Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181523
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
923 AM CST THU DEC 18 2014

.UPDATE...
ISSUED AT 920 AM CST THU DEC 18 2014

BASED ON INCOMING COOP SNOWFALL REPORTS AND RADAR TRENDS...HAVE
LOWERED SNOWFALL AMOUNTS BY SEVERAL TENTHS OF AN INCH AND REMOVED
SNOW NORTH OF I-80 FOR THE REST OF TODAY. DRY LAYER VERY PROMINENT
ON THE 12Z 0AX SOUNDING IS RESULTING IN A SHARP CUTOFF IN THE
PRECIP BAND JUST SOUTH OF DSM. IT IS QUITE POSSIBLE THAT THE
METRO MAY SEE ONLY FLURRIES BY THE TIME THIS EVENT IS ALL SAID
AND DONE WITH THE BEST DGZ FORCING ALREADY SHIFTING TOWARDS SE
IOWA AND RADAR ECHOS DECREASING OVER THE SW CWA.

&&

.SHORT TERM /TODAY/...
ISSUED AT 336 AM CST THU DEC 18 2014

AN UPPER LEVEL SHORT WAVE IS MOVING ACROSS MISSOURI THIS MORNING
WITH THE NORTHERN FRINGE OF KINEMATIC FORCING REACHING INTO SOUTHERN
IOWA. A BRANCH OF THE WARM CONVEYOR BELT IS WRAPPING AROUND THE
NORTH SIDE WITH MOISTURE OVERRUNNING THE SURFACE COLDER AIR. THE
COMBINATION OF MOISTURE AND FORCING HAS RESULTED IN A BAND OF SNOW
LIFTING NORTHEAST THROUGH NORTHEAST KANSAS AND NORTHERN MISSOURI.
THIS BAND OF SNOW WILL MOVE INTO SOUTHERN IOWA EARLY THIS MORNING
HOWEVER ITS NORTHEAST EXTENT WILL BE LIMITED BY DRY AIR ASSOCIATED
WITH A RIDGE OF HIGH PRESSURE SITUATED OVER NORTHEAST IOWA. IN
ADDITION...FORCING WILL WEAKEN AS THE UPPER LEVEL SYSTEM DEPARTS TO
THE EAST AND LEAVES THE SNOW BAND BEHIND TO DIMINISH. SNOWFALL
AMOUNTS OF 1 TO 3 INCHES ARE POSSIBLE OVER THE SOUTHERN 2 TIERS OF
COUNTIES WITH MINOR ACCUMULATIONS OF A FEW TENTHS POSSIBLE AS FAR
NORTH AS INTERSTATE 80. POTENTIAL FOR FREEZING DRIZZLE BY THIS
AFTERNOON IS LOW. MOISTURE DEPTHS WITH RELATIVE HUMIDITIES GREATER
THAN 75 PERCENT REMAIN RELATIVELY HIGH AND COLD FOR ICE TO REMAIN
INTRODUCED AND RESULTING IN LIGHT SNOW OR FLURRIES COMPARED TO
FZDZ.

.LONG TERM /TONIGHT THROUGH WEDNESDAY/...
ISSUED AT 336 AM CST THU DEC 18 2014

OUR WEATHER PATTERN IS SET TO BECOME MORE ACTIVE OVER THE COMING
DAYS. INITIALLY A SERIES OF 500 MB TROUGHS WILL CROSS THE
REGION...THE FIRST AROUND FRIDAY NIGHT AND ANOTHER AROUND
SATURDAY NIGHT/SUNDAY...GRADUALLY CARVING OUT BROAD CYCLONIC FLOW
ACROSS THE CENTRAL U.S. CONCURRENT WITH A LARGE 500 MB HIGH
PRESSURE DOME DEVELOPING OVER THE EASTERN PACIFIC. WITH THE FIRST
TROUGH PASSAGE OVER IOWA ON FRIDAY NIGHT THERE IS A POTENTIAL FOR
SOME STRATUS AND DRIZZLE...BUT FORECAST SOUNDINGS INDICATE THAT
LOW LEVEL MOISTURE AND LIFT WILL PROBABLY BE INSUFFICIENT TO
GENERATE PRECIPITATION. WITH THE SECOND TROUGH ON SATURDAY NIGHT
INTO SUNDAY THERE IS A BETTER SIGNAL FOR SOME LIGHT
PRECIPITATION...PROBABLY IN THE FORM OF DRIZZLE OR FREEZING
DRIZZLE. IT IS LIKELY THAT TEMPERATURES WILL FALL BELOW FREEZING
SATURDAY NIGHT...HOWEVER...DEWPOINTS AND TEMPERATURES MAY RECOVER
TO JUST ABOVE FREEZING BEFORE DRIZZLE DEVELOPS.
NEVERTHELESS...GIVEN THE POSSIBILITY OF FREEZING DRIZZLE AND ITS
ASSOCIATED IMPACTS HAVE ADDED A CHANCE INTO THE OUTGOING FORECAST
PACKAGE. WE WILL BE CLOSELY MONITORING THIS POSSIBILITY OVER THE
NEXT COUPLE OF DAYS.

AS MENTIONED EARLIER...BY THE LATTER PART OF THE WEEKEND A BROAD
500 MB TROUGH WILL HAVE BEEN CARVED OUT OVER THE CENTRAL U.S. A
COMPACT BUT ENERGETIC SHORTWAVE WILL THEN OVERTOP THE EASTERN
PACIFIC RIDGE AND ROCKET SOUTHEASTWARD OVER THE NORTHERN ROCKIES
ON SUNDAY...SUBSEQUENTLY DIGGING AND CLOSING OFF AS A LOW OVER
MINNESOTA AND IOWA BY MONDAY NIGHT. THIS LOW WILL THEN WOBBLE OVER
THE REGION...OR SLIGHTLY TO THE EAST OVER WISCONSIN AND LAKE
MICHIGAN...BEFORE FINALLY EJECTING EASTWARD AROUND CHRISTMAS EVE.
IN TERMS OF SENSIBLE WEATHER...THE INITIAL APPROACH OF THE LOW ON
WILL SPREAD A SWATH OF WARM ADVECTION REGIME PRECIPITATION ACROSS
IOWA ON SUNDAY NIGHT INTO MONDAY...THEN AS THE LOW MOVES NEARBY OR
OVERHEAD A LARGE STRATUS AND PRECIPITATION SHIELD WILL AFFECT MOST
OF THE REGION AND EVENTUALLY CHANGE OVER TO SNOW. THERE IS STILL
SOME UNCERTAINTY AS TO THE LOCATION OF THE LOW PRESSURE CENTER AND
ATTENDANT TEMPERATURE FIELDS...BUT IT SEEMS LIKELY THAT INITIALLY
THE PRECIPITATION WILL BE MOSTLY IN THE FORM OF RAIN WITH A WINTRY
MIX ACROSS NORTHERN IOWA...THEN BECOME LIGHTER BY THE TIME A
CHANGE TO SNOW OCCURS LATER IN THE EVENT. THIS WILL LIMIT SNOW
ACCUMULATIONS ACROSS MOST OF IOWA...BUT IN THE NORTH THERE COULD
BE SOME DECENT AMOUNTS DEPENDING ON HOW THE TEMPERATURES SET UP.
CERTAINLY TOO EARLY FOR ANY HEADLINE DIAGNOSIS BUT CERTAINLY ALL
EYES WILL BE ON THIS LARGE STORM SYSTEM IN THE COMING DAYS.

&&

.AVIATION...18/12Z
ISSUED AT 555 AM CST THU DEC 18 2014

BAND OF SNOW MOVING INTO SOUTHERN IOWA WILL DROP VISIBILITIES TO
MVFR AT TIMES AT KOTM/KDSM THIS MORNING BEFORE DIMINISHING. CIGS
WILL DROP TO MVFR AS WELL IN THAT AREA. PRIMARILY VFR CONDITIONS
AT OTHER SITES THROUGH MUCH OF THE DAY. CIGS ARE EXPECTED TO LOWER
LATE TODAY AND THROUGH TONIGHT WITH WIDESPREAD MVFR OVERNIGHT.
LIGHT WINDS ARE EXPECTED THROUGH THE PERIOD.


&&

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

$$

UPDATE...SKOW
SHORT TERM...DONAVON
LONG TERM...LEE
AVIATION...DONAVON


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