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000 FXUS63 KMPX 250001 AAA AFDMPX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN 655 PM CST TUE NOV 24 2009 .UPDATED FOR AVIATION DISCUSSION BELOW... .DISCUSSION... 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.. && .AVIATION... /06Z TAF ISSUANCE/ SFC TROUGH OVER EASTERN MN WILL REMAIN QUASISTATIONARY DURING THE FCST PERIOD AS UPPER LOW MOVES SLOWLY TO THE EAST. WEAK FORCING INTO SOUTHEAST MN CONTINUES GENERATING SOME LIGHT RAIN/DRIZZLE OVER EC MN INTO SRN PORTION OF THE FA. SHOULD AFFECT MAINLY KMSP THIS EVENING...BEFORE GENERATING FARTHER EAST INTO WI LATE TONIGHT AND WED. CIGS/VSBYS SHOULD REMAIN LOW OVER THE CENTRAL FA INTO THE NIGHT WITH SOME DRYING NOTED OVER THE WESTERN AREAS...AS SECONDARY WAVE DROPS SOUTHEAST. TIMING OF THE WAVE AND SFC CDFNT BRINGS SYSTEM INTO THE WESTERN FA AFTER 07Z...AND AFFECTING THE KAXN/KRWF/KSTC REGION THROUGH 12Z WED. SHOULD SEE MAINLY SNOW OVER THE FAR WEST WITH A PERIOD OF -RASN OVER CENTRAL PORTIONS OF THE FCST AREA THROUGH 18Z WED. THEN WARMER LOW LEVEL AIR DOES ROTATE OVER THE EASTERN FA AND SHOULD KEEP MAINLY LIQUID HERE THROUGH 00Z THU. GFS INDICATING SOME ISENTROPIC LIFT OVER EC MN INTO THE AFTERNOON WITH THE ANOTHER SHOT OF FORCING OVER THE SOUTHERN FA. BEST PV FORCING OVER THE MN RIVER VALLEY TO SOUTHEAST MN THROUGH EARLY WED EVENING. TIMING OF PCPN OVER TO SNOW CRITICAL IN FCST OF ACCUMS. SFC TEMPS SHOULD START OFF ON THE WARM SIDE HOWEVER...LIMITING ACCUM SOMEWHAT. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...NONE. WI...NONE. && $$ JLT/DWE
