Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 211711
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1211 PM EST WED JAN 21 2015


.AVIATION...

A SLOW WEAKENING IN THE INTENSITY OF THE SNOW WILL OCCUR DURING THE
COURSE OF THE AFTERNOON PRIOR TO IT COMING TO AN END THIS EVENING.
DURING THIS PERIOD...THE MORE PERSISTENT SNOWFALL WILL IMPACT FNT
AND PTK. MBS WILL REMAIN ON THE NORTHERN FRINGE OF THE SNOW AND
SHOULD ACTUALLY SEE IT COME TO AN END OVER THE NEXT 2 TO 3 HOURS.
ADDITIONAL SNOW ACCUMULATIONS AT FNT AND MBS WILL BE AN INCH OR LESS
THROUGH THE AFTERNOON. THERE WILL HOWEVER BE QUITE A BIT OF
VARIABILITY IN CIGS AND VSBY WITH BRIEF INTERVALS OF HIGHER
INTENSITY SNOWFALL PERSISTING DURING THE AFTERNOON.

FOR DTW...THE LOWER VISIBILITIES IMPACTING METRO DETROIT ARE MORE
THE RESULT OF FOG THAN INTENSE SNOWFALL. WHILE SNOW SHOWERS WILL
PERSIST THROUGH THE AFTERNOON...ADDITIONAL ACCUMULATIONS SHOULD JUST
BE A DUSTING. ALTHOUGH SOME LOW LEVEL MOISTURE WILL ATTEMPT TO HOLD
INTO TONIGHT...THE DEGREE OF LOW LEVEL DRY AIR ADVECTION FROM THE
N-NE TONIGHT WILL REMAIN SUPPORTIVE OF THE CURRENT FORECAST IN
HOLDING CIGS ON THE LOW END VFR.

//DTW THRESHOLD PROBABILITIES...

* HIGH FOR CEILINGS BELOW 5000 FEET TODAY...MEDIUM TONIGHT.

* HIGH FOR PRECIPITATION AS ALL SNOW.

&&

.PREV DISCUSSION...ISSUED 1021 AM EST WED JAN 21 2015

UPDATE...

THE REGION OF WIDESPREAD SNOW HAS BEEN SLOWLY BUT STEADILY LIFTING
NORTHWARD OVER THE LAST COUPLE OF HOURS. RADAR AND SATELLITE ALONG
WITH THE LATEST RAP AND 12Z NAM INDICATE THIS IS BEING FORCED
WITHIN A RIBBON OF DEFORMATION JUST NORTH OF A SECONDARY MID
LEVEL VORTICITY MAX NOW ROTATING ACROSS THE SW MI/NRN INDIANA
BORDER. THIS MID LEVEL CIRCULATION IS EXPECTED TO SHEAR APART AS
IT WORKS INTO NW OHIO THIS AFTERNOON ALONG WITH THE LARGER MID
LEVEL VORTICITY CENTER TO THE SOUTH. THE TRACK OF THIS FEATURE AND
RECENT RADAR TRENDS SUGGEST THAT THE NORTHWARD PUSH TO THE BACK
EDGE OF THE HEAVIER SNOW WILL HALT AS IT ROTATES INTO THE NRN
DETROIT SUBURBS OVER THE NEXT COUPLE OF HOURS. THE MID LEVEL
DEFORMATION WILL THEN CONTRACT BEFORE ENDING DURING THE
AFTERNOON...FOCUSING THE BETTER LIFT BETWEEN THE I 96/696 AND I
69 CORRIDORS. THIS REGION IS LIKELY TO PICK UP AROUND AN
ADDITIONAL INCH THROUGH MID AFTERNOON. SOME ADDITIONAL SNOWFALL
ACCUMS UP TO TWO INCHES DO HOWEVER SEEM PROBABLE IN ANY LOCAL
WHERE THE MID LEVEL FORCING HOLDS ON JUST A BIT LONGER /MOST
LIKELY SOMEWHERE IN OAKLAND...MACOMB OR SOUTHERN ST CLAIR
COUNTIES/.

SFC OBS SUGGESTED SOME PATCHY FREEZING DRIZZLE NEAR THE OHIO
STATE LINE THIS MORNING. MID LEVEL MOISTURE APPEARS TO BE
INCREASING AGAIN SOUTH OF THE I 94 CORRIDOR. THIS SHOULD END ANY
FREEZING DRIZZLE AND SUPPORT JUST SNOW SHOWERS THROUGH EARLY
AFTERNOON...WITH LITTLE ADDITIONAL ACCUMS EXPECTED.

THE GOING FORECAST STILL HAS A GOOD HANDLE ON THE SITUATION
/ALTHOUGH SOME OF THE NORTHERN DETROIT SUBURBS MAY END UP WITH
TOTAL SNOWFALL AMOUNTS UP TO OR SLIGHTLY OVER THREE INCHES/. AN
UPDATE WILL BE ISSUED MAINLY TO MAKE SOME ADJUSTMENTS TO REFLECT
LATEST TRENDS ON TIMING.

PREV DISCUSSION...ISSUED 353 AM EST WED JAN 21 2015

SHORT TERM... TODAY AND TONIGHT

THE NEXT WAVE OF LOW PRESSURE MOVING THROUGH THE SOUTHERN GREAT
LAKES IS ON TRACK TO SPREAD ANOTHER PATTERN OF SNOW OVER SE MICHIGAN
DURING THE MORNING. MOST OF THE ACCUMULATION WILL ONCE AGAIN FALL
SOUTH OF MIDLAND...BAY CITY...AND THE NORTHERN THUMB WITH HIGHEST
AMOUNTS AROUND 2 INCHES FROM THE OHIO BORDER UP TO ABOUT THE I-96
CORRIDOR AND THEN TAPERING OFF TO THE NORTH FROM THERE. SIMILAR TO
YESTERDAY...EXPECT A PEAK IN RATES FROM ABOUT MID MORNING THROUGH
EARLY AFTERNOON BUT WITH LIGHT ACCUMULATION LINGERING LONGER TOWARD
EVENING. THE LONGER DURATION AND POTENTIAL FOR A MORE FOCUSED BURST
SHOULD RESULT IN A FEW MORE 2 INCH TOTALS COMPARED TO YESTERDAY.

SNOW WILL SPREAD NORTH OF THE OHIO BORDER SHORTLY AFTER PRESS TIME
AS THE FIRST PHASE OF FORCING GETS UNDERWAY. SATELLITE IMAGERY AND
MODEL ANALYSIS FIELDS INDICATE THE LEADING MID LEVEL CIRCULATION
OVER THE OHIO VALLEY BEFORE SUNRISE WILL HELP GET ISENTROPIC LIFT
STARTED OVER SE MICHIGAN AS THE MID LEVEL THETA-E RIDGE MOVES
NORTHWARD. MODEL SOUNDINGS AND CROSS SECTIONS OF THETA-E INDICATE
THIS PROCESS WILL OCCUR WITH MODERATE MOISTURE TRANSPORT AND
FAVORABLY LOW STABILITY THAT WILL SUPPORT A RAPID INCREASE IN
COVERAGE AND INTENSITY OF SNOW ALONG THE LEADING EDGE OF THE THETA-E
RIDGE AROUND 700 MB. THE MOISTURE AXIS WILL SPREAD NORTHWARD OVER
ABOUT THE SOUTH HALF OF THE AREA DURING THE MORNING BEFORE BEING
DRAWN BACK WESTWARD INTO THE PRIMARY MID LEVEL CIRCULATION. THIS
WILL SET UP A RESPECTABLY ORGANIZED TROWAL AXIS WITHIN WHICH A
HEAVIER BURST OF SNOW IS EXPECTED DURING MID TO LATE MORNING AS THE
MAIN UPPER CIRCULATION SHEARS THROUGH INDIANA AND OHIO. THE
STRONGEST RESPONSE SHOULD OCCUR ON THE SOUTH FLANK OF THE TROWAL
WHERE THE STABILITY PROFILE WILL BE MOST FAVORABLE AND DEFORMATION
STRONGEST...ROUGHLY ALONG AND SOUTH OF I-96 CORRIDOR AS THE BEST
ESTIMATE FOR NOW. IT IS ALSO IMPORTANT TO POINT OUT THE STRONGEST
FORCING WILL BE BRIEF AS THE TRACK OF THE MID LEVEL CIRCULATION WILL
CARRY FAVORABLE SYSTEM RELATIVE FLOW EASTWARD AND CAUSE THE TROWAL
TO WEAKEN OVER SE MICHIGAN DURING THE AFTERNOON. EXPECT SNOW WILL
NOT END COMPLETELY BUT WILL DIMINISH CONSIDERABLY TOWARD EVENING.
CONSIDERING THE THERMAL PROFILE AND SURFACE TEMPERATURES ARE SIMILAR
TO YESTERDAY...A 15 TO 1 RATIO IS USED AGAIN FOR PEAK ACCUMULATION
TOTALS AROUND 2 INCHES WHICH IS AT OR SLIGHTLY LESS THAN MODEL 12 HR
QPF OF 0.15 INCHES TO FACTOR IN THE UNCERTAINTY ON DURATION.

THERE IS A CHANCE OF LIGHT SNOW OR FLURRIES LINGERING INTO THE
EVENING BUT FORCING WILL BECOME MUCH MORE NEBULOUS WITHIN THE
REMNANT LARGER SCALE UPPER TROUGH AXIS. NEUTRAL DYNAMIC SUPPORT WILL
BE COUPLED WITH WEAK BUT NEGATIVE THETA-E ADVECTION IN THE 850 TO
700 MB LAYER OVERNIGHT WHICH WILL LEAD TO SOME BREAKS IN THE CLOUD
COVER AND TEMPS SETTLING INTO THE TEENS TO LOWER 20S BY SUNRISE
THURSDAY.

LONG TERM...

SW-NE UPPER LEVEL TROUGH/SHEAR AXIS WILL BE SWINGING THROUGH THE
GREAT LAKES REGION ON THURSDAY...BUT WITH A LARGE AREA OF SURFACE
HIGH PRESSURE TUCKED UNDERNEATH...NO BIG CONCERNS FOR MEASURABLE
PRECIPITATION...BUT WOULD NOT RULE OUT A FEW FLURRIES WITH FAIRLY
COLD AIRMASS (925 MB TEMPS IN THE UPPER NEGATIVE SINGLE NUMBERS/850
MB TEMPS AROUND -10 C). THIS COLD LOW LEVEL AIRMASS/SURFACE RIDGE
WILL LINGER INTO THURSDAY EVENING...WITH WARM ADVECTION/RETURN FLOW
AROUND THE DEPARTING HIGH/RIDGE AXIS BEGINNING TO INCREASE OVERNIGHT
THURSDAY...BUT STILL STRUGGLING TOWARD THE OHIO BORDER...AND
EXPECTING TEMPS TO FALL PREDOMINATELY INTO THE TEENS...WITH LOWER
20S OVER THE URBAN HEAT ISLAND OF DETROIT AND TOWARD TRI-CITIES
REGION...WHERE CONVERGENT SOUTHWEST FLOW EXPECTED TO HELP FACILITATE
DEVELOPING LOW CLOUDS OFF LAKE MICHIGAN. WITH STRENGTHENING WARM
ADVECTION SEEN AT THE 850 MB LEVEL...RISING TOWARD ZERO...BECOMING
MORE PESSIMISTIC FOR FRIDAY...AS EURO/NAM/GFS ALL SHOWING A GOOD
DEAL OF MOISTURE UNDERNEATH (950-925 MB)...AND WILL BE INCREASING
CLOUD COVER...BUT STILL THINK THE LOW LEVEL WARM ADVECTION WILL BE
ENOUGH TO WARM TEMPS JUST ABOVE THE FREEZING MARK AS SURFACE COLD
FRONT APPROACHES. THE WEAK COLD FRONT LOOKS TO BE SLIDING THROUGH
THE CENTRAL GREAT LAKES ON SATURDAY WITH LITTLE FANFARE/MARGINAL
UPPER LEVEL SUPPORT. LIGHT WESTERLY FLOW AND WEAK COLD ADVECTION
SATURDAY AFTERNOON WILL BE OFFSET A BIT BY DIURNAL IMPACT...WITH 925
MB TEMPS (-2.5 TO - 3 C) SUGGESTING MID TO UPPER 30S IS OBTAINABLE.

MARINE...

A RIDGE OF HIGH PRESSURE EXTENDING FROM HIGH PRESSURE OVER THE
MIDWEST WILL TRACK THROUGH THE CENTRAL GREAT LAKES OVER THE NEXT 24
TO 36 HOURS.  AS THIS OCCURS...A BRIEF UPTICK IN NORTHWEST WINDS
LOOKS TO OCCUR OVER THE NORTHERN THIRD OF LAKE HURON THURSDAY
MORNING...WITH GUSTS UP AROUND 20 KNOTS. WINDS WILL SHIFT TO THE
SOUTHWEST THURSDAY NIGHT...AND INCREASE AS WE HEAD INTO FRIDAY AHEAD
OF A COLD FRONT. SUSTAINED WINDS OF 20 TO 25 KNOTS ARE EXPECTED OVER
THE CENTRAL PORTIONS OF LAKE HURON. HOWEVER...GUSTS SHOULD NOT BE
MUCH HIGHER AS WARMER AIR STREAMS IN...LEADING TO STABLE LOW LEVEL
PROFILES BY FRIDAY EVENING.

&&

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

$$

AVIATION.....SC
UPDATE.......SC
SHORT TERM...BT
LONG TERM....SF
MARINE.......SF


YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE
AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).


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