FXUS63 KMPX 072157
AFDMPX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
357 PM CST MON DEC 7 2009
.DISCUSSION...
THE FORECAST CHALLENGES CENTER ON THE FIRST WINTER STORM OF THE
SEASON WITH WARNINGS AND ADVISORIES NOW IN EFFECT FOR MUCH OF THE
FORECAST AREA.
NUMERICAL MODELS CONTINUE IN SURPRISINGLY GOOD AGREEMENT WITH THIS
SYSTEM...ESPECIALLY CONSIDERING ITS DEPTH AND INTENSIFICATION
FORECAST DURING TUESDAY NIGHT AND WEDNESDAY. THIS SYSTEM IS UNDER
THE CLASSIFICATION OF A PANHANDLE HOOKER...WHICH CAN OFTEN BRING
6 PLUS INCHES OF SNOW TO THE UPPER MIDWEST. COMPARING THE SFC
LOW AND 850MB LOW TRACKS TO THE IDEAL TRACK FOR KMSP...THIS SYSTEM
IS A BIT FURTHER SOUTH. HENCE...FAR SRN MN...SERN MN...AND WI
REMAIN HEAVILY FAVORED FOR SNOWFALL AMOUNTS IN EXCESS OF SIX
INCHES. THE SNOW AMOUNTS IN EXCESS OF 6 INCHES ARE ALSO FAVORED
SLIGHTLY NORTH TOWARDS KMSP BECAUSE OF INTERACTION WITH A NRN
STREAM JET MAX AND TROUGH. WITH THE TRACK OF THE PRIMARY SYSTEM
BEING FROM SE KS TO NEAR CHICAGO AND THE COLD AIR MASS IN
PLACE...MEDIUM TO HIGH SNOW TO LIQUID RATIOS ARE ALSO FAIRLY
CONFIDENT. FOR QPF HAVE BLENDED CURRENT FORECAST WITH HPC QPF AND
INCREASED IN AREAS WHERE BANDED POTENTIAL IS LIKELY. THE GREATEST
SNOWFALL RATES APPEAR TO BE PRIMARILY DURING TUE NIGHT AND WED
MORNING. WITH THE CONSISTENT PRESSURE PATTERN BETWEEN THE
MODELS...THE DERIVED SURFACE WINDS ALSO ARE VERY SIMILAR FROM
MODEL TO MODEL. THE STRONGEST WINDS IN THE FORECAST AREA ARE
FOCUSED OVER THE MINNESOTA RIVER VALLEY AND SOUTH CENTRAL
MINNESOTA. OVERALL CHANGES ARE MINIMAL...JUST INCREASED SNOW
AMOUNTS AND POPS AND TRIED TO FOCUS WHERE SNOW MAY BE HEAVY AT
TIMES.
THE STRONG SYSTEM IS TAKING SHAPE OVER THE SW CONUS. THIS WAS
BASICALLY HALF ON-SHORE TODAY FOR 12Z RAOB ANALYSIS...WITH 00Z
DATA LIKELY TO CONTAIN THE FULL MID-LEVEL CIRCULATION.
STILL...MODEL CONSISTENCY FOR A COUPLE DAYS WOULD POINT AWAY FROM
A SIGNIFICANT MODEL SHIFT TONIGHT. ISENTROPIC ASCENT BEGINS ALOFT
LATE TONIGHT AND INCREASES THROUGH THE DAY TUE IN THE WARM AIR
ADVECTION PATTERN AHEAD OF THE EVOLVING SYSTEM. SATURATION WILL
INCREASE RAPIDLY DURING TUE AFTN AS MOISTURE TRANSPORT INCREASES.
THIS IS DRAWN IN BY RIGHT ENTRANCE REGION OF JET BRANCH OVER
SOUTHERN CANADA. SLIGHT COUPLED STRUCTURE EXISTS THROUGH TUE NIGHT
KEEPING SNOW CONSTANT WITH NO BREAKS EXPECTED. EXPECT THE SNOWFALL
TO BE MAINLY STEADY FROM NEAR NOON THROUGH MIDNIGHT WITH PERIODIC
W TO E BANDS OF HEAVIER SNOW WHERE WARM FRONTAL F-GEN WILL
INEVITABLY DEVELOP. BUT THESE SHOULD MOVE QUICKLY NORTH WHERE THEY
DEVELOP. SNOW RATIOS WILL START HIGH WITH THE COLD AIR IN PLACE.
AMOUNTS OF 1 TO 2 INCHES ARE IN THE FORECAST BY 6 PM. AS THE
SFC LOW MOVES EAST ACROSS MO TUE EVENING...DRAWN IN MIXING
RATIOS WILL INCREASE TO 1.5 TO 2 G/KG IN SE MN AND WRN WI. WHILE
RATIOS WILL BE AT THEIR LOWEST AT THIS TIME...12 TO 1 TO 15 TO
1...SNOWFALL AMOUNTS SHOULD STILL ACCORDINGLY INCREASE BECAUSE OF
MOISTURE AND FORCING. AS THE UPPER TROUGH BECOMES NEGATIVELY
TILTED OVERNIGHT TUE NIGHT AND WED MORNING...AND THE SFC LOW
DEEPENS NEAR 15 MB/12 HOURS...THE DEFORMATION AND THERMAL/MOISTURE
CONVERGENCE WILL BECOME FOCUSED OVER ERN MN AND WRN AND CTRL WI.
THIS IS WHERE SLOW MOVING BANDED SNOW IS LIKELY. THE OMEGA ALSO
DECREASES IN HEIGHT TO NEAR -15C ALOFT...WHICH WILL BE FAVORABLE FOR
LARGER DENDRITIC SNOW FLAKES. ENSEMBLE PROBABILITIES FOR HEAVIER
RATES ALSO INCREASE OVER SERN MN AND WRN WI. THE BIG QUESTION IS
WHETHER OUR MOISTURE FEED WILL DROP TOO QUICKLY BY WED MORNING AS
CONVECTION IN THE WARM SECTOR DRIVES THE LLJ EASTWARD TOWARDS THE
EAST COAST. BUT AT THIS TIME...BELIEVE THE STRENGTH OF THE SYSTEM
WILL CONTINUE TO YIELD MODERATE SNOW IN THE ERN FORECAST AREA.
SNOW SHOULD WIND DOWN QUICKLY BY WED AFTERNOON ACROSS MN AND WED
EVE IN WI. TOTAL SNOWFALL AMOUNTS IN EXCESS OF A HALF A FOOT ARE
EXPECTED IN ERN AND SRN MN AND WRN WI. FROM ALBERT LEA TO THE ERN
TWIN CITIES METRO AND ALL OF WRN WI...SCATTERED AMOUNTS OF 9
INCHES TO ONE FOOT ARE BECOMING MORE FAVORED. THESE AMOUNTS MATCH
UP WELL WITH EMPIRICAL TECHNIQUES SUCH AS THE GARCIA METHOD DURING
THE WARM AIR ADVECTION PATTERN OF THIS SYSTEM AND THE COOK METHOD
FOR THE UPPER LEVEL WARMING DURING THE DURATION OF THIS SYSTEM.
THE RAPID DEEPENING OF THIS SYSTEM TO OUR SE WILL LEAD TO A FAIRLY
RAPID INCREASE IN WINDS DURING OVERNIGHT TUE NIGHT AND CONTINUING
THROUGH WED AND WED EVE. THE COLD ADVECTION PATTERN WILL STEEPEN
THE LOW LEVEL LAPSE RATES AND ALLOW MIXING OF GUSTS TO 35 TO 45
MPH IN THE MN RIVER VALLEY TO SOUTH CENTRAL MN. WITH THE HIGH
RATIO SNOW...WIDESPREAD BLOWING WITH LOW VISIBILITIES ARE
EXPECTED IN THAT AREA. OTHER OPEN AREAS THROUGHOUT THE FORECAST
AREA WILL SEE SOME BLOWING SNOW AS WELL. GIVEN THE CONFIDENCE AND
A LEAN TOWARDS IT BEING THE FIRST APPRECIABLE EVENT...HAVE
UPGRADED TO BLIZZARD WARNING IN SOUTH CENTRAL MN. BLIZZARD
CONDITIONS AGAIN ARE NOT EXPECTED TO DEVELOP UNTIL MIDNIGHT TUE
NIGHT...BUT THE ACCUMULATING SNOW WILL BEGIN BY NOON TUE.
THE LONG TERM PORTION OF THE FORECAST (FRIDAY THROUGH MONDAY)
FEATURES A SURGE OF VERY COLD ARCTIC AIR PUSHING ACROSS THE FA
DURING THE WEEKEND. THIS IS IN RESPONSE TO THE POLAR VORTEX DROPPING
SOUTH INTO SASKATCHEWAN AND MANITOBA. SOME MODERATION IS HINTED AT
ON MONDAY AS THE UPPER LOW MOVES OFF TOWARD QUEBEC. THE TREND TODAY
WAS TO STAY WITH LOW/HIGH TEMPERATURES WELL BELOW HPC GUIDANCE.
READINGS USED WERE MUCH CLOSER TO THE MEX GUIDANCE. THEREFORE THE
PERIOD IS QUITE COLD AND DRY. AN AREA OF SURFACE LOW PRESSURE IS
PROJECTED OVER THE CENTRAL PLAINS EARLY ON MONDAY AS A NEW SHORT
WAVE EMANATES FROM THE SOUTHWESTERN UNITED STATES. PRESENT
INDICATIONS ARE FOR LIGHT SNOW TO BE JUST SOUTH OF THE FA FROM
MONDAY INTO TUESDAY.
&&
.AVIATION.../18Z TAF ISSUANCE/
WK SFC BNDRY ACRS SW MN HELPING TO MAINTAIN MVFR CIGS AND SCT
-SN...BUT THIS FEATURE WL WKN AS HI PRES NOSES IN THIS AFTN AND ERLY
EVE. THUS DO NOT XPCT MUCH EXPANSION OF THESE CLDS BEYOND RWF AND
AXN. MAIN WX FEATURE WL BE STG LOW PRESSURE LIFTING OUT OF OK
PANHANDLE ON TUESDAY. ISENTROPIC LIFT AHD OF THIS FEATURE WL SPRED
-SN INTO MOST TAFS BFR 18Z TUES..BUT SIG SNOW NOT UNTIL AFT TAF
PRD.
&&
.MPX WATCHES/WARNINGS/ADVISORIES...
MN...WINTER STORM WARNING FROM NOON TUESDAY 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 TUESDAY TO 6 PM CST WEDNESDAY
FOR BENTON-ISANTI-KANABEC-KANDIYOHI-MCLEOD-MEEKER-MILLE
LACS-REDWOOD-RENVILLE-SHERBURNE-STEARNS-WRIGHT.
BLIZZARD WARNING FROM NOON TUESDAY TO MIDNIGHT CST WEDNESDAY
NIGHT FOR BLUE EARTH-FARIBAULT-FREEBORN-MARTIN-STEELE-
WASECA-WATONWAN.
WI...WINTER STORM WARNING FROM NOON TUESDAY TO MIDNIGHT CST WEDNESDAY
NIGHT FOR BARRON-CHIPPEWA-DUNN-EAU CLAIRE-PEPIN-PIERCE-POLK-
RUSK-ST. CROIX.
&&
$$
