Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 162344
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
544 PM CST TUE DEC 16 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 330 PM CST TUE DEC 16 2014

MAIN CHALLENGES TONIGHT WILL BE CLOUD COVER AND TEMPERATURES.
BRIEF BREAK IN CLOUDS THIS AFTERNOON WEST AS NOSE OF COLD AIR
ADVECTION MOVED SOUTHEAST ACROSS THE REGION. AS THE LOWEST
PORTIONS OF THE BOUNDARY LAYER WARMED...CLOUDS AGAIN FORMED ON THE
TOPS OF THE THERMALS...THEN BACKFILLING TO SOME EXTENT ACROSS THE
REGION. TONIGHT...MAY SEE THE CLOUDS DISSIPATE BRIEFLY AS THE
PROCESS REVERSES WITH SUNSET AND AN AREA OF CLEARING MAY DEVELOP
ONCE AGAIN WEST/CENTRAL. CONFIDENCE ON HOW LONG OR HOW WIDESPREAD
ANY CLEARING WILL BE IS LOW...GIVEN THE TENDENCY FOR MORE STRATUS
TO REDEVELOP AS THE RIDGE WEST OF THE AREA BEGINS TO MOVE EAST
OVERNIGHT. THE OTHER FACTOR REGARDING CLOUD COVER IS A WEAK WAVE
OVER COLORADO THIS AFTERNOON...WHICH IS CAUSING A RETURN OF WARM
AIR ADVECTION OVER THE WESTERN PLAINS. THIS IS GENERATING AN AREA
OF MID LEVEL CLOUDS WHICH SHOULD BEGIN ADVECTING EAST OVER THE
H700 RIDGE TOWARD IOWA OVERNIGHT. CONSEQUENTLY...OVERNIGHT WILL
MOST LIKELY SEE A MIXTURE OF SOME CLEARING EARLY AND CLOUDS MOST
AREAS LATE INTO SUNRISE. WINDS ALOFT WILL CONTINUE TO DECOUPLE BY
EVENING AND LIGHTER NORTHWEST WINDS ARE EXPECTED TONIGHT.
TEMPERATURES WILL BE QUITE CHILLY...AND REGARDLESS OF THE CLOUD
COVER...THE PERSISTENT LOW LEVEL COLD AIR ADVECTION TODAY WILL
RESULT IN OVERNIGHT LOWS NEAR 5 FAR NORTHWEST TO NEAR 20 IN THE
SOUTH. FETCH OF COLD AIR SUGGESTS THE WEST/NORTHWEST WILL BE
COLDER DUE TO ADVECTION TODAY.

.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
ISSUED AT 330 PM CST TUE DEC 16 2014

A PROGRESSIVE BUT RELATIVELY CALM WEATHER PATTERN WILL BE IN
PLACE OVER THE REGION THROUGH THE WEEKEND WITH WITH TWO MAIN
WEATHER SYSTEMS OF NOTE ON THURSDAY AND EARLY NEXT WEEK. THE
BEGINNING OF THE PERIOD WILL SEE AN INVERTED 500 MB TROUGH AND
ASSOCIATED PV ANOMALY SWING THROUGH THE UPPER MISSISSIPPI RIVER
VALLEY ON THE BACKSIDE OF A DEPARTING LOW. A SURFACE RIDGE AXIS
WILL ALSO LAY OUT ACROSS THE WESTERN PORTION OF THE STATE ON
WEDNESDAY WITH LIGHT NORTHERLY FLOW OVER THE CWA. THE BIGGEST
QUESTION FOR TOMORROW WILL BE THE EXTENT AND DURATION OF CLOUD
COVER. NAM 0.5-1 KM MOISTURE PROFILES KEEP STRATUS OVER THE
NORTHEAST THROUGH MUCH OF THE DAY TOMORROW AND LEANED TOWARDS THIS
GUIDANCE FOR SKY GRIDS...BUT GIVEN THE UNCERTAINTY IN ANY CLEAR
SLOT POSITIONING HAVE KEPT AT LEAST PARTLY CLOUDY WORDING
THROUGHOUT THE CWA.

LATE WEDNESDAY INTO THURSDAY MORNING WILL SEE A SHARP MID TO
UPPER LEVEL SHORTWAVE SHIFT OFF THE FRONT RANGE AND INTO THE
MISSOURI RIVER VALLEY. THE WAVE WEAKENS AS IT RUNS UP AGAINST THE
DOWNSTREAM RIDGE...BUT PORTIONS OF SOUTHERN IOWA SHOULD SEE
MEASURABLE SNOW THURSDAY MORNING. ALL MEDIUM RANGE SOLUTIONS PLACE
A NARROW BUT STRONG AREA OF DGZ FORCING BETWEEN 09Z AND 18Z ALONG
AND NORTH OF THE MISSOURI BORDER. HOWEVER...NAM SOUNDINGS ARE
NOTABLY DRIER IN THE DGZ COMPARED TO THE GFS AND EC. THE SYSTEM
WILL ALSO BE FIGHTING A WEDGE OF LOW-LEVEL DRY AIR THAT SHOULD
RESULT IN A SHARP PRECIP CUTOFF ALONG THE NORTHERN SIDE OF THE
LINE. FOR THE FORECAST PACKAGE...DID NOT CHANGE COVERAGE TOO MUCH
BUT INCREASED POPS TO HIGH CHANCES OVER THE SOUTH WITH THE
EXPECTATION OF THESE BEING UPPED TO LIKELIES OR CATEGORICAL ONCE
THE LOCATION OF THE GREATEST FORCING AND MOISTURE PROFILES ARE
BETTER RESOLVED. TOTAL SNOWFALL AMOUNTS LOOK TO BE AROUND OR LESS
THAN AN INCH GIVEN THE TRANSIENT NATURE OF THE SYSTEM AND INITIAL
SURFACE DRY LAYER.

FRIDAY AND INTO THE WEEKEND LOOK TO REMAIN DRY WITH WEAK SOUTHERLY
FLOW/WAA BRINGING HIGHS BACK ABOVE FREEZING. SATURDAY WILL SEE A
AN UPPER LEVEL TROUGH SWING THROUGH THE REGION...BUT FORECAST
SOUNDINGS REMAIN MOSTLY DRY AND CONFIDENCE WAS NOT HIGH ENOUGH TO
INTRODUCE POPS IN ANY GIVEN LOCATION. A FEW LIGHT SNOW SHOWERS ARE
CERTAINLY NOT OUT OF THE REALM OF POSSIBILITIES WITH THE PASSAGE
OF THIS SYSTEM.

HOWEVER...THE WEATHER PATTERN TAKES A MORE ACTIVE TURN EARLY NEXT
WEEK AS A LONGWAVE TROUGH DIGS OVER THE ROCKIES AND SLOWLY
MIGRATES EAST TOWARDS THE CWA BY THE END OF THE PERIOD. THE GFS
AND EC ARE IN SOMEWHAT GOOD AGREEMENT WITH THE RESULTANT SURFACE
CYCLONE AND TAKE IT ON A COURSE NORTH OF THE CWA...WITH IOWA
REMAINING IN THE WARM SECTOR AND SEEING A MIX OF RAIN AND SNOW
ALONG THE ATTENDANT WARM/COLD FRONTAL BOUNDARIES.

&&

.AVIATION...17/00Z
ISSUED AT 544 PM CST TUE DEC 16 2014

STATUS DECK CONTINUES ACROSS MUCH OF CENTRAL IOWA ALTHOUGH BOTH
RAP AND HRRR INDICATE THIS DECK WILL ERODE OVERNIGHT. WILL
CONTINUE WITH THE STATUS THROUGH THE EVENING BUT GO WITH SOME
GRADUAL CLEARING LATE TONIGHT AS SURFACE RIDGE APPROACHES. WINDS
WILL DIMINISH THIS EVENING AND REMAINING LIGHT TO MODERATE
OVERNIGHT INTO WEDNESDAY.

&&

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

&&

$$

SHORT TERM...REV
LONG TERM...SKOW
AVIATION...COGIL



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