FXUS63 KMPX 241840
AFDMPX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
1240 PM CST THU DEC 24 2009
.UPDATE...
ADDED AVIATION DISCUSSION FOR 18Z TAFS BELOW...
&&
.DISCUSSION...
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.../18Z TAF ISSUANCE/
THE TERMINALS REMAIN IN A BIT OF A LULL PERIOD BEFORE THE NEXT
ROUND OF INTENSE PRECIPITATION ARRIVES LATE THIS AFTERNOON/THIS
EVENING. SFC INVERTED TROUGH BISECTS THE AREA EXTENDING FROM LOW
PRESSURE CENTERED NORTHWARD THROUGH CENTRAL MISSOURI...IOWA...AND
MINNESOTA. UPPER LEVEL LOW IS JUST STARTING TO TURN NORTHWARD OVER
NORTHEASTERN TEXAS WITH ANOTHER UPPER LOW DROPPING SOUTHWARD INTO
THE WESTERN HIGH PLAINS. WAVES OF SNOW SHOWERS WILL CONTINUE TO
DEVELOP AND STREAM NORTHWARD ACROSS THE TERMINALS THIS AFTERNOON
WITH MIXED PRECIP AT KEAU WHERE WARMER AIR IS IN PLACE. EXPECT A
GRADUAL INCREASE IN COVERAGE AND INTENSITY THROUGH LATE AFTERNOON
WITH THE MOST SIGNIFICANT PRECIPITATION ARRIVING THIS EVENING.
HAVE SLOWED ARRIVAL OF SUB 1SM VSBY TYPE PRECIP A FEW HOURS.
MODERATE TO OCCASIONALLY HEAVY SNOW CONTINUES TONIGHT AS BANDS
WORK FROM EAST TO WEST AS THE LOW WRAPS UP. NORTH TO NORTHEAST
WINDS INCREASE THIS EVENING AND FRIDAY MORNING CAUSING SOME
BLOWING AND DRIFTING OF SNOW. IMPROVEMENT WILL ARRIVE FROM EAST TO
WEST STARTING FROM MID TO LATE FRIDAY MORNING BUT SNOW SHOWERS
WILL CONTINUE...JUST AT A LIGHTER INTENSITY. DETAILS BELOW...
KMSP...WAVES OF SNOW SHOWERS WILL CONTINUE THROUGH THE AFTERNOON
RESULTING IN VARIABLE VISIBILITIES. EXPECT 4 TO 6 SM WITH LIGHT
SNOW WITH REDUCTIONS TO 2 TO 3 SM IN SNOW SHOWERS. CIGS SHOULD BE
PRETTY STEADY EVEN IN THE SNOW WITH A LOWERING TREND THROUGH THE
AFTERNOON. SNOW COVERAGE AND INTENSITY WILL INCREASE LATE THIS
AFTERNOON AND EARLY THIS EVENING WITH...ESPECIALLY AFTER 00Z.
HEAVIEST PERIOD OF SNOW LOOKS TO GET STARTED AROUND 03Z OR SO.
THIS IS A BIT SLOWER THAN EARLIER EXPECTED AND WILL HAVE TO WATCH
FOR ANY FURTHER ADJUSTMENTS. PEAK INTENSITY EXPECTED AFTER 06Z
FORA SEVERAL HOUR PERIOD WHERE 1/4SM IS POSSIBLE. PEAK SNOWFALL
RATES OF AROUND AN INCH PER HOUR ARE EXPECTED AS WELL. THE
HEAVIEST BAND OF SNOW WILL SHIFT WEST FRIDAY MORNING AND HAVE
INDICATED IMPROVEMENT TO 3SM VSBY AT 16Z BUT THIS MAY BE TOO
OPTIMISTIC. LOWER CONDITIONS MAY ALSO BE POSSIBLE THAN CURRENTLY
FORECAST IN THE 12 TO 16Z PERIOD. MAY SEE AN INCH OR 2 OF
ACCUMULATION THIS AFTERNOON WITH ANOTHER 6 TO 8 OVERNIGHT WITH
ANOTHER FEW INCHES FRIDAY MORNING.
KRWF/KAXN/KSTC...WAVES OF SNOW SHOWERS WILL CONTINUE TO LIFT
NORTHWARD THROUGH THE AFTERNOON RESULTING IN VARIABLE VISIBILITY
AND CEILINGS. EXPECT PRIMARILY LOW END MVFR CEILINGS WITH BOUTS OF
IFR POSSIBLE WITH VSBY IN THE 4 TO 7SM RANGE AT KRWF/KAXN BUT
LOWER AT KSTC WHERE SNOW SHOWERS WILL BE MORE PERSISTENT. SNOW
COVERAGE WILL EXPAND LATER THIS AFTERNOON WITH KSTC CONDITIONS
SIMILAR TO THOSE AT KMSP. KRWF AND KAXN WILL SEE THE HEAVIEST SNOW
A BIT LATER AND CONTINUE LATER INTO FRIDAY MORNING AND POSSIBLY
UNTIL MIDDAY FRIDAY BEFORE DECREASING IN INTENSITY. EXPECT 1/2 TO
3/4 SM VSBY TO PREVAIL WITH THE HEAVIER SNOW TONIGHT AND FRIDAY
MORNING WITH SOME 1/4SM POSSIBLE...SUPPORTING MAX SNOWFALL RATES
OF AROUND AN INCH PER HOUR AT TIMES. ANOTHER 8 TO 12 INCHES OF
ACCUMULATION LOOKS POSSIBLE INTO EARLY FRIDAY AFTERNOON.
KRNH/KEAU...KRNH WILL BE SIMILAR TO KMSP OVERALL BUT SEE HEAVIEST
SNOW ARRIVE AND DEPART A BIT SOONER. SNOW SHOWERS WILL PERSIST
THROUGH THE AFTERNOON KEEPING VSBY VARIABLE BUT CIGS SHOULD BE
FAIRLY STEADY WITH A LOWERING TREND THROUGH THE AFTERNOON. KEAU
WILL BE MUCH TOUGHER WITH WARM LAYER ALOFT IN CLOSE PROXIMITY TO
THE AIRPORT. MIXED PRECIP WILL BE THE PROBLEM WITH
SLEET...FREEZING RAIN...AND SLEET. SFC TEMPS RIGHT AT FREEZING AND
ARE EXPECTED TO WARM A FEW DEGREES SO IF THE LOWER ATMOSPHERE
STAYS WARM ENOUGH THEN PLAIN RAIN MAY EVEN OCCUR. WILL KEEP THE
MIX GOING WITH THE TEMPERATURE PROFILE RIGHT ON THE THRESHOLD OF
THE VARIOUS TYPES. THE ATMOSPHERE IS EXPECTED TO COOL THIS EVENING
AS FORCING INCREASES SO A TRANSITION TO PRIMARILY SNOW SHOULD
OCCUR LATE THIS EVENING BUT THIS WILL HAVE TO BE WATCHED
CAREFULLY. MAY SEE SOME SLEET MIX IN FRIDAY MORNING. ..MDB..
&&
.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.
&&
$$
MTF/MDB
