Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 180541
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1141 PM CST WED DEC 17 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 245 PM CST WED DEC 17 2014

A QUIET AFTERNOON IS IN STORE FOR THE CWA WITH SURFACE HIGH PRESSURE
ANCHORED ALONG THE MISSOURI RIVER VALLEY. A SHARP 300 MB SHORTWAVE
TROUGH CURRENTLY EJECTING FROM THE FOUR CORNERS REGION WILL ENTER
SOUTHWEST IOWA BY 12Z THURSDAY. NAM/GFS ARE TOO SLOW WITH RETURNING
MID-LEVEL MOISTURE (MANIFESTING ITSELF AS AN ALTOSTRATUS DECK) NOW
WELL INTO SW IOWA AS WELL AS WITH PRECIP COVERAGE WITH THIS SYSTEM
IN OKLAHOMA. THE HRRR BEST REFLECTS REALITY IN TERMS OF PRECIP
COVERAGE BUT IS ABOUT 3 HOURS BEHIND THE CURVE WITH CLOUD PLACEMENT.
HAVE SUBSEQUENTLY MANUALLY INCREASED CLOUD COVER OVER THE SW HALF OF
THE CWA AND USED THE HRRR FOR TIMING THE ONSET OF SNOW IN THE
SOUTHWESTERN CWA. LOW TEMPERATURES TONIGHT...ESPECIALLY OVER THE
NORTH...WILL BE DEPENDENT ON HOW QUICKLY CLOUDS CAN ADVECT OVERHEAD.
THESE WILL NEED TO BE MONITORED THIS EVENING AND ADJUSTED UPWARD
SHOULD THE CLOUDS ADVANCE FASTER THAN CURRENTLY FORECAST.

THE FASTER MOISTURE RETURN THROWS JUST ANOTHER WRENCH IN TIMING AND
AMOUNTS OF SNOWFALL IN SOUTHERN IOWA LATE TONIGHT. 12/18Z MODEL BUFR
SOUNDINGS DEPICT A STOUT 925-850MB DRY WEDGE ATTENDANT WITH THE
SURFACE RIDGE...BUT THIS MIDLEVEL MOISTURE MAY SATURATE THE COLUMN
FASTER THAN PROGGED AND LEAD TO HIGHER SNOWFALL TOTALS. OVERALL MADE
LITTLE CHANGES TO THE FORECAST IN THIS RESPECT...BUT DID TIGHTEN THE
POP GRADIENT AS IT ADVANCES NORTHEAST FROM 09 TO 12Z THINKING THAT
THE LOW LEVEL DRY AIR WILL RESULT IN A SHARP CUTOFF IN ACCUMULATING
SNOW.

.LONG TERM /THURSDAY THROUGH WEDNESDAY/...
ISSUED AT 245 PM CST WED DEC 17 2014

A FEW CHALLENGES OVER THE EXTENDED PERIOD...WITH A NUMBER OF WEAK
SYSTEMS TO AFFECT PORTIONS OF THE AREA THROUGH THE PERIOD. SPLIT
FLOW CONTINUES TO BE THE MAIN DRIVING FORCE BEHIND THE RATHER
QUIESCENT WEATHER FOR MID DECEMBER. RATHER HIGH AMPLITUDE RIDGE OF
HIGH PRESSURE STILL ANCHORED FROM THE NORTHERN PLAINS TO THE GULF
COAST NEXT FEW DAYS. FIRST WAVE CURRENTLY OVER THE SOUTHERN
PLAINS TODAY AND SHOULD MOVE NORTHEAST THROUGH THU MORNING. AHEAD
OF THE WEAK SYSTEM WARM AIR ADVECTION AND LIMITED MOISTURE WILL BE
FORCED UPWARD IN AN AREA OF ISENTROPIC LIFT OVER SOUTHERN IOWA
THURSDAY. THE LIFT AND MOISTURE WILL BE ATTEMPTING TO OVERCOME THE
DRY AIRMASS CURRENTLY IN PLACE...LIMITING OVERALL AMOUNTS. THIS
WILL RESULT IN SOME RATHER LIGHT SNOW DURING THE MORNING HOURS
WITH MINOR ACCUMULATIONS GENERALLY LESS THAN AN INCH. DURING THE
EARLY AFTERNOON HOURS...BUFR SOUNDINGS SHOW THAT WE LOSE ICE
INTRODUCTION ACROSS THE SOUTHWEST AND SOUTH. HAVE INTRODUCED SOME
PATCHY FREEZING DRIZZLE ALONG WITH VERY LIGHT SNOW DURING THE
AFTERNOON HOURS...THOUGH EXPECTATIONS ARE THAT ANY PRECIPITATION
BY THAT TIME WILL BE VERY LIGHT AND LITTLE IF ANY ICE ACCUMULATION
IS EXPECTED. NEXT SYSTEM ARRIVES BY SUNDAY AND MONDAY AND EXTENDS
INTO TUESDAY. LOWER CONFIDENCE MOVING FORWARD WITH A CONSENSUS
SOLUTION AS THE GFS IS QUITE MILD WHILE THE EURO IS NOW SUGGESTING
A SOLUTION WITH MORE SIGNIFICANT DYNAMIC COOLING AS THE PRECIP
ARRIVES INTO MONDAY. THE EURO SUGGESTS MORE SNOW THAN RAIN/SNOW
MIX. GIVEN THE EVENT IS STILL 5 DAYS AWAY...WILL HOLD ONTO CURRENT
MIX TRENDS FOR NOW...AND MONITOR DURING SUBSEQUENT MODEL PACKAGES.
THE ONE TREND THAT IS BEGINNING TO EMERGE IS COLDER AIR AS NEXT
WEEK PROGRESSES. AS A STRONG PACNW STREAM WAVE ADVANCES EAST INTO
THE WESTERN STATES...MIDWEST RIDGE FINALLY BEGINS TO BREAK DOWN
AND BECOME DISPLACED BY A LONG WAVE TROUGH. THIS SHOULD SPELL A
TRANSITION TO A MORE ACTIVE AND COLDER PATTERN TOWARD THE END OF
THE EXTENDED. AS THE WAVE TUESDAY DEEPENS...A RATHER STRONG SFC
WAVE WILL EJECT NORTH NORTHEAST OUT OF EASTERN TEXAS TO EASTERN
MICHIGAN WEDNESDAY. GIVEN THE CURRENT GFS/EURO PACKAGES...THE MAIN
H850 MOISTURE PLUME...BETTER FORCING...AND HIGHER PRECIPITATION
AMOUNTS WILL PROBABLY END UP EAST OF THE REGION. WILL MONITOR FOR
GENERAL WESTWARD DRIFT NEXT SEVERAL PACKAGES...THIS SOMETIMES
OCCURS WITH WINTER STORM EVOLUTION IN MODEL PACKAGES...FIRST
PROJECTING THE EVENT TOO FAR EAST THEN PULLING IT BACK AS THE
EVENT NEARS.

&&

.AVIATION...18/06Z
ISSUED AT 1140 PM CST WED DEC 17 2014

SYSTEM ACROSS PLAINS WILL CONTINUE TO LIFT NORTHEAST THROUGH
THURSDAY. CLOUDS WILL MOVE INTO IOWA WITH PRIMARILY MID AND HIGH
CLOUDINESS OVERNIGHT AND VFR CONDITIONS. AS DEEPER SATURATION
APPROACHES FROM THE SOUTH TOWARD DAYBREAK...MVFR TO IFR CONDITIONS
NEAR THE MISSOURI BORDER ARE EXPECTED ALTHOUGH DRIER LOW LEVEL AIR
WILL KEEP LOCATIONS FARTHER NORTH VFR. THERE WILL BE SOME LOW
LEVEL CLOUDS NEAR KMCW OVERNIGHT NEAR RIDGE AXIS AND THIS MAY
LINGER INTO THURSDAY MORNING AS FLOW REMAINS QUITE LIGHT. WITH
PASSAGE OF SYSTEM ON THURSDAY...THREAT OF PRECIPITATION IN THE
SOUTH WILL END AFTER MIDDAY. HOWEVER...LOW TO MID LEVEL MOISTURE
WILL REMAIN WITH WIDESPREAD STATUS EXPECTED TO PERSIST THROUGH THE
END OF THE FORECAST.

&&

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

&&

$$

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


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