FXUS63 KMPX 241215
AFDMPX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
615 AM CST THU DEC 24 2009
.UPDATE...FOR 12Z AVIATION DISCUSSION BELOW
&&
.DISCUSSION.../ISSUED 440 AM CST THU DEC 24 2009/
THE ONGOING WINTER STORM CONTINUES TO OFFER MANY CHALLENGES TO THE
FORECAST.
THE FIRST WAVE OF SNOW PUSHED THROUGH LAST NIGHT AND THIS MORNING
ON A STRONG LEAD SHORT WAVE ASSOCIATED WITH A BURST OF WARM AIR
ADVECTION. SNOWFALL AMOUNTS BETWEEN TWO AND SIX INCHES WERE
WIDESPREAD. TIMING OF NEXT SURGE STILL LOOKS ON TRACK FOR LATE
THIS AFTERNOON INTO TONIGHT...WITH THE THIRD ONE SOON ON ITS HEELS
CHRISTMAS MORNING AND EARLY AFTERNOON. NEXT SURGE ASSOCIATED WITH
MOIST CONVEYOR BELT OF SYSTEM AS SFC LOW TRACKS NORTH AND THE
FINAL ONE AS THE MID-LEVEL LOWS PHASE TO OUR SOUTH. EACH OF THESE
SHOULD HAVE STRONGER FORCING AND PERSIST LONGER THAN THE FIRST
WAVE. THIS WINTER STORM REMAINS CHARACTERIZED BY ITS IMPRESSIVE
LONGEVITY AND HIGH AMOUNTS OF MOISTURE. FORECAST STORM TOTAL
SNOWFALL AMOUNTS ARE SIMILAR. THESE WERE INCREASED SLIGHTLY OVER
THE SRN TWIN CITIES METRO SOUTHWARD TOWARDS THE IOWA STATE LINE.
PART OF THIS WAS DUE TO EARLIER OBSERVED HIGHER SNOWFALL AMOUNTS
FROM LAST NIGHT AND EARLY THIS MORNING. STORM TOTAL SNOWFALL
TOTALS BY DAYBREAK SATURDAY FORECAST TO BE A FOOT NEAR EAU CLAIRE
TO 16 TO 22 INCHES IN MN COUNTIES. THIS HAS THE POTENTIAL TO BE
NEAR A RECORD IN SOME COMMUNITIES.
THE SRN MID-LEVEL LOW IS COMING ACROSS TEXAS THIS MORNING AND
ABOUT TO LIFT NORTHEAST. SECONDARY BUT DEEPER AND COLDER MID-LEVEL
LOW DROPPING SOUTHEAST FROM ERN MT...ON A COLLISION COURSE WITH THE
SRN ONE WHICH WILL LEAD TO A QUICK INTENSIFICATION TONIGHT AND
CHRISTMAS. THE MORNING LULL IS ASSOCIATED WITH NVA BEHIND LEAD
IMPULSE. THIS WILL GENERALLY PREVAIL MUCH OF THE LATER MORNING AND
EARLY AFTN...HOWEVER THERE WILL BE AT LEAST LIGHT ACCUMULATING
SNOW AND BRIEF POCKETS OF MODERATE SNOW AND SLEET...ALMOST SHOWERY
LIKE. LATE THIS AFTERNOON AND TONIGHT MOISTURE TRANSPORT ON ALL
MODELS RAMPS UP ON THE ISENTROPIC SURFACES INTO THE FORECAST AREA.
AS THE WAVE LIFTS NORTH...BEING BROUGHT UP BY THE CIRCULATION OF
THE STRONGER ONE TO THE NW....MIXING RATIOS WILL STAY AT 4 G/KG.
LAST EVES RAOBS HAD PWATS 175 TO 250 PERCENT OF NORMAL ALREADY
FROM HERE THROUGH CTRL IL. IT IS LIKELY THERE WILL BE SOME
INSTABILITY BROUGHT UP...AS THERE ARE POCKETS OF NEGATIVE EPV ON
MULTIPLE HEIGHT FIELDS. EVEN UPRIGHT ELEVATED INSTABILITY IS
PROGGED NOT FAR TO THE SOUTHEAST. HAVE ADDED HEAVY SNOW MENTION
IN FOR TONIGHT. THE LEAD IMPULSE HAD HEAVY SNOW AND EXPECT
TONIGHT TO BE SIMILAR JUST LONGER LASTING. WE CONTINUE WITH A
MENTION OF SLEET AND SMALL CHANCE FOR FREEZING RAIN IN WRN WI.
EXPECT THIS TO BE MAINLY IN THE EVENING AS COLUMN DIABATICALLY COOLS
DUE TO HEAVY PRECIP. COULD BE SOME MINOR ACCUMS NEAR EAU. DOMINANT
PRECIP TYPE WILL REMAIN SNOW THOUGH ACROSS THE FORECAST AREA.
HEAVIEST SNOW ACCUMS TONIGHT SHOULD BE IN ERN AND SRN MN AND FAR
WRN WI...JUST TO THE EAST OF H5 HEIGHT FALLS AND ALONG AND TO THE
WEST /UPSLOPE DIRECTION/ OF THE AXIS OF GREATEST CONVERGENCE.
MODELS BEGIN TO DISAGREE TO SOME REGARD ON CHRISTMAS DAY AS LOWS
PHASE AND HOW THAT EFFECTS TROWAL AND DRY SLOT INFLUENCES IN OUR
AREA. OVERALL THOUGH...THEY AGREE ON HEAVY SNOW BEING MOST LIKELY
DURING THE MORNING AT THE NOSE OF THE PV BOOT. THIS IS IN THE MN
COUNTIES. THIS WILL BE THE BEST DYNAMIC FORCING OF THE
EVENT...WITH AN ELONGATED W TO E DEFORMATION AREA LIFTING SLOWLY
BACK TO THE W. THE SFC LOW IS FORECAST BY ALL GUIDANCE TO WRAP
AROUND THE CLOSED UPPER LEVEL LOW SOMEWHERE OVER IA. VERY
IMPRESSIVE...AND THE SYSTEM WILL REMAIN SLOW TO FULLY OCCLUDE
WITH DUMBBELL TYPE CHARACTERISTICS AND A TROWAL THROUGH EARLY
SAT...INDICATING ACCUMULATING SNOW BEING SLOW TO END.
TOTAL QPF FOR THE EVENT STILL ON THE ORDER OF 1.25 TO 2 INCHES ON
ALL GUIDANCE. RATIOS WERE OBSERVED NEAR 10 TO 1 LAST NIGHT AND
EXPECT THAT TO BE SIMILAR TONIGHT AND CREEP UP DURING CHRISTMAS.
THIS WAS USED IN THE SNOWFALL FORECASTS.
BLOWING SNOW WILL OCCUR IN OPEN AREAS AS THE PRESSURE GRADIENT
REMAINS SIMILAR TO WHAT IT HAS BEEN...TIGHTENING MORE SO TONIGHT
AND FRIDAY. HAVE HIGHLIGHTED MORE IN THE GRIDS OVER WRN MN. ITS
POSSIBLE A BLIZZARD HEADLINE WILL BE NEEDED FOR PART OF WRN MN
DURING CHRISTMAS BUT AT THIS TIME TOO EARLY TO TELL IF BEST WINDS
WILL OR WILL NOT ARRIVE WITH FALLING SNOW AND THUS REALLY ENHANCE
THE BLIZZARD LIKELIHOOD.
SATURDAY SHOULD SEE OFF AND ON SNOW FOR MOST OF THE AREA WITH
MINOR ADDITIONAL ACCUMULATIONS. IMPACTS SHOULD BE MINIMAL SO
CONTINUE TO END WARNING AT 12Z. ITS POSSIBLE IT MAY NEED TO BE
EXTENDED IF WEAKENING DEFORMATION AREA HAS ENOUGH STRENGTH.
&&
.AVIATION.../12Z TAF ISSUANCE/
LIFR/VLIFR CIGS AND VSBYS ASSOCIATED WITH AN AREA OF MODERATE TO
HEAVY INTENSITY SNOWFALL OVER EAST CENTRAL MN AND WEST CENTRAL WI
TAF SITES /KSTC-KMSP-KRNH-KEAU/ AT THE START OF THE TAF
PERIOD WILL CONTINUE SHIFTING TO THE NORTH THIS MORNING. A RESPITE
FROM THE HEAVY SNOWFALL WILL ENSUE...WITH AN ATTENDANT LULL IN THE
ACCUMULATING SNOWFALL AT THE TAF SITES THIS MORNING. ALTHOUGH
OCCASIONAL MVFR CIG/VSBYS AND SCATTERED SNOW SHOWERS WILL BE
POSSIBLE TODAY...THE NEXT BOUT OF HEAVY SNOW IS NOT EXPECTED TO
ARRIVE UNTIL AFTER 21Z THIS AFTERNOON. ONCE THE HEAVY SNOW RE-
DEVELOPS...LIFR/VLIFR CIGS AND VSBYS WILL PREVAIL THROUGH 12Z
FRIDAY. ADDITIONALLY...SLEET AND FREEZING DRIZZLE WILL BE POSSIBLE
THIS MORNING...AND AT KEAU THROUGH THE TAF PERIOD. AS THE LOW
PRESSURE CENTER OF INTEREST NEARS...WINDS WILL SHIFT TO
NORTHEASTERLY IN DIRECTION...AND NORTH/NORTHWESTERLY AT WESTERN
SITES /KAXN-KRWF/. GUSTS WILL ALSO BEGIN DEVELOPING TONIGHT AS THE
SYSTEM STRENGTHENS.
&&
.MPX WATCHES/WARNINGS/ADVISORIES...
MN...WINTER STORM WARNING UNTIL 6 AM CST SATURDAY FOR ANOKA-BENTON-
BLUE EARTH-BROWN-CARVER-CHIPPEWA-CHISAGO-DAKOTA-DOUGLAS-
FARIBAULT-FREEBORN-GOODHUE-HENNEPIN-ISANTI-KANABEC-
KANDIYOHI-LAC QUI PARLE-LE SUEUR-MARTIN-MCLEOD-MEEKER-MILLE
LACS-MORRISON-NICOLLET-POPE-RAMSEY-REDWOOD-RENVILLE-RICE-
SCOTT-SHERBURNE-SIBLEY-STEARNS-STEELE-STEVENS-SWIFT-TODD-
WASECA-WASHINGTON-WATONWAN-WRIGHT-YELLOW MEDICINE.
WI...WINTER STORM WARNING UNTIL 6 AM CST SATURDAY FOR BARRON-CHIPPEWA-
DUNN-EAU CLAIRE-PEPIN-PIERCE-POLK-RUSK-ST. CROIX.
&&
$$
LS/MTF
