Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 240551
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1151 PM CST TUE DEC 23 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 313 PM CST TUE DEC 23 2014

PRECIP TRENDS WILL REMAIN THE PRIMARY FOCUS AGAIN TONIGHT. LIGHT
SNOW CONTINUES TO DRIFT NE-SW ACROSS THE NW HALF OF IA DRIVEN
MAINLY BY MID LEVEL KINEMATIC FORCING WITH WEAK DEFORMATION AND
DIFFERENTIAL POSITIVE VORTICITY ADVECTION NORTH OF IA PV ANOMALY
NOTED IN RAP ANALYSIS AND SUGGESTED BY PIVOT POINT IN WATER VAPOR
IMAGERY. THERE IS LITTLE THERMODYNAMIC COMPONENT AT THIS POINT.
PRECIP IS QUITE LIGHT WITH WEAK REFLECTIVITY...EVEN IN LONG
PULSE/VCP 31. WEBCAMS SHOW CURRENT VISIBILITY RESTRICTIONS ARE AS
MUCH DUE TO FOG AS SNOW WITH LITTLE SNOW NOTED. THIS WEAK FORCING
WILL REMAIN ESSENTIALLY STEADY STATE AS IT DRIFTS SLOWLY SOUTH AND
EAST THROUGH THE NIGHT BEFORE THE NEXT SHORT WAVE/PV ANOMALY
ROUNDS THE BASE OF THE TROUGH AND RE-ENERGIZES SYSTEM AGAIN TO OUR
SOUTH AND EAST TOWARD 12Z. SE FRINGE OF PRECIP SHIELD WILL BE A
RAIN/SNOW MIX WITH RAIN/SNOW LINE DRIFTING SE. ONLY LIGHT QPF
AMOUNTS AND TEMPERED SNOW RATIOS WILL RESULT ANY ADDITIONAL ACCUMS
BEING UNDER AN INCH AND MAINLY ON GRASSY OR ELEVATED SURFACES.
ROAD TEMPS REMAIN WET AND 35F PLUS SO MUCH OF SNOW WILL MELT ON
ROADS. MID LEVEL MOISTURE WILL BE LACKING SE WITH LITTLE ICE
INTRODUCTION SO PRECIP WILL GENERALLY REMAIN LIQUID/DRIZZLE SOUTH
AND EAST OF DES MOINES.

.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
ISSUED AT 313 PM CST TUE DEC 23 2014

THE LONG TERM PORTION OF THE FCST PD WILL BE CHARACTERIZED BY
MINIMAL PRECIPITATION...ALONG WITH ABOVE NORMAL TEMPERATURES LATE
THIS WEEK FOLLOWED BY TEMPERATURES GOING BELOW NORMAL BEGINNING
THIS WEEKEND AND CONTINUING INTO NEXT WEEK.

MODELS IN GOOD AGREEMENT WITH THE CENTRAL CONUS SHORTWAVE ENERGY
DEVELOPING A SFC LOW ACROSS THE SOUTHERN U.S. TONIGHT...BRINGING
IT NORTH-NORTHEAST INTO THE EASTERN GREAT LAKES AND DEEPENING BY
THURSDAY. GREATEST SYNOPTIC FORCING AND MOISTURE WILL REMAIN TO
THE EAST OF THE CWA. MODELS SHOWING REMNANT AREA OF WEAK LIFT AND
MOISTURE IN E/SE CWA THRU WED MORNING...SO HAVE KEPT CHC/SLIGHT
CHANCE POPS THERE. PCPN IS EXPECTED TO BE LIGHT.

ATTENTION THEN TURNS TO THE NEXT WX SYSTEM TO AFFECT OUR REGION.
CENTRAL U.S. SHORTWAVE LIFTS OUT AND ANOTHER SHORTWAVE ENTERS THE
WESTERN U.S. BY THURSDAY. MODELS BEGIN DIVERGING ON THE SPEED AND
STRENGTH OF THIS SHORTWAVE ONCE IT ARRIVES. GFS PRESENTS A FASTER
AND WEAKER SOLUTION...WITH THE ECMWF HOLDING THE ENERGY BACK FOR
LONGER. THE DIVERGING SOLUTIONS IMPACT SFC FEATURES AS WELL...WITH
THE GFS BRINGING A STRONGER SFC SYSTEM THROUGH OUR REGION AND
FASTER VS. THE ECMWF. HAVE TRENDED SLOWER THAN THE GFS GIVEN ITS
RECENT PERFORMANCE...BRINGING -SN INTO THE CWA BEGINNING 12Z FRI.
PRIOR TO THE PRECIP/S ARRIVAL...THOUGH...STRONG WAA WILL PUSH
THURSDAY/S MAXES INTO THE MIDDLE 30S AND 40S. ATTM THE BULK OF THE
PRECIP IS EXPECTED TO REMAIN NW OF THE CWA...WITH BEST COMBINATION
OF FORCING AND MOISTURE MOVING ACROSS THE CWA FRI AFTN AND FRIDAY
NIGHT. ALTHOUGH POPS ARE RELATIVELY LOW...THERE IS THE POTENTIAL
FOR A BRIEF PERIOD OF LIFT TO COINCIDE WITH ELEVATED RH LEVELS
WITHIN THE DGZ ESPECIALLY ACROSS NW IOWA.

MODEL SOLUTIONS DIVERGE FURTHER WITH TIME INTO THE WEEKEND SO
FORECAST CONFIDENCE DECREASES. GFS KEEPS OUR REGION IN CYCLONIC
FLOW WITH WEAK SHORTWAVES WHILE THE ECMWF BRINGS MORE FOCUSED
SHORTWAVE ENERGY THROUGH OUR REGION MONDAY. HAVE TENDED TOWARD THE
ECMWF AND HAVE SLIGHT CHANCE POPS FOR THIS TIME PD. LITTLE IF ANY
SNOWFALL ACCUMULATIONS ARE AGAIN EXPECTED GIVEN THE BEST FORCING
BEING OUT OF PHASE WITH ELEVATED MOISTURE LEVELS. TEMPERATURES
WILL BEGIN A DOWNWARD TEND AS NORTHWEST FLOW ALOFT PREVAILS.

&&

.AVIATION...24/06Z
ISSUED AT 1146 PM CST TUE DEC 23 2014

BAND OF LIFR CONDITIONS IN SNOW AND STRATUS WILL CONTINUE TO SINK
SOUTHEAST WITH CONDITIONS BECOMING MVFR FROM THE NORTHWEST LATE
TONIGHT AND WED. VFR CONDITIONS WILL REACH THE FAR WEST LATE IN
THE TAF PERIOD AS HIGH PRESSURE RIDGE AXIS REACHES CENTRAL IA.

OUTLOOK...VFR CONDITIONS WILL OVERSPREAD THE AREA WEDNESDAY NIGHT
AND CHRISTMAS DAY WITH VFR CONTINUING INTO FRIDAY.

&&

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

$$

SHORT TERM...SMALL
LONG TERM...ZOGG
AVIATION...MS DEC 14


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