FXUS63 KMPX 080839
AFDMPX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
239 AM CST TUE DEC 8 2009

.DISCUSSION...

TWO PRIMARY CONCERNS WITH THIS FORECAST PACKAGE AS THE WINTER
STORM BEGINS TO DEVELOP ACROSS THE PLAINS LATER TODAY. 

FIRST...THE INITIAL ONSET OF PRECIPITATION WILL BE DELAY A FEW HRS
IN THE FAR N/NE CWA AS DRIER ARCTIC AIRMASS REMAINS IN PLACE AND
WILL HINDER ACCUMULATING SNOWFALL. HOWEVER...THIS AIRMASS WILL
BE QUICKLY MODIFIED AS STRONG MOISTURE ADVECTION FROM THE PLAINS
COMMENCES BY MID/LATE AFTN. CURRENT SFC ANALYSIS WITH HRLY
PRECIPITATION TOTALS CONTINUE TO INCREASE ALONG THE MO/IA BORDER
WITH VSBYS LOWERING TO LESS THAN TWO MILES. REFLECTIVITY PATTERN
WITH THE HIGHEST RETURNS HAVE BEEN CONCENTRATED ACROSS SOUTHERN
IA/NORTHERN MO OVERNIGHT. HOWEVER...THERE HAS BEEN AN OVERALL
INCREASE IN HIGHER REFLECTIVITY RETURNS IN NE NEBRASKA WHICH WILL
EVENTUALLY SATURATE THE ATMOSPHERE THIS MORNING IN SW MN. MAY
MODIFY MORNING POPS/WX GRIDS BASED ON REFLECTIVITY PATTERN BY 4
AM. 

SECONDLY...THIS CURRENT COLD/DRY AIRMASS WILL ALSO ENHANCE
SNOWFALL RATIOS. THIS MAY LEAD TO RATIOS IN THE 15 OR 20-1 RANGE 
WHICH WILL PRODUCE HIGHER SNOWFALL AMTS...EVEN IF THE QPF ISN'T
AS HIGH. CURRENT SNOWFALL AMTS WERE BASED ON HPC QPF BUT MODIFIED
THEM FROM THE LATEST GFS 18-1 SNOWFALL RATIO. THIS WILL LEAD TO
SLIGHTLY HIGHER AMTS...WITH EIGHT INCHES OR MORE LIKELY FROM NEAR
ST. JAMES...NORTHEAST TO THE TWIN CITIES AND LADYSMITH WISCONSIN.
THESE AMTS WILL LIKELY BE MODIFIED AS LATER FORECASTERS BEGIN TO 
FINE TUNE THE HIGHEST SNOWFALL RATES AND WHERE THE MAXIMIZED
OMEGAS WILL LIE BASED ON RADAR TRENDS. LATEST 06Z NAM
INITIALIZATION SEEMS TO HAVE A GOOD HANDLE ON THE DRIER AIRMASS IN
PLACE...WHICH DOESN'T INCREASE THE DEEPEST MOISTURE UNTIL AFT
18Z...ESPECIALLY NORTH OF I-94. THIS WILL LEAD TO OCCASIONAL
FLURRIES OR LIGHT SNOW...WITH MODERATE SNOW HOLDING OFF UNTIL LATE
IN THE DAY.

BASED ON STRONG FORCING AND MOISTURE ADVECTION FROM SEVERAL MODELS
WHICH HAVE BEEN FAIRLY CONSISTENT...SN WITH PERIODS OF +SN WILL
LIKELY DEVELOP ACROSS SOUTH CENTRAL MN LATE THIS AFTN...THEN
SPREAD RAPIDLY N/NE ACROSS THE TWIN CITIES...AND INTO WEST CENTRAL
WISCONSIN DURING THE OVERNIGHT PERIOD. SNOWFALL RATES WILL LIKELY
RANGE FROM ONE HALF...TO LOCALLY ONE INCH PER HOUR BETWEEN 03 TO
09Z WEDNESDAY WHICH IS RELATED TO THE BEST FORCING AND MOISTURE
FIELDS. WESTERN WISCONSIN WILL HOLD ONTO THIS SCENARIO LONGER
WHICH MAY LEAD TO LOCALLY 10+ INCHES...ESPECIALLY AROUND EAU
CLAIRE. 

ANOTHER ASPECT TO THIS SYSTEM IS THE RAPID INTENSIFICATION OF THE
SFC LOW WHICH WILL DEEPEN ACROSS NORTHERN MO TODAY...AND BE NEAR
MKE BY 12Z WED. NEARLY 20 MB DROP ON THE SFC LOW IS PROGGED BY THE
LATEST NAM WITH A SPAN OF 1012 MB ALONG THE WESTERN MN BORDER...TO
988 NEAR THE FAR SE CORNER. THIS WILL LEAD TO STRONG WINDS
DEVELOPING LATER THIS EVENING...WITH BLIZZARD CONDITIONS IN SOUTH
CENTRAL MN BY LATE EVENING. WITH THE FLUFFY NATURE OF THIS
SNOWFALL...VSBY WILL LIKELY FALL BELOW 1/4 OF A MILE FOR SEVERAL
HRS DURING THE NIGHT AND WEDNESDAY BEFORE THE PRESSURE GRADIENT
BEGINS TO LESSEN. 

AFTER WEDNESDAY EVENING...THERE REMAINS SOME CONCERNS WITH THE
AMOUNT OF COLD AIR FOR LATER IN THE WEEK...AND FOR THE UPCOMING
WEEKEND. HAVE LEAN TOWARD COLDER TEMPERATURES...ESPECIALLY FOR THE
OVERNIGHT PERIOD WITH NEW SNOW COVER AND TEMPERATURES ALREADY -40C
JUST OVER THE BORDER OF MT THIS MORNING. NOT UNTIL LATER NEXT WEEK
WILL ANY TYPE OF MODIFICATION OF THIS AIR MASS TAKE PLACE.
PLUS...DUE TO THE ANTICIPATION SNOW COVER ACROSS IA/MO...ANY TYPE
OF WARMER AIR RETURNING NORTHWARD FROM THE SOUTHERN PLAINS...WILL 
LIKELY BE MODIFIED (HENCE...LOW CLOUD COVER WITH FOG!). ..JLT..

&&

.AVIATION.../06Z TAF ISSUANCE/ 

ALL EYES ON MAJOR WINTER STORM THAT WILL HAVE SIGNIFICANT IMPACTS
ON ALL TAF SITES AT SOME POINT IN THE NEXT 24/30 HRS...ESPECIALLY
AT RWF/MSP/RNH/EAU. CONDITIONS THOUGH FOR THE FIRST FEW HOURS
LOOK TO REMAIN RATHER TRANQUIL...AS BEST WAA AND ISENTROPIC LIFT
LOOK TO HOLD OFF INTO IOWA UNTIL AFTER 12Z. THIS WILL KEEP MAINLY
VFR CONDITIONS GOING AT MOST TAF SITES THROUGH THE LATE
MORNING...THOUGH RWF WILL LIKELY SEE MVFR CIGS MOVE IN BEFORE
12Z...CONSIDERING CIGS OF THIS NATURE ARE ALREADY BEING SEEN ON
THE BUFFALO RIDGE. EXPECT CONDITIONS TO BEGIN TO DETERIORATE AT
MSP THIS AFTERNOON...WITH CIGS AND VSBYS FALLING THROUGH
MVFR...NAM/GFS SHOW CONSIDERABLE AGREEMENT ON STRONGEST
FORCING...IN TERMS OF OMEGA...CENTERED AROUND 06Z WEDNESDAY AT
MSP...SO DROVE VSBYS AND CIGS DOWN TO IFR LEVELS...THOUGH WOULD
NOT BE SURPRISED IF EVEN LOWER CONDITIONS ARE SEEN TUESDAY NIGHT.
FINAL BIG ISSUE THIS TAF PERIOD WILL BE WINDS...AS LOW PASSES TO
THE SOUTH AND DEEPENS...PRESSURE GRADIENT WILL INCREASE
CONSIDERABLY TUESDAY NIGHT...LIKELY SENDING SUSTAINED NORTH WINDS
INTO THE 15-20+ KT RANGE. MIXING DOWN ON NAM BUFKIT SOUNDINGS AT
ALL SITES INDICATING GUST POTENTIAL OVER 30 KTS BY END OF TAF
PERIOD...LIKELY LEADING TO BLSN ISSUES...SO HAVE KEPT VSBYS DOWN AT
ALL SITES WITH THIS IN MIND.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...WINTER STORM WARNING FROM NOON TODAY TO MIDNIGHT CST WEDNESDAY 
     NIGHT FOR ANOKA-BROWN-CARVER-CHISAGO-DAKOTA-GOODHUE-
     HENNEPIN-LE SUEUR-NICOLLET-RAMSEY-RICE-SCOTT-SIBLEY-
     WASHINGTON.

     WINTER WEATHER ADVISORY FROM NOON TODAY TO 6 PM CST WEDNESDAY 
     FOR BENTON-ISANTI-KANABEC-KANDIYOHI-MCLEOD-MEEKER-MILLE 
     LACS-REDWOOD-RENVILLE-SHERBURNE-STEARNS-WRIGHT.

     BLIZZARD WARNING FROM NOON TODAY TO MIDNIGHT CST WEDNESDAY NIGHT 
     FOR BLUE EARTH-FARIBAULT-FREEBORN-MARTIN-STEELE-WASECA-
     WATONWAN.

WI...WINTER STORM WARNING FROM NOON TODAY TO MIDNIGHT CST WEDNESDAY 
     NIGHT FOR BARRON-CHIPPEWA-DUNN-EAU CLAIRE-PEPIN-PIERCE-POLK-
     RUSK-ST. CROIX.

&&

$$

JLT/MPG