FXUS63 KMPX 251219
AFDMPX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
619 AM CST FRI DEC 25 2009
.UPDATE...FOR 12Z AVIATION DISCUSSION BELOW
&&
.DISCUSSION.../ISSUED 402 AM CST FRI DEC 25 2009/
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.../12Z TAF ISSUANCE/
PRECIPITATION TYPE CONTINUES TO BE THE MAIN CONCERN THIS TAF
PERIOD. WARM AIR INTRUSION CIRCA 5K FEET HAS WORKED INTO WEST
CENTRAL WI THIS MORNING. RESULTANT PRECIP TYPE AT KEAU AND KRNH
WAS RAIN OVERNIGHT. EXPECT PREDOMINATELY RAIN AT KEAU THIS
MORNING...WHILE KMSP AND KRNH FLUCTUATE BETWEEN SNOW/SLEET.
PROGGED SOUNDINGS INDICATE SUFFICIENT COOLING IN THE LOW/MID
LEVELS TO CHANGE PRECIP TYPE TO PREDOMINATELY SNOW OVER ERN MN BY
18Z...AND ACROSS WEST CENTRAL WI BY 00Z SAT. DO NOT ANTICIPATE
IMPROVEMENT IN CIGS...AND EXPECT LOW-END MVFR CIGS TO PREVAIL.
VSBYS AT MN SITES ARE GENERALLY EXPECTED TO RANGE FROM 1SM TO 3SM
TODAY...WHILE RAIN IN WEST CENTRAL WI YIELDS HIGHER VSBYS IN THE
VFR CAT AT KEAU. WINDS WILL BE NORTH/NORTHWEST AT WESTERN SITES
/KAXN-KRWF/...WHILE THE REMAINDER OF THE SITES ARE EAST/NORTHEAST.
GUSTS INTO THE LOW/MID 20S /KNOTS/ WILL ALSO PERSIST.
&&
.MPX WATCHES/WARNINGS/ADVISORIES...
MN...WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR ANOKA-CARVER-
CHISAGO-DAKOTA-FREEBORN-HENNEPIN-ISANTI-LE SUEUR-RAMSEY-
RICE-SCOTT-STEELE-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 GOODHUE.
WI...WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR BARRON-POLK-
ST. CROIX.
WINTER WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR CHIPPEWA-
DUNN-EAU CLAIRE-PEPIN-PIERCE-RUSK.
&&
$$
LS/MTF
