Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 220555
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1155 PM CST SUN DEC 21 2014

.UPDATE...
ISSUED AT 1141 PM CST SUN DEC 21 2014

PRECIP TRENDS CONTINUE TO BE DIFFICULT. ANOTHER WEAK SHORTWAVE IN
THE BROADER FLOW IS LIFTING ACROSS IOWA. A VORT MAX TIED TO THIS
IS PRODUCING ENOUGH LIFT TO KICK OFF SOME SHOWERS THAT CURRENTLY
EXTEND FROM WEBSTER CITY TO JUST WEST OF THE METRO TO ABOUT
OSCEOLA. THE PRECIP IS LIFTING NNE AND WILL EXIT THE CWA IN A
COUPLE HOURS. IN THE BROADER PICTURE...A SECONDARY TROUGH PUSHING
OUT OF THE ROCKIES INTO NEBRASKA IS PRODUCING SOME FORCING THAT IS
CAUSING PRECIP TO BLOSSOM OVER EASTERN NEBRASKA. THIS PRECIP IS
ALSO MOVING NNE BUT BECAUSE IT IS EXPANDING...SHOULD MOVE INTO THE
NWRN CWA IN THE NEXT FEW HOURS. THESE AFOREMENTIONED FEATURES WILL
WILL DIVE SOUTHEAST AND ACROSS NORTHERN MO BEFORE WRAPPING BACK
AROUND INTO CENTRAL IOWA DURING THE DAY MONDAY. THE SYSTEM IS
SLOWING DOWN AND THUS PRECIP WILL BE GOING LONGER THEN FIRST
THOUGHT. THE BEST FORCING SHIFTS INTO SRN/CEN IA BY 18Z AND
REMAINS OVER CEN INTO NRN IA THROUGH 00Z AND INTO NRN IA BY 06Z.
THIS IS ABOUT 6 HOURS SLOWER THAN THE PREVIOUS FORECAST. PRECIP
SHOULD BE MOSTLY RAIN THROUGH 00Z. HOWEVER...SOUNDINGS STILL HINT
AT A POSSIBLE MIX FAR NORTH AS PROFILES HOVER THE 0 DEGREE LINE.
ANY MIX WOULD BE BRIEF THOUGH AS TEMPS GO ABOVE FREEZING BY 18Z.

.UPDATE...
.ISSUED AT 1004 PM CST SUN DEC 21 2014

A SMALL VORT MAX IS LIFTING OUT OF SW IA AND WILL CONTINUE NE
ACROSS CENTRAL IOWA. THIS VORT MAX IS PRODUCING ENOUGH LIFT FOR
SOME SCATTERED SHOWERS THAT ARE CURRENTLY FROM LAMONI TO WEST OF
WINTERSET TO GUTHRIE CENTER. THESE WILL NOT PRODUCE MUCH PRECIP
AND THEY SHOULD BE SHORT LIVED. IN THE BIGGER PICTURE THE SYSTEM
AS A WHOLE IS SLOWING DOWN AND I STILL AM FORECASTING THE MAIN
PRECIP EVENT TO BE TIED TO THE FORCING WHICH IS STILL PROGGED TO
SHIFT ACROSS SE NEBRASKA/NORTHERN MO THEN WRAP BACK INTO IA FROM
12Z NOW THROUGH MONDAY. INITIALLY I THOUGHT IT WOULD BE NORTH BY
21Z BUT IT APPEARS IT WILL BE MORE LIKE 23/06Z. I DID HAVE TO
ADJUST POPS INTO MID MONDAY MORNING TO BLEND WITH THE SLOWER TREND
IN PRECIP.

.UPDATE...
ISSUED AT 705 PM CST SUN DEC 21 2014

WEAK WAVE EXITING NE IA AND PRECIP SHOULD EXIT NORTHERN LOCATIONS
IN THE NEXT HOUR OR SO. ATTENTION SHIFT TO THE WEST WHERE PRECIP
IS TRYING TO BLOSSOM THOUGH CURRENTLY IS HAVING A HARD TIME. LATER
THIS EVENING/TONIGHT AS MOISTURE DEEPENS AND QG FORCING INCREASES
A BIT THE PRECIP SHOULD PUSH INTO THE WEST/NORTH AND AS MENTIONED
IN THE SHORT TERM DISCUSSION...IT SHOULD BEGIN AS DRIZZLE THEN
RAIN. TOWARDS 12Z ACROSS THE NORTH TEMPS MAY BE COLD ENOUGH FOR A
RAIN SNOW MIX BUT MOST OTHER LOCATIONS SHOULD REMAIN ALL LIQUID.

&&

.SHORT TERM /TONIGHT/...
ISSUED AT 326 PM CST SUN DEC 21 2014

SITUATION WILL BEGIN MUCH LIKE THE PAST SEVERAL NIGHTS WITH
PERSISTENT STRATUS BUT INCREASING SYNOPTIC SCALE LIFT EVENTUALLY
BEGINNING TO PLAY A PART LATE AS WELL. LONG WAVE TROUGH WITH
ASSOCIATED WEAK LIFT AND PRECIP MAINLY ALOFT IS IN PLACE FROM TX
ACROSS IA INTO THE UPPER MS VALLEY. MINOR SHORT WEAK WAVES
EMBEDDED IN THE TROUGH WILL MOVE EAST TONIGHT BUT GENERAL AXIS
WILL REMAIN UNCHANGED AS JET MAX AND ASSOCIATED PV TROUGH DIVES
FROM ERN BITTERROOTS INTO WY/NE/CO/KS COMMON AREAS BY DAYBREAK.
LIFT AHEAD OF THESE FEATURES...CURRENTLY OVER WRN NE...WILL SLOWLY
MATURE AND EXPAND EWD INTO THE MO VALLEY AND EVENTUALLY IA BY 12Z.
LIFT APPEARS TO MAINLY BE KINEMATIC WITH MINIMAL BAROCLINICITY
NOTED IN 285-295K ISENT LAYER. ONLY WEAK TO AT TIMES MODERATE
H85/H7 THETA-E ADVECTION IS NOTED AS WELL. THUS EXPECT PRECIP
PROGRESSION TO BEGIN AS LIGHT DRIZZLE EXPANDING LATER THIS EVENING
AS LOW LEVEL MOISTURE DEPTH INCREASES FROM JUST BELOW 1KM TO 2KM
OR MORE BY 09Z. THIS SHOULD LEAD INTO HIGHER BASED LIGHT RAIN
ADVANCING WEST-EAST INTO THE STATE DUE TO AFOREMENTIONED SYNOPTIC
LIFT.

PRECIP TYPE WILL MAINLY BE RAIN...BUT AT LEAST A RAIN/SNOW MIX MAY
START DEVELOPING NE FROM MASON CITY TOWARD WATERLOO JUST BEFORE 12Z.
BUFR SOUNDINGS SHOW NEARLY ISOTHERMAL WEAK WARM LAYER TO 900M JUST
A DEGREE OR SO ABOVE 0C SO CONFIDENCE IN WHICH WAY THIS SWINGS
IS NOT HIGH. ANY SNOW THAT DOES FALL WOULD BE WET AND SLUSHY. WITH
NEARLY SATURATED HOMOGENEOUS AIRMASS IN PLACE FROM IA
UPSTREAM...SLY WINDS WILL KEEP OVERNIGHT TEMPS/DEWPOINTS FAIRLY
STEADY WITH LITTLE FALL.

.LONG TERM /MONDAY THROUGH SUNDAY/...
ISSUED AT 326 PM CST SUN DEC 21 2014

SEVERAL FORECAST CONCERNS WITH PRECIPITATION CHANCES AND TYPE
MONDAY INTO TUESDAY...AND WITH ADDITIONAL POPS LATE IN THE WEEK.
NAM SEEMS A LITTLE TOO FAST WITH THE UPPER LEVEL LOW MONDAY INTO
TUESDAY...WHILE THE ECMWF/SREF/GFS ARE IN BETTER AGREEMENT WITH
ONE ANOTHER AND UTILIZED A BLEND OF THOSE THREE DURING THE
AFOREMENTIONED TIME. STILL SOME SIGNIFICANT DISCREPANCIES LATE IN
THE WORK WEEK WITH A FAIRLY STRONG SHORTWAVE TO CUT ACROSS THE
REGION LATE THURSDAY INTO FRIDAY. THE GFS IS MUCH FURTHER NORTH
THAN THE MORE CONSISTENT TRACK OF THE ECMWF...EVEN THOUGH THE GFS
WAS THE FIRST MODEL TO HINT ON THE FRIDAY SYSTEM AS AN OPEN WAVE.

MONDAY INTO TUESDAY...THE BETTER FORCING ON MONDAY LOOKS TO
TRANSITION ACROSS CENTRAL TO SOUTHERN IOWA BEFORE A WEAK TROWAL
DEVELOPS AND PUSHES THE STRONG FORCING FOR ASCENT INTO NORTHERN
IOWA AND SOUTHERN MINNESOTA BY AROUND 00Z TUESDAY. THE WAA AT THE
SURFACE TO MID-LEVELS MAKES FOR A PROBLEMATIC PRECIP TYPE FORECAST
ACROSS THE NORTHEAST. FORECAST SOUNDINGS TEETER ON THE RAIN/SNOW
AT MCW TO AS FAR SOUTHEAST AS ALO MONDAY MORNING THROUGH AROUND
17-18Z...AND EVEN HINT ON SNOW POSSIBLY INTO THE AFTERNOON AT MCW.
CERTAINLY SOUNDINGS ARE DEEPLY SATURATED BUT FORCING WITHIN THE
DENDRITIC LAYER IS FAIRLY WEAK AND HAVE LOW CONFIDENCE IF THE
FALLING SNOW ALOFT COULD COOL THE COLUMN TO BELOW FREEZING NEAR
THE SURFACE LAYER TO KEEP PRECIP ALL SNOW. ON THE OTHER HAND...THE
SURFACE WARM LAYER MAY NOT BE DEEP ENOUGH TO COMPLETELY MELT THE
SNOW FALLING MONDAY MORNING AT MCW...THUS KEPT THE RAIN/SNOW
MENTIONED ACROSS THE NORTHEAST PORTIONS OF THE FORECAST AREA. IF
IT DOES STAY ALL SNOW...IT WILL BE A VERY SLUSHY WET SNOW AND
ACCUMS UP TO AROUND 2 INCHES ARE POSSIBLE AROUND MCW AREA. FOR
WHAT ITS WORTH...THE 18Z NAM IS ALL RAIN AT MCW SO THE MODEL
DISCREPANCY CONTINUES. AT ANY RATE...TWEAKED UP SNOW AMOUNTS
MONDAY MORNING IN THIS AREA...BUT ONLY HAVE UP TO A HALF INCH
GOING ATTM AS THINKING THERE SHOULD BE ENOUGH MELTING NEAR THE
SURFACE TO KEEP ACCUMS DOWN TOMORROW. HAVE A BRIEF BREAK IN THE
SNOW MONDAY AFTERNOON BEFORE MENTIONING BACK IN THE FORECAST OVER
FAR NORTHERN IOWA MONDAY EVENING.

THE DRY SLOT CONTINUES TO PUNCH WELL INTO CENTRAL IOWA BY THE
AFTERNOON MONDAY INTO MONDAY NIGHT AND WENT DRY OVER THE SOUTHERN
PORTIONS OF THE FORECAST AREA OVERNIGHT MONDAY NIGHT INTO TUESDAY.
WRAP-AROUND MOISTURE AND CAA MOVE INTO WESTERN IOWA PAST 06Z
TUESDAY AND MORE LIKELY CLOSER TO 12Z TUESDAY. HAVE LIKELY POPS
GOING ACROSS THE FAR NORTH 12-18Z TUESDAY AS SOUNDINGS BECOME
DEEPLY SATURATED AGAIN DURING THIS TIME FRAME. STILL NOT MUCH
FORCING WITH THE SNOW GROWTH REGION...BUT ENOUGH TO TWEAK SNOW
AMOUNTS OVER THE NORTH RESULTING IN AN INCH TO AROUND 2 INCHES
CLOSE TO THE MINNESOTA BORDER. LIGHT SNOW POSSIBLE ELSEWHERE BUT
SNOW AMOUNTS WILL BE LESS THAN A HALF INCH ATTM BY TUESDAY NIGHT.

CHRISTMAS EVE THROUGH FRIDAY...STRONG CAA CONTINUES ON WEDNESDAY
WITH LIGHT SNOW SHOWERS POSSIBLE EAST OF I-35 THROUGHOUT THE DAY
WEDNESDAY. ONLY A DUSTING OF SNOW IS ANTICIPATED ON CHRISTMAS EVE IN
THE EAST BUT WINDY AND LIGHT FALLING SNOW SHOWERS MAY STILL BE
ENOUGH TO CAUSE SOME MINOR TRAVEL CONCERNS.

DRY AND SLIGHTLY WARMER ON CHRISTMAS DAY. THE SUN LOOKS TO EVEN PEAK
THROUGH THE STRATUS ACROSS SOUTHERN IOWA. BY LATE THURSDAY NIGHT
INTO FRIDAY...THE NEXT STRONGER SHORTWAVE CUTS ACROSS THE FORECAST
AREA PER 12Z ECMWF AND BRINGS A DECENT AMOUNT OF SNOW AND WIND TO
THE CENTRAL AND NORTHERN SECTIONS OF THE FORECAST AREA. THE GFS
STILL PEGS MINNESOTA WITH THIS SHORTWAVE...SO WITH THE MODEL
DISCREPANCIES...LEANED TOWARD THE SLIGHTLY MORE
CONSISTENT ECMWF FOR NOW.

&&

.AVIATION...22/06Z
ISSUED AT 1141 PM CST SUN DEC 21 2014

WIDESPREAD MAINLY IFR CONDITIONS EXPECTED THROUGH THE FORECAST
PERIOD PRIMARILY DUE TO CIGS WITH PERIODS OF DRIZZLE OR LIGHT
RAIN. PRECIP WILL BECOME MORE WIDESPREAD AS RAIN AFT 22/10Z AND
OVERSPREAD SRN TAF LOCATIONS THROUGH THE MORNING. AFT 23/02Z
PRECIP SHOULD SHIFT INTO NORTHERN IA. VSBYS MAY DROP TO 1SM AT
TIMES DUE TO A COMBO OF PRECIP AND FOG WITH DRIZZLE AT TIMES. SFC
FLOW WL BE GENERALLY SRLY BUT BECOME MORE EASTERLY AFT 23/03Z.

&&

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

$$

UPDATE...FAB
SHORT TERM...SMALL
LONG TERM...PODRAZIK
AVIATION...FAB


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