FXUS63 KDTX 262311
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
611 PM EST THU NOV 26 2009

.AVIATION...

MVFR CLOUD DECK WILL CONTINUE TO HOLD OVER THE AREA TONIGHT AS A 
COLD FRONT MOVES THROUGH THE TAF SITES. THIS FRONTAL
BOUNDARY...CURRENTLY ACROSS CENTRAL MICHIGAN...WILL MOVE THROUGH THE 
TAF SITES BETWEEN 01Z AND 05Z. BEHIND THE FRONT...WINDS WILL FLIP TO 
THE NORTHWEST AND GUST BETWEEN 20 AND 25 KNOTS...AND LIGHT RAIN 
SHOWERS WILL BEGIN TO MIX WITH SNOW. SNOW DURATION AND ACCUMULATION 
WILL BE VERY BRIEF AND LIGHT AS UPSTREAM SITES ARE STILL ALL RAIN. 
CURRENT THINKING CALLS FOR SOME MIX AFTER 03/04Z AT 
PTK/FNT/MBS...BEFORE DRIER AIR HINDERS ANY PRECIPITATION FROM 
FALLING AFTER 07Z. GUSTY NORTHWEST WINDS AND MVFR CEILINGS WILL 
REMAIN FOR FRIDAY AFTERNOON...WITH SOME SITES POPPING INTO THE VFR 
CATEGORY.

&&

.PREV DISCUSSION...ISSUED 341 PM EST THU NOV 26 2009 

SHORT TERM...TONIGHT

MAIN FORECAST ISSUE FOR THE SHORT TERM PERIOD WILL BE THE POTENTIAL 
FOR PRECIPITATION TO CHANGE OVER TO SNOW...AND IF SO...WHETHER OR 
NOT THERE WILL BE ACCUMULATIONS.

WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL LOW PRESSURE SYSTEM LIFTING 
NORTHEAST FROM LOWER MICHIGAN TOWARDS LAKE HURON...WHILE A STRONG 
SHORTWAVE DIVES DOWN ACROSS WESTERN LAKE SUPERIOR AND THE UPPER 
PENINSULA. MODELS INDICATE THESE TWO FEATURES WILL MERGE TOGETHER 
TONIGHT...AND MOVE INTO SOUTHERN ONTARIO BY 12Z FRIDAY. AT THE 
SURFACE...THE LOW PRESSURE SYSTEM OVER THE STRAITS OF MACKINAC WILL 
LIFT INTO ONTARIO OVERNIGHT. ASSOCIATED SURFACE TROUGHING AND A COLD 
FRONT SHOWN BY OBS OVER WESTERN MICHIGAN WILL PUSH ACROSS SOUTHEAST 
MICHIGAN TONIGHT...WITH PRECIPITATION CURRENTLY ASSOCIATED WITH 
THESE FEATURES MOVING ACROSS THE AREA AS WELL. THE HEAVIEST 
PRECIPITATION IS EXPECTED TO OCCUR NORTH OF I-69...WHERE THE TROUGH 
WILL BE MOST EFFECTIVE AT FILLING IN AND DEFORMATION WILL BE A 
LITTLE STRONGER. ADVERTISED DRY AIR THAT IS EXPECTED BEHIND THE 
FRONT AT THE MID LEVELS CAN BE SEEN ON WATER VAPOR MOVING EAST INTO 
LAKE MICHIGAN...AND THIS SHOULD BE EFFECTIVE AT CUTTING OFF 
PRECIPITATION DURING THE LATTER PARTS OF THE OVERNIGHT 
PERIOD...ESPECIALLY ANY SNOW AS MOISTURE IS STRIPPED OUT OF THE 
DENDRITIC GROWTH ZONE. 

THERE ARE BOTH PROS AND CONS CONCERNING THE POTENTIAL FOR RAIN TO 
MIX WITH OR CHANGE TO SNOW OVERNIGHT. UPSTREAM OBSERVATIONS OVER 
NORTHERN LOWER MICHIGAN DO SHOW SNOWFALL CURRENTLY MIXING IN ALONG 
THE FRONT...AND FORECAST MODEL SOUNDINGS AFTER ABOUT 2Z (9PM EST) DO 
AT LEAST WEAKLY SUPPORT SNOWFALL. MOST OF THE THERMAL SUPPORT WOULD 
COME FROM LOW-LEVEL WETBULB EFFECTS...HOWEVER BOTH NAM AND GFS ARE 
ALREADY TOO LOW WITH SURFACE DEWPOINTS ACROSS LOWER MICHIGAN THIS 
AFTERNOON. GIVEN ONLY WEAK LIFT...WEAKLY SUPPORTIVE THERMAL PROFILES 
AND A TIMING BATTLE BETWEEN THE ARRIVAL OF POST FRONTAL COLD AIR 
ADVECTION AND MID-LEVEL DRY AIR...WILL KEEP SNOW ACCUMULATIONS LOW. 
FORECAST SNOW AMOUNTS ARE SIMILAR TO PREVIOUS FORECAST PACKAGE WITH 
HIGHEST TOTALS OVER THE THUMB...WHICH ARE JUST SHY OF AN INCH AT 
MOST. FOR THE REMAINDER OF THE AREA...TOTALS RANGE FROM ZERO NEAR 
THE OHIO BORDER TO ONE TO TWO TENTHS ELSEWHERE.

LONG TERM...FRIDAY THROUGH NEXT THURSDAY

THE FLOW ACROSS NORTH AMERICA THROUGH THE REMAINDER OF THE HOLIDAY 
WEEKEND IS FORECAST TO EVOLVE INTO A SLIT FLOW PATTERN...FEATURING A 
PROGRESSIVE NORTHERN STREAM. THIS WILL ALLOW THE UPPER LOW NOW 
SITUATED OVER EASTERN LOWER MI TO RAPIDLY LIFT OFF INTO THE MID 
ATLANTIC/NEW ENGLAND ON FRIDAY. COLD AIR ADVECTION WILL BE ONGOING 
DURING THE MORNING AS 850MB TEMPS FALL TO -8C UNDER BRISK NORTHWEST 
WINDS. LOW LEVEL CYCLONIC FLOW AND INSTABILITY OFF THE GREAT LAKES 
MAY ACCOUNT FOR SOME LINGERING SNOW SHOWERS OR FLURRIES DURING THE 
MORNING. INVERSION HEIGHTS ARE EXPECTED TO FALL DURING THE AFTERNOON 
UNDER PERSISTENT MID LEVEL SUBSIDENCE...WHICH SHOULD JUST RESULT IN 
SOME LINGERING STRATO CU. UNDER THE COLD AIR ADVECTION AND EXPECTED 
CLOUD COVER...MAX TEMPS SHOULD REMAIN STUCK IN THE 30S ON FRIDAY 
/WITH BLUSTERY NW WINDS KEEPING WIND CHILLS IN THE 20S\. 

MID LEVEL RIDGING IS THEN FORECAST TO QUICKLY BUILD INTO THE 
SOUTHERN GREAT LAKES/OHIO VALLEY FRI NIGHT THROUGH SATURDAY. LOW 
LEVEL WARM AIR ADVECTION WILL STRENGTHEN AND LOWER INVERSION 
HEIGHTS...A PATTERN WHICH TYPICALLY YIELDS AN ABUNDANCE OF STATUS. 
HOWEVER...GIVEN THE LOW LEVEL DRY AIR POISED TO THE SOUTHWEST OF THE 
AREA...PLAN TO ACCOUNT FOR SOME POSSIBLE PEAKS OF SUN ON SATURDAY 
/ESPECIALLY FROM METRO DETROIT SOUTH/. THE LOW LEVEL WARM AIR 
ADVECTION WILL ALSO ALLOW TEMPS TO REBOUND INTO THE 40S ON SAT. 

THE 12Z MODEL SUITE ARE IN REASONABLE AGREEMENT IN INDICATING A COLD 
FRONT SLIDING ACROSS THE STATE ON SUNDAY IN ADVANCE OF A FAST MOVING 
NORTHERN STREAM TROUGH. UPPER LEVEL JET SUPPORT WILL STRENGTHEN THE 
LOWER TO MID TROPOSPHERIC FRONTAL DYNAMICS ALONG AND BEHIND THE 
ACTUAL SURFACE FRONT. THERE HAS BEEN A NOTICEABLE TREND IN THE 12Z 
SOLUTIONS IN CARRYING THE BEST LOW LEVEL MOISTURE TRANSPORT AND 
LARGE SCALE ASCENT TO THE SOUTHEAST OF MICHIGAN /WITH FAIRLY LOW QPF 
OVER THE FORECAST AREA/. GIVEN THE UPPER JET SUPPORT...WOULD NOT BE 
SURPRISED IF A STRONGER SURFACE WAVE DEVELOPS ALONG THE FRONT AND 
ADVECTS THE DEEPER MOISTURE A LITTLE FARTHER NORTH INTO SOUTHERN 
MICHIGAN. FOR THIS REASON...PLAN NOT TO STRAY TOO FAR FROM THE 
CURRENT FORECAST WHICH FEATURES A CHANCE OF PRECIP SUNDAY INTO 
MONDAY. THERE HAS ALSO BEEN A TREND TOWARD A LITTLE STRONGER COLD 
AIR ADVECTION SUN NIGHT INTO MONDAY /850MB TEMPS NOW FORECAST TO 
DROP DOWN TO -9C BY MONDAY/. THIS WOULD ALLOW ANY LINGERING PRECIP 
TO CHANGE OVER TO SNOW SUN NIGHT. THE DEPTH OF THE EQUILIBRIUM 
LEVELS ON MODEL SOUNDINGS WOULD ALSO SUPPORT SOME POTENTIAL LAKE 
EFFECT PRECIPITATION EARLY IN THE WEEK. 

THE MEDIUM RANGE SUITE CONTINUE TO INDICATE A SLOW MOVING SOUTHERN 
STREAM UPPER LOW GETTING EJECTED INTO THE EASTERN US BY AN 
AMPLIFYING NORTHERN STREAM WAVE OVER THE NORTH-CENTRAL US NEXT WEEK. 
TIMING AND DEGREE OF PHASING OF THESE TWO SYSTEMS WILL DETERMINE THE 
STRENGTH OF EASTERN US SURFACE DEVELOPMENT. MODEL SOLUTIONS HAVE 
BEEN SHOWING EXTREMELY POOR RUN TO RUN CONTINUITY WITH THIS SYSTEM 
/WITH SOME SOLUTIONS SHOWING ONLY A WEAK WAVE WITH LIGHT QPF WHILE 
OTHER SOLUTIONS HAVING DEVELOPED A STRONG EAST COAST OR GREAT LAKES 
STORM SYSTEM/. THE END RESULT WILL BE A CONTINUED HIGHER DEGREE OF 
UNCERTAINTY IN THE MID TO LATE WEEK PERIOD NEXT WEEK. AT THIS 
TIME...THERE IS ENOUGH SUPPORT TO ADD JUST A CHANCE OF RAIN OR SNOW 
TO NEXT THURSDAYS FORECAST.

MARINE...

A COLD FRONT WILL DROP ACROSS THE CENTRAL GREAT LAKES TONIGHT AS AN 
AREA OF LOW PRESSURE LIFTS FROM THE STRAITS OF MACKINAC INTO CENTRAL 
ONTARIO. NORTHWESTERLY WIND GUSTS BETWEEN 25 AND 30 KNOTS WILL 
DEVELOP BEHIND THE FRONT LATE TONIGHT AND CONTINUE INTO TOMORROW AS 
COLD AIR CONTINUES TO STREAM OVER THE RELATIVELY WARM WATERS. THE 
STRONG WIND GUSTS WILL SUBSIDE BY TOMORROW EVENING. IN ADDITION TO 
THE GUSTY WINDS...A LONG FETCH OVER THE WATER WILL ALLOW WAVES TO 
BUILD AS HIGH AS 8 FEET OVER THE NEARSHORE WATERS OF LAKE HURON BY 
LATE FRIDAY MORNING. LARGE WAVES OVER LAKE HURON WILL CONTINUE INTO 
SATURDAY MORNING BEFORE SUBSIDING AS HIGH PRESSURE BUILDS INTO THE 
AREA.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.

LAKE HURON...
     SMALL CRAFT ADVISORY...INNER SAGINAW BAY...FROM 11 PM THURSDAY TO 6 
     PM FRIDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS FROM PORT AUSTIN TO PORT 
     HURON...FROM 3 AM FRIDAY TO 10 AM SATURDAY.

     SMALL CRAFT ADVISORY...OUTER SAGINAW BAY...FROM 11 PM THURSDAY TO 
     10 AM SATURDAY.

LAKE ST CLAIR...
     SMALL CRAFT ADVISORY...FROM 11 PM THURSDAY TO 6 PM FRIDAY.

MI WATERS OF LAKE ERIE...
     SMALL CRAFT ADVISORY...FROM 11 PM THURSDAY TO 6 PM FRIDAY.

&&

$$

AVIATION.....KURIMSKI
SHORT TERM...HLO
LONG TERM....SC
MARINE.......HLO

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