Issued by NWS Detroit/Pontiac, MI
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000 FXUS63 KDTX 202340 AFDDTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI 640 PM EST FRI NOV 20 2009 .AVIATION... SUSTAINED AREA OF LOW LEVEL MOISTURE TRAPPED BENEATH A STRONG INVERSION WILL MAINTAIN A PERSISTENT STRATUS DECK THROUGHOUT THE TAF PERIOD. CONDITIONS PREDOMINATELY IN MVFR THIS EVENING /IFR AT PTK/ WITH A TREND TOWARD IFR AT ALL TERMINALS OVERNIGHT AS THE INVERSION HEIGHT LOWERS. A DIMINISHING WIND FIELD WOULD ALSO SUGGEST SOME POTENTIAL FOR FOG OVERNIGHT...BUT THE BKN/OVC CANOPY OF STRATUS SHOULD LIMIT THE VISIBILITY RESTRICTIONS TO MAINLY MVFR/BRIEF IFR. CEILINGS WILL SLOWLY RISE IN RESPONSE TO WEAK DAYTIME MIXING ON SATURDAY...BUT PLENTY OF LOW LEVEL MOISTURE WILL ENSURE MVFR STRATUS HOLDS THROUGH THE AFTERNOON. && .PREV DISCUSSION...ISSUED 313 PM EST FRI NOV 20 2009 SHORT TERM...TONIGHT LOW LEVEL MOISTURE (AIDED BY THE WESTERLY FLOW OFF LAKE MICHIGAN) LOOKS TO REMAIN TRAPPED OVER SOUTHEAST MICHIGAN...MAINTAINING CLOUDY SKIES THROUGH THE NIGHT...PREVENTING MUCH OF A DIURNAL SPREAD IN TEMPERATURES...AND WILL MAINTAIN THE MIN TEMPS IN THE LOWER 40S/AROUND 40...IN LINE WITH LATEST MAV GUIDANCE. 500 MB SHORTWAVE RIDGING THIS AFTERNOON IN PLACE OVER THE CENTRAL GREAT LAKES WILL BE REPLACED BY THE BROAD TROUGHING CURRENTLY OVER THE WESTERN GREAT LAKES WITH THE MAIN UPPER WAVE OVER IOWA TRACKING ALONG THE SOUTHERN MICHIGAN BORDER. EVEN SO...BOTH NAM/GFS INDICATING ONLY MODEST LIFT AND A FAILURE TO SATURATE MUCH ABOVE 850/800 MB...AND WILL THUS MAINTAIN A DRY FORECAST WITH THE LACK OF CLOUD DEPTH (AFTER AFTERNOON SPRINKLES/DRIZZLE CONTINUES TO COME TO AN END)...BUT WOULD NOT BE SHOCKED IF A FEW SPRINKLES REDEVELOP OVERNIGHT. DID ADD SOME PATCHY FOG TO THE ZONES HOWEVER AND CONCERNED ABOUT THE LOW CLOUD CLEARING TREND OVER WESTERN LOWER MICHIGAN AT THE PRESENT TIME. IF THIS WORKS EAST...WE CAN EXPECT A BIT MORE WIDESPREAD FOG AND COLDER TEMPERATURES OVERNIGHT. SUSPECT ONCE THE SUN SETS THE LOW STRATUS WILL BUILD BACK TO THE WEST A BIT. LONG TERM...SATURDAY THROUGH NEXT FRIDAY A MID LEVEL WAVE NOW SHOWN OVER THE UPPER MISSISSIPPI VALLEY WILL TRACK ACROSS THE STATE ON SATURDAY. LIFT ASSOCIATED WITH THIS WAVE IS NOT EXPECTED TO BE VERY STRONG. THE 12Z UPPER AIR ANALYSIS DID SHOW AN AREA OF HIGHER MID LEVEL MOISTURE ACROSS WISCONSIN...WHICH IS FORECAST TO SLIDE ACROSS LOWER MI IN ADVANCE OF THE UPPER WAVE ON SATURDAY. MODEL SOUNDINGS INDICATE RESIDUAL LOW LEVEL MOISTURE ALONG WITH THE MID LEVEL MOISTURE WILL RESULT IN A FAIRLY CLOUDY START TO THE DAY SATURDAY. MID LEVEL RIDGING ADVANCING INTO THE WESTERN GREAT LAKES SAT AFTERNOON IN THE WAKE OF THE SHORT WAVE WILL HELP SCOUR OUT THE MID LEVEL MOISTURE LATE IN THE DAY. SHALLOW WARM AIR ADVECTION WILL ALSO HELP WEAKEN THE INVERSION. THESE FACTORS MAY ALLOW FOR SOME BREAKS IN THE CLOUD COVER /OR AT LEAST SOME THINNING OF THE CLOUDS/ LATE IN THE DAY. THIS ALONG WITH THE MILD LOW LEVEL AIRMASS LINGERING OVER THE REGION SHOULD BE ABLE TO BOOST MAX TEMPS INTO THE 50S ON SATURDAY. THE 12Z MODEL SUITE WERE WELL INITIALIZED WITH THE UPPER WAVE NOW OVER TEXAS AND CONTINUE TO SHEAR THIS WAVE APART AS IT TRACKS ACROSS THE EASTERN US LATE SUN INTO MON. THIS WILL LEAVE WEAK HIGH PRESSURE IN PLACE ACROSS THE EASTERN GREAT LAKES. GIVEN THE RELATIVE HIGH LOW LEVEL MOISTURE ACROSS THE REGION /SURFACE DEWPOINTS FROM THE UPPER 30S TO LOWER 40S/...FOG AND/OR LOW CLOUDS MAY BE PROBLEMATIC SAT NIGHT/SUN MORNING AND AGAIN SUN NIGHT/MON MORNING. A LIGHT EASTERLY FLOW IS ALSO FORECAST TO DEVELOP...WITH MOISTURE FLUX OFF LAKE ERIE ENHANCING THE FOG/LOW CLOUD POTENTIAL. THE PREVIOUS FORECAST SHIFT ADDED A MENTION OF FOG FOR THESE TIME PERIODS AND THE LATEST MODEL GUIDANCE REMAINS SUPPORTIVE OF THIS. TEMPERATURES WILL REMAIN ON THE MILD SIDE THROUGH THE REST OF THE WEEKEND INTO EARLY NEXT WEEK. SINCE ANY MORNING FOG OR LOW CLOUDS WILL LIKELY TAKE INTO THE AFTERNOON TO MIX OUT GIVEN THE LOW SUN ANGLE...HAVE KEPT MAX TEMPS ON THE CONSERVATIVE SIDE /NEAR 50/ SUN AND MON. THE 12Z MODEL SUITE CONTINUE TO SHOW DISCREPANCIES WITH RESPECT TO TIMING AND AMPLITUDE OF A MID LEVEL WAVE FORECAST TO ADVANCE INTO THE PACIFIC NORTHWEST TOWARD THE END OF THE WEEKEND...THEN INTO THE CENTRAL US EARLY NEXT WEEK. THE 12Z ECMWF/CANADIAN/UKMET ALL SHOW MUCH MORE AMPLIFICATION WITH THIS WAVE AS IT TRAVERSES THE ROCKIES ON MONDAY IN COMPARISON TO THE GFS. THE MORE AMPLIFIED SOLUTIONS THEN CUT THE UPPER LOW OFF OVER THE CENTRAL PLAINS TUES INTO WED...WHILE THE GFS CUTS THE SYSTEM OFF OVER THE WESTERN GREAT LAKES DURING THIS TIME PERIOD. WITH THE SUPPORT OF THE CANADIAN AND UKMET AND GIVEN THE STILL LARGE SPREAD AMONG GFS ENSEMBLE MEMBERS /A FEW OF WHICH ARE IN LINE WITH THE ECMWF/...WILL LEAN THE FORECAST TOWARD THE SLOWER AND MORE AMPLIFIED ECMWF. THIS WILL SUPPORT CONTINUED DRY AND MILD WEATHER THROUGH TUESDAY...WITH INCREASING CHANCES OF RAIN BY WED/WED NIGHT. THE ECMWF DOES CONTINUE TO INDICATE SOME PHASING OF NORTHERN STREAM ENERGY BY THURS...BRINGING A MUCH COLDER AIRMASS INTO THE GREAT LAKES /WARRANTING THE CHANCE OF SNOW THURS AND FRI/. MARINE... WESTERLY WINDS OF 10 TO 20 KNOTS WILL CONTINUE TO DIMINISH THIS EVENING AND TONIGHT...BECOMING LIGHT SATURDAY AND SUNDAY AS HIGH PRESSURE BECOMES FULLY ESTABLISHED OVER THE CENTRAL GREAT LAKES DURING THE WEEKEND. THIS HIGH WILL SLOWLY RETREAT EARLY NEXT WEEK...BUT SURFACE RIDGING WILL PERSIST ON MONDAY...MAINTAINING LIGHT WINDS FOR AT LEAST ANOTHER DAY. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. LAKE HURON...NONE. LAKE ST CLAIR...NONE. MI WATERS OF LAKE ERIE...NONE. && $$ AVIATION.....MR SHORT TERM...SF LONG TERM....SC MARINE.......SF YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).
