FXUS63 KMPX 251811
AFDMPX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
1211 PM CST FRI DEC 25 2009

.UPDATE...

ADDED AVIATION DISCUSSION FOR 18Z TAFS BELOW...

&&

.DISCUSSION...

THE FORECAST CHALLENGES THIS CHRISTMAS DAY CENTER ON DEPARTING
WINTER STORM AND WRAPAROUND WARM AND COLD AIR CONVEYOR
BELTS...WHICH WILL DICTATE PRECIP TYPES AND AMOUNTS.

THE SYNOPTIC SCALE UPPER LOWS ARE PHASING INTO ONE GIANT SYSTEM
TO OUR SOUTH...WITH BAROCLINIC LEAF STRUCTURE EXPANDED OVER THE
ENTIRE UPPER MIDWEST AND GREAT LAKES AND EVEN MID-ATLANTIC. THE 
MOIST CONVEYOR BELT IS STILL VERY APPARENT ON SATELLITE CHANNELS
AND IS AND WILL CONTINUE TO FEED CONSTANT PRECIP ACROSS THE AREA.
THE QUESTION FOR ERN MN AND WRN WI IS PRECIP TYPE. THE LOW LEVEL
WARM NOSE OBVIOUSLY MADE IT FURTHER NORTHWEST THAN THOUGHT OVER
THE PAST FEW DAYS...DESPITE HEAVY RAIN UPSTREAM /ACROSS THE ENTIRE
LOWER AND MID MISSISSIPPI VALLEY/ MUCH OF YESTERDAY. THIS IS
QUITE AMAZING FOR TEMPS HERE TO WARM IN THE LOW LEVELS WITH
ONGOING PRECIP IN LATE DECEMBER AND WITH THE MAIN SFC LOW SO FAR
SOUTH. IT LIKELY CAN BE CONTRIBUTED IN SOME PART TO A FEW THINGS.
FOR ONE THE LEAD WAVE...LIFTING NORTH ACROSS MO THIS MORNING...IS
FURTHER WEST THAN PROGGED BY MOST GUIDANCE AND DEVELOPED THE
INVERTED TROUGH FURTHER WEST AND AT A MORE NW TO SE
ORIENTATION...KEEPING THE ERN TWO-THIRDS OF THE FORECAST AREA
WARM. ALSO...A WARM STARTING POINT IN LOW LEVEL TEMPS /NOT FAR
BELOW FREEZING/ BEFORE THIS SYSTEM HELPED TO START THINGS MILD
BEFORE WAA. FINALLY...A VIGOROUS LOW LEVEL JET VERY LIKELY OFFSET
DIABATIC COOLING. TO SHOW HOW RARE THIS TYPE OF SYSTEM IS...WITH
THE PHASING AND LONG-LIVED OCCLUSION...THE CIPS ANALOG GUIDANCE
THE PAST TWO DAYS HAS PREDOMINATELY REPORTED NO MATCHES FROM THE
PAST 30 YEARS WITH THIS WEATHER PATTERN. THE ONLY ONE COMING CLOSE
IN THE PAST 48 HOURS WAS THE HALLOWEEN BLIZZARD OF 1991. A TRICKY
SITUATION...THAT IS BY NO MEANS FINISHED.

THE HIGH RES NAM AND RUC HAVE A GOOD HANDLE ON THE SFC LOW AND
INVERTED TROUGH. HAVE USED THE MAXIMUM TEMPERATURE IN THE LOWEST
KILOMETER AS A PROXY FOR WHICH PRECIP TYPE TO MENTION AS THE
DOMINANT TODAY. THIS HAS WORKED WELL WITH OBS TRENDS. THE WARM
CORRIDOR OVER ERN MN AND WRN WI WILL HOLD IN PLACE THIS MORNING.
THIS WILL BEGIN TO RETREAT NORTHWARD IN THE MID-LEVELS THIS AFTN
AS FIRST COLD CONVEYOR BELT WRAPS AROUND THE SYSTEM AND APPROACHES
SE MN THIS AFTN. THUS PRECIP TYPE AS SNOW STILL EXPECTED TO BE
PREVAILING BY AFTN...THOUGH FORCING BEGINS TO WANE IN ERN MN AND
WRN WI. HAVE DROPPED ACCUMULATIONS TO TWO TO FOUR INCHES IN MOST
OF THESE AREAS. VERY GOOD FORCING ALL DAY IN SRN TO SWRN MN AND
WEST CTRL MN...WITH ISENTROPIC UPGLIDE AND DEEP UPPER LEVEL PV
ADVECTION FROM THE SOUTH. THIS SHOULD SUPPORT BOUTS OF HEAVY
SNOW...WITH AN ADDITIONAL FOUR TO SEVEN INCHES ACROSS THOSE AREAS.
TO THE WEST OF THE INVERTED TROUGH...MAINLY ARE FAR WEST CENTRAL
COUNTIES...WILL CONTINUE TO MENTION BLOWING SNOW. SNOWFALL HAS
BEEN IN THE LOWER TEENS AND EVEN HIGH SINGLE DIGITS EARLY
TODAY...SO EXPECT A LIGHTER SNOW MORE READILY ABLE TO BLOW.
THE SMALL BLIZZARD THREAT THOUGH HAS WANED CONSIDERABLY. THE SNOW
WILL CONTINUE TONIGHT BUT BE LIGHTER AS THE MOIST CONVEYOR BELT
SHIFTS WELL EAST AND WE NOW DEPEND ON WRAPAROUND MOISTURE. DO
STILL EXPECT LIGHT ACCUMULATIONS...POTENTIALLY ONE TO TWO INCHES.

FOR HEADLINES...AT 6 AM WILL DOWNGRADE TO AN ADVISORY FOR AT LEAST
PART OF WRN WI AND POSSIBLY SOME PART OF ERN MN. WILL ALSO MOVE
UP THE END TIME TO THE WARNING SOME IN ERN MN. NO CHANGES ARE
PLANNED TO THE WRN THIRD OF THE FORECAST AREA.

CONTINUE WITH LIKELY POPS FOR MOST AREAS ON SAT AND SAT NIGHT.
WHILE IT IS ONLY 24 TO 36 HOURS...IT IS STILL DIFFICULT TO PLACE
DRY SLOTS AND TROWAL WITH THIS OCCLUDING SYSTEM AS IT SLOWLY SHIFTS
NE. WHAT CAN BE SAID IS THAT PRECIP SHOULD BE MAINLY LIGHT...WITH
ACCUM POSSIBILITIES OF 1 TO 2 INCHES NEAR THE F-GEN THAT DEVELOPS
ON WARM AND COLD SIDES OF THE TROWAL NOSE. THIS MAY BE JUST WEST
OF THE FORECAST AREA.

LITTLE CHANGES MADE TO SUN THROUGH TUES. MAIN ONE WAS TO LOWER
TEMPS CONSIDERABLY MON NIGHT AND SOME TUE. THIS WILL BE THE FIRST
TIME A SFC HIGH IS OVER THE FRESH SNOW. ALL SIGNS POINT TOWARDS
BELOW ZERO TEMPS IN THE MN CWA ON MON NIGHT IF THE SKY IS CLEAR.
AGAIN...IT IS LATE DEC SO NOTHING CRAZY. EARLY SIGNS FOR THE FINAL
DAYS OF 2009 ARE FOR A WNW FLOW SYSTEM OR TWO TO RIDE THE MID-
LEVEL BAROCLINIC ZONE ACROSS THE UPPER MIDWEST. ALL SEEM FAIRLY
PROGRESSIVE.

&&

.AVIATION.../18Z TAF ISSUANCE/ 

PERIODS OF PRECIPITATION AND LOW CIGS/VSBYS WILL CONTINUE THROUGH
MUCH OF THE PERIOD AS STRONG LOW PRESSURE OVER CENTRAL IOWA MOVES
LITTLE BUT BEGINS TO WEAKEN. WAVES OF PRECIPITATION WILL CONTINUE
TO ROTATE WESTWARD ACROSS THE TERMINALS THIS AFTERNOON WITH MIXED
PRECIPITATION EAST AND ALL SNOW WEST. TREND WILL BE TO FOCUS SNOWFALL
ACROSS WEST CENT MN THIS EVENING AND TONIGHT WITH LIGHTER PRECIP
EAST. MVFR/IFR CIGS WILL CONTINUE WITH MOSTLY MVFR VSBY EXCEPT FOR
IFR AND LOWER VSBY IN MORE INTENSE SNOW BANDS. MIXED PRECIP WILL
END THIS EVENING AS COLDER AIR ARRIVES FROM THE SOUTHEAST WITH
SNOWFALL COVERAGE AND INTENSITY DECREASING WITH TIME EAST AS WELL. STRONG
NORTHEAST WINDS WILL SLACKEN THIS EVENING BUT BLOWING SNOW WILL
CONTINUE IN THE WEST UNTIL THIS OCCURS. DETAILS BELOW...

KMSP...THE TERMINAL SITES RIGHT ON THE FRINGE OF WARMER AIR IN THE
LOW LEVELS SO MIXED PRECIP CONTINUES. TREND WILL BE FOR MAINLY SNOW
BUT AT LEAST A LITTLE RAIN AND SLEET WILL CONTINUE TO MIX IN
EARLY THIS AFTERNOON...BUT FOR SHORTER AND SHORTER PERIODS.
PRECIP INTENSITY WILL START TO DECREASE AS THE CURRENT BAND SHIFTS
WESTWARD. ENOUGH FORCING WILL BE PRESENT INTO THE EVENING TO KEEP
SNOW SHOWERS GOING SO KEPT LOWERED VSBY IN LIGHT SNOW AND MIST
THROUGH TONIGHT. EXPECT CIGS OF 1000-1500 FT THROUGH TONIGHT WITH
SOME IMPROVEMENT EXPECTED SATURDAY. SNOW SHOWERS WILL CONTINUE
SATURDAY BUT WILL BE MUCH LESS PERSISTENT.

KAXN/KRWF/KSTC...BAND OF BETTER FORCING MOVING INTO THESE SITES AT
18Z BRINGING A HEAVIER PERIOD OF SNOW. TOUGH TO GET A GOOD
HANDLE ON VSBY AS THE PRECIP IS CHANGING FROM RAIN TO SNOW AS IT
APPROACHES BUT EXPECT REDUCTIONS TO AROUND 3/4SM AND POSSIBLY
LOWER THIS AFTERNOON. BANDS OF SNOW WILL CONTINUE ON AND OFF
THROUGH THE NIGHT WITH VSBY FALLING TO LESS THAN 1SM IN THE
HEAVIEST BANDS INTO SATURDAY MORNING. BLOWING SNOW WILL LIKELY
OCCUR AS THE NEW SNOW FALLS AND WINDS STAY UP BUT WINDS WILL
SLACKEN THIS EVENING SO BLOWING SHOULD BE MINIMIZED BY EARLY SATURDAY.

KRNH/KEAU...WARMER AIR REMAINS IN THE LOW LEVELS WHICH CONTINUES TO
BRING MIXED PRECIPITATION TO THESE SITES. TENDENCY WILL BE FOR THE
ATMOSPHERE TO COOL BUT PRECIP COVERAGE WILL BEGIN TO WANE TOO.
TRANSITION FROM MIX TO ALL SNOW WILL OCCUR THIS AFTERNOON WITH
ENOUGH ENERGY TO KEEP LIGHT PRECIP FALLING. WITH ABUNDANT LOW
LEVEL MOISTURE HAVE KEPT MIST IN AS WELL. TOUGH T0 TIME ANY
SIGNIFICANT REDUCTIONS IN VSBY BUT THEY SHOULD BE SHORT LIVED WITH
THE FORCING NOT THAT WELL FOCUSED. CIGS WILL STAY DOWN TO AROUND
1000 OR 1500 FT WITH SOME IMPROVEMENT POSSIBLE FOR SATURDAY. SNOW
SHOWERS WILL CONTINUE ON AND OFF FOR SATURDAY. ..MDB..

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR ANOKA-CARVER-
     CHISAGO-HENNEPIN-ISANTI-LE SUEUR-RAMSEY-SCOTT-WASECA-
     WASHINGTON.

     WINTER STORM WARNING UNTIL 6 AM CST SATURDAY FOR BENTON-BLUE 
     EARTH-BROWN-CHIPPEWA-DOUGLAS-FARIBAULT-KANABEC-KANDIYOHI-
     LAC QUI PARLE-MARTIN-MCLEOD-MEEKER-MILLE LACS-MORRISON-
     NICOLLET-POPE-REDWOOD-RENVILLE-SHERBURNE-SIBLEY-STEARNS-
     STEVENS-SWIFT-TODD-WATONWAN-WRIGHT-YELLOW MEDICINE.

     WINTER WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR DAKOTA-
     FREEBORN-GOODHUE-RICE-STEELE.

WI...WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR BARRON-POLK.

     WINTER WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR CHIPPEWA-
     DUNN-EAU CLAIRE-PEPIN-PIERCE-RUSK-ST. CROIX.

&&

$$

MTF/MDB