FXUS63 KARX 232157
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
355 PM CST WED DEC 23 2009
.SHORT TERM...TODAY THROUGH SATURDAY
WINTER STORM WAS A CHALLENGE TODAY WITH CONSISTENT YET VARYING MODEL
SOLUTIONS ON THE OUTCOME AND EVOLVING PRECIPITATION TYPES. OVERALL...
HAVE TRIED TO MOVE THE FORECAST TOWARD THE MORE LIKELY OUTCOMES.
THIS MEANS THE FORECAST IN THE NORTH WILL BE LESS FREEZING RAIN
MENTION AND MORE SLEET INCLUSION. THE FREEZING RAIN INCLUSION WAS
BASED ON THE BREAKS IN THE DEEP SATURATION WHICH SEEM TO BE MORE
LIMITED IN MULTI-SHORTWAVE TROUGH SYSTEM.
CURRENT REGIONAL RADAR IS SHOWING A BROAD BAND OF FZRA/IP/SN
HEADING NORTHWARD ROUGHLY ALONG I-80. THIS BAND INITIALLY /PER RUC
THERMAL PROFILE AND OBSERVATIONS/ IS PRODUCING A MIX OF
PRECIPITATION TYPES BUT ONCE INTO THE ECHO /OVER 20 DBZ/ AND DEEP
SATURATION OCCURS...SN IS THE MAIN PRECIPITATION TYPE OVER NERN
IA AND SWRN WI. THIS BAND IS RIDING ON WITHIN A -1 TO 3C WARM LAYER
WITH THE -1C ISOTHERM ON THE NORTH SIDE. THUS...THE BAND SHOULD
TRANSITION TO SNOW RAPIDLY...THEN A MIX OF SNOW/SLEET AND FINALLY
ACROSS THE SOUTH PART OF THE BAND...TOWARD FREEZING RAIN. GOES WATER
VAPOR SHOWING THE FORCING FOR THIS IN THE MID-UPPER LEVELS AS A
SHORTWAVE TROUGH NEAR KANSAS CITY KS COMING AT THE AREA. THERE IS A
NOTICEABLE BREAK BEHIND THIS ENERGY HEADING NORTHEAST. THE ALL
IMPORTANT 32F LINE IS JUST TO THE SOUTH OF THE FORECAST AREA.
23.12Z GFS/ECMWF/NAM CAME IN WITH CONSISTENT SOLUTIONS TO WHAT THEY
HAD BEEN FORECASTING WITH THE NAM THE WARMEST OVERALL AND THE GFS
THE COLD SOLUTION. WENT WITH A MORE NAM WEIGHTED FORECAST TOWARD THE
23.09Z SREF PACKET OF SURFACE LOWS WORKING NORTHWARD TO NEAR KMCW.
23.12Z ECMWF ALSO ON TRACK WITH THIS SOLUTION WHICH MAINLY PROVIDES
THE FORECAST AREA WITH A PROLONGED WARM AIR ADVECTION REGIME.
BECAUSE OF THIS...HAVE MOVED TOWARD A WARMER SOLUTION THAN PREVIOUS
FORECASTS INCLUDING INCLUSION OF RAIN ACROSS THE SOUTHERN FORECAST
AREA.
TONIGHT IS FIRST SHOT OF INTENSE WARM ADVECTION/ISENTROPIC LIFT AND
MOISTURE TRANSPORT IN THE LOW-LEVELS IN RESPONSE TO THE KANSAS CITY
SHORTWAVE TROUGH. THIS WILL BRING A WARM NOSE NORTHWARD WHICH WILL
THEN HOLD IN PLACE PER CONSENSUS MODEL GUIDANCE. THUS...A DEEPLY
SATURATE BAND NOW OVER IA SHIFTS NORTH WITH A COLD ACCUMULATING
SNOW. A BAND OF SLEET AND SNOW /2-3 INCHES/ THEN SETS UP ALONG THE I-
90 CORRIDOR IN THE 0-2C WARM LAYER ALOFT LATE TONIGHT. BY THAT SAME
TIME THE WARM LAYER ALOFT WILL HAVE WET-BULB TEMPERATURES NEAR 3C SO
FREEZING RAIN AND SLEET COULD REALLY DOMINATE NORTHEAST IOWA AND
SOUTHWEST WISCONSIN AFTER MIDNIGHT. THINKING AROUND 1/4-1/2 INCH OF
INCH ACCUMULATION THERE BY THU MORNING WITH SLEET AND FZRA.
A SLOW WARMING OCCURS THURSDAY UNDER CONTINUED WARM ADVECTION AND
SURGES IN MOISTURE TRANSPORT CONVERGENCE AND LOW-LEVEL JET
INTENSIFICATION WITH DEVELOPING LOW IN ARKANSAS MOVING NORTHWARD.
THIS IS WHEN A 225 PERCENT NORMAL SOURCE REGION OF PRECIPITABLE
WATER OVER KSTL IS POINTING TOWARD THE AREA...WITH JUST UNDER 1 INCH
INTO THE FORECAST AREA /WHICH IS AROUND 200 PERCENT NORMAL LOCALLY/.
THERMAL ORIENTATION AND WARM ADVECTION SUGGESTS A CHANGE TO RAIN
ACROSS THE FAR SOUTH THURSDAY BUT SNOW IN THE FAR NORTHWESTERN
COUNTIES...SOMETIMES MIXED WITH SLEET. DEPENDING ON SURFACE
TEMPERATURES IN THE SOUTH...SOME FREEZING RAIN COULD STILL OCCUR SO
HAVE NOT DROPPED IT FROM THE FORECAST. HOWEVER...WARM ADVECTION
BATTLES DIABATIC COOLING FROM PRECIPITATION THURSDAY AND THURSDAY
NIGHT TO HOLD THE WARM LAYER ALOFT...WITH THE SREF 0C LINE MOVING SE-
NW BASED ON THIS BATTLE BUT ROUGHLY BISECTING THE FORECAST AREA IN
HALF FROM KISW-KCCY. THIS IS IN PLACE AS THE LARGEST FORCING MOVES
THROUGH THURSDAY NIGHT AS NORTHWEST FLOW SHORTWAVE TROUGH AND
SOUTHERN STREAM SHORTWAVE TROUGH MERGE. FORCING IS DEEP AND THERMAL
RIBBON REMAINS IN PLACE DURING IT WITH RAIN IN THE SOUTH AND HEAVY
SNOW NW.
OVERALL...WHILE THE CONFIDENCE IS NOT HIGH ON THE EXACT THERMAL
STRUCTURE /AND THIS FORECAST AREA IS IN THE MOST CRITICAL ZONE FOR
PRECIPITATION TYPE/...DEEPEST SNOWFALL SHOULD BE FAR WRN SERN MN
COUNTIES AND WRN WI COUNTIES. THE WORST ICE ACCUMULATIONS WILL
OCCUR ACROSS NERN IA AND SWRN WI. IN BETWEEN...A MIX OF ICE /MAINLY
AS SLEET/ AND SNOW. WITH SO MUCH LIQUID WITH THIS STORM /NEAR 2
INCHES/...A CONSERVATIVE SNOWFALL OF NEAR 18 INCHES FOR A 60 HOUR
DURATION SEEMS EASY. IF LESS SLEET WOULD MIX INTO THE ENVIRONMENT...
OR THE SURFACE TEMPERATURES WARM MORE...THOSE SNOW NUMBERS WILL BE
LOWER. ICE ACCUMULATIONS LOOK TO BE MOST PREFERRED TONIGHT AND THEN
THE SOUTHERN FORECAST AREA MAY TRANSITION TO RAIN. THIS MAY ALLOW FOR
EARLY CANCELLATION OF THE WARNING THURSDAY...FUTURE SHIFTS WILL HAVE
TO WATCH THIS.
STRONG COLD ADVECTION COMES IN FROM THE SOUTH ON FRIDAY AND
CONTINUES THE SNOW THREAT AND CHANGES ALL PRECIPITATION TYPES TO
SNOW. HOWEVER...IT WILL BE VERY COLD AND TEMPERATURES DURING THE DAY
MAY NOT BE LOW ENOUGH AS IT COOLS DOWN WITH -15C AT 850MB COMING IN
FRIDAY.
LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY
LITTLE ATTENTION WAS PLACED INTO THIS PERIOD WITH MAJOR SHORT TERM
CHALLENGES. HAVE SLOWED THE EASTWARD PROGRESSION OF THE MIDWEST
STORM AND INCLUDED HIGHER SNOW CHANCES ON SATURDAY NIGHT. THIS
LOOKS LIKE A LIGHT SNOW PERIOD WITH TAPERING OFF FROM WEST TO EAST ON
SUNDAY.
ANOTHER WEATHER SYSTEM IS PROJECTED BY THE 23.00Z AND 23.12Z ECMWF
SOLUTION AROUND MIDWEEK. THIS SYSTEM IS STRONGER THAN THE 23.06Z GFS
SOLUTION AND WILL WARRANT SOME CLOSER INSPECTION IN FUTURE SHIFTS
ONCE HOLIDAY STORM MOVES ON. HAVE INCLUDED A SMALL CHANCE OF SNOW
WEDNESDAY.
&&
.AVIATION...PRIMARY FORECAST CONCERNS REMAIN CENTERED ON
PRECIPITATION TYPE AND LOWERING OF FLIGHT CATEGORIES ASSOCIATED WITH
POTENT STORM SYSTEM TO IMPACT AIRPORTS ACROSS THE ENTIRE AREA THROUGH
THE END OF THE WEEK. LARGE MAGNITUDE OF PRECIPITATION BOTH TEMPORALLY
AND SPATIALLY SUGGEST THERE SHOULD BE SIGNIFICANT IMPACTS ON AVIATION
COMMUNITY. THIS ESPECIALLY INCLUDES EFFECTS ON DEICING AIRCRAFT AND
SNOW/ICE REMOVAL ON AIRPORT SURFACES.
ENORMOUS AMOUNT OF MOISTURE BEING PULLED NORTHWARD INTO THE UPPER
MISSISSIPPI VALLEY THIS AFTERNOON AHEAD OF SURFACE AND UPPER LEVEL
LOW PRESSURE BEGINNING TO EJECT OUT OF THE SOUTHERN PLAINS. THIS WAS
EVIDENT BY EXPANSIVE PRECIPITATION SHIELD MOVING NORTHWARD ACROSS
IOWA AND ILLINOIS...ATTENDED BY LARGE EXPANSE OF IFR CONDITIONS.
WITH REGARD TO PRECIPITATION TYPE...SNOW EXPECTED TO ONSET THIS
EVENING AT BOTH TAF SITES. WARMER AIR ALOFT BEING PULLED NORTHWARD
AHEAD OF STORM SYSTEM SHOULD RESULT IN A MIX OF SNOW AND FREEZING
RAIN OVERNIGHT...THEN MAINLY FREEZING RAIN THURSDAY MORNING. THERE
IS CERTAINLY A POTENTIAL FOR SLEET DURING CURRENT VALID TAF
PERIOD...ALTHOUGH CONFIDENCE WAS NOT HIGH ENOUGH ON WHEN IT WOULD
OCCUR. THUS...CHOSE NOT TO INCLUDE SLEET IN THE TAFS AT THIS TIME.
WITH RESPECT TO WINDS AND LOWERING OF FLIGHT CATEGORIES...STRONG
EAST WINDS EXPECTED TO CONTINUE THROUGH TONIGHT AND THURSDAY. GUSTS
UP TO 30KTS COULD OCCUR...ESPECIALLY AT WIND PRONE SITES IN
SOUTHEAST MN AND NORTHEAST IOWA...INCLUDING THE KRST TAF SITE.
FORECAST CONFIDENCE WAS HIGH THAT ALL AIRPORTS IN THE AREA WILL HAVE
PREVAILING IFR CONDITIONS TONIGHT THROUGH CHRISTMAS DAY.
&&
.HYDROLOGY...WITH THE LARGE AMOUNT OF LIQUID EQUIVALENT EXPECTED...
HAVE SOME CONCERNS IN THE RAIN AREA ACROSS THE FAR SOUTH THAT WATER
FLOWS COULD BECOME ELEVATED. AM MOSTLY CONCERNED WITH MELTING THE 8-
9 INCH SNOWPACK AS TEMPERATURES RISE THURSDAY AND THURSDAY
NIGHT...AND THEN THE ADDED RAINFALL OF NEARLY ONE INCH. THERE IS
STILL SOME CONFIDENCE ISSUE ON TYPE OF PRECIPITATION BUT HAVE THE
HIGHEST CONFIDENCE IS IN THE FAR SOUTH WARMING ABOVE FREEZING AND
SEEING RAIN. FOR NOW WILL ISSUE A DISCUSSION ON IT /ESF/...BUT FUTURE
SHIFTS WILL NEED TO THINK ABOUT A NEED FOR A FLOOD WATCH.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...WINTER STORM WARNING TO 6 PM CST FRIDAY FOR WIZ054-WIZ055-
WIZ061.
WINTER STORM WARNING FROM 6 PM THIS EVENING TO 6 PM CST FRIDAY
FOR WIZ041-WIZ042-WIZ043-WIZ044-WIZ053.
WINTER STORM WARNING FROM 9 PM THIS EVENING TO 6 PM CST FRIDAY
FOR WIZ017-WIZ029-WIZ032-WIZ033-WIZ034.
MN...WINTER STORM WARNING FROM 6 PM THIS EVENING TO 6 PM CST FRIDAY
FOR MNZ086-MNZ087-MNZ088-MNZ094-MNZ095-MNZ096.
WINTER STORM WARNING FROM 9 PM THIS EVENING TO 6 PM CST FRIDAY
FOR MNZ079.
IA...WINTER STORM WARNING TO 6 PM CST FRIDAY FOR IAZ008-IAZ009-
IAZ010-IAZ011-IAZ018-IAZ019-IAZ029-IAZ030.
&&
$$
SHORT/LONG TERM...BAUMGARDT
AVIATION..........THOMPSON
