Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 241622
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1122 AM CDT TUE MAR 24 2015

.UPDATE...
ISSUED AT 1122 AM CDT TUE MAR 24 2015

SEVERAL REPORTS OF A LIGHT GLAZING ACROSS PORTIONS OF WEST CENTRAL
AND SOUTH CENTRAL IOWA WITH LIGHT FREEZING RAIN OR DRIZZLE.
OBSERVATIONS ALL SHOW TEMPERATURES RIGHT AROUND FREEZING...WITH
ROAD AND BRIDGE DECK TEMPERATURES ALL ABOVE FREEZING. THIS IS
CREATING MAINLY WET DRIVING CONDITIONS...WITH FREEZING OCCURRING
ON ELEVATED SURFACES SUCH AS CARS...TREES...DECKS...ETC. A FEW
HUNDREDTHS OF AN INCH OF ICE ACCUMULATION WITH A FEW ISOLATED
SITES SEEING AS MUCH AS ONE TENTH OF AN INCH OF ICE ACCUMULATION
IS POSSIBLE ON THESE ELEVATED SURFACES THROUGH THE NEXT FEW HOURS
UNTIL TEMPERATURES WARM ABOVE FREEZING A FEW DEGREES. THIS AREA OF
PRECIPITATION WILL MOVE UP THROUGH CENTRAL IOWA THROUGH EARLY
AFTERNOON AS WELL. AS PRECIPITATION MOVES NORTH...MAINLY LIGHT
SNOW IS EXPECTED ACROSS NORTHERN IOWA LATE THIS AFTERNOON AND
EVENING.

&&

.SHORT TERM /TODAY/...
ISSUED AT 336 AM CDT TUE MAR 24 2015

A MULTI FACETED FORECAST AGAIN TODAY AS A POTENT UPPER LEVEL SYSTEM
MOVES INTO THE REGION. THIS SYSTEM HAS TRENDED FURTHER SOUTH THE
PAST 24 HRS WITH THE SFC LOW PRESSURE MOVING THROUGH CENTRAL
MISSOURI TO NORTHERN ILLINOIS. THIS WILL TRANSLATE TO A SLOWER
ARRIVAL OF PRECIPITATION TO CENTRAL AND NORTHERN IOWA BUT ALSO WILL
LEAD TO A STRONGER INTRUSION OF COLDER AIR INTO THE STATE LEADING TO
MORE SNOW ACROSS NORTHERN IOWA. CURRENTLY THE UPPER PV ANOMALY WAS
MOVING INTO WESTERN COLORADO. FURTHER EAST...AN INTENSE LOW LEVEL
JET HAS DEVELOPED THROUGH OKLAHOMA AND THROUGH EASTERN KANSAS WITH
AN AREA OF SHOWERS AND THUNDERSTORMS EXTENDING FROM SOUTHERN
NEBRASKA TO NORTHERN MISSOURI. THIS ACTIVITY WILL GRADUALLY SHIFT
NORTHEAST THIS MORNING INTO SOUTHERN AND WESTERN IOWA. STILL SOME
CONCERN FOR A BRIEF PERIOD OF FREEZING RAIN WITH CURRENT
TEMPERATURES IN THE LOWER 30S AND SOURCE REGION DEWPOINTS IN THE MID
TO UPPER 20S. TEMPERATURES MAY COOL A DEGREE OR TWO WITH
PRECIPITATION ONSET DUE TO EVAPORATIVE COOLING BEFORE WARMING BACK
ABOVE FREEZING DUE TO ADVECTION AND LATENT HEAT RELEASE.  LATEST
SREF FREEZING RAIN PROBS SUGGEST POTENTIAL WILL EXIST ALONG AND
SOUTH OF HIGHWAY 30 BETWEEN 9-15Z THIS MORNING. THEREFORE ANY ICING
DURING THIS PERIOD COULD CAUSE A SIGNIFICANT IMPACT WITH MORNING
COMMUTE. DO NOT HAVE ENOUGH CONFIDENCE FOR A HEADLINE AT THIS TIME
BUT WILL MONITOR TRENDS.

THE STRONGER KINEMATIC FORCING ARRIVES THIS AFTERNOON WITH
PRECIPITATION BECOMING MORE WIDESPREAD. THUNDERSTORMS WILL BECOME
MORE PREVALENT OVER FAR SOUTHERN IOWA. MUCAPES TO NEAR 500 J/KG
STILL ON TARGET OVER THE FAR SOUTHEAST AND GIVEN THERMAL
PROFILES...AN ISOLATED CELL MAY PRODUCE QUARTER SIZED HAIL. FARTHER
NORTH...A QUICK TRANSITION TO SNOW LOOKS MORE PLAUSIBLE AS PROFILES
COOL. SOME QUESTION ON THE LOW LEVEL THERMAL PROPERTIES AND
POTENTIAL FOR NEAR SURFACE MELTING. GIVEN THE AFOREMENTIONED SOURCE
REGION DEWPOINTS...EXPECT SNOW TO DEVELOP TO THE SURFACE. IN
ADDITION...THE SNOW WILL LIKELY BE CONVECTIVE AND COULD HAVE A
PERIOD OF THUNDERSNOW WITH LAPSE RATES STEEPENING ABOVE 700 MB AND
MUCAPES OVER 100 J/KG. ANY RESIDUAL WARM LAYER ALOFT WILL BE
CONVECTIVELY COOLED HELPING LEAD TO THE TRANSITION TO SNOW. COULD BE
A BRIEF PERIOD OF SNOWFALL RATES IN EXCESS OF 1 INCH PER HOUR BY MID
TO LATE AFTERNOON OVER NORTH CENTRAL IOWA. GIVEN THE TIMING AND
POTENTIAL IMPACT...WILL BE ISSUING A WINTER WEATHER ADVISORY FOR
PORTIONS OF NORTH CENTRAL IOWA BEGINNING AT 20Z. TEMPERATURES TODAY
WILL BE COOL AND GENERALLY IN THE 30S.

.LONG TERM /TONIGHT THROUGH MONDAY/...
ISSUED AT 336 AM CDT TUE MAR 24 2015

MAIN FORECAST CONCERN WILL BE WITH EXPECTED ONGOING PRECIPITATION
FOR BEGINNING OF PERIOD. MAIN CONCERN WILL BE WITH PRECIPITATION
TYPE AND LOCATION OF HEAVIEST PRECIPITATION...ESPECIALLY SNOW.
EXPECT PRECIPITATION TO BE ONGOING ACROSS AREA...THOUGH WILL SEE
SYSTEM PUSH EAST QUICKLY BEGINNING AFTER 00Z...WITH PRECIPITATION
ENDING ACROSS MOST OF THE AREA BY 06Z OR A LITTLE LATER AS SYSTEM
PUSHES EAST. EXPECT PRECIPITATION TO BE SNOW ACROSS THE NORTH WITH
RAIN FURTHER SOUTH...AND MAY SEE GOOD SNOWFALL RATES ACROSS THE
NORTH WITH STRONG FORCING...ESPECIALLY NEAR 00Z. FORCING
DIMINISHES QUICKLY...BUT COULD STILL SEE AN INCH OR MORE OF
ACCUMULATION ESPECIALLY NEAR THE MN BORDER. INSTABILITY IS NOTED
GENERALLY ACROSS CENTRAL AND EASTERN CWA AT 00Z...THOUGH THIS
AGAIN QUICKLY DIMINISHES AFTER. HOWEVER...HAVE INCLUDED THUNDER
MENTION ACROSS MUCH OF SOUTHERN CWA...AND CANNOT RULE OUT
POTENTIAL FOR INCREASED SNOWFALL RATES WITH THIS INSTABILITY.
HOWEVER...HAVE NOT FACTORED THIS INTO SNOWFALL TOTALS GIVEN
UNCERTAINTIES ATTM. WINDS WILL BE ANOTHER CONCERN...THOUGH THE
GFS IS MUCH MORE AGGRESSIVE WITH HIGHER WINDS...10 TO 15KTS...AND
HAVE KEPT WINDS CLOSER TO OTHER MODELS...LESS THAN 10KTS THOUGH
COULD SEE SOME HIGHER GUSTS ESPECIALLY VERY EARLY IN THE PERIOD.
LAST CONCERN WILL BE POTENTIAL FOR FZRA/FZDZ AS LOW LEVEL MOISTURE
REMAINS WITH ICE INTRODUCTION LOST AS SYSTEM DEPARTS. BIGGEST
CONCERN FOR THIS WILL BE EAST OF INTERSTATE 35. GIVEN ALL OF
THESE FACTORS WILL EXTEND WINTER WEATHER ADVISORY THROUGH 06Z WITH
THE WORST CONDITIONS ARE EXPECTED TO HAVE ENDED ACROSS THE
AREA...THOUGH MAY NEED TO BE EXTENDED IN TIME IF FZRA/FZDZ
PERSISTS.

BEHIND RETREATING SYSTEM BOUNDARY WILL PUSH SOUTH LATE IN THE DAY
WEDNESDAY. HAVE CUT BACK ON TEMPERATURES SLIGHTLY FOR WEDNESDAY
WITH CLOUDS EXPECTED AND POTENTIAL FOR FRESH SNOWFALL IN THE
NORTH. MODELS ATTEMPT TO PRODUCE PRECIPITATION ACROSS THE CWA WITH
FROPA...THOUGH HAVE KEPT PRECIPITATION CHANCES FOCUSED TO THE
SOUTHERN PORTION OF THE CWA IN THE AFTERNOON WHERE BEST MOISTURE
IS. SOUNDINGS INDICATE DECENT DRY LAYER AT LOW AND MID LEVELS
ELSEWHERE WHICH SHOULD HELP LIMIT NORTHERN EXTENT OF
PRECIPITATION. IN THE AFTERNOON WEAK MUCAPE NOTED IN THE SOUTH AND
HAVE INCLUDED ISOLATED THUNDER MENTION THERE AGAIN WEDNESDAY.

HIGH PRESSURE SYSTEM WILL DROP SLOWLY SOUTH FROM CANADA BEHIND
BOUNDARY. THIS WILL KEEP CONDITIONS PRIMARILY DRY...THOUGH
TEMPERATURES WILL BE COOLER. COLDEST READINGS WILL COME FRIDAY AS
HIGH PASSES OVER IOWA...WITH BELOW NORMAL TEMPERATURES. BEYOND
FRIDAY INTO THE WEEKEND WILL SEE A RETURN TO WARMER/ABOVE NORMAL
CONDITIONS AS WAA RETURNS. HOWEVER...WILL ALSO SEE A RETURN FOR
PRECIPITATION CHANCES. MODELS DROP WAVE SOUTH NEAR MISSOURI RIVER
VALLEY SATURDAY...WHICH MAY PUSH PRECIPITATION CHANCES INTO
WESTERN CWA...THOUGH ONLY HAVE SLIGHT CHANCES POPS FOR NOW WITH
UNCERTAINTIES IN TIMING/LOCATION. BETTER CHANCES WILL COME
SATURDAY INTO SUNDAY WITH ANOTHER BOUNDARY PUSHING SOUTH ACROSS
THE AREA. DO NOT HAVE HIGH CONFIDENCE IN PRECIPITATION CHANCES FOR
NOW...WITH MODELS IN NOT IN GOOD AGREEMENT WITH TIMING OF SYSTEM.
WARMER TEMPERATURES WILL PERSIST BEYOND PERIOD.

&&

.AVIATION...24/12Z
ISSUED AT 631 AM CDT TUE MAR 24 2015

WIDESPREAD IFR TO MVFR CONDITIONS THROUGH THE DAY WITH SHOWERS AND
THUNDERSTORMS DEVELOPING SOUTH AND RAIN CHANGING TO SNOW NORTH.
VISIBILITIES MAY DROP TO LESS THAN 1SM AT KMCW AT TIMES DUE SNOW
LATE AFTERNOON THROUGH EVENING. BRISK SOUTHEAST WINDS WILL PREVAIL
THROUGH THE DAY THE BECOME WESTERLY OVERNIGHT. PRECIPITATION WILL
END WITH THE WIND SHIFT.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 3 PM THIS AFTERNOON TO 1 AM CDT
WEDNESDAY FOR BREMER-BUTLER-CERRO GORDO-EMMET-FRANKLIN-HANCOCK-
HUMBOLDT-KOSSUTH-PALO ALTO-POCAHONTAS-SAC-WINNEBAGO-WORTH-WRIGHT.

&&

$$

UPDATE...BEERENDS
SHORT TERM...DONAVON
LONG TERM...AWB
AVIATION...DONAVON



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