Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 310205
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1005 PM EDT MON MAR 30 2015

.UPDATE...

VERY INTERESTING FORECAST SETTING UP FOR THE OVERNIGHT HOURS AS A
SMALL BUT VIGOROUS SHORTWAVE SHEARS THROUGH THE AREA ALONG THE FAR
SOUTHERN EDGE OF POLAR UPPER LOW POSITIONED OVER/EAST OF HUDSON
BAY. WITH THE UPPER LOW SO FAR NORTH...THE AREA WILL REMAIN ON THE
EDGE OF A RAIN VERSUS SNOW SCENARIO...MAKING SNOW ACCUMULATION
FORECASTS RATHER ELUSIVE OVERNIGHT.

SEVERAL FACTORS SUPPORT ENOUGH COOLING TO CHANGE A FAIR AMOUNT OF
THIS PRECIPITATION TO SNOW OVERNIGHT. FIRST...THE SYSTEM WILL MOVE
THROUGH THE REGION DURING THE DIURNAL TEMPERATURE MINIMUM. IN
FACT...TEMPERATURES OVER THE THUMB THIS EVENING ARE ALREADY AOB
FREEZING...WITH TEMPERATURES FURTHER SOUTH WHERE BEST PRECIPITATION
BANDING IS EXPECTED NOW AROUND 40F.

THE COOLING PROCESS WILL BE AIDED BY EVAPORATIONAL COOLING AS THE
LEAD BAND OF PRECIPITATION WITHIN A BAND OF ISENTROPIC LIFT FALLS
INTO A RATHER DRY LAYER OF AIR FROM H85 TO THE SURFACE AND ALSO
FROM A GENERAL AREA OF DYNAMIC LIFT FROM THE SYSTEM ITSELF THAT
WILL COOL THE ENTIRE COLUMN OVER THE AREA WITH TIME SLIGHT DURING
THE OVERNIGHT. THIS SHOULD BE NOTICEABLE GIVEN PRECIPITATION
RATES OF NEAR ONE-TENTH INCH AN HOUR FOR 3 OR 4 HOURS.

WITH SHORT TERM MODELS PINPOINTING A BAND OF ONE THIRD/ONE HALF
AN INCH OF QPF FOCUSING GENERALLY BETWEEN I 69 AND I 94...THE
FINAL QUESTION WILL BE WHICH LOCATIONS WILL SEE THAT FINAL 1000
FEET OF THE ATMOSPHERE COOL ENOUGH TO SUPPORT ACCUMULATING SNOW.
THE GENERAL TERRAIN OVER THE AREA SUPPORTS AREAS ACROSS NORTHERN
OAKLAND COUNTY BACK TOWARDS WEST/NORTHWEST ALONG THE M-59 AND I-69
CORRIDOR FROM LAPEER/FLINT WEST. THIS IS A VERY COMMON DELINEATIONTHIS
LATE IN THE SEASON FOR THE AREA.

SO...ADJUSTING QPF TOTALS/POSITIONING SLIGHTLY BASED ON SHORT TERM
TRENDS THIS EVENING AND REFINING THE MOST LIKELY CORRIDOR FOR A
NOTABLE PERIOD OF SNOWFALL WITH SNOW TO LIQUID RATIOS APPROACHING
10:1...HAVE NARROWED A 2 TO 3 INCH AREA OF SNOWFALL FROM AROUND
HOWELL...OWOSSO AREA INTO NORTHERN OAKLAND COUNTY.

AMOUNTS SHOULD TAPER OFF TO A TRACE OR LESS SOUTH OF I-94 WHERE
THE BOUNDARY LAYER WILL BE JUST A BIT TOO MILD FOR SIGNIFICANT
SNOW ACCUMULATION. TO THE NORTH...TEMPERATURES WILL BE COLD ENOUGH
IN THE BOUNDARY LAYER...BUT THE NORTHERN EDGE OF THE HEAVIEST
PRECIPITATION BAND WILL LIKELY BYPASS THE NORTHERN SAGINAW VALLEY
AND THUMB WITH AMOUNTS FORECAST TO TAPER FROM 1 OR 2 INCHES JUST
NORTH OF I-69 TO PERHAPS A DUSTING ALONG THE FAR NORTHERN FRINGE
OF THE CWA.

&&

.AVIATION...ISSUED 651 PM EDT MON MAR 30 2015

VIGOROUS SHORTWAVE DISTURBANCE WILL BRING A BRIEF...BUT FAIRLY
INTENSE...PERIOD OF FGEN TO THE AREA OVERNIGHT...PARTICULARLY KFNT
SOUTHWARD. WHILE LOCATION OF THIS BAND WILL BE TRICKY...THE PRECIP
TYPE FOR ANY GIVEN TAF WILL BE EVEN MORE DIFFICULT TO ASCERTAIN.
THIS PRECIPITATION WILL BEGIN AS RAIN. HOWEVER...DIURNAL COOLING
WILL COMBINE WITH BOTH DYNAMIC COOLING FROM THE SYSTEM/S LIFT AND
EVAPORATIVE COOLING AS THE RAIN FALLS INTO INITIALLY DRY LOW LEVELS.
THIS WILL LEAD TO A SWITCH TO SNOW...WHICH MAY BE RATHER IMPRESSIVE
FOR A TIME TONIGHT. WILL MAINTAIN CURRENT FORECAST MORE OR LESS AS
RAPID CYCLE GUIDANCE STILL SUGGESTS BEST FGEN BANDING FROM NEAR THE
I 69 CORRIDOR TO AROUND I 94. KFNT/KPTK SHOULD CHANGE TO SNOW A BIT
QUICKER AS HIGHER ELEVATIONS ALLOW LOW LEVELS TO COOL FASTER. IN
FACT...THE I 94 TAFS MAY REMAIN PRIMARILY RAIN. MVFR TO IFR CIGS AND
VSBYS CAN BE EXPECTED WITH STEADY IMPROVEMENT ON TUESDAY AS THIS
SYSTEM RACES OFF TO THE EAST.

FOR DTW...FGEN WILL BRING RAIN TO THE TERMINAL BY 04Z-05Z WITH THIS
CONTINUING UNTIL 10Z-12Z. MVFR TO OCCASIONAL IFR CIGS/VSBYS CAN BE
EXPECTED. SOME SNOW MAY MIX WITH THE RAIN OVERNIGHT...BUT PRECIP AT
THIS TIME APPEARS TO BE PRIMARILY RAIN.

//DTW THRESHOLD PROBABILITIES...

* HIGH FOR CEILING AOB 5000 FEET OVERNIGHT AND TUESDAY MORNING.

* LOW CONFIDENCE IN BRIEF PERIOD OF SNOW AS PTYPE 9-12Z.

&&

.PREV DISCUSSION...ISSUED 359 PM EDT MON MAR 30 2015

SHORT TERM...TONIGHT

EXPANDING RADAR ECHOES OVER WISCONSIN AT PRESS TIME INDICATE THAT
285K-295K ISENTROPIC ASCENT HAS BEGUN TO STRENGTHEN IN ADVANCE OF
THE COMPACT WAVE OVER NORTHERN MINNESOTA THIS AFTERNOON. THIS ASCENT
WILL INCREASE CONSIDERABLY AS THE EVENING WEARS ON AND RELEASE OF
BAROCLINIC INSTABILITY DUE TO THE RESIDENT POLAR FRONT GETS
UNDERWAY. THE FAST FORWARD SPEED OF THE CLIPPER AND AND A DEVELOPING
50KT LOW-LEVEL JET WILL PRODUCE MODERATE TO STRONG 925MB-850MB FGEN
ALONG THE LEAD EDGE OF THE THETA-E RIDGE. CONSENSUS HAS INCREASED
CONSIDERABLY WITH REGARD TO BOTH THE LOW TRACK AND MODELED LOCATION
OF HIGHEST QPF ASSOCIATED WITH ACTIVE FGEN ON THE ELEVATED FRONTAL
SLOPE. RAP/WRF-NMM REMAIN OUTLIERS TO THE SOUTH WHILE THE
ECMWF/GFS/NAM/WRF-ARW AND 4KM SPC (NMM CORE) RUN COMPRISE THE
TIGHTLY PACKED CONSENSUS UPON WHICH THE OFFICIAL FORECAST IS BASED.

CROSS-SECTION ANALYSIS REVEALS THAT ASCENT UP THE FRONTAL SLOPE WILL
ENCOUNTER VERY LOW STATIC STABILITY WITH POCKETS OF EMBEDDED
CONVECTIVE INSTABILITY OVER THE CENTRAL/SOUTHERN CWA. THERMODYNAMIC
PROFILES REMAIN VERY MARGINAL, BUT WET-BULB EFFECTS COMBINED WITH
STRONG ASCENT THAT IS UNLIKELY TO BE FULLY CAPTURED AT THE NWP SCALE
SUGGEST A HIGH LIKELIHOOD FOR THE COLUMN TO COOL ENOUGH FOR A BURST
OF MODERATE TO HEAVY SNOW CENTERED ON THE 4-10Z TIME FRAME. AS THE
SURFACE LOW TRACKS JUST SOUTH OF THE OHIO BORDER BETWEEN 06-12Z,
FORCING WILL BECOME INCREASINGLY DOMINATED BY OUTSTANDING DEEP
LAYER DEFORMATION THAT MAY ULTIMATELY BE SUFFICIENT TO SUSTAIN
APPRECIABLE PRECIP RATES WITHIN A NARROW BAND THROUGH 12Z.

THE AFOREMENTIONED MODELS ARE REASONABLE IN THEIR QPF DEPICTIONS
GIVEN THE LOCATION OF LOWEST STATIC STABILITY NOTED IN CROSS-
SECTIONS, AND THEY HAVE HONED IN ON AN OWOSSO-HOWELL-DETROIT LINE
WHERE FORECAST QPF IS AROUND 0.35". THE NAM IS QUALITATIVELY PART OF
THE CONSENSUS, THOUGH IT IS SUFFERING FROM TYPICAL OVERZEALOUS
CONVECTIVE OUTPUT. ELEVATED WARM ADVECTION WILL INHIBIT ANY
CHANGEOVER TO SNOW SOUTH OF ABOUT I-96, BUT WET-BULBING AND STRONG
LIFT SHOULD PRODUCE A BAND OF 1-2, PERHAPS LOCALLY 3", ALONG THE
NORTHERN PERIPHERY OF THE STRONGEST FORCING. AT THIS TIME, EXPECTING
AN ACCUMULATION OF WET SNOW ALONG AN MBS-FNT-PTK-CLINTON TOWNSHIP
LINE, OR ROUGHLY BISECTING THE CWA FROM NW TO SE. CAN CERTAINLY
ENVISION A SCENARIO IN WHICH 1"/RATES DEVELOP FOR JUST AN HOUR OR
TWO BEFORE DIMINISHING OR EVEN CHANGING BACK OVER TO A LIGHT RAIN.
SUCH A SCENARIO WOULD BE MOST LIKELY TO UNFOLD DURING PEAK
FORCING...AROUND 09Z.

LONG TERM...

LOOKS LIKE THERE WILL BE A SHORT PERIOD OF POST WAVE SUBSIDENCE LATE
TOMORROW MORNING/EARLY TOMORROW AFTERNOON...BEFORE YET ANOTHER UPPER
WAVE STREAKS SOUTHEAST THROUGH LOWER MICHIGAN. 500 MB COLD POOL OF
-32 C PROGGED TO MOVE THROUGH THE STRAIGHTS TOWARD EARLY EVENING.
ENOUGH SUNSHINE EXPECTED TO BOOST MAXES INTO THE 40S...COOLER OVER
NORTHERN THUMB REGION WITH THE NORTHEAST FLOW OFF LAKE HURON.

QUESTION MARK IS WHETHER THE LOW LEVELS WILL HAVE DRIED OUT ENOUGH
TO PREVENT A SENSIBLE WEATHER RESPONSE OVER SOUTHEAST MICHIGAN.
SEEMS LIKE SLIGHT CHANCE POPS ARE WARRANTED AS THERE IS SOME
SEMBLANCE OF A WEAK SURFACE/LOW LEVEL REFLECTION/CIRCULATION SHOWING
UP TUESDAY EVENING...PER 12Z GFS/GEM/NAM...WITH TRAILING INVERTED
SURFACE TROUGH OVERNIGHT.  MUCH LIKE TONIGHT...THERMAL PROFILES WILL
BE MARGINAL...HOVERING AROUND ZERO C IN A DEEP LAYER...TEETERING ON
RAIN/SNOW LINE. 12Z EURO REMAINS ADAMANT THERE WILL BE NO MEASURABLE
QPF.

PRONOUNCED UPPER LEVEL RIDGE BUILDING INTO THE GREAT LAKES REGION
WEDNESDAY-THURSDAY WILL PROMOTE A BIG WARMUP. HOWEVER...WARM FRONT
WILL LIKELY STRUGGLE TO LIFT NORTH OF SOUTHEAST MICHIGAN DURING THE
DAYTIME HOURS AS SURFACE WINDS REMAIN BACKED OUT OF THE
SOUTHEAST...MAINTAINING COOLER INFLUENCE OFF LAKE ERIE/ST CLAIR IN
PLAY. STILL...925 MB TEMPS OF 2 TO 5 C SUGGESTS MAXES AROUND
50/LOWER 50S (COOLER THUMB REGION).

STRONG LOW LEVEL JET (IN EXCESS OF 50 KNOTS IN THE 925-850 MB LAYER)
WILL PUMP A GOOD DEAL OF WARM AND MOIST (850 MB DEW PTS IN THE HIGH
SINGLE NUMBERS) AS WEAK HEAD INTO THURSDAY. HIGHS ON THURSDAY WILL
BE DEPENDENT ON WHEN THE RAIN SHOWERS ARRIVE AHEAD OF THE COLD
FRONT...WHICH IS EXPECTED TO MOVE THROUGH AROUND OR JUST BEFORE 00Z
FRIDAY. REGARDLESS...MAXES IN EXCESS OF 60 DEGREES APPEARS TO BE A
GOOD BET...BUT THE POTENTIAL TO HIT 70 DEGREES IS THERE WITH
FAVORABLE SOUTHWEST FLOW AND 850 MB TEMPS UP AROUND 10 C.

VERY LOW CONFIDENCE IN THE THURSDAY NIGHT/FRIDAY TIME FRAME WITH 12Z
GFS/GEFS MEAN PORTRAYING THE COLD FRONT CLEANLY CLEARING SE MI WITH
A DRYING TREND...WHILE GEM/ECMWF DEVELOP A CIRCULATION ALONG THE
FRONT WHICH WOULD KEEP PRECIP IN THE FORECAST. MIDDLE ROAD OF
LOW/MID CHANCE POPS IS THE BEST OPTION UNTIL MORE MODEL
AGREEMENT/CONTINUITY CAN BE ESTABLISHED...HIGHEST POPS ACROSS THE
SE. TEMPS RETURN TO NEAR/A BIT BELOW NORMAL FOR THE WEEKEND BENEATH
LONGWAVE TROUGH. MAINLY DRY FOR THE WEEKEND ALTHOUGH 12Z SUITE
DEPICTS A WEAK COLD FRONT SAT/SAT NIGHT WHICH COULD CAUSE SOME LIGHT
PRECIP. WARMER FOR BEGINNING OF NEXT WEEK WITH RISING HEIGHTS AS THE
UPPER TROUGH TRANSLATES EASTWARD.

MARINE...

GALE WARNINGS FOR NORTH HALF OF LAKE HURON WILL BE CANCELED AS GOOD
PUSH OF LOW LEVEL COLD AIR IS OVER...AND OBSERVATIONS SUPPORTS WIND
SPEEDS 30 KNOTS OR LESS. LOW PRESSURE EXPECTED TO TRACK NEAR THE
SOUTHERN MICHIGAN BORDER LATE TONIGHT INTO TOMORROW MORNING.
ALTHOUGH WINDS WILL BE COMING OUT OF THE NORTH IN THE LOWS WAKE
TOMORROW...WIND SPEEDS ARE ONLY EXPECTED TO TOP OUT AROUND 25
KNOTS...AS THERE IS LITTLE ADDITIONAL PUSH OF COLD AIR BEHIND THE
LOW.

HIGH PRESSURE IN PLACE TUESDAY NIGHT AND WEDNESDAY WILL PROVIDE
LIGHT WINDS AND LOW WAVES. SOUTHERLY WINDS WILL INCREASE WEDNESDAY
NIGHT AHEAD OF A COLD FRONT...WHICH IS EXPECTED TO MOVE THROUGH LATE
THURSDAY. WARM PUSH OF AIR AHEAD OF THE COLD FRONT WILL LEAD TO
STABLE PROFILES...WITH WIND SPEEDS LIKELY REMAINING MOSTLY BELOW 25
KNOTS.

&&

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

$$

UPDATE.......DG
AVIATION.....DG
SHORT TERM...JVC
LONG TERM....SF/DT
MARINE.......SF


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