FXUS63 KDMX 241754 AAA
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1155 AM CST THU DEC 24 2009
...UPDATED AVIATION AND PUBLIC DISCUSSION...
.SHORT TERM.../TODAY/
HAVE UPDATED THE WEST FOR SNOWFALL AMOUNTS THIS AFTERNOON.
SUBJECTIVE ANALYSIS THIS AM SHOWS RAPIDLY FORMING H850 FRONT AND GFS
HAS A GOOD HANDLE ON THE IMMEDIATE SHORT TERM FORCING AND QPF THERE.
THE TRANSITION MAY BE A BIT SLOWER FOR THE CENTRAL SECTIONS AND HAVE
TRIMMED THE SHORT TERM SNOW AMOUNTS A BIT. TEMPERATURES BEGINNING
TO FALL WITH EVAPORATIVE COOLING. ANOTHER ITEM TO NOTE. ARCTIC
RIDGE NOW BULGING SOUTHEAST AND COLD AIR SQUEEZING INTO WESTERN IA
FASTER THAN PROGGED ALONG MO VALLEY. LATEST GUIDANCE TAKES THE LOW
TRACK MORE SOUTHEAST OF THE REGION THROUGH EARLY AM FRIDAY...THEN
BACK OVER US WITH TIME. FURTHER REFINEMENTS FOR TNT FORECAST
POSSIBLE...AS FOR SNOWFALL AMTS AND DETAILS. THIS LOOKS TO BE AN
ONGOING PROCESS. /REV
.PREVIOUS SHORT TERM.../TODAY/
INVERTED TROUGH EXTENDING NORTH THROUGH IOWA EARLY THIS MORNING WITH
AT TO ABOVE FREEZING TEMPERATURES BEING REPORTED AT NEARLY ALL
STATIONS EXCEPT THE FAR NORTHWEST WHERE ESTHERVILLE IS AT 26 DEGREES
AND SNOW. DESPITE THE WARMER TEMPERATURES...SOME POCKETS OF WINTRY
MIX REMAIN INCLUDING MINOR ADDITIONAL ICE ACCUMULATIONS TO POWER
LINES AND TREES. LIGHT PRECIPITATION IS SCATTERED ABOUT THE FORECAST
AREA AND THAT WILL REMAIN THE CASE UNTIL THE NEXT PIECE OF ENERGY
AND CLOSED LOW ALONG WITH ITS ATTENDANT MOISTURE PLUME LIFTS
NORTH OUT OF THE SOUTHERN PLAINS AND INTO THE STATE LATE THIS
MORNING AND INTO THE AFTERNOON HOURS.
AS THE NEXT ROUND OF MOISTURE LIFTS NORTHWARD...COLDER AIR WILL BE
FILTERING IN FROM THE WEST AS THE POLAR UPPER LOW NEARS FROM THE
NORTHWEST AND DIVES INTO THE WESTERN DAKOTAS. THE ENTIRE WARM LAYER
ALOFT SHOULD BE ERASED BY 00Z THIS EVENING FOR ALL AREAS EXCEPT THE
VERY FAR SOUTHEAST NEAR BLOOMFIELD. THIS WILL LEAD TO A CHANGE OF
PRECIPITATION FROM RAIN AND FREEZING RAIN TO ALL SNOW MOST LOCATIONS
BY THIS TIME. THE AMES...DES MOINES AND WATERLOO AREAS SHOULD HAVE
MAINLY SNOW AFTER 18Z. NEW SNOW ACCUMULATIONS UP TO 2 INCHES WILL BE
POSSIBLE BY 00Z. SOME ADDITIONAL ICE ACCUMULATIONS MAINLY LESS THAN
ONE TENTH OF AN INCH ARE ALSO POSSIBLE HOWEVER...THE WARMER SFC
TEMPERATURES SHOULD BE ABLE TO SURVIVE LONGER THAN THE WARM LAYER
ALOFT DURING THE DAY. THUS IT WILL BE DIFFICULT TO TRANSITION BACK
FROM RAIN TO FREEZING RAIN IN ALL BUT THE NORTHWEST FRINGES.
.LONG TERM /TONIGHT THROUGH MONDAY/...MAJOR WINTER STORM WILL BE THE
BLUNT OF THE FORECAST ISSUES AS A PAIR OF UPPER LEVEL LOWS MERGE
WITH ONE ANOTHER BY FRIDAY MORNING. ECMWF AND GFS ARE IN VERY GOOD
AGREEMENT...WITH THE ECMWF SLIGHTLY HIGHER WITH THE QPF AMOUNTS. NAM
SEEMED TO BE PUSHING THE LOW TOO FAR EAST AND THEN BACK AROUND AGAIN
BY SATURDAY. SO...GENERALLY CONSENSUS WAS TO LEANED TOWARD A
GFS/ECMWF SOLUTION THROUGH SATURDAY.
TONIGHT THROUGH FRIDAY...COLDER TEMPS FILTER INTO MUCH OF THE CWA
DURING THE EVENING SWITCHING THE MAJORITY OF THE PRECIP TO ALL
SNOW. THERE WILL BE A FINE LINE AGAIN FROM ALL SNOW TO THE WINTRY
MIX...WHICH SETS UP FROM AROUND ALO TO TNU TO LWD WITH DSM BEING
ALL SNOW BY 00Z FRIDAY. THE SOUTHEAST LOOKS TO SEE MAINLY
RAIN/FZRA/SLEET THROUGH 06Z FRIDAY WITH THE CHANGE OVER DURING THE
OVERNIGHT HOURS INTO FRIDAY MORNING. EQUIVALENT POTENTIAL
TEMPERATURE AT 700MB BY 12Z FRIDAY HAS MUCH WARMER AIR ADVECTING
UP AROUND THE LOW PRESSURE SYSTEM INTO NORTHEASTERN IOWA. THIS
TROWAL FEATURE MAY POSE AN ISSUE ACROSS THE FAR NORTHEAST DURING
THE MORNING HOURS FRIDAY WITH REGARDS TO PRECIP TYPE. GFS BUFR
SOUNDINGS SHOWING SHALLOW LAYER OF WARM AIR BY 15Z FRI...WITH EVEN
PUSHING THE SURFACE TEMPS ABOVE FREEZING. HOWEVER...THINKING
STRONG ENOUGH LATENT HEAT RELEASE TO OVERCOME THIS FEATURE SO KEPT
WITH MENTION OF ALL SNOW. IF THIS BECOMES TRUE...MAY GET A
SLEET/SNOW MIX FOR A FEW HOURS LATE FRIDAY MORNING IN AND AROUND
ALO...WHICH WOULD DECREASE SNOWFALL TOTALS SLIGHTLY. 00Z COBB
OUTPUT ONLY GIVING ALO AROUND 2 INCHES TOTAL FOR THE EVENT.
NO SHORTAGE OF ISENTROPIC LIFT ACROSS THE CWA FROM TONIGHT THROUGH
FRIDAY NIGHT...AND EVEN INTO SATURDAY MORNING ACROSS THE EASTERN
SECTIONS OF THE CWA. ABUNDANT THETA-E ADVECTION WITH THIS
SYSTEM...AND BUFKIT SOUNDINGS DEPICTING A DEEP LAYER OF MOISTURE
THROUGHOUT MUCH OF THE AFOREMENTIONED TIME FRAME. 00Z COBB OUTPUT
STILL CONSISTENT WITH THE TOTALS ACROSS THE NORTHWEST...BUT HAVE
INCREASE THE CENTRAL AND SOUTHEAST A BIT WITH THE LOW PROGGED TO
BE A LITTLE FURTHER EAST LEADING TO A LONGER DURATION OF SNOWFALL.
SHARP PRESSURE GRADIENT BEHIND THE UPPER LOW AS IT STALLS ACROSS
SOUTH CENTRAL IOWA BY TONIGHT. THIS WILL AFFECT MUCH OF THE
WESTERN CWA TONIGHT THROUGH MUCH OF THE DAY FRIDAY. UNCERTAINTY
WITH THE AMOUNT OF DRIFTING WITH THE CURRENT RAIN/FZRA ONGOING
ACROSS THE AREA...AS MAY AID IN SNOW STINKING MORE EASILY TO
SURFACES. HOWEVER...ANY FALLING SNOW WILL REDUCE VISIBILITY.
SATURDAY AND SATURDAY NIGHT...ISENTROPIC LIFT AND THETA-E
ADVECTION CONTINUES THROUGH MUCH OF THE DAY. SO INCREASE POPS AND
SNOW AMOUNTS TO FIT THIS TREND. NO MAJOR CHANGES TO THE ONGOING
FORECAST SUNDAY AND MONDAY.
&&
.AVIATION...
24/18Z...NO MAJOR CHANGES GO PREVIOUS FORECAST. LOW CIGS TO
CONTINUE...IFR TO VLIFR CONDITIONS WILL BE WIDESPREAD DURING THE
PERIOD. A MIX OF PRECIPITATION IS ANTICIPATED THROUGH EARLY
AFTERNOON WEST WITH A TRANSITION TO SNOW LATE THIS AFTERNOON AND
EVENING. THE EASTERN TAF SITES...KALO AND KOTM WILL TAKE INTO THE
MID TO LATE EVENING TO FULLY TRANSITION TO ALL SNOW. THE MOST
INTENSE PRECIPITATION WILL OCCUR OVERNIGHT TONIGHT AND CONTINUE INTO
EARLY FRIDAY. EXPECT WINDS TO BECOME MORE NORTHERLY DURING THE
PERIOD AND INCREASE OVER THE NORTHWEST...CREATING HAZARDOUS
CONDITIONS.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FOR THE ENTIRE STATE EXCEPT THE SOUTHEAST
PORTIONS THROUGH 12Z SAT.
WINTER STORM WATCH FOR SOUTHEAST IOWA THROUGH 12Z SAT.
&&
$$
SHORT TERM...DONAVON/REV
AVIATION...DONAVON/REV
LONG TERM...PODRAZIK
