Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 301701
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1201 PM EST FRI JAN 30 2015


.AVIATION...

BRISK NORTHWEST WINDS GUSTING AT OR SLIGHTLY ABOVE 20 KNOTS HAS
USHERED IN ENOUGH DRY LOW LEVEL AIR TO SUPPORT MOSTLY CLEAR-PARTLY
CLOUDY SKIES OVER MUCH OF SOUTHEAST MICHIGAN EARLY THIS AFTERNOON.
HOWEVER...LOW LEVEL MOISTURE...HIGH MVFR/LOW VFR CLOUDS REMAIN
LURKING OVER WESTERN LOWER MICHIGAN THIS AFTERNOON. WITH WINDS
BACKING TOWARD THE WEST...THESE CLOUDS WILL ATTEMPT TO MOVE EAST.
BANKING ON THE DAYTIME MIXING TO BE SUFFICIENT TO CONTINUE TO ERODE
THE CLOUDS AS THEY ATTEMPT TO PUSH EAST...BUT CONFIDENCE LOWERS AS
WE HEAD TOWARD SUNSET. WITH WEAK WARM FRONT LIFTING THROUGH AND VERY
LIGHT SOUTHWEST WINDS...WILL ALLOW FOR LOWER CEILINGS (AROUND 3000
FEET) TO WORK THROUGH SOUTHEAST MICHIGAN THIS EVENING/TONIGHT.

FOR DTW...THE STRONGEST NORTHWEST WINDS ARE BEHIND US...BUT FEW MORE
GUST AROUND 20 KNOTS ARE EXPECTED THIS AFTERNOON WITH WINDS QUICKLY
DROPPING OFF AROUND SUNSET. EXACT TIMING AND DURATION OF LOW CLOUDS
RETURNING IS LOW.

//DTW THRESHOLD PROBABILITIES...

* LOW CONFIDENCE IN CIGS BELOW 5000 FT THIS AFTERNOON INTO
  THIS EVENING...MEDIUM TONIGHT...LOW AGAIN TOMORROW MORNING.

&&

.PREV DISCUSSION...ISSUED 400 AM EST FRI JAN 30 2015

SHORT TERM...LATE THIS AFTERNOON AND TONIGHT

A CONTRACTING MIDLEVEL TROUGH AXIS AND ATTENDENT SHORTWAVE IS
DIGGING THROUGH SOUTHEASTERN MICHIGAN AT 8Z. INCREASED MIXING DEPTHS
AND STRONG GRADIENT FLOW IN THE WAKE OF THE COLD FRONTAL PASSAGE HAS
LED TO BLUSTERLY CONDITIONS WITH WIDESPREAD NORTH TO NORTHEASTERLY
WINDS IN THE 25 TO 40 MPH RANGE.

STEEP LOW LEVEL LAPSE RATES IN ADDITION TO A REASONABLY DEEP
ISOTHERMAL LAYER HANGING AROUND -15C RESULTED IN A WIDESPREAD
COVERAGE OF LIGHT SNOW FOR THE EASTERN 1/2 TO 2/3RDS OF THE CWA.
GAUGING FROM SURFACE OBSERVATIONS AND REFLECTIVITY...SUSPECT THAT
ACCUMULATIONS HAVE BEEN VERY LIGHT AND LIKELY EVEN INDISCERNIBLE
GIVEN THE BLOWING COMPONENT. THE DEPENDENCE OF THE SETUP ON
THERMODYNAMICS AND MOISTURE SUGGESTS THIS SNOW WILL RAPIDLY WIND
DOWN THIS MORNING AS DRY AIR ADVECTING INTO THE AREA DRIES OUT THE
800-900MB LAYER. EXPECTATIONS ARE FOR THIS TO OCCUR BY 8AM. LAST FEW
REFLECTIVITY IMAGES...SHOW SOME BANDING OF THE SNOW. THIS APPEARS TO
BE A DOWNSTREAM RESPONSE TO MASS CONVERGENCE IN ADVANCE OF YET
ANOTHER THETA E DISCONTINUITY. OVERALL STRUCTURE AND APPEARANCE
SUGGESTS A SQUALL LIKE BEHAVIOR. THE PROBLEM IS...HAVE NOT SEEN ANY
SIGNIFICANT REDUCTION IN SFC OBSERVATIONS...AND IF THIS IS IN FACT
THE FORCING...LONGEVITY OF THE SETUP IS VERY SHORT TERM...OVER THE
NEXT COUPLE OF HOURS OR LESS. CURRENT THINKING IS THIS WILL END
BEFORE THE START OF THE MORNING COMMUTE. WILL MONITOR FOR NOW AND
ADDRESS WITH SPS IF NEED BE.

THE TROUGH AXIS WILL QUICKLY TURN TO A NEGATIVE TILT PUSHING WELL
EAST OF LOWER MICHIGAN TODAY ALLOWING SURFACE HIGH PRESSURE TO
AGGRESSIVELY BUILD EASTWARD. EXPECT BREEZY NORTHERLIES THROUGH THE
MORNING HOURS...THEN QUICKLY DIMINISHING LATE THIS AFTERNOON.
TEMPERATURES TODAY WILL BE COLD...RANGING BETWEEN 15 AND 20 DEGREES
FOR HIGHS.

A FLAT SHORTWAVE RIDGE IS FORECASTED TO ZIP ACROSS THE WESTERN GREAT
LAKES TONIGHT NUDGING THE THETA E GRADIENT UP AGAINST SOUTHEASTERN
MICHIGAN VERY LATE TONIGHT. WEAK SHORTWAVE ENERGY ON THE BACKSIDE OF
THE RIDGE WILL ARRIVE...BUT UNDER A HEAVY DOSE OF
SHEARING/CONFLUENCE ALOFT. QUALITATIVELY...THE AREA WILL BE IN THE
CORRECT SPOT FOR ASCENT DUE TO ISENTROPIC ASCENT ALOFT. FOR THIS
REASON WILL INCLUDE A LOW CHANCE POP TONIGHT...BUT MOISTURE BELOW 10
KFT AGL WILL BE LACKING. NO SNOW ACCUMULATIONS ARE EXPECTED.
MASSAGED TEMPERATURES DOWNWARD GIVEN THE PROJECTED
DEWPOINTS...CURRENT FORECAST WILL HAVE LOWS RANGING IN THE SINGLE
DIGITS ABOVE ZERO.

LONG TERM... SATURDAY THROUGH THURSDAY

PRIMARY FOCUS DURING THIS LONG TERM PERIOD CENTERS ON THE SUNDAY AND
SUNDAY NIGHT PERIODS...WITH A SIZABLE SHIFT IN EXPECTATIONS TOWARD A
GREATER POTENTIAL OF A LONG DURATION ACCUMULATING SNOWFALL IMPACTING
A PORTION OF SE MICHIGAN.

COMPLEX UPPER LEVEL PATTERN HEADING INTO THE WEEKEND...FEATURING A
CLOSED MID LEVEL CIRCULATION NOW CENTERED OVER BAJA TUCKED BENEATH
AN ENERGETIC WEST-NORTHWEST NORTHERN STREAM.  MODEST HEIGHT FALLS
ATTENDANT TO A LEAD CLIPPER SYSTEM PIVOTING ACROSS SOUTH-CENTRAL
CANADA WILL SHIFT AN ARCTIC FRONTAL BOUNDARY SOUTH AND ACROSS THE
REGION SATURDAY NIGHT.  MODEST THERMAL RESPONSE ON SATURDAY AS THE
THE PRE-FRONTAL SOUTHWEST GRADIENT TEMPORARILY TAKES HOLD.  THIS
WILL ALLOW TEMPERATURES TO RECOVER INTO THE 30 DEGREE RANGE.  A
BRIEF INCREASE IN MOISTURE QUALITY IMMEDIATELY ALONG THE FRONTAL
SLOPE MAY BE SUFFICIENT TO GENERATE SOME LIGHT SNOW SHOWERS LATE
SATURDAY/SATURDAY EVENING...LOW POTENTIAL FOR ACCUMULATION GIVEN THE
LIMITED DURATION AND INTENSITY.

A PIECE OF THE SOUTHWEST CLOSED LOW WILL SHEAR EAST-NORTHEAST
COINCIDENT WITH THE ARRIVAL OF A STRONGER WAVE EMANATING OUT OF THE
GULF OF ALASKA.  THE INTERACTION OF THIS ENERGY YIELDING
CONSIDERABLE VARIANCE WITHIN THE MODEL SUITE IN RECENT DAYS...BUT
HAVE LARGELY CONVERGED ON A SOLUTION SPACE WHICH NOW PROVIDES A
NOTEWORTHY NORTHWARD ADJUSTMENT IN THE POSITIONING OF THE GOVERNING
DYNAMICS AND A CORRESPONDING SHIFT TOWARD GREATER SNOWFALL
POTENTIAL.  THIS PROCESS WILL INITIATE AN AGGRESSIVE NORTHEASTWARD
ADVECTION OF THE SOUTHERN PACIFIC MOISTURE PLUME CURRENTLY TIED TO
THE SOUTHWEST CONUS LOW.   THIS MOISTURE WILL QUICKLY WORK ACROSS
THE ELEVATED PORTION OF THE EXITING ARCTIC FRONT...RESULTING IN AN
EXPANDING AREA OF SNOWFALL BY LATE SATURDAY NIGHT.  THIS SETUP
POINTS TOWARD A LONG DURATION OF LIGHT TO MODERATE SNOWFALL SUNDAY
INTO EARLY SUNDAY NIGHT...FORCED THROUGH A SUSTAINED PERIOD OF MID
LEVEL FRONTOGENESIS UNDER FAVORABLE ENTRANCE REGION UPPER JET
DYNAMICS AND INCREMENTAL SPIKES IN CVA.  A DRY NORTHEAST FLOW AND
DIMINISHING ASCENT WITH NORTHWARD EXTENT WILL MAINTAIN A FIRM
NORTHERN GRADIENT TO THIS SNOWFALL...PROVIDING A HIGHER DEGREE OF
UNCERTAINTY ON THE POTENTIAL ACROSS THE TRI-CITIES AND THUMB
CORRIDOR.

MODEL MOISTURE FIELDS ALLOW SPECIFIC HUMIDITY WITHIN THE 850-700 MB
LAYER TO REACH 3 G/KG DURING THE PEAK OF THE EVENT.  THE LONG
RESIDENCE TIME OF THIS ENVIRONMENT TRANSLATES INTO A SIGNIFICANT
UPTICK IN QPF WITHIN THE NAM/GFS/ECMWF.  AT THIS STAGE AND GIVEN THE
SIZABLE MODEL ADJUSTMENT...GOING FORECAST WILL LEAN IN THE DIRECTION
OF THE NAM...WITH A LEERY EYE ON THE HIGHER END AMOUNTS NOW SHOWN ON
THE ECMWF /.4" M-59 CORRIDOR TO .7" OHIO BORDER/.  THE ENSUING POST-
FRONTAL COLD AIR ADVECTION SATURDAY NIGHT WILL LEAVE A COLDER
THERMODYNAMIC PROFILE IN PLACE...SUGGESTING SNOW RATIOS WILL ARRIVE
ABOVE CLIMO /BALLPARK 16-17:1 AT THIS STAGE/.  A BROADER 3 TO 6 INCH
TYPE SWATH LOOKS ATTAINABLE...WITH A 6+ INCH BAND NOW WITHIN THE
ENVELOP OF POSSIBILITIES.

MARINE...

ONGOING NORTHWEST GALES ACROSS LAKE HURON THIS MORNING WILL EASE
THROUGH THE AFTERNOON PERIOD AS HIGH PRESSURE BUILDS INTO THE
REGION.  WINDS WILL SHIFT TO SOUTHWESTERLY ON SATURDAY IN THE WAKE
OF THIS HIGH.  ARCTIC FRONT SHIFT THROUGH SATURDAY NIGHT WILL THEN
BRING AN INCREASING NORTHEAST WIND INTO SUNDAY.  THIS WILL RESULT IN
SOME GUSTS TO 30 KNOTS.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...HEAVY FREEZING SPRAY WARNING UNTIL 7 PM EST THIS EVENING FOR LHZ362-
     363-441-462>464.

LAKE ST CLAIR...NONE.
MICHIGAN WATERS OF LAKE ERIE...NONE.
&&

$$

AVIATION.....SF
SHORT TERM...CB
LONG TERM....MR
MARINE.......MR


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AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).


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