEEP FORCING PRESENT FOR ISOLATED SEVERE POTENTIAL EARLY
IN THE EVENING. OTHERWISE...LOWERED POPS PRIOR TO 00Z SUNDAY ACROSS
THE SOUTHEAST AS ANY FORCING HAS PUSHED SOUTHEAST INTO MISSOURI. LOW
TEMPERATURES TONIGHT SHOULD COOL DOWN BEHIND THE SHORTWAVE TONIGHT
AND LEANED TOWARD THE COOLER NAM12/SREF.

.LONG TERM /SUNDAY THROUGH SATURDAY/...
ISSUED AT 332 PM CDT SAT SEP 20 2014

OVERALL THE LONG TERM FORECAST IS PRETTY QUIET...BUT WITH AN
INTERESTING INTERLUDE TOWARD THE MIDDLE OF NEXT WEEK. FIRST...A
STRONG NORTHERN STREAM SHORTWAVE APPROACHING TONIGHT WILL SWING
ACROSS THE UPPER MIDWEST AND GREAT LAKES ON SUNDAY...PUSHING
THROUGH A REINFORCING SHOT OF COOLER AIR THAT WILL KEEP OUR HIGH
TEMPERATURES IN CHECK. THE PASSAGE OF THIS SYSTEM WILL PRODUCE
BREEZY CONDITIONS ACROSS THE AREA DURING THE DAYTIME AND ALSO
GENERATE SCATTERED SHOWERS AND THUNDERSTORMS...BUT THESE SHOULD
REMAIN JUST NORTH OF OUR FORECAST AREA. A SURFACE HIGH PRESSURE
AREA WILL THEN BUILD ACROSS IOWA ON SUNDAY NIGHT BRINGING LIGHT
WINDS AND COOL TEMPERATURES...BUT WITH LOWS REMAINING IN THE 40S.

BY MONDAY THE 500 MB PATTERN WILL BE CHARACTERIZED BY A BROAD
RIDGE EXTENDING ACROSS THE WESTERN HALF OF THE U.S. AND UP INTO
CENTRAL CANADA...HOWEVER A WEAKNESS IN THE FORM OF A BROAD TROUGH
WILL BE CAUGHT UP BENEATH THE RIDGE ACROSS THE CENTRAL AND
NORTHERN ROCKIES BY MONDAY EVENING. THE SUBSEQUENT EVOLUTION OF
THIS PATTERN LOOKS TO BE COME SOMEWHAT ATYPICAL. BY LATE TUESDAY
THE TROUGH FEATURE WILL HAVE PROGRESSED TO THE DAKOTAS AND
NEBRASKA BUT ON TUESDAY NIGHT A SHORTWAVE PASSING BY FURTHER
NORTH ACROSS CANADA SHOULD PHASE WITH IT AND DIG MOST OF THE
ENERGY OUT...ALLOWING THE RIDGE TO REBUILD ACROSS THE ROCKIES FROM
WEDNESDAY ONWARD. THE CATCH IS THAT NEARLY ALL LARGE SCALE MODELS
NOW DEPICT A SCENARIO WHERE SOME OF THE VORTICITY FROM THE TROUGH
IS ABANDONED AND BECOMES TRAPPED BENEATH THE DEVELOPING
RIDGE...RESULTING IN ANOMALOUSLY LOW PRESSURE HEIGHTS AND CREATING
A PROBLEMATIC TEMPERATURE AND PRECIPITATION FORECAST.

INITIALLY THE EARLY PART OF THE WEEK WILL BE QUIET WITH THE
SURFACE HIGH DEPARTING AND THE BROAD 500 MB TROUGH NOT HAVING YET
ARRIVED. THE DRY CONDITIONS WILL HOLD THROUGH TUESDAY...BUT THEN
AS THE TROUGH MOVES OVERHEAD TUESDAY NIGHT AND IS LARGELY SCOOPED
OUT ON WEDNESDAY SCATTERED SHOWERS AND THUNDERSTORMS CAN BE
EXPECTED ESPECIALLY ACROSS THE NORTHWEST HALF OF THE FORECAST
AREA. THE SYNOPTIC PATTERN IS NOT SUPPORTIVE OF A SEVERE WEATHER
OR HEAVY RAIN THREAT. AFTER THE BULK OF THE TROUGH IS SCOOPED OUT
WEDNESDAY NIGHT BUT SOME REMAINS LEFT BEHIND AS DISCUSSED
ABOVE...THE FORECAST PARTICULARLY FOR FRIDAY AND SATURDAY IS MORE
UNCERTAIN. HOWEVER...IT APPEARS THAT WITH THE CHARACTERISTICS OF
THE ABANDONED LOW PRESSURE ALOFT BEING SLOWLY WASHED OUT BENEATH
THE RIDGE PRECIPITATION WILL BE PRETTY UNLIKELY...BUT TEMPERATURES
WILL PROBABLY BE HELD BACK FROM WARMING AS MUCH AS THEY NORMALLY
WOULD BENEATH A RIDGE AS PRONOUNCED AS THE ONE BUILDING LATER IN
THE WEEK. THEREFORE...AT THIS TIME THE PRIMARY IMPACT ON THE
FORECAST IS TO KEEP HIGH TEMPERATURES IN THE MID TO POSSIBLY UPPER
70S LATER IN THE WEEK AS OPPOSED TO GOING UP INTO THE 80S...AND
ALSO TO HOLD SOME LINGERING CLOUDS IN FOR THESE DAYS.

&&

.AVIATION...21/06Z
ISSUED AT 1203 AM CDT SUN SEP 21 2014

VFR CONDITIONS EXPECTED WITH NORTHWESTERLY WINDS PICKING UP DURING
THE DAY SUNDAY. WINDS THEN EXPECTED TO DIMINISH AND BECOME MORE
NORTHERLY SUNDAY EVENING.

&&

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

&&

$$

SHORT TERM...PODRAZIK
LONG TERM...LEE
AVIATION...BEERENDS


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