Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 230009
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
610 PM CST MON DEC 22 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 325 PM CST MON DEC 22 2014

MAIN CONCERN TODAY WILL OBVIOUSLY BE PRECIP TRENDS WITH RAIN
TRANSITIONING TO SNOW NORTH.  SHORT WAVES EMBEDDED WITHIN CENTRAL
CONUS TROUGH OVER SD AND SERN NE CONTINUE TO PRODUCE AREAS OF
PRECIP ACROSS THE MIDWEST WITH THE LATTER DIRECTLY IMPACTING IA.
FORCING IS DEEP BUT RELATIVELY WEAK AND ONLY RESULTING IN LIGHT
RAIN FOR THE TIME BEING. KS PV ANOMALY ROTATING THROUGH TROUGH
WILL STRENGTHEN MID LEVEL CIRCULATION AND ASSOCIATED FORCING
THROUGH THE NIGHT HOWEVER. WIDESPREAD PRECIP SHOULD BE IN PLACE
NORTH OF THE CYCLONE BY EARLY TUESDAY BUT LACK OF LIFT AND
ROTATION OF NE/KS DRY SLOT SHOULD EVENTUALLY REDUCE PRECIP SOUTH.

QUESTION NORTH IS A FUNCTION OF TYPE MORE THAN ANYTHING. SOUNDINGS
DEPICT FADING WARM LAYER NORTH RIGHT NOW WITH ONLY MID 30S SURFACE
WET BULB TEMPS KEEPING THINGS LIQUID. FORECAST SOUNDINGS
EVENTUALLY COOL SURFACE TEMPS AND LOWEST KM OR SO SUFFICIENTLY
TO FLIP TO ALL SNOW OVERNIGHT BUT THE TRANSITION WILL BE QUITE
SLOW. DENDRITIC GROWTH ZONE FORCING IS FAIRLY STRONG 06-12Z NOTED
AROUND 600MB OR 300K ISENTROPICALLY. THIS WILL LIKELY RESULT IN
FAIRLY TYPICAL LOWER TEEN SNOW RATIOS EVENTUALLY...BUT SLOW
TRANSITION WITH A MIX/MELTING AND LOWER RATIOS WILL LIKELY EAT AT
TOTALS A BIT AND REDUCE CONFIDENCE AS WELL. THUS ONLY HAVE UP TO
AN INCH OR SO FAR NW THROUGH 12Z WITH LESSER AMOUNTS AS FAR SOUTH
AS CARROLL-FORT DODGE- HAMPTON LINE. SOUNDINGS IN LOWEST 1KM ARE
NEARLY ISOTHERMAL HOVERING AROUND 0C SO THE SNOW WILL BE QUITE WET
AND LIKELY END UP SLUSHY ON ROADS. CURRENT NRN IA ROAD TEMPS ARE
IN THE 30S AND NOT EXPECTED TO CHANGE MUCH INTO THE EVENING AS
THEY REMAIN WET.

.LONG TERM /TUESDAY THROUGH MONDAY/...
ISSUED AT 325 PM CST MON DEC 22 2014

FORECAST REMAINED FOCUSED ON SNOW AND RAIN TUESDAY INTO TUESDAY
EVENING. LEANED TOWARD A SREF/GFS/ECMWF/HIRES_NMM_EAST BLEND AS
NAM SEEMED TO BE TOO COLD AND AGGRESSIVE WITH SNOW AMOUNTS
TUESDAY.

TUESDAY INTO WEDNESDAY...BY 12Z TUESDAY THE SURFACE LOW LOOKS TO
BE CENTERED OVER EAST-CENTRAL IOWA WITH THE UPPER LOW TILTED BACK
OVER SOUTHWEST PORTIONS OF THE STATE. THERE REMAINS A SHARP
GRADIENT BETWEEN RAIN/SNOW LINE TUESDAY OVER CENTRAL TO SOUTHEAST
IOWA. HOWEVER...A DECENT PUNCH OF DRY AIR LOOKS TO ADVECT INTO THE
CENTRAL TO SOUTHEAST SECTIONS OF THE STATE TUESDAY AND SHOULD HELP
MITIGATE ANY PRECIP CHANCES ON TUESDAY. KEPT WITH CHANCE POPS
GOING LATE IN THE MORNING INTO THE AFTERNOON. THERE MAY BE A BRIEF
CHANGE OVER TO ALL SNOW AS FAR SOUTHEAST AS DES MOINES DURING THE
MORNING HOURS TUESDAY...BUT SNOW ACCUMULATIONS SHOULD BE MITIGATED
BY THE WARM SURFACE WET BULBS NEAR OR SLIGHTLY ABOVE 32 DEGREE
RANGE AND LIKELY WILL MELT AS IT REACHES THE GROUND.

FURTHER WEST TO NORTHWEST...BETTER FORCING AND THE DEEPER
MOISTURE WRAPS AROUND ON THE BACKSIDE OF THE SURFACE LOW TUESDAY
MORNING AND LINGERS INTO THE LATE AFTERNOON HOURS. HAVE A NON-
DIURNAL TEMPERATURE TREND GOING FOR MUCH OF THE FORECAST AREA
TUESDAY. MOISTURE TRANSPORT ALONG THE 285-300K ISENTROPIC SURFACE
COINCIDES WITH THE NICE TROWAL SETTING UP OVER WISCONSIN TO
SOUTHERN MINNESOTA INTO NORTH-CENTRAL IOWA BY TUESDAY MORNING.
EXPECTING PRECIPITATION TO SWITCH OVER ENTIRELY TO SNOW B/T
12-14Z...IF NOT ALREADY BY THAT TIME...OVER NORTHERN TO WESTERN
SECTIONS OF THE FORECAST AREA. A WET SNOW IS LIKELY AS THE LOW
SNOW RATIOS OF 7:1 TO 10:1 ARE EXPECTED RESULTING IN ROUGHLY 1 TO
UNDER 2 INCHES THROUGH 18Z...WITH AN ADDITIONAL HALF INCH POSSIBLE
BY 00Z WEDNESDAY.

THE CAA AND LIGHT SNOW CHANCES CONTINUE TUESDAY NIGHT INTO
WEDNESDAY...BUT MUCH OF THE FORCING AND BEST MOISTURE MOVES EAST
WITH THE SURFACE LOW. STRONG WINDS WILL BE THE MAIN FACTOR ON
WEDNESDAY WITH WINDS ATOP THE MIXED LAYER RANGE AROUND 30-35 KNOTS
THROUGHOUT THE DAY. KEPT LINGERING POPS ACROSS THE SOUTHEAST AFTER
12Z WEDNESDAY.

CHRISTMAS INTO FRIDAY...THE OPERATIONAL GFS IS MORE IN LINE WITH
THE PARALLEL GFS AND ECMWF WITH A SHORTWAVE AND SURFACE LOW
PRESSURE SYSTEM TO PUSH ACROSS THE REGION LATE THURSDAY NIGHT
INTO FRIDAY. THE SURFACE LOW TRACKS FROM EASTERN NEBRASKA INTO
INTO SOUTHERN WISCONSIN PLACING A SNOW BAND OVER WEST-CENTRAL TO
NORTHERN IOWA ON FRIDAY.

SATURDAY THROUGH MONDAY...HIGH PRESSURE BUILDS INTO THE REGION OVER
THE WEEKEND AND PROVIDES SOME COOL AND DRY WEATHER...WITH SUN
POTENTIALLY SHOWING FOR MUCH OF THE WEEKEND. BIG DISCREPANCIES EARLY
NEXT WEEK BETWEEN THE ECMWF/GFS WITH TIMING OF A DEEP TROUGH MOVING
INTO THE CENTRAL U.S. ONE THING THEY DO AGREE UPON IS COLDER
TEMPERATURES BY NEW YEARS.

&&

.AVIATION...23/00Z
ISSUED AT 606 PM CST MON DEC 22 2014

WIDESPREAD STRATUS THAT HAS BEEN WITH US FOR DAYS WILL CONTINUE TO
KEEP IFR CONDITIONS IN PLACE.  PERIODS OF RAIN AND DRIZZLE WILL ALSO
PERIODS WITH A TRANSITION TO SNOW NORTH OF HWY 20 BETWEEN 10Z AND
14Z.  A TRANSITION TO SNOW WILL OCCUR ACROSS THE SOUTH BUT LIKELY
NOT UNTIL AFT 16Z.  LOCALLY DENSE FOG IS NOTED ACROSS SERN IA MAINLY
AFFECTING KOTM.  I DO HAVE A PERIOD OF 1/2SM TEMPO 1/4 IN FG.  THIS
MAY NEED TO BE EXTENDED FOR A COUPLE MORE HOURS AND WILL BE WATCHED
FOR EXPANSION.

&&

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

&&

$$

SHORT TERM...SMALL
LONG TERM...PODRAZIK
AVIATION...FAB



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