FXUS63 KDTX 282038
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
338 PM EST SAT NOV 28 2009

.SHORT TERM...TONIGHT

RIDGE OF HIGH PRESSURE THAT BROUGHT DRY AND QUIET WEATHER TODAY WILL 
SHIFT EAST TONIGHT AS AN AREA OF LOW PRESSURE LIFTS FROM THE CENTRAL 
PLAINS INTO THE WESTERN GREAT LAKES. A GOOD AMOUNT OF MOISTURE BELOW 
900MB IS EXPECTED TO ADVECT INTO THE AREA TONIGHT PRECEDING THIS 
SYSTEM...AS IT RIDES ALONG AN EXISTING LOW LEVEL THERMAL BOUNDARY 
ARCED FROM SOUTH CENTRAL MINNESOTA DOWN THROUGH SOUTHEAST MICHIGAN. 
GFS/NAM MODEL SOUNDINGS AND CROSS SECTIONS SHOW STRONG SUPPORT FOR 
STRATUS DEVELOPMENT OVERNIGHT...AS THIS MOISTURE FILLS IN UNDERNEATH 
A STRONG INVERSION. UPSTREAM OBSERVATIONS FROM ACROSS MINNESOTA AND 
WISCONSIN SHOW SOME AREAS OF LOW STRATUS AND LIGHT FOG HAVE ALREADY 
DEVELOPED. EXPECT A GREATER COVERAGE TONIGHT OVER MICHIGAN...AS MEAN 
RH INCREASES IN THE LOWEST LEVELS OF THIS AIRMASS AFTER ITS JOURNEY 
ACROSS LAKE MICHIGAN AND AS INVERSIONS STRENGTHEN AND LOWER 
OVERNIGHT. THERE IS SOME POTENTIAL FOR FOG TO DEVELOP AS OPPOSED TO 
THE STRATUS...ESPECIALLY IF THE WINDS GO COMPLETELY CALM. THE 
GREATEST POTENTIAL FOR THIS TO HAPPEN WOULD BE NORTH OF INTERSTATE 
69. HAVE CONTINUED WITH THE IDEA OF SKIES BECOMING MOSTLY CLOUDY 
OVERNIGHT AND WILL HAVE TO MONITOR THE POTENTIAL FOR THIS CLOUD 
LAYER TO LOWER TO THE GROUND. THE EXTRA MOISTURE AND SOME CLOUDINESS 
WILL ALLOW FOR SOME MODERATION TO OVERNIGHT LOWS...WITH MINS IN THE 
LOW TO MID 30S EXPECTED.

&&

.LONG TERM...SUNDAY THROUGH NEXT SATURDAY

THE MILD AIR RESPONSIBLE FOR THE LOW CLOUDS AND FOG WILL GIVE A
BOOST TO DAYTIME TEMPERATURES FOR A WHILE ON SUNDAY. THIS WILL BE
DISRUPTED BY INCREASED RAIN POTENTIAL SUNDAY AFTERNOON ALONG AND
BEHIND A COLD FRONT MOVING THROUGH THE REGION. WHILE THE RAIN
POTENTIAL IS ON THE RISE, IT DOES STILL LOOK LIKE THE BULK OF IT
WILL BE OVER BEFORE ENOUGH COLD AIR MOVES IN FOR ANY MEANINGFUL
CHANGE TO SNOW SUNDAY NIGHT.

ONE OF THE DEVELOPMENTS FAVORABLE FOR SHOWERS BY SUNDAY AFTERNOON IS 
INCREASED LOW LEVEL MOISTURE TRANSPORT INTO THE FRONTAL ZONE OVER 
LOWER MICHIGAN. THIS AIR IS FROM GULF MODIFIED HIGH PRESSURE AND IS 
SHOWN TO BRING 850 MB DEWPOINT UP TO AROUND 2C WHICH IS ADEQUATE TO 
FUEL A RESPECTABLE FRONTAL CIRCULATION DEVELOPING IN OUR AREA. 
JUDGING FROM THETA-E AND FGEN FIELDS, THERE IS A NOTABLE FGEN 
RESPONSE TO THE POSITIONING OF THE ENTRANCE REGION IN THE UPPER JET 
OVER THE GREAT LAKES. THE ADDED DYNAMIC SUPPORT AND INCREASED 
MOISTURE WILL MAKE SHOWERS LIKELY ALONG AND BEHIND THE FRONT DURING 
THE AFTERNOON HOURS OVER MOST OF SE MICHIGAN. 

COLD AIR WILL MOVE BACK INTO THE REGION SUNDAY NIGHT AS THE FRONTAL  
SYSTEM EXITS TO OUR EAST. ANY LIGHT PRECIPITATION LEFT OVER FROM THE 
FRONT WILL TURN TO SNOW DURING THE EVENING BEFORE TRANSITIONING TO A 
LAKE EFFECT COMPONENT AFTER MIDNIGHT. 850 MB TEMPERATURE IS SHOWN BY 
BOTH THE NAM AND GFS TO DROP DOWN TO NEAR -10C FOR A LAKE DELTA-T 
AVERAGING ABOUT 18C. ENOUGH OF A NORTHERLY WIND IS THEN SHOWN IN THE 
NAM FOR A CHANCE OF LAKE EFFECT SNOW SHOWERS BRUSHING THE THUMB 
BEFORE THE WIND BACKS TO THE WEST TOWARD SUNRISE MONDAY AND 
ACTIVATES LAKE MICHIGAN. THE GFS REMAINS CONSIDERABLY STRONGER WITH 
THE NEXT NW FLOW SHORT WAVE AND IS THEREFORE MORE BULLISH ON THE 
POTENTIAL FOR SNOW SHOWERS FROM LAKE MICHIGAN DURING MONDAY AND 
MONDAY NIGHT. WE WILL MAINTAIN THE CHANCE OF SNOW SHOWERS IN OUR 
GOING FORECAST UNTIL MODEL PERFORMANCE ON THE STRENGTH OF THIS 
FEATURE CAN BE BETTER EVALUATED. ALL THINGS REMAINING THE SAME IN 
THE NEXT FEW FORECAST CYCLES MAY REQUIRE AN INCREASE IN THE POP 
FORECAST DURING THIS TIME.  

A QUIET STRETCH OF MID WEEK WEATHER WILL FEATURE ANOTHER BRIEF 
WARMING TREND WITH HIGHS REACHING THE LOWER TO MID 40S BY WEDNESDAY, 
BUT ATTENTION THEN QUICKLY TURNS TO THE QUESTION OF THE NEXT LOW 
PRESSURE SYSTEM TO IMPACT THE GREAT LAKES LATE WEDNESDAY AND 
THURSDAY. THIS SYSTEM WILL BE CAPABLE OF THE FIRST SUBSTANTIAL 
SNOWFALL OF THE SEASON IN SE MICHIGAN. THE LATEST GLOBAL MODELS HAVE 
A FAIR AMOUNT OF AGREEMENT ON THE OVERALL SCENARIO WITH THE USUAL 
TIMING AND TRACK DIFFERENCES. FOR THIS PATTERN, THE DIFFERENCES IN 
THE TRACK OF THE LEAD SYSTEM APPEAR SENSITIVE TO THE TIMING AND 
LOCATION OF THE NEXT ROUND OF HEIGHT FALLS OVER THE NORTHERN PLAINS. 
GIVEN THE STRENGTH AND BREADTH OF THE LONG WAVE TROUGH OVER THE 
NORTH PACIFIC, AND THE UPPER RIDGE THAT SHOULD BE FIRMLY IN PLACE 
OVER THE WEST COAST, PREFERENCE IS FOR SOLUTIONS THAT SHOW HEIGHT 
FALLS OCCURRING FARTHER WEST IN THE PLAINS AND CENTRAL CANADA. THE 
12Z RUNS OF THE GFS AND PARALLEL GFS HAVE TRENDED IN THAT DIRECTION 
WHILE THE ECMWF HAS GIVEN UP A LITTLE OF THE IDEA. ADDITIONAL 
COMPLICATING MATTERS ARE WITH THE AMOUNT OF COLD AIR IN PLACE OVER 
LOWER MICHIGAN BOTH IN ADVANCE OF THE SYSTEM AND AS IT MOVES TO OUR 
SOUTH AND EAST ON THURSDAY. THE GFS KEEPS THE BOUNDARY LAYER ON THE 
MILD SIDE DUE TO SURFACE PRESSURE FALLS OVER THE NORTHERN LAKES AND 
THIS COULD VERY WELL TURN OUT TO BE THE CASE IF THE UPPER LEVEL 
TROUGH AXIS REMAINS TO OUR WEST. THIS WILL HAVE TO BE REFINED AS THE 
DETAILS OF THE SMALL SCALE FEATURES BECOME CLEARER OVER THE NEXT 
COUPLE OF DAYS. 

CONTINUED INTERACTION BETWEEN THE UPPER LEVEL FEATURES IS SHOWN WITH 
GOOD AGREEMENT IN THE MODELS TO FORM A LARGE AND COLD UPPER LOW 
CENTERED NEAR JAMES BAY BY FRIDAY. THIS WILL KEEP CONDITIONS COLD 
AND UNSETTLED OVER THE GREAT LAKES WITH SNOW SHOWERS ON THE ACTIVE 
SIDE GIVEN POTENTIAL LAKE INTERACTION. HIGH TEMPERATURES IN THE MID 
AND UPPER 30S EXPECTED INTO SATURDAY ARE ONLY A FEW DEGREES BELOW 
NORMAL FOR THE FIRST WEEK OF DECEMBER.

&&

.MARINE...

LIGHT WINDS WILL CONTINUE ACROSS THE AREA TONIGHT EVEN AS HIGH 
PRESSURE BEGINS TO SHIFT EAST AND A LOW PRESSURE SYSTEM LIFTS INTO 
THE WESTERN GREAT LAKES. COLD AIR WILL FILTER DOWN INTO THE REGION 
LATE SUNDAY NIGHT AND MONDAY IN THE WAKE OF THIS SYSTEM'S PASSAGE 
ACROSS MICHIGAN...BRINGING AN INCREASE IN BOTH WINDS AND WAVES. 
SMALL CRAFT ADVISORIES MAY BE NEEDED FOR THE LAKE HURON NEARSHORE 
WATERS SUNDAY NIGHT AND MONDAY AS NORTHWESTERLY WINDS GUST TO 25 
KNOTS.

&&

.PREV DISCUSSION...ISSUED 1228 PM EST SAT NOV 28 2009 

AVIATION...
VFR CONDITIONS WILL PREVAIL THROUGH THE AFTERNOON AND EVENING
BEFORE CONDITIONS DETERIORATE OVERNIGHT. CONCERN IS BECOMING HIGH
THAT LOW STRATUS WILL FILL THE SKIES OVER SOUTHEAST MICHIGAN AFTER
MIDNIGHT...DROPPING TO IFR OR LOWER BY 10Z. LOW-LEVEL MOISTURE
PRECEDING A LOW PRESSURE SYSTEM IS EXPECTED TO RIDE DOWN AN
EXISTING LOW-LEVEL THERMAL BOUNDARY THIS EVENING. THIS BOUNDARY
IS CURRENTLY IN PLACE FROM SOUTH CENTRAL MINNESOTA ARCING DOWN
THROUGH SOUTHEAST MICHIGAN. UPSTREAM OBSERVATIONS ALONG THE EDGE
OF THIS BOUNDARY FROM WISCONSIN AND MINNESOTA ALREADY SUPPORT LOW
STRATUS IDEA...WITH SEVERAL SITES NOW REPORTING CEILINGS LESS THAN
700 FEET. MODEL FORECAST SOUNDINGS SUPPORT IFR TO LIFR CONDITIONS DEVELOPING
OVERNIGHT...SHOWING ABUNDANT MOISTURE BECOMING TRAPPED BENEATH
THE INVERSION BY 6Z. GIVEN MODEL DATA...AND TAKING INTO ACCOUNT
UPSTREAM OBSERVATIONS AND ADDITIONAL MOISTURE THAT MAY BE PICKED
UP OVER LAKE MICHIGAN...HAVE ADDED PREVAILING MVFR GROUPS AFTER
5/6Z AND IFR GROUPS AFTER 10Z TO ALL SITES FOR THE 18Z PACKAGE. THERE IS
SOME POTENTIAL THAT LIGHTER WINDS COULD CAUSE GROUND FOG TO FORM
INSTEAD OF THE LOW STRATUS AT MBS AND POSSIBLY FNT...AND THERE IS
POTENTIAL FOR CEILINGS TO DROP TO LIFR AT ALL SITES. WILL
CONTINUE TO MONITOR TRENDS AND INCOMING MODEL DATA TO SEE IF
ADDITIONAL CHANGES ARE NEEDED.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.

&&

$$

SHORT TERM...HLO
LONG TERM....BT
MARINE.......HLO
AVIATION.....HLO

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