Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171741
AFDDMX

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

.UPDATE...
ISSUED AT 1145 AM CST WED DEC 17 2014

MINOR SKY ADJUSTMENTS WERE MADE FOR THE AFTERNOON FORECAST TO
REFLECT THE CLEARER CONDITIONS IN NORTHERN IOWA. STRATOCUMULUS
DECK IS TRYING TO REFORM OVER THE FAR NE CWA BUT HAS BEEN
STRUGGLING TO ADVANCE MUCH INTO BREMER AND BLACK HAWK COUNTIES.

&&

.SHORT TERM /TODAY/...
ISSUED AT 344 AM CST WED DEC 17 2014

NO SIGNIFICANT WEATHER OF CONCERN TODAY...BUT TEMPS MAY BE A TAD
PROBLEMATIC. RIGHT ENTRANCE REGION OF IA/IL JET SEGMENT AND WEAK
FORCING DRIVEN BY WRN KS SHORT WAVE AHEAD OF SWRN CONUS TROUGH
CONTINUE TO SPREAD TOKEN MID CLOUDS AND PRECIP ALOFT INTO SWRN
IA. FORCING REMAINS OF SIMILAR MAGNITUDE THROUGHOUT THE DAY...IS
MAINLY 3KM AND ABOVE...AND CONTINUES TO ADVANCE INTO SURFACE RIDGE
SO NO PRECIP IS ANTICIPATED THROUGH VALID PERIOD.

MODELS ARE HAVING SOME DIFFICULTY HANDLING CURRENT TEMP REGIME.
TEMPS HAVE COOLED BELOW SOME MODELS WHERE SKIES HAVE CLEARED. HIGH
RES MODELS ARE DOING BETTER THAN TRADITIONAL OPERATIONAL RUNS AS
HRRR...RAP AND GEM REGIONAL MODELS BETTER RESEMBLE REALITY.
QUESTION IS WHETHER TO EXTRAPOLATE THEIR SOLUTIONS INTO THE FUTURE
AS THEY VARY QUITE A BIT. RAP AND GEM REGIONAL KEEP THINGS QUITE
COOL WITH MINIMAL REBOUND WHILE HRRR IS BACK UP TOWARD MODEL
CONSENSUS BY AFTERNOON. EVEN THOUGH WINDS WILL DIMINISH SOMEWHAT
IN THE SURFACE RIDGE...STILL EXPECT SOME MIXING WITHOUT SNOW COVER
SO HAVE ONLY GONE JUST A TOUCH BELOW MODEL CONSENSUS WHICH IS NOT
DISSIMILAR TO MOS SUGGESTIONS.

.LONG TERM /TONIGHT THROUGH MONDAY/...
ISSUED AT 344 AM CST WED DEC 17 2014

A WEAK 500 MB TROUGH WILL PROGRESS ACROSS KANSAS AND MISSOURI ON
WEDNESDAY NIGHT AND THURSDAY...SPREADING LIGHT PRECIPITATION THAT
MAY MAKE IT INTO MAINLY SOUTHERN IOWA. HOWEVER...THERE WILL BE A
SURFACE HIGH PRESSURE RIDGE BUILD ACROSS THE STATE DURING THAT
TIME RESULTING IN LOW LEVEL DRY AIR ISSUES AS DEPICTED BY MOST
FORECAST SOUNDINGS. IT APPEARS PROBABLE THAT THERE WILL AT LEAST
BE SOME FLURRIES OR VERY LIGHT SNOW ACROSS OUR SOUTHERN
COUNTIES...BUT FURTHER NORTH PRECIPITATION IS MUCH LESS LIKELY AND
EVEN IN THE SOUTH ANY ACCUMULATIONS SHOULD BE LIGHT. HAVE
MAINTAINED CHANCE POPS LATE WEDNESDAY NIGHT AND THURSDAY MORNING
WITH SNOWFALL AMOUNTS AROUND AN INCH OR LESS AND GENERALLY LIMITED
TO I-80 SOUTHWARD. THERE IS A POSSIBILITY OF FREEZING DRIZZLE AT
TIMES IF THE CLOUD LAYER IS SUFFICIENTLY SHALLOW/WARM...HOWEVER AT
THIS TIME THAT IS AN UNLIKELY SCENARIO AND IT HAS NOT BEEN
INCLUDED IN THE FORECAST DUE TO THE LOW PROBABILITY OF
OCCURRENCE.

FROM THURSDAY NIGHT THROUGH SATURDAY THE AFOREMENTIONED SURFACE
HIGH PRESSURE RIDGE WILL REMAIN ANCHORED JUST TO OUR EAST...FROM
THE GREAT LAKES DOWN ACROSS ILLINOIS...RESULTING IN VERY LIGHT
SOUTHERLY FLOW AND LIKELY SEVERAL MORE DAYS OF LOW STRATUS CLOUDS
THAT WILL LIMIT TEMPERATURE FLUCTUATIONS. HAVE INCREASED CLOUD
COVER FORECAST AND COMPRESSED DIURNAL TEMPERATURE SPREADS
ACCORDINGLY. IT IS PROBABLE THAT THIS WILL MANIFEST AS MORE
DRY BUT CLOUDY AND DREARY WEATHER TO END THE WORK WEEK.

BY SATURDAY A LONGWAVE 500 MB TROUGH WILL BE CROSSING THE ROCKIES
AND SHAPING UP TO BE A BIG PLAYER IN OUR WEATHER FOR THE FIRST
HALF OF NEXT WEEK. THE TROUGH WILL REACH THE HIGH PLAINS AND THE
MIDWEST BY SUNDAY AND SUBSEQUENTLY DEEPEN AS IT IS REINFORCED BY A
SERIES OF SUCCESSIVE SHORTWAVE IMPULSES ENTERING THE TROUGH FROM
WESTERN CANADA. BY TUESDAY BOTH THE ECMWF AND GFS ARE NOW CARVING
OUT A MASSIVE 500 MB TROUGH ACROSS MOST OF THE CONUS...WITH A
CENTRAL CLOSED LOW SOMEWHERE OVER THE UPPER MIDWEST. FOR OUR
FORECAST AREA THIS SCENARIO WILL TRANSLATE INTO A PROLONGED PERIOD
OF PRECIPITATION CHANCES...BEGINNING SOMETIME AROUND SUNDAY OR
SUNDAY NIGHT AND PERSISTING AT INTERVALS AT LEAST THROUGH TUESDAY.
HOWEVER...PRECIPITATION TYPE WILL BE HEAVILY DEPENDENT ON THE
LOCATION OF THE UPPER AND SURFACE LOW PRESSURE CENTERS AND THEIR
ASSOCIATED THERMAL PROFILES. CURRENT LONG RANGE MODEL SOLUTIONS
SUPPORT A FAIRLY LONG PERIOD OF RAIN WITH SNOW MORE LIMITED TO
NORTHERN IOWA AND LATER IN THE EVENT...LIKELY RESULTING IN LOW
SNOWFALL AMOUNTS. HOWEVER...WITH THE SYSTEM STILL FOUR OR FIVE
DAYS AWAY AND NOT EVEN ENTERING THE RAOB NETWORK UNTIL AROUND
FRIDAY...THE SOLUTIONS ARE HARDLY STABLE AND A WIDE RANGE OF
EVENTUAL OUTCOMES IS POSSIBLE SO THE FORECAST WILL LIKELY BE
FLUCTUATING IN THE COMING DAYS. ALL INTERESTS WITH TRAVEL PLANS
AROUND CHRISTMAS SHOULD CONTINUE TO MONITOR THESE TRENDS AND HOPE
THAT TEMPERATURES STAY WARM ENOUGH FOR THE PRECIPITATION TO FALL
PREDOMINANTLY AS RAIN.

&&

.AVIATION...17/18Z
ISSUED AT 1145 AM CST WED DEC 17 2014

VFR CONDITIONS ARE EXPECTED TODAY AND TONIGHT...THOUGH PATCHES OF
MVFR STRATUS ARE POSSIBLE AROUND MCW AND ALO THIS AFTERNOON BUT
CONFIDENCE WAS NOT HIGH ENOUGH TO MENTION IN THE TAFS. LOWER CIGS
AND SNOW MOVE INTO SOUTHERN IOWA AFTER 06Z...THOUGH HOW FAR NORTH
THE SNOW PROGRESSES WILL BE A CHALLENGE FOR THE DSM/FOD TAFS.
BEST CHANCE OF MVFR CIGS/VSBYS WILL BE AT OTM FROM 12Z TO THE END
OF THE TAF PERIOD.

&&

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

$$

UPDATE...SKOW
SHORT TERM...SMALL
LONG TERM...LEE
AVIATION...SKOW


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