Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182132
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
332 PM CST THU DEC 18 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 324 PM CST THU DEC 18 2014

SNOW HAS ENDED OVER THE SOUTHEASTERN CWA AS THE CULPRIT UPPER LEVEL
WAVE HAS RAPIDLY WEAKENED AND DGZ FORCING HAS WANED. BASED ON WEBCAM
IMAGES AND A FEW SPOTTER/COOP REPORTS...MOST AREAS IN THE SOUTHERN
THREE TIERS OF COUNTIES SAW LESS THAN AN INCH OF SNOW. ATTENTION NOW
TURNS UPSTREAM TO A LARGE STRATUS PLUME STRETCHING FROM NORTHWEST
MONTANA TO THE TEXAS PANHANDLE...WITH THE EASTERN FRONT NOW JUST
ABOUT TO ENTER THE WESTERN CWA. LIGHT WEST TO SOUTHWEST FLOW IN THE
LOWEST 1 KM OF THE ATMOSPHERE BEHIND A DEPARTING SURFACE RIDGE
SHOULD ADVECT THIS CLOUD MASS OVER CENTRAL IOWA TONIGHT AND KEEP
SKIES OVERCAST ALL NIGHT AND INTO TOMORROW MORNING. THE CLOUDS
SHOULD ALSO BOLSTER LOW TEMPS TONIGHT COMPARED TO PREVIOUS NIGHTS
AND DIDN`T DEVIATE MUCH FROM GUIDANCE IN THIS RESPECT.

THE BIGGEST UNCERTAINTY FOR TONIGHT IS THE POSSIBILITY OF FLURRIES
OR FREEZING DRIZZLE WEST OF I-35. SHORT-RANGE MODEL SOUNDINGS (RUC
AND HRRR IN PARTICULAR) SATURATE THE LOWEST 5 KFT OF THE ATMOSPHERE
IN THIS REGION TONIGHT WITH NEARLY ISOTHERMAL PROFILES IN THIS LAYER
HOVERING BETWEEN -4 AND -7C. WITH THIS CONDITIONAL ICE PROBABILITY
...WENT WITH A MIX OF OCCASIONAL FLURRIES AND PATCHY FZDZ. FORCING
SHOULD BE MARGINAL AT BEST AND NO ACCUMULATION OF SNOW OR ICE IS
ANTICIPATED. ANY LIFT WITH AN UPPER LEVEL TROUGH ON WATER VAPOR
IMAGERY NW OF THE CWA SHOULD STAY NORTH AND EAST OF THE AREA.
ISENTROPIC UPGLIDE IN THE LOWEST 100MB OF THE ATMOSPHERE IS
PRESENT...BUT AGAIN VERY WEAK.

.LONG TERM /FRIDAY THROUGH THURSDAY/...
ISSUED AT 324 PM CST THU DEC 18 2014

THE WEATHER IN THE LONG TERM WILL IN GENERAL BE MORE ACTIVE THEN
WHAT WE HAVE SEEN IN A WHILE.  FRIDAY THROUGH SUNDAY THERE WILL BE
NUMEROUS RELATIVELY WEAK SHORT WAVES MOVE ACROSS IOWA IN A GENERALLY
ZONAL FLOW.  THROUGH SATURDAY MOISTURE WILL EITHER BE SPARSE OR
SHALLOW SO I AM NOT EXPECTING MUCH WITH THESE SHORTWAVES IN TERMS OF
SENSIBLE WEATHER EXCEPT FOR CLOUD COVER.  TEMPS SHOULD BE RATHER
MILD FOR MID TO LATE DECEMBER.

SATURDAY NIGHT THROUGH SUNDAY A LITTLE STRONGER SHORT WAVE MOVES IN
AND MOISTURE DEPTH INCREASES SLIGHTLY ALTHOUGH IT IS STILL VERY
SHALLOW.  SOUNDINGS DO NOT INDICATE ICE INTRODUCTION AND WHILE THERE
ARE MODEL DIFFERENCES AS TO WHERE ANY WEAK FORCING MIGHT OCCUR...IT
DOES APPEAR THAT THE NORTHERN HALF OF IOWA IS MORE LIKELY TO SEE
THIS THAN SOUTHERN IOWA.  AS A RESULT I KEPT THE MENTION OF FREEZING
DRIZZLE CONFINED TO THE NORTH.  THE SOUNDING DOES SATURATE
OR BEGIN TO SATURATE SUFFICIENTLY LATE SATURDAY NIGHT THROUGH
SUNDAY MORNING THROUGH THE DENDRITIC ZONE THAT WE COULD TRANSITION
TO SOME LIGHT SNOW UP NORTH...ESPECIALLY SUNDAY MORNING. THAT
SHORTWAVE EXITS DURING THE EARLY AFTERNOON AND TEMPS WARM
SUFFICIENTLY FOR A WINTERY MIX CHANGING TO RAIN ACROSS AT LEAST
SOME OF THE AREA.

SUNDAY NIGHT THROUGH MONDAY A MORE POTENT SHORTWAVE MOVES INTO
IOWA...AFFECTING THE WEST SUNDAY NIGHT AND THE REST OF THE AREA
THROUGH TUESDAY.  HERE MODELS DIVERGE ON HOW TO HANDLE THIS SYSTEM.
THE GFS KEEP THE UPPER LEVEL WAVE OPEN AND DEVELOPS A CLOSED SURFACE
LOW OVER CENTRAL TO NORTHEAST IOWA.  THE EURO DRASTICALLY DEPEND
THE UPPER WAVE TO OUR WEST THEN CONTINUES TO DEVELOP IT INTO A
CLOSED LOW MONDAY NIGHT AND TUESDAY ACROSS EASTERN IOWA.  THE
RESULTANT CLOSED SURFACE LOW IS OVER EASTERN IOWA AS WELL AND
FURTHER SOUTHWEST THAN THE GFS HAS.  THE GFS REALLY DEVELOPS THE QG
FORCING ACROSS CENTRAL IOWA SUNDAY NIGHT THEN IS SIGNIFICANTLY
WEAKER THROUGH TUESDAY.  SOUNDINGS SHOW A PROFILE THAT IS PRETTY
WARM THROUGH MONDAY AFTERNOON WHEN THE PROFILE COOLS TO BELOW
FREEZING.  PRECIP THROUGH EARLY MONDAY AFTERNOON SHOULD BE RAIN WITH
AN OCCASIONAL WINTERY MIX ESPECIALLY UP NORTH.  DURING THE AFTERNOON
WE WILL BE TRANSITIONING TO A RAIN/SNOW MIX BEFORE GOING TO ALL SNOW
MONDAY NIGHT.  IT IS TOO FAR OUT TO CONSIDER SNOWFALL AMOUNTS YET
AND GIVEN THE MODEL DISCREPANCIES IT WOULD BE HARD TO DO AT THIS
POINT.

THAT SYSTEM PULLS OUT BY CHRISTMAS EVE LEAVING US WITH SEASONAL
TEMPS AND MAINLY DRY.  ANOTHER SYSTEM PUSHES IN FOR THURSDAY NIGHT
AND FRIDAY AND GIVEN THE STRENGTH OF THE TROUGH OVER US THAT FAR OUT
WE WILL GET SOME PRECIP OUT OF THE SYSTEM BUT MODEL DISCREPANCIES
ARE TOO BIG TO GIVE MUCH IN THE WAY OF TIMING AND AMOUNT DETAILS.

&&

.AVIATION...18/18Z
ISSUED AT 1141 AM CST THU DEC 18 2014

SNOW IS WINDING DOWN ACROSS SOUTHERN IOWA THIS AFTERNOON WITH IFR
TO LOW MVFR CEILINGS LINGERING OVER THE AREA. IFR/LOW MVFR STRATUS
IS ALSO HANGING ON ACROSS PORTIONS OF NE IOWA. EXPECT LITTLE
IMPROVEMENT THROUGHOUT THE DAY WITH A LARGE SHIELD OF MVFR TO IFR
STRATUS OVER NEBRASKA AND WESTERN IOWA ADVECTING OVER CENTRAL IOWA
TONIGHT AND LINGERING THROUGH THE END OF THE TAF PERIOD.


&&

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

$$

SHORT TERM...SKOW
LONG TERM...FAB
AVIATION...SKOW



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