Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 181932 AFDDTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI 332 PM EDT THU SEP 18 2014 .SHORT TERM...
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THROUGH TONIGHT SURFACE COLD FRONT DROPPED DOWN THROUGH LOWER MICHIGAN EARLY THIS MORNING...HELPED BY HIGH PRESSURE SETTLING INTO THE NORTHERN GREAT LAKES. THIS IS EVIDENT BY NORTHEAST FLOW SHOWN ON SURFACE OBSERVATIONS. MODELS INDICATE HOWEVER THAT THE ELEVATED PORTION OF THE FRONT (925MB) IS LAGGING BEHIND THE SURFACE FEATURE...AND IS NOW SITUATED FROM WESTERN WISCONSIN TO SOUTH-CENTRAL LOWER MICHIGAN TO NEW YORK STATE. THIS LINES UP WELL WITH A STRATUS DECK SHOWN ON SATELLITE IMAGERY. THIS UPPER FRONT WILL SLOWLY SAG SOUTHWARD TONIGHT...AND BEND TO TAKE ON A MORE NORTHWEST TO SOUTHEAST ORIENTATION BY LATE AS SURFACE HIGH PRESSURE SLIDES FROM EAST OF LAKE SUPERIOR SOUTHEAST THROUGH QUEBEC. CLOUD FORECAST FOR TONIGHT IS VERY TRICKY WITH THE BOUNDARY REMAINING OVERHEAD...AND AS A SUBSIDENCE INVERSION STRENGTHENS UNDER HIGH PRESSURE. UNCERTAINTY WITH THE CLOUD FORECAST THEN LEADS TO UNCERTAINTIES IN MIN TEMPS AND FROST THREAT. LIGHT NORTHEAST FLOW OFF LAKE HURON WILL WEAKEN THIS EVENING THEN TURN MORE EASTERLY OVERNIGHT. MEANWHILE...ON THE WEST SIDE OF THE STATE FLOW WILL BEGIN TO TURN MORE SOUTHWEST AS A SURFACE TROUGH PROGRESSES THROUGH THE UPPER MISSISSIPPI RIVER VALLEY. BELIEVE WE SHOULD INITIALLY SEE A DECREASE IN CLOUD COVER TONIGHT AS WE LOSE DIURNAL HEATING...BEFORE INCREASING LOW-LEVEL CONVERGENCE OVER THE CENTER OF THE STATE ALLOWS STRATUS TO FILL BACK IN UNDER THE INVERSION OVERNIGHT. BEEFED UP CLOUD COVER OVER THE WESTERN HALF OF THE FORECAST AREA...WHILE ALSO ADDING A MENTION OF FROST OVER THE THUMB WHERE CLEARING WILL ALLOW MIN TEMPS TO DROP INTO THE MID 30S. PATCHY FOG WILL ALSO BE POSSIBLE OFF LAKES ERIE AND ST CLAIR TOWARDS SUNRISE AS LIGHT EASTERLY FLOW COMES OFF THE LAKE.
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&& .LONG TERM...
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BURGEONING HEIGHT FALLS AS WE HEAD INTO THE WEEKEND...WITH MAX 500 MB HEIGHT FALLS OCCURRING SUNDAY MORNING AS A STRONG UPPER LEVEL WAVE/TROUGH AXIS SWINGS THROUGH THE CENTRAL GREAT LAKES. AS THE NORTHERN STREAM FLOW BEGINS TO BUCKLE OVER THE NORTHERN ROCKIES TOMORROW...INCREASING SOUTH TO SOUTHWEST FLOW/WARM ADVECTION WILL TAKE PLACE INTO SATURDAY MORNING AHEAD OF A COLD FRONT. MODEL DISCREPANCIES DEAL WITH THE UPPER LEVEL LOW/TROUGH ALONG THE WEST COAST AND THE AMOUNT OF UPPER LEVEL ENERGY WHICH SCOOTS OFF TO THE NORTHEAST...AS THE LOW CLOSES OFF AND LOSES THE LINKAGE WITH THE STRONG WESTERLIES. THIS IMPACTS THE AMOUNT OF LOW LEVEL MOISTURE WHICH ADVECTS UP FROM TEXAS/NEW MEXICO. THIS IS IMPORTANT AS AIRMASS WILL INITIALLY BE EXTREMELY DRY TOMORROW OVER SOUTHEAST MICHIGAN...AS 700 MB DEW PT DEPRESSIONS RESIDE IN THE 30 TO 50 DEGREE RANGE. THERE IS MODEST CONCERN SURFACE/NEAR SURFACE FOG/CLOUDS COULD BECOME TRAPPED UNDER DEVELOPING INVERSION TOMORROW...AS WISCONSIN IS DEEPLY SOCKED IN TODAY...BUT THAT WAS STRONGLY INFLUENCED BY THE FLOW OFF LAKE MICHIGAN THIS MORNING. AS ELEVATED WARM AIR (8C AT 700 MB) STREAMS IN FRIDAY NIGHT-SATURDAY MORNING...MID LEVEL LAPSE RATES WILL BE IMPRESSIVE (7+ C AT 700-500 MB)...BUT THAT COLUMN WILL BE DRY...AND AS MOISTURE AXIS ARRIVES SATURDAY AFTERNOON/EARLY EVENING...MOISTENING UP OF THE COLUMN WILL DWINDLE THE LAPSE RATES TO MORE MANAGEABLE 6.0-6.5 C/KM. MOISTURE FEED IS NARROW...BUT SOLID WITH 700 MB DEW PTS IN THE 3-5 C RANGE...WITH PW VALUES JUST UNDER 1.75 INCHES...PER BLEND NAM/GEM/GFS. STILL...NOT TOTALLY CONVINCED ON COVERAGE/DEVELOPMENT DUE TO LIMITED INSTABILITY...AS MUCAPES REMAIN BELOW 1000 J/KG...IF NOT UNDER 500 J/KG. CONCERN IS EVEN WITH MEAGER INSTABILITY...STRONG WIND FIELDS...45-50 KNOTS AT 850 MB DRAWS CONCERN FOR STRONG WINDS OF 45-60 MPH WITH ANY STRONGER CONVECTIVE ACTIVITY...BUT NEAR SURFACE INVERSION STILL WILL PROBABLY BE TOUGH TO OVERCOME TO GET WINDS TO THE GROUND...ESPECIALLY IF THE FRONT IS SLOWER...POST DAYLIGHT HOURS. MORE VIGOROUS HEIGHT FALLS WILL OCCUR FOR THE SECOND HALF OF THE WEEKEND AS STRONG UPPER WAVE EJECTS OUT OF THE GULF OF ALASKA. A STRONG MID LEVEL DRY SLOT WILL DEVELOP AND TRACK THROUGH SUNDAY MORNING BEFORE SECONDARY COLD FRONT AND COLD ADVECTION SETS IN DURING THE DAY...LIKELY LEADING TO SCATTERED SHOWERS WITH THE STEEP LOW LEVEL LAPSE RATES...WITH AN ISOLATED THUNDERSTORM NOT OUT OF THE QUESTION AS SOUNDINGS DO SHOW EQL LEVELS UP AROUND -20 C ISOTHERM. AS THE UPPER WAVE MOVES EAST OF THE REGION...HIGH PRESSURE WILL FILL IN FOR SEVERAL DAYS. THIS WILL KEEP ALL WEATHER HIGHLIGHTS AWAY FROM THE CWA FOR THE EXTENDED PERIOD. AS WE NEAR CLOSER TO EARLY NEXT WEEK...EXPECT A FEW MENTIONS OF AM FOG. TEMPERATURES WILL GRADUALLY BE ON THE INCREASE THROUGH THE WEEK AS WELL WITH 500MB HEIGHTS SLOWLY RISING FROM 570 TO 586 DAM.
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&& .MARINE...
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SURFACE HIGH SETTLED OVER THE CENTRAL GREAT LAKES TONIGHT WILL LEAD TO LIGHT WINDS. SOUTHERLY FLOW WILL BEGIN TO INCREASE TOMORROW...AND ESPECIALLY TOMORROW NIGHT AHEAD OF A COLD FRONT...WHICH WILL TRACK THROUGH SATURDAY NIGHT. ALTHOUGH WARM AIR WILL STREAM INTO THE GREAT LAKES REGION...LEADING TO MORE STABLE LOW LEVEL PROFILES...SMALL CRAFT ADVISORIES STILL MAY BE NEEDED OVER THE WEEKEND...ESPECIALLY OVER SAGINAW BAY. DUE TO THIS WARM AIR...WINDS ARE EXPECTED TO HOLD IN THE 25 TO 30 KNOT RANGE OVER THE OPEN WATERS OF LAKE HURON.
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&& .AVIATION...
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ISSUED 130 PM EDT THU SEP 18 2014 //DISCUSSION... STRATUS OVER SOUTHEAST MICHIGAN WILL CONTINUE TO EXPAND SOUTHWARD THIS AFTERNOON AS MOISTURE FILLS IN BEHIND A WEAK COLD FRONT AND IS MAINTAINED BY NORTHEAST FLOW OFF LAKE HURON. DAYTIME HEATING WILL ALLOW FOR SOME MIXING TODAY AND SHOULD ALLOW FOR A GRADUAL RISE IN CEILINGS...BUT STILL EXPECTING MVFR CONDITIONS FROM PTK NORTHWARD. LOW CONFIDENCE FORECAST THEN FOR TONIGHT AS FLOW OFF LAKE HURON WEAKENS AND SHIFTS TO THE SOUTHEAST LATE. CURRENTLY...CLOUDS CAN BE SEEN ALIGNED FROM NORTHERN WISCONSIN DOWN THROUGH EASTERN ONTARIO....BEHIND A FRONTAL BOUNDARY AND ALONG THE PERIPHERY OF SURFACE HIGH PRESSURE CENTERED EAST OF LAKE SUPERIOR. THIS MOISTURE SHOULD SHIFT EASTWARD TONIGHT WITH THE HIGH...WHILE NORTHEAST FLOW REMAINS EARLY OVER EASTERN MICHIGAN AND SOUTHWEST FLOW BEGINS TO DEVELOP OVER THE WESTERN GREAT LAKES. THIS SHOULD PROVIDE SOME LOW-LEVEL CONVERGENCE ALONG THE ELEVATED PORTION OF THIS BOUNDARY WHICH WILL REMAIN OVER MICHIGAN...ALLOWING FOR SOME STRATUS RE-DEVELOPMENT TONIGHT UNDER A SUBSIDENCE INVERSION. BEST POTENTIAL FOR BKN MVFR CEILINGS WILL BE ALONG A NORTH-NORTHWEST TO SOUTH-SOUTHEAST AXIS THROUGH THE CENTER PART OF THE STATE...WITH MBS INCLUDED. IN ADDITION...VERY LIGHT SOUTHEAST FLOW OFF LAKES ERIE AND ST CLAIR WILL ALSO BRING POTENTIAL FOR BR TO FORM NEAR THE DETROIT TERMINALS. DID ADD A SMALL WINDOW OF MVFR VISIBILITIES FRIDAY MORNING FOR DTW...DET...AND YIP. FOR DTW...LOWER CLOUDS ARE PROGRESSING SOUTHWARD TOWARDS THE TERMINAL EARLY THIS AFTERNOON. DAYTIME HEATING SHOULD ALLOW FOR HIGHER CEILINGS HEIGHTS AND MORE BREAKS IN THE CLOUD DECK THAN SEEN UPSTREAM SO FAR TODAY...AND STILL HAVE JUST MEDIUM CONFIDENCE WITH CEILINGS BKN BELOW 5000 FEET. SOME UNCERTAINTY WHETHER LIGHT FOG WILL DEVELOP BEFORE SUNRISE FRIDAY MORNING...BUT FELT GOOD ENOUGH WITH CALM CONDITIONS AND SOUTHEAST FLOW OFF LAKE ERIE TO INTRODUCE A PERIOD OF MVFR VIS. //DTW THRESHOLD THREATS... * MEDIUM CONFIDENCE WITH CEILINGS BELOW 5000 FEET THROUGH 00Z.
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&& .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. LAKE HURON...NONE. LAKE ST CLAIR...NONE. MI WATERS OF LAKE ERIE...NONE. && $$ SHORT TERM...HLO LONG TERM....SF/MM MARINE.......SF AVIATION.....HLO YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).

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