FXUS63 KMPX 081818
AFDMPX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
1218 PM CST TUE DEC 8 2009

.UPDATE...

ADDED DISCUSSION FOR 18Z TAFS BELOW...

&&

.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.../18Z TAF ISSUANCE/ 

DETERIORATING CONDITIONS UNDERWAY...

UPPER LEVEL LOW PRESSURE MOVING ACROSS THE ROCKIES IS HELPING TO
STRENGTHEN A SFC LOW ACROSS COLORADO WHICH WILL WORK EAST-
NORTHEAST ACROSS KANSAS...IOWA...AND NORTHERN ILLINOIS INTO EARLY
WEDNESDAY. LIGHT SNOW ACROSS MUCH OF SOUTHERN MN AND WESTERN WI AT
18Z WILL EXPAND NORTHWARD AND INTENSIFY THROUGH THE AFTERNOON FROM
SOUTH TO NORTH. THE BRUNT OF THE SNOWFALL WILL OCCUR THIS EVENING
AND TONIGHT WITH NORTHEAST WINDS INCREASING THIS EVENING THEN
TURNING NORTHWEST OVERNIGHT AND BECOMING VERY STRONG. STRONGEST
WINDS LOOKING HIGHER THAN EARLIER FORECAST AND EXPECTED FROM AROUND
DAYBREAK THROUGH MIDDAY. SUSTAINED SPEEDS OF 20 TO 25 KTS WITH
GUSTS OF 30 TO 35 KTS ARE EXPECTED WITH THE STRONGEST SPEEDS
ACROSS EAST/SOUTH CENT MN AND WEST CENT WI. SLIGHTLY HIGHER
SUSTAINED SPEEDS AND GUSTS ARE POSSIBLE. CHALLENGES WILL BE
DETERMINING PERIOD OF HEAVIEST SNOW/LOWEST VSBY AND ALSO VSBY
LEVELS IN BLOWING SNOW LATE TONIGHT THROUGH THE DAY WEDNESDAY.
DETAILS FOLLOW...

KAXN...SNOW WILL BE MOVING IN OVER THE NEXT HOUR OR TWO AND STEADILY
INCREASE INTO THE EVENING BRINGING VSBY DOWN TO AROUND 3/4SM THIS
EVENING AND TONIGHT. SNOW WILL WILL TAPER OFF LATE WEDNESDAY
MORNING BUT BLSN WILL CONTINUE. SNOW ACCUMULATION OF 3 TO 5
INCHES IS EXPECTED.

KSTC...SNOW INTENSITY WILL VARY THROUGH 21Z OR SO THEN INTENSIFY
INTO EARLY EVENING. HEAVIEST SNOWFALL RATES ARE EXPECTED TO RANGE
FROM 1/2 TO 3/4 IN/HR FROM MID EVENING THROUGH THE OVERNIGHT. SNOW
INTENSITY WILL DECREASE AFTER DAYBREAK BUT ADDITIONAL
ACCUMULATIONS LOOK TO OCCUR THROUGH LATE MORNING. TOTAL
ACCUMULATIONS AROUND 6 INCHES EXPECTED.

KRWF...SNOW WILL STEADILY INCREASE IN INTENSITY THROUGH THE
AFTERNOON WITH VSBY DROPPING TO AROUND 3/4SM. FURTHER
INTENSIFICATION WILL OCCUR THIS EVENING WITH 1/4 TO 1/2SM VSBY
EXPECTED BY LATE EVENING AND CONTINUING TOWARDS DAYBREAK...WITH
1/2 TO 1 IN/HR RATES. SNOW WILL TAPER OFF TOWARDS MID MORNING BUT BLSN
WILL CONTINUE TO REDUCE VSBY. TOTAL SNOWFALL OF 6 TO 8 IN.

KMSP...SNOW INTENSITY WILL VARY THROUGH 20 OR 21Z WITH VSBY
REDUCTIONS TO JUST UNDER 2SM AT TIMES BUT PREDOMINANTLY IN THE 3
TO 5SM RANGE. SNOW INTENSITY IS EXPECTED TO INCREASE MARKEDLY
AROUND 23Z WITH 3/4 TO 1 1/2 VSBY PREVAILING WITH SNOWFALL RATES
OF AROUND A 1/2 IN/HR POSSIBLE. HEAVIEST SNOW EXPECTED FROM 02 OR
03Z THROUGH 09Z WITH 1/2 TO 1 IN/HR RATES AND VSBY DROPPING TO
1/2SM WITH REDUCTIONS TO 1/4SM POSSIBLE IN SNOW AIDED BY BLOWING
SNOW WHICH WILL BECOME MORE PREVALENT AFTER 05Z OR SO. ACCUMULATING
SNOW LOOKS TO CONTINUE AFTER 09Z THROUGH THE MORNING BUT AT A
LIGHTER RATE WITH CONTINUED VSBY REDUCTIONS. TOTAL SNOWFALL OF 7
TO 9 IN.

KEAU...DURATION OF INTENSE SNOWFALL WILL BE LONGEST HERE. SNOW
WILL VARY IN INTENSITY THROUGH LATE AFTERNOON BEFORE INCREASING
THIS EVENING. VSBY THIS EVENING EXPECTED TO FALL TO 1/2 TO 3/4SM
WITH PERIODS OF 1/4SM LATE THIS EVENING AND TONIGHT...WITH HELP
FROM BLSN. SNOWFALL RATES OF 1/2 TO 1 IN/HR EXPECTED THIS EVENING
AND TONIGHT. SNOW WILL LIKELY REMAIN INTENSE THROUGH 15Z OR SO WEDNESDAY
BEFORE STARTING TO WANE. SNOW ACCUMULATION AROUND A FOOT IS
EXPECTED AND VSBY WILL LIKELY REMAIN REDUCED TO 1SM OR LESS
THROUGH THE MORNING.

KRNH...MUCH THE SAME SITUATION AS KMSP BUT AMOUNTS OF 8 TO 10
INCHES EXPECTED. SNOW INTENSITY WILL VARY THROUGH 20Z OR SO WITH A
RANGE OF VISIBILITIES. INTENSIFICATION WILL OCCUR LATE THIS
AFTERNOON AFTER ABOUT 20Z WITH A FURTHER INCREASE AROUND AND AFTER
00Z. INTENSITY WILL BE GREATEST THIS EVENING AND TONIGHT AND LAST
LATER INTO LATE MORNING...WITH RATES OF 1/2 TO 1 IN/HR. ..MDB..

&&

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

     WINTER WEATHER ADVISORY UNTIL 6 PM CST WEDNESDAY FOR BENTON-
     CHIPPEWA-ISANTI-KANABEC-KANDIYOHI-LAC QUI PARLE-MCLEOD-
     MEEKER-MILLE LACS-MORRISON-REDWOOD-RENVILLE-SHERBURNE-
     STEARNS-SWIFT-WRIGHT-YELLOW MEDICINE.

     BLIZZARD WARNING UNTIL MIDNIGHT CST WEDNESDAY NIGHT FOR BLUE 
     EARTH-FARIBAULT-FREEBORN-MARTIN-STEELE-WASECA-WATONWAN.

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

&&

$$

JLT/MDB