Heavy Snow in Iowa, January 16 to 17, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
655 AM CST WED JAN 16 2008

.UPDATED 16/12Z AVIATION DISCUSSION

.DISCUSSION...
TWO PERIODS OF CONCERN WITH FORECAST. FIRST BEING THE NEAR TERM
POTENTIAL FOR MODERATE SNOWFALL AND THE SECOND BEING THE PASSING OF
AN ARCTIC BOUNDARY ON FRIDAY. FOR TODAY COLD FRONT HAS ENTERED FAR
NORTHWEST IOWA WHILE THE PRIMARY UPPER LEVEL SYSTEM IS LAGGING OVER
WESTERN COLORADO. THE COLD FRONT WILL MOVE SLOWLY EAST OVER IOWA
TODAY AND WILL PROVIDE A LARGE TEMPERATURE GRADIENT ACROSS THE STATE
WITH FALLING TEMPERATURES OVER THE NORTHWEST AND UNSEASONABLY WARM
TO THE SOUTHEAST. THE UPPER LEVEL SHORT WAVE WILL BEGIN TO EJECT OUT
INTO THE PLAINS LATE THIS AFTERNOON AND THEN PASS THOUGH THE REGION
OVERNIGHT TONIGHT. SOME SNOWFALL IS ANTICIPATED TO BEGIN BY THIS
AFTERNOON WITH UP TO AN INCH POSSIBLE OVER THE SOUTHWEST.

THE 12Z AND 18Z MODEL RUNS WERE PRETTY GENEROUS REGARDING
PRECIPITATION AMOUNTS OVER ONE HALF INCH OVER MUCH OF CENTRAL IOWA.
HOWEVER HAVING DIFFICULTY FINDING ENOUGH AVAILABLE MOISTURE TO FEED
INTO THIS SYSTEM AS A STRONGER AREA OF LOW PRESSURE TRACKS ACROSS
THE NORTHEN GULF OF MEXICO...EFFECTIVELY LIMITING THE MOISTURE
SUPPLY TO THE MIDWEST. THE 00Z AND 06Z MODEL RUNS SEEM TO BE
CATCHING ON TO THIS AND HAVE BEEN GRADUALLY SCALING BACK THE QPF AND
FALLS MORE IN LINE WITH THE FAVORED EASTNMM4 WITH QPF IN THE .3 TO
.4 TENTH OF AN INCH RANGE. THESE QPF AMOUNTS COMBINED WITH SNOWFALL
RATIOS OF 11-13 TO 1 YIELDS A BAND OF 4 TO 6 INCHES. THE 00Z GFS AND
NAM EACH FAVORED PEAK LIFT/OMEGA BELOW THE DEDRITIC ZONE...FAVORING
SMALL PLATE GROWTH INSTEAD AND SUGGESTING LESSER SNOW AMOUNTS UNTIL
THE PRIMARY SHORT WAVE PUSHES THROUGH THE AREA LATE TONIGHT. THE 06Z
NAM NOW SUGGESTING A MORE PROLONGED PERIOD OF DENDRITES SO
UNCERTAINTY STILL EXISTS. ANOTHER FACTOR IS THE POTENTIAL DRY SLOT
AFFECTING A PORTION OF THE SOUTHEAST TONIGHT AS THE SHORT WAVE
ARRIVES. OVERALL EXPECT A PROLONGED PERIOD OF LIGHT SNOW
TONIGHT...BECOMING MODERATE AT TIMES THEN BEST CHANCE FOR WIDESPREAD
MODERATE SNOW OCCURRING BETWEEN 06Z AND 12Z. WILL LEAVE THE WINTER
STORM WATCH INTACT THOUGH DELAYED AN HOUR TO COINCIDE WITH NEW
FORECAST PACKAGE. CURRENTLY EXPECT A HIGH END ADVISORY EVENT. WITH A
PROLONGED SNOW EVENT...WARNING CRITERIA BECOMES 8 INCHES IF TIME
PERIOD EXCEEDS 12 HOURS AND NOT CONFIDENT 6 INCHES WILL OCCUR IN ANY
12 HOUR PERIOD WITH EVENT. BY LEAVING AS A WATCH...WILL GIVE DAY
SHIFT ANOTHER FEW MODELS RUNS TO LOOK AT AS WELL AS THE DEVELOPING
SITUATION AND YET ALLOW FOR A LARGE LEAD TIME YET IF A WARNING WOULD
NEED TO BE ISSUED. OVERALL THE LACK OF A MOISTURE SUPPLY MAY VERY
WELL BE THIS EVENTS DOWNFALL BUT THE PROLONGED PERIOD OF SNOWFALL
CAN NOT BE DISCOUNTED.

THURSDAY THE SNOW WILL COME TO AN END FROM WEST TO EAST AND IT WILL
BE COLD. WARMED LOWS FOR THURSDAY NIGHT AS WARM ADVECTION COMMENCES
AHEAD OF THE APPROACHING POLAR BOUNDARY. ALSO WARMED HIGHS ON FRIDAY
SOMEWHAT THOUGH THEY WONT BE AROUND LONG AS STRONG COLD ADVECTION
SWEEPS ACROSS THE STATE IN THE MORNING RESULTING IN FALLING
TEMPERATURES. LIGHT SNOW UP TO ONE INCH CAN ALSO BE EXPECTED. THE
TWO CONCERNS WITH THIS SYSTEM LATE FRIDAY AND FRIDAY NIGHT WILL BE
THE WIND CHILLS AND THEN BLOWING SNOW FROM NORTHWEST WINDS GUSTING
TO 40 MPH. COULD VERY WELL EXCEED WIND CHILL WARNING CRITERIA OF
-30F BELOW ZERO AND APPROACH BLIZZARD CONDITIONS WITH ALL OF THE NEW
FLUFF ON THE GROUND. WILL HIGHLIGHT IN THE HWO. BITTERLY COLD ON
SATURDAY WITH MANY LOCATIONS STAYING BELOW ZERO.

&&

AVIATION...16/12Z
MVFR CIGS WILL CONTINUE TO PUSH SOUTH ACROSS IOWA THIS MORNING AHEAD
OF -SN EXPECTED TO BEGIN BY MID TO LATE AFTERNOON. KALO/KDSM STILL
IN LINE TO GET HIGHER INTENSITY SNOW OVERNIGHT AND HAVE PERIODS OF
1/2SM VSBY AND MODERATE SNOW PRIOR TO 12Z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM WEDNESDAY AFTERNOON THROUGH THURSDAY
AFTERNOON FOR ADAIR-ADAMS-APPANOOSE-AUDUBON-BLACK HAWK-BOONE-
BREMER-BUTLER-CASS-CERRO GORDO-CLARKE-DALLAS-DAVIS-DECATUR-
FRANKLIN-GREENE-GRUNDY-GUTHRIE-HAMILTON-HANCOCK-HARDIN-HUMBOLDT-
JASPER-LUCAS-MADISON-MAHASKA-MARION-MARSHALL-MONROE-POLK-
POWESHIEK-RINGGOLD-STORY-TAMA-TAYLOR-UNION-WAPELLO-WARREN-WAYNE-
WEBSTER-WORTH-WRIGHT.

&&

SHORT TERM/AVIATION...KINNEY
LONG TERM...DONAVON

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