FXUS63 KDTX 201105 AFDDTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI 705 AM EDT SAT SEP 20 2008 .AVIATION... MVFR AND OCCASIONAL IFR VISIBILITIES IN FOG HAVE BEEN IMPACTING THE DETROIT TERMINALS THIS MORNING. A LITTLE STRONGER GRADIENT HAS BEEN ABLE TO KEEP VSBY IN VFR RANGE AT FNT AND MBS. MODEL SOUNDINGS SUGGEST THE FOG IS VERY SHALLOW AND THUS WOULD EXPECT VISIBILITIES TO RETURN TO VFR WITHIN THE FIRST HOUR OR TWO OF TAF ISSUANCE. A COLD FRONT IS EXPECTED TO REACH MBS AROUND 00Z...THEN MOVE ACROSS THE DETROIT TERMINALS AROUND 06Z. THE MAIN IMPACT OF THIS FRONT WILL JUST BE A SHIFT IN THE WINDS FROM WEST-SOUTHWEST TO NORTH-NORTHEAST. BOTH NAM AND GFS MODEL SOUNDINGS STRONGLY SUGGEST AN INCREASE IN LOW LEVEL MOISTURE AS SHALLOW COLD AIR ADVECTION COMMENCES IN THE WAKE OF THE SURFACE FRONT. THUS WILL INTRODUCE SOME IFR POST FRONTAL STRATUS OVERNIGHT. && .PREV DISCUSSION...ISSUED 351 AM EDT SAT SEP 20 2008 SHORT TERM...TODAY OVERALL...THE 00Z NCEP SUITE IS IN GOOD AGREEMENT WITH RESPECT TO THE TIMING AND STRENGTH OF THE COLD FRONT CURRENTLY DRAPED ACROSS THE UPPER MIDWEST AND NORTHERN GREAT LAKES. THIS FRONT IS IN ASSOCIATION WITH A RATHER DEEP UPPER TROUGH AND SURFACE LOW OVER HUDSON BAY. THE FRONT IS FORECAST TO DROP INTO THE SAGINAW VALLEY AND THUMB REGION BY 00Z THIS EVENING. CONVERGENCE ALONG THE FRONT HOWEVER IS EXPECTED TO WEAKEN AS THE FLOW ALOFT BECOMES WEAKER AND ORIENTED MORE PARALLEL TO THE FRONTAL BOUNDARY. A NARROW RIBBON OF HIGH 850MB THETA E VALUES WILL ACCOMPANY FROPA. SURFACE BASED CAPE IS ALSO EXPECTED TO REACH 1200 J/KG BY LATE AFTERNOON. GIVEN THESE FACTORS...WILL CONTINUE TO MENTION THE POSSIBILITY OF ISOLATED CONVECTION ACROSS THE TRI CITIES AND NORTHERN THUMB AFTER 21Z. THE LIMITING FACTOR FOR MORE WIDESPREAD CONVECTION HOWEVER WILL BE THE LACK OF LARGE SCALE FORCING...WHICH WILL MAKE IT DIFFICULT TO OVERCOME AN 800-700MB CAPPING INVERSION. THE REST OF THE FORECAST AREA WILL BE TOO FAR REMOVED FROM THE FRONT TO WARRANT AFTERNOON POPS. A BROAD UPPER LOW WOBBLING OVER MISSOURI CONTINUES TO ADVECT QUITE A BIT OF HIGH LEVEL MOISTURE INTO SOUTHERN MI...WHICH WILL AGAIN RESULT IN A LOT OF CIRRUS DURING THE DAY. DESPITE THIS...LOW LEVEL WARM AIR ADVECTION WITHIN STRENGTHENING WEST-SOUTHWESTERLY LOW LEVEL FLOW AHEAD OF THE SURFACE FRONT SHOULD STILL BE ABLE TO BOOST DAYTIME HIGHS TO AROUND 80. LONG TERM...SATURDAY NIGHT THROUGH NEXT FRIDAY. WILL CONTINUE TO CARRY A LOW CHANCE OF THUNDERSTORMS FOR THIS EVENING ACROSS THE NORTHERN COUNTIES TO ABOUT PORT HURON WITH THE BETTER FRONTAL FORCING PASSING TO OUR NORTHEAST. DESPITE SATELLITE IMAGERY SHOWING LOW LEVEL MOISTURE PUSHING NORTH INTO SOUTHERN ILLINOIS...THIS MOISTURE IS NOT EXPECTED TO REACH SOUTHEAST MICHIGAN WITH A LOW LEVEL RIDGE AXIS HOLDING FIRM OVER THE SOUTHERN LAKES REGION. 850-500MB RH STRUGGLES TO INCREASE FROM ABOUT 40 PERCENT THIS AFTERNOON TO BARELY 60 PERCENT ACROSS OUR SOUTHERN COUNTIES AS THE FRONT ARRIVES. GIVEN THE LIMITED MOISTURE AND DECREASING INSTABILITY HAVE REMOVED THE SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS ALONG AND SOUTH OF M-59...LEAVING ONLY A CHANCE OF SPRINKLES LATE TONIGHT. MODELS HAVE TRENDED SLIGHTLY WARMER BEHIND THE FRONT ON SUNDAY WITH 850MB TEMPS ONLY BOTTOMING OUT AROUND 8C OVER THE THUMB. HIGHS IN THE LOW 60S ARE ONLY EXPECTED FOR AREAS IMMEDIATELY ADJACENT TO LAKE HURON UNDER NORTHEASTERLY FLOW WHILE INLAND AREAS WILL RANGE FROM THE UPPER 60S NORTH TO LOW 70S SOUTH. A COOL NIGHT IS STILL EXPECTED ON SUNDAY NIGHT WITH SOME OF THE COOLER LOCATIONS ACROSS THE NORTH DROPPING BELOW 40 DEGREES. HIGH PRESSURE WILL THEN DOMINATE THE WEATHER PATTERN FOR MUCH OF THE WEEK WITH WINDS GRADUALLY VEERING FROM EASTERLY TO SOUTHERLY. THE 00Z GFS CONTINUES TO BE A LITTLE MORE PROGRESSIVE WITH THE LEAD TROUGH MOVING THROUGH THE UPPER MIDWEST BY WEDNESDAY. ALTHOUGH THE 00Z/20 ECMWF TRENDED IN THIS DIRECTION...PREFER THE 12Z/19 UKMET/ECMWF AND 00Z/20 GEM WHICH POINT TOWARDS A STRONGER UPPER LOW SETTING UP OVER CENTRAL CANADA. THIS SOLUTION ALLOWS THE RIDGE TO REMAIN OVER MICHIGAN THROUGH MOST OF THE WEEK WITH ADDITIONAL SHORTWAVES PASSING TO OUR NORTH. GIVEN THE PREFERRED DEEPER/SLOWER SOLUTION...WILL LEAVE THE FORECAST DRY UNTIL THURSDAY NIGHT/FRIDAY. MARINE... RECENT OBSERVATIONS INDICATE SUSTAINED WINDS OF 10 TO 15 KNOTS ACROSS MUCH OF LAKE HURON WITHIN A SOUTHWESTERLY GRADIENT IN ADVANCE OF A COLD FRONT OVER LAKE SUPERIOR. GUSTS HAVE BEEN GENERALLY IN THE 20 TO 25 KNOT RANGE AS THE AIRMASS IS WARM ENOUGH TO PROMOTE LIMITED MIXING OF STRONGER WINDS OVER THE LAKE. THE FRONT WILL MOVE ACROSS LAKE HURON THIS AFTERNOON...THEN ACROSS SOUTHERN MICHIGAN THIS EVENING. WINDS WILL VEER TO THE NORTHEAST IN THE WAKE OF THE FRONT. SHALLOW COLD AIR ADVECTION OVER LAKE HURON WILL PROMOTE MIXING OF HIGHER WINDS TO THE LAKE SURFACE. THIS WILL BOOST WAVE HEIGHTS INTO SMALL CRAFT ADVISORY CONDITIONS TONIGHT. THESE CONDITIONS WILL PERSIST THROUGH MOST OF SUNDAY BEFORE THE NORTHEASTERLY GRADIENT WEAKENS. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. LAKE HURON... SMALL CRAFT ADVISORY...OUTER SAGINAW BAY AND THE NEARSHORE WATERS FROM PORT AUSTIN TO HARBOR BEACH...UNTIL 4 AM MONDAY. SMALL CRAFT ADVISORY...INNER SAGINAW BAY...UNTIL 8 PM SUNDAY. SMALL CRAFT ADVISORY...NEARSHORE WATERS FROM HARBOR BEACH TO PORT HURON...FROM 8 PM SATURDAY TO 4 AM MONDAY. LAKE ST CLAIR...NONE. MI WATERS OF LAKE ERIE...NONE. && $$ AVIATION.....SC SHORT TERM...SC LONG TERM....KEC MARINE.......SC YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).