Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211140
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
540 AM CST SUN DEC 21 2014

.SHORT TERM /TODAY/...
ISSUED AT 332 AM CST SUN DEC 21 2014

HIGHER DEWPOINTS ADVECTING NORTH INTO THE STATE HAS RESULTED IN
AREAS OF DENSE FOG OVER CENTRAL AND NORTHERN IOWA THIS MORNING. THE
DENSE FOG WILL GRADUALLY MIGRATE NORTH OVER THE NEXT SEVERAL HOURS
WHILE IMPROVED VISIBILITIES ARRIVE OVER THE SOUTH. WILL CONTINUE TO
ADJUST THE DENSE FOG ADVISORY AND REMOVE SOUTHERN AREAS AS
CONDITIONS IMPROVE. DRIZZLE WILL ACCOMPANY AREAS WITH FOG. CURRENT
TEMPERATURES ARE GENERALLY 32 DEGREES OR ABOVE WITH THE EXCEPTION OF
THE VERY FAR NORTH. STILL POTENTIAL FOR A FEW ICY PATCHES ON ROADS
OVER AREAS WITH TEMPERATURES AT 33 DEGREES OR BELOW WITH PAVEMENT
TEMPERATURES A DEGREE OR TWO COOLER THAN THE AIR TEMPERATURE. THE
ICING THREAT WILL END SOON AS THE HIGH DEWPOINTS ARRIVE AND INCREASE
SURFACE WETBULBS ABOVE 32.

IN ADDITION TO DRIZZLE...A BAND OF THETA-E ADVECTION PASSING ACROSS
THE STATE MAY PRODUCE VERY LIGHT RAIN OR SNOW ACROSS NORTHERN IOWA
THIS MORNING THEN VERY LIGHT RAIN THIS AFTERNOON. OTHERWISE...THE
AREAS OF FOG WILL DIMINISH THROUGH THE DAY HOWEVER EXPECT ANOTHER
CLOUDY AND DREARY DAY. HIGH TEMPERATURES TODAY WILL BE MID 30S NORTH
TO LOWER 40S SOUTH.

.LONG TERM /TONIGHT THROUGH SATURDAY/...
ISSUED AT 332 AM CST SUN DEC 21 2014

SURFACE LOW WILL BEGIN TO AFFECT THE REGION TONIGHT...AS SYSTEM
SHIFTS EAST AND PHASES WITH DEEPENING UPPER LEVEL LOW. THIS SYSTEM
WILL BE THE PRIMARY DRIVER FOR WEATHER THROUGH THE BEGINNING OF THE
PERIOD. WILL SEE LIGHT LINGERING RAIN ACROSS MUCH OF THE AREA FOR
THE BEGINNING OF THE PERIOD...BEFORE MAIN INFLUENCES FROM LOW
APPROACH FROM THE WEST. MODELS ARE BEGINNING TO INDICATE BEST
FORCING ARRIVING A BIT SLOWER AND HAVE SLOWED POPS...KEEPING THEM
CONFINED TO AFTER 06Z. HOWEVER...ONCE FORCING ARRIVES...SYSTEM
WILL PUSH THROUGH QUICKLY AND HAVE SPREAD UP END OF PRECIPITATION
IN THE WEST MONDAY. MODELS CONTINUE TO BRING DRY SLOT ACROSS
SOUTHERN IOWA MONDAY INTO TUESDAY...THOUGH THERE IS SUFFICIENT
DISAGREEMENT IN THE MODELS ABOUT THE PLACEMENT OF THE LOW...AND
HAVE THEREFORE KEPT SLIGHT CHANCES A BIT LONGER THAN MAY BE
NECESSARY. HAVE TRENDED TOWARDS A NAM/GFS BLEND WHICH KEEP LOW
SLIGHTLY FURTHER SOUTH INTO IOWA BEFORE LIFTING QUICKLY BACK
NORTHEAST. WRAP AROUND PRECIPITATION WILL CONTINUE TO AFFECT MUCH
OF CWA THROUGH WEDNESDAY BEFORE SYSTEM FINALLY PULLS EAST.

PRECIPITATION TYPE WILL BE MAIN CONCERN WITH THE SYSTEM.
PRECIPITATION SHOULD BE PRIMARILY RAIN FOR BEGINNING...THOUGH
SOUNDINGS INDICATING ENOUGH SATURATION INTO DENDRITIC GROWTH ZONE
ACROSS THE NORTH WHICH MAY ALLOW FOR ICE INTRODUCTION AND SNOW IN
THE NORTH EARLY. HOWEVER...BEST FORCING WILL REMAIN IN THE LOWER
LEVELS AND HAVE ONLY INCLUDED A SMALL AREA OF RAIN/SNOW MIX ACROSS
THE NORTH. GIVEN SLIGHTLY FURTHER SOUTH SOLUTION...MAY SEE RAIN
CHANGING OVER TO SNOW SOONER ACROSS THE CWA AS COLD AIR FILTERS
IN...AND HAVE THEREFORE SPED UP ONSET OF SNOW MONDAY NIGHT INTO
TUESDAY. NEXT CONCERN WILL BE WITH PRECIPITATION TYPE AS SYSTEM
DEPARTS AND ICE INTRODUCTION IS LOST. CURRENT MODEL TRENDS
INDICATE A LOSS OF ICE INTRODUCTION BEGINNING TUESDAY NIGHT INTO
WEDNESDAY. WITH TEMPERATURES BELOW FREEZING...THIS COULD RESULT IN
FREEZING RAIN ACROSS MUCH OF THE AREA. HOWEVER...AGAIN GIVEN
UNCERTAINTY IN MODEL SOLUTIONS...HAVE KEPT PRECIPITATION AS
RAIN/SNOW OR SNOW THROUGH THIS TIME. HOWEVER...THIS WILL NEED TO
BE MONITORED FOR POTENTIAL FOR FREEZING RAIN/FZDZ. ADDITIONAL
CONCERNS WILL BE WITH WINDS FOR TUESDAY INTO WEDNESDAY.
NORTHWESTERLY WINDS WILL INCREASE AS SYSTEM LIFTS NORTHEAST.
HOWEVER...MODELS CONTINUE TO BACK OFF OF STRONGER WINDS...THOUGH
MAY SEE GUSTS NEAR 25KTS WHICH WOULD LIKELY BE SUFFICIENT TO CAUSE
VISIBILITY RESTRICTIONS WHEN COMBINED WITH ANY FALLING SNOW OR
FRESH FALLEN SNOW.

SYSTEM WILL DEPART BRINGING A BRIEF BREAK IN PRECIPITATION CHANCES
FOR WEDNESDAY BEFORE ANOTHER SYSTEM BEGINS TO AFFECT THE REGION.
MODELS HAVE HAD POOR CONSISTENCY ON HANDLING OF SECOND
SYSTEM...THOUGH ECMWF HAS BEGUN TO COME IN LINE WITH GFS. CURRENT
SOLUTION IS FOR UPPER LEVEL TROUGH TO DROP SOUTH INTO PLAINS
BEGINNING TO INFLUENCE IOWA THURSDAY INTO FRIDAY. SYSTEM IS QUICK
MOVING...THOUGH TIMING AND LOCATION IS STILL UNCERTAIN. HAVE
INCREASED POPS FOR THURSDAY ACROSS THE NORTH AND INTO FRIDAY
ACROSS THE CWA. TEMPERATURES WILL BE COLD ENOUGH FOR SNOW...AND
WINDS WILL BE STRONG WITH SYSTEM. THOUGH SYSTEM WILL BE QUICK
MOVING...WILL NEED TO BE MONITORED CLOSELY GIVEN MODEL UNCERTAINTY
AND POTENTIAL IMPACTS ON HOLIDAY TRAVEL FOR END OF WEEK.

&&

.AVIATION...21/12Z
ISSUED AT 540 AM CST SUN DEC 21 2014

AREAS OF DENSE FOG HAVE MOVED INTO NORTHERN IOWA AND WILL MAINLY
IMPACT KMCW EARLY IN THE PERIOD. VSBYS HAVE IMPROVED OR WILL BE
IMPROVING SHORTLY AT OTHER SITE. WIDESPREAD IFR TO LIFR CIGS EARLY
WITH THE EXCEPTION OF KOTM. CIGS SHOULD BE PRIMARILY IFR NORTH AND
MAY POSSIBLE RISE TO MVFR AT KDSM. AREAS OF RAIN ARE EXPECTED TO
DEVELOP OVERNIGHT AND IMPACT TERMINALS LATE. THE WIND WILL BE
BREEZY FROM THE SOUTH TO SOUTHEAST.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR BLACK HAWK-
BREMER-BUTLER-CALHOUN-CERRO GORDO-EMMET-FRANKLIN-GRUNDY-HAMILTON-
HANCOCK-HARDIN-HUMBOLDT-KOSSUTH-PALO ALTO-POCAHONTAS-SAC-WEBSTER-
WINNEBAGO-WORTH-WRIGHT.

&&

$$

SHORT TERM...DONAVON
LONG TERM...AWB
AVIATION...DONAVON


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