FXUS63 KDMX 232158
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
358 PM CST WED DEC 23 2009

...PROLONGED WINTER STORM TO HAVE SIGNIFICANT IMPACT ON IOWA...

.SHORT TERM.../REST OF TODAY THROUGH TONIGHT/
FIRST PUSH OF WARM AIR ADVECTION IS HEADING OVER THE NORTHERN 
PORTIONS OF THE STATE WITH MIXTURE OF FREEZING RAIN AND 
SNOW...TRANSITIONING TO RAIN AS IT THE WARMER CONDITIONS BUILD 
NORTH. TEMPERATURES GENERALLY ABOVE FREEZING NOW SOUTH. NEXT 
UPSTREAM WAVE ALREADY IMPACTING EASTERN KS WITH CONVECTION WHICH IS 
WEAKENING WITH TIME...BUT WILL STILL MAKE IT ACROSS THE SOUTHERN 
COUNTIES THIS EVENING AND WILL CONTINUE THUNDER CHANCES THERE. FAR 
NORTH LOOKS MAINLY TO CONTINUE AS SNOW WITH SEVERAL INCHES OF ACCUMS 
TNT ALONG WITH A STRIP OF FREEZING RAIN AGAIN ALONG HIGHWAY 20 AFTER 
06Z. HAVE UPDATED AUDUBON AND GUTHRIE COUNTIES TO WINTER STORM 
WARNING WITH OTHER COUNTIES TO THE NORTH DUE TO FREEZING RAIN ACCUMS 
AND NOW SEEING SOME POWER ISSUES WITH OUTAGES IN THE AREA. ROAD 
CONDITIONS NOT BEING IMPACTED TOO MUCH BUT POWER OUTAGES AND SOME 
FALLING TREES HAVE RAISED THE DANGER IN THESE COUNTIES FOR NOW. 
OTHERWISE AS THE NEXT WAVE ARRIVES WITH ANOTHER ROUND OF STRONG 
MOISTURE TRANSPORT...WILL BE LOOKING AT PRECIP CHANGES ACROSS THE 
NORTH THROUGH MORNING AS WARM AIR ATTEMPTS TO SCOUR THE COLD SFC 
AIR. TEMPERATURES WILL HOLD STEADY OR RISE SLOWLY WITH TIME.

.LONG TERM /THURSDAY THROUGH WEDNESDAY/...
FORECAST MODELS CONTINUE TO HAVE PROBLEMS PREDICTING THE INTERACTION 
BETWEEN THE LARGE MID/UPPER CYCLONES TRAVELING THROUGH THE NORTHERN 
AND SOUTHERN STREAMS. THEY HAVE SHIFTED SUBSTANTIALLY SINCE 
YESTERDAY MORNING...WHEN THERE WAS NEARLY UNANIMOUS AGREEMENT IN THE 
SOUTHERN VORTEX BEING THE MORE ENERGETIC AND SWINGING 
NORTHEASTWARD...WITH THE ASSIMILATION OF THE NORTHERN VORTEX 
DRAGGING THE THEN CONSOLIDATED GYRE FURTHER NORTHWEST INTO 
SOUTHWESTERN IOWA. NOW THERE IS NEARLY UNANIMOUS AGREEMENT IN THE 
NORTHERN VORTEX BEING THE MORE ENERGETIC...WITH THE SOUTHERN VORTEX 
SWINGING MUCH WIDER IN ITS ARC AROUND THE SOUTHEASTERN PERIPHERY OF 
THE CONUS-SCALE TROUGH...AND ASSIMILATING THE SOUTHERN VORTEX LATER 
RESULTING IN A MORE NORTHWESTERLY TRACK OF THE CONSOLIDATED GYRE. IN 
OTHER WORDS...THINGS ARE TRENDING COLDER /THAT IS...PRECIP SWITCHING 
TO SNOW SOONER ACROSS OUR CENTRAL AREAS/ WITH PRECIP LINGERING 
LONGER.

HAVE MAINTAINED NEAR 100 POPS FOR THE FIRST SEVERAL DAYS OF THE LONG 
TERM FORECAST...TAPERING OFF SATURDAY AND SATURDAY NIGHT. BEYOND 
THAT ECMWF CONTINUES TO TRY TO GENERATE ANOTHER STORM APPROACHING 
FROM OUR SOUTHWEST...BUT KEPT THE FORECAST DRY AS THERE IS WAY TOO 
MUCH UNCERTAINTY WITH THIS SYSTEM FIRST.

SO FOR THE BIG STORM THE BIGGEST ISSUE REMAINS P-TYPES...THEIR 
TRANSITIONS...AND RESULTING EFFECTS ON ACCUMULATIONS ACROSS THE 
AREA. WITH THE THINKING NOW BEING A SOONER TRANSITION TO SNOW ACROSS 
MOST OF OUR AREA THE RESULTING FORECAST SNOW ACCUMULATIONS ARE 
HIGHER THAN ON PREVIOUS SHIFTS...EXCEPT IN THE NORTHWEST WHERE 
THINKING HAS NOT CHANGED MUCH. OUTGOING STORM TOTAL SNOWFALLS NOW 
RANGE FROM AROUND 6 INCHES IN THE SOUTHEAST...TO 9 TO 13 INCHES 
CENTRAL...AND 14 TO 17 INCHES IN OUR FAR WESTERN AND NORTHWESTERN 
COUNTIES. AT FIRST BLUSH THESE AMOUNTS APPEAR A BIT HIGH BUT ONE 
MUST REMEMBER THAT THEY ARE 84-PLUS HOUR TOTAL ACCUMULATIONS. THERE 
REMAINS SOME QUESTION AS TO WHETHER ANY OF THE CENTRAL AREAS WILL BE 
ABLE TO GET 6 INCH SNOW ACCUMULATIONS IN ANY 12 HOUR WINDOW THUS 
MEETING WARNING CRITERIA. THERE ARE ALSO QUESTIONS OF WHETHER OR NOT 
BLIZZARD CONDITIONS WILL MAKE AN APPEARANCE IN OUR CWA. HOWEVER...IT 
APPEARS THAT THE CONSOLIDATED SURFACE LOW WILL STALL SOMEWHERE OVER 
OUR AREA FOR MANY HOURS FROM FRIDAY THROUGH SATURDAY WHICH MAY 
RESULT IN WEAKER WINDS OVER MUCH OF THE AREA.

FOR NOW...THE WINTER STORM WATCH WILL BE LEFT AS IS BEGINNING AT 18Z 
THURSDAY. THERE IS SOME CONCERN THAT FZRA ACCUMULATIONS MAY BE HIGH 
ENOUGH TO WARRENT A WARNING A BIT SOONER IN THE SOUTHWEST TOMORROW 
MORNING...BUT CONFIDENCE IS NOT HIGH ENOUGH TO TAKE THE 
WATCHES/WARNINGS THAT WAY AT THIS TIME.

&&

.HYDROLOGY...
SIGNALS ARE BECOMING STRONGER OF A MODERATE TO HEAVY RAINFALL EVENT 
ACROSS OUR SOUTHERN AND SOUTHEASTERN CWA THROUGH THURSDAY NIGHT. 
CONFIDENCE IS AT OR ABOVE THE 50 PERCENT THRESHOLD FOR FLOODING IN 
THIS AREA...THUS HAVE DECIDED ON A FLOOD WATCH THERE.

PRECIP TYPE WILL BE A MAJOR DRIVING FORCE BEHIND THE HYDROLOGIC 
RESPONSE. WITH PRIMARILY LIQUID PRECIP EXPECTED THROUGH THURSDAY 
NIGHT IN THE ABOVE AREA OF CONCERN...HYDRO ISSUES ARE DEFINITELY A 
CONCERN. RIVER FLOODING MAY RESULT FROM THE SHEER VOLUME OF 
RUNOFF...ALONG WITH SNOW MELT. ANY WATER THAT MAKES ITS WAY INTO 
STREAMS WOULD IMPART ENOUGH ENERGY TO BEGIN THE PROCESS OF BREAKING 
UP RIVER ICE. SINCE THE RIVER ICE IS RELATIVELY THIN...THOUGH...WE 
EXPECT ANY FLOODING FROM ICE JAMS TO BE LOCALIZED AND LESS SEVERE 
THAN WOULD TYPICALLY OCCUR LATE IN THE WINTER.

IN ADDITION...FLOODING MAY OCCUR IN OTHER AREAS...SUCH AS TOWNS AND 
CITIES...WHOSE NORMAL DRAINAGE SYSTEMS WOULD HAVE LOWER THAN NORMAL 
CONVEYANCE DUE TO SNOW AND ICE OBSTRUCTED INTAKES.

ANY RISES THAT OUR STREAMS DO EXPERIENCE SHOULD BE QUICKLY REDUCED 
AS COLDER TEMPERATURES WORK THEIR WAY IN TO OUR REGION ON FRIDAY.

&&

.AVIATION...
23/18Z...NO TIME FOR UPDATE...WILL BE MAINTAINING PREVIOUS PACKAGE 
TRENDS AS AREA OF LIGHT RAIN AND FZRA MOVES NORTH. THUNDER HAS 
DIMINISHED AND WILL NOT INCLUDE ANY MENTION IN AFTERNOON TAF.  
PREVIOUS...HAVE HIGH CONFIDENCE THAT IFR/LIFR CONDITIONS WILL REMAIN 
SOCKED INTO ALL TAFS SITES THROUGH DURATION OF THE VALID PERIOD.  
FOD/MCW/ALO WILL SEE FREEZING OR FROZEN PRECIP TODAY AND TONIGHT... 
AND COULD SEE AT LEAST MODERATE ICE AND SNOW ACCUMULATIONS LATER 
THIS AFTERNOON AND EVENING.  HOWEVER KDSM/KOTM SHOULD TRANSITION TO 
ALL LIQUID BY THIS AFTERNOON.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER WX ADVY KAMW/KMIW/KALO AREAS TIL 06Z
WINTER STORM WARNING ALONG AND N OF KAUD/KEBS/KOLZ LINE TIL 06Z SAT 
WINTER STORM WATCH SWRN IA...THROUGH CENTRAL IA...TO NERN IA 18Z 
THU TO 06Z SAT
FLOOD WATCH ALONG AND SE OF ALO...TO DSM...TO LWD LINE FROM 12Z THU 
TO 12Z FRI

$$

SHORT TERM...REV
AVIATION...REV
HYDROLOGY...ZOGG
LONG TERM...LEE