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000 FXUS63 KMPX 250559 AFDMPX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN 1159 PM CST TUE NOV 24 2009 .AVIATION... /06Z TAF ISSUANCE/ AN INVERTED TROUGH OVER FAR WESTERN WI WILL BE THE FOCUS FOR IFR/LOW MVFR CONDITIONS AT KRNH AND KEAU THROUGH MUCH OF THE UPCOMING TAF PERIOD. DRIZZLE WILL BE COMMON THROUGH THE MORNING THEN MORE OF A STEADY RAIN WILL DEVELOP AS A STRONG LOW PRESSURE SYSTEM ALOFT APPROACHES FROM NORTH DAKOTA. PROFILE DATA INDICATES THE RAIN SHOULD MIX WITH SNOW TOWARD EVENING WITH A PERIOD OF LIGHT SNOW BEFORE IT ENDS WEDNESDAY NIGHT. IFR/LIFR CONDITIONS ACROSS EASTERN MN...INCLUDING KMSP...SHOULD IMPROVE SOME OVERNIGHT AS DRIER AIR WORKS IN BEHIND THE INVERTED SURFACE TROUGH TO THE EAST. SOME POTENTIAL THAT KMSP COULD BE VFR BETWEEN 12Z AND 15Z BEFORE THE NEXT UPSTREAM SYSTEM BRINGS A LOWERING OF CEILINGS AND VISIBILITIES. LIGHT RAIN SHOULD DEVELOP AT KMSP BY MID MORNING AND PERSIST THROUGH THE AFTERNOON. AS COLDER AIR FILTERS IN...THE RAIN SHOULD MIX WITH OR CHANGE TO SNOW LATE IN THE AFTERNOON AND LAST INTO THE EARLY EVENING BEFORE ENDING. KSTC IS CURRENTLY ON THE VERGE OF CLEARING. HOWEVER...POTENTIAL IS HIGH FOR IFR/LIFR IN FOG WITH VERY MOIST LOW LEVEL CONDITIONS IN PLACE. CLEARING WILL NOT LAST LONG AS MID LEVEL MOISTURE SPREADS IN BEFORE DAYBREAK WITH LIGHT SNOW LIKELY DURING THE MORNING AS A STRONG LOW PRESSURE SYSTEM ALOFT PASSES BY. IMPROVING CONDITIONS FOR KSTC DURING THE LATE AFTERNOON ALONG WITH NORTHWEST WINDS AT 15 TO 25 KNOTS. ALTHOUGH KAXN AND KRWF ARE CURRENTLY VFR...THEY ARE ALREADY SEEING MID/HIGH LEVEL MOISTURE INCREASE AHEAD OF THE UPPER LOW OVER NORTH DAKOTA. LIGHT SNOW SHOULD DEVELOP/SPREAD INTO BOTH SITES BETWEEN 09Z AND 12Z AND THEN CONTINUE DURING THE MORNING WITH IFR/LOW MVFR CONDITIONS OCCURRING. NORTHWEST WINDS WILL BE ON THE INCREASE BY 17Z WITH 16-18 KNOTS SUSTAINED GUSTING TO 30 KNOTS OR SO FOR THE AFTERNOON. ONLY FLURRIES EXPECTED FOR THE AFTERNOON WITH CEILINGS LIFTING TO NEAR 3000 FEET. && .DISCUSSION... /ISSUED 138 PM CST TUE NOV 24 2009/ ALTHOUGH CURRENT TRENDS INDICATE A STRONG AND DEEPENING AREA OF LOW PRESSURE ACROSS THE GREAT LAKES IN THE NEXT 24/48 HRS...THERE REMAINS SOME SUBTLE DIFFERENCES IN THE THERMAL PROFILE THAT WILL HIGHLY INFLUENCE SNOWFALL AMTS AS THE -RA/-DZ CHANGES OVER TO MAINLY -SN LATE TONIGHT...AND INTO WEDNESDAY. CURRENT RADAR AND SATELLITE TRENDS INDICATED A PERSISTENT AREA OF -RA/-DZ ACROSS MOSTLY THE EASTERN 1/2 OF CWA. SFC LOW AND ASSOCIATED AREA OF LIFT THAT HAS CONTRIBUTED TO AMPLE CONVECTIVE ALONG AND AHEAD OF A FRONTAL BOUNDARY ACROSS EASTERN MO...THERE REMAINS A WEAK AREA OF CONVERGENCE ACROSS EASTERN MN WHICH WILL WEAKEN OVERNIGHT AS THE SFC REFLECTION MOVES EASTWARD ACROSS IL...AND INTO THE SOUTHERN GREAT LAKES. A PERIOD OF SUBSIDENCE IN THE WAKE OF THIS SYSTEM...AND THE NEXT SYSTEM ALREADY DIVING SOUTHEAST ACROSS THE NORTHERN ROCKIES AND NORTHERN PLAINS...WILL CONTRIBUTE TO MAINLY CLOUDY SKIES IN THE WESTERN 1/2 OF CWA. CURRENT SFC ANALYSIS INDICATED -SN ALREADY DEVELOPING ACROSS NORTHERN MONTANA...WITH BRISK N/NW WINDS. USUALLY THESE TYPE OF SYSTEMS (ALBERTA CLIPPER) ARE FAST AND PRODUCE TWO TO FOUR INCHES OF -SN ALONG AND JUST NORTH OF THE SFC LOW. HOWEVER...THERE ARE TWO PROBLEMS WITH THIS AMT OF SNOWFALL EXPECTED BY WEDNESDAY NIGHT. FIRST...IT HAS BEEN EXTREMELY WARM FOR THIS TIME OF YEAR IN TERMS OF LOW LEVEL THERMAL FIELDS. SECONDLY...THE SFC REFLECTION IS WEAK AND THIS SYSTEM IS MORE ASSOCIATED WITH THE 70H/50H TROUGH AXIS. THEREFORE...DUE TO THESE CONDITIONS...SNOWFALL AMOUNTS WILL BE LIGHT AND UNDER TWO INCHES. THE ONLY CHANGE IN THE SNOWFALL AMTS WILL BE ASSOCIATED WITH CONVECTIVE INSTABILITY ALOFT WHICH IS NOT FORECAST BY ANY SHORT TERM MODEL AT THIS TIME. PLUS...THERE REMAINS TOO MANY MODEL DIFFERENCES WITH THE DEPTH OF COLD AIR. ONE ELEMENT THAT IS IN OUR FAVOR FOR THE DEVELOPMENT OF HEAVIER SNOWFALL RATES IS THE JET DYNAMICS...AND HOW THE NOSE OF THE PVU (1.5 OR GREATER)...AND ASSOCIATED DRY SLOT WILL ENHANCE LARGE SCALE LIFT. CURRENTLY THE GFS/EC HAS EASTERN MN FAVOR FOR THIS TYPE OF SCENARIO...WITH THE NAM/WRF FAVORING WEST-CENTRAL WISCONSIN. DUE TO THE SPEED OF THIS SYSTEM...MOST OF THE AREA WILL BE PRECIPITATION FREE BY LATE WEDNESDAY NIGHT...WITH THURSDAY/FRIDAY DRY. THE EXTENDED PERIOD FROM SATURDAY THRU NEXT TUESDAY REMAINS FAVORABLE FOR A LONG WAVE TROUGH DEVELOPING ACROSS THE CENTRAL U.S. OVER THE WEEKEND/EARLY NEXT WEEK. AS PER THE LATEST GFS/EC ...THE SOUTHERN JET WILL BE MORE ACTIVE...ALLOWING FOR ONLY SUBTLE/WEAK WAVES AFFECTING OUR REGION. HOWEVER...IT SHOULD BE COOL ENOUGH FOR MOSTLY SNOW...OR AT LEAST A MIXTURE DEPENDING UPON THE EXACT TRACK OF THE SFC REFLECTION OF THE WAVES. ..JLT.. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...NONE. WI...NONE. && $$ RAH/JLT
