Heavy Snow in Minnesota, December 13 to 14, 2005

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
248 PM CST TUE DEC 13 2005

.DISCUSSION...TONIGHT THROUGH FRIDAY
VERY INTERESTING AND QUITE DIFFICULT FORECAST IN THE SHORT TERM.
ELONGATED SFC LO CURRENTLY STRETCHING SWRD FROM SERN MANITOBA THRU
THE CNTRL DAKOTAS AND INTO NWRN KS ATTM. UPR TROF EVIDENT IN WV
IMAGERY DROPPING SWRD INTO WRN MT/WRN WY...WHICH WILL SERVE TO
KICK THE SFC LO EWRD AND FURTHER ORGANIZE IT AS IT APPROACHES THE
FA THIS EVENING AND OVRNGT. VERY MOIST SFC AIR CURRENTLY OVER MN
AND WI...WITH STRONG SELY SFC WINDS BRINGING UPR 20S AND LO 30S
DEW PTS INTO THE AREA. HOWEVER...THE MOISTURE IS VERY SHALLOW /AS
EXPECTED/...WITH LATEST ACARS PROFILES OVER MSP SHOWING A FAIRLY
DEEP DRY LAYER STARTING AT AROUND 900MB...WHICH WILL HAVE
IMPLICATIONS PERTAINING TO PCPN TYPE AT THE ONSET OF THE UPCOMING
WINTER EVENT.

S/WV TROF...CURRENTLY EVIDENT IN WV IMAGERY SITTING IN ERN
WY...WILL SERVE AS THE INITIAL KICK FOR PCPN LATE THIS EVENING AND
OVRNGT AS IT MOVES EWRD INTO THE NRN PLAINS. DESPITE THE RATHER
WARM LAYER JUST ABOVE THE SFC EVIDENT ON THE UPSTREAM 12Z RAOB
DATA...THE CURRENT EXPECTATION IS THAT THE MAJORITY OF THE PCPN
WILL FALL AS SNOW DUE TO THE DRY AIR CONTAINED IN THAT LAYER. MUCH
OF THE PCPN WILL EVAPORATE AND COOL THE LAYER TO BELOW FREEZING
PRIOR TO THE BULK OF THE PCPN MAKING IT TO THE SFC. HOWEVER...HAVE
LEFT A SHORT TIME PERIOD THIS EVENING WITH A MENTION OF VERY LGT
FRZ RAIN...IN CASE SOME OF THIS MAKES IT TO THE SFC. THE FRZ RAIN
IS NOT EXPECTED TO CAUSE ANY SIGNIFICANT PROBLEMS. WITH CURRENT
ACARS DATA AND THE 12Z KMPX RAOB SHOWING PROFILES BELOW
FREEZING...HAVE NOT EXTENDED THE FRZ RAIN THREAT INTO THE
CITIES...BUT KEPT IT ACRS THE FAR SWRN AND S CNTRL MN COUNTIES
THRU LATE EVENING.

THE BIG QUESTIONS WITH THIS SYSTEM ARE HOW MUCH SNOW WILL
FALL...OVER WHAT DURATION...AND IN WHAT AREAS. ATTM...THE MOST
FAVORABLE AREAS ARE W CNTRL WI THRU THE DAY WED AND INTO WED NGT.
THERE ARE SEVERAL REASONS FOR THIS. AN AREA OF ENHANCED PV
ADVECTION WILL SWING INTO W CNTRL WI DURG THIS TIME FRAME...AS
WELL AS A DEEP AREA OF LO AND MID LEVEL WAA AND H8-H7
FRONTOGENESIS MOVES INTO SRN AND INTO CNTRL WI DURG THE DAY WED.
IN ADDITION...NAM CROSS-SECTIONS SHOW A NEARLY VERTICAL
AGEOSTROPHIC CIRCULATION...IN AN AREA OF VERY LOW /AND AT TIMES
NEGATIVE/ EPV...SO WITH ALL INGREDIENTS COMBINED...HIGHEST
SNOWFALL RATES ARE SUPPORTED IN THIS RGN. AS A RESULT...UPGRADED
PREVIOUS WINTER STORM WATCH TO A HEAVY SNOW WARNING...WITH AROUND
8 INCHES OF SNOW EXPECTED IN THE 24 HR PERIOD.

ACRS THE REMAINDER OF THE FA...ENHANCED SNOWFALL RATES WILL BE
HARD TO COME BY DUE TO THE BAROTROPIC NATURE OF THE ATMOSPHERE
OVER THE CNTRL MN COUNTIES WITH THIS SYSTEM...MAKING IT UNLIKELY
THAT A PROLONGED TIME PERIOD OF ENHANCED LIFT WILL DEVELOP. THIS
IS ILLUSTRATED IN THE LACK OF CONCENTRATED H7 OMEGA IN THE WRN
HALF TO TWO-THIRDS OF THE FA. ALTHO THE DURATION OF SNOWFALL WILL
ALLOW THE ACCUMULATIONS TO BE FAIRLY HIGH IN MOST AREAS /EXCEPTION
BEING SWRN MN COUNTIES/...THE RATES SHUD NOT BE ENUF TO REACH
HEAVY SNOW CRITERIA. THERE WILL BE SOME HIGHER AMTS ACRS THE NRN
PART OF THE FA...IN THE RGN OF WRAP AROUND MOISTURE AS THE MID LVL
LO PASSES...AND ALSO NEAR THE TWIN CITIES...AS THIS AREA WILL FEEL
THE EFFECTS OF THE FORCING IN W CNTRL WI. AS A RESULT...LEFT THE
REMAINDER OF THE FA IN A SNOW ADVISORY...WITH TOTAL ACCUMULATIONS
RANGING FROM ABOUT 5 INCHES IN SWRN MN TO AROUND 8 INCHES AROUND
AXN...LXL...CBG AND MSP.

HEAVIEST SNOWFALL SHUD CUT OFF BY THU AND THU NGT...BUT CONTINUED
IMPULSES ROTATING AROUND THE UPR LVL LO CNTR WILL PROVIDE ENUF
LIFT FOR LGT SNOW TO CONT THRU THE REMAINDER OF THE WEEK.

MANY THANKS TO ALL THE SURROUNDING WFOS FOR THEIR COORDINATION
THIS MRNG AND AFTN.

LONGER TERM...FRIDAY NIGHT THROUGH TUESDAY
LOW PRES OVER WRN GREAT LAKES FRI NIGHT WILL MOVE INTO SE ONTARIO
SAT. TROF EXTENDING WESTWARD FROM LOW LINGERS ACROSS NRN/CENTRAL MN
INTO SAT EVE WHICH KEEPS SOME LOW LVL MOISTURE AND WEAK FORCING OVER
AREA THRU PD. SLIGHT CHANCE POPS FOR -SN STILL WARRENTED CWA WIDE
FRI NIGHT AND ACROSS NRN 1/2 CWA SAT WITH FEATURE. DRIER AIR PUSHES
IN FROM THE N SUN AND LINGERS THRU MON NIGHT WITH CHILLY NW FLOW.
HIGHS IN THE TEENS WITH LOWS NEAR ZERO SUN/MON. WEAK SHORT WAVE FCST
TO SWING SEWARD THRU CWA TUE BUT LIFT/MOISTURE LIMITED WITH
FEATURE...SO FOR NOW WILL KEEP FCST DRY WITH PASSAGE.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...SNOW ADVISORY FROM MIDNIGHT TONIGHT TO 6 AM CST THURSDAY FOR
     ANOKA-BENTON-CARVER-CHISAGO-DAKOTA-DOUGLAS-HENNEPIN-ISANTI-
     KANABEC-LE SUEUR-MCLEOD-MEEKER-MILLE LACS-MORRISON-POPE-
     RAMSEY-RICE-SCOTT-SHERBURNE-SIBLEY-STEARNS-TODD-WASHINGTON-
     WRIGHT COUNTY(IES).

     SNOW ADVISORY FROM MIDNIGHT TONIGHT TO 6 PM CST WEDNESDAY FOR
     BLUE EARTH-BROWN-CHIPPEWA-FARIBAULT-FREEBORN-KANDIYOHI-LAC
     QUI PARLE-MARTIN-NICOLLET-REDWOOD-RENVILLE-STEELE-STEVENS-
     SWIFT-WASECA-WATONWAN-YELLOW MEDICINE COUNTY(IES).

     HEAVY SNOW WARNING FROM 6 AM WEDNESDAY TO 6 AM CST THURSDAY FOR
     GOODHUE COUNTY(IES).

WI...HEAVY SNOW WARNING FROM 6 AM WEDNESDAY TO 6 AM CST THURSDAY FOR
     BARRON-CHIPPEWA-DUNN-EAU CLAIRE-PEPIN-PIERCE-POLK-RUSK-ST.
     CROIX COUNTY(IES).

&&

KAT/MARGRAF

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