907 
FXUS63 KDTX 212323
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
623 PM EST THU FEB 21 2013

.AVIATION...

//DISCUSSION...

SUSTAINED SUBSIDENCE INTO A DRYING SUB-INVERSION LAYER WILL WORK 
TOWARD MAINTAINING THE LATE DAY CLEARING TREND THROUGH THE EARLY 
PORTION OF THE NIGHT.  A LOWER PROBABILITY DOES EXIST FOR THE 
LINGERING LAKE HURON ENHANCED STRATUS DECK ANCHORED ACROSS THE 
THUMB TO DRIFT SOUTHWEST WITH TIME...BRINGING IT BACK IN THE 
VICINITY OF DETROIT OVERNIGHT.  OTHERWISE...CLEAR SKIES EXPECTED 
BELOW 15K FT THROUGH MUCH OF THE NIGHT WITH A THICKENING CIRRUS DECK 
FILLING IN DOWNSTREAM OF AN APPROACHING PLUME OF DEEPER MOISTURE.  
THIS MOISTURE WILL RAPIDLY TRANSLATE ACROSS ALL TERMINALS 12-16Z
FRIDAY MORNING LEADING TO A GOOD 2-3 HOUR BURST OF LIFR/IFR VSBY
SNOWFALL DURING THAT TIMEFRAME. DEEPER LOW LEVEL SATURATION MAY
PROMOTE FURTHER DETERIORATION IN CEILINGS AS THE MID LEVEL DRY
SLOT WORKS IN BEHIND THIS SNOWFALL. THIS WILL INTRODUCE THE
POSSIBILITY FOR SOME PATCHES OF LIGHT DZ/FZDZ...ALTHOUGH LOW
CONFIDENCE IN BOTH TIMING AND COVERAGE WILL PRECLUDE A SPECIFIC
MENTION AT THIS TIME.

FOR DTW...RECENT SATELLITE AND OBSERVATIONAL TRENDS SUGGEST THE 
LINGERING STRATUS TO THE EAST WILL HOLD IN THAT POSITION.  
HOWEVER...GIVEN AN EAST WIND THE POTENTIAL DOES EXIST FOR THE 
WESTERN FRINGES OF MVFR STRATUS TO DRIFT BACK TOWARD THE TERMINAL 
YET TONIGHT.  PROBABILITY REMAINS TOO LOW TO INCLUDE...AND WILL 
CONTINUE TO MONITOR TRENDS.  SNOWFALL EXPECTED TO BEGIN 
11-12Z...RAPIDLY INCREASING IN INTENSITY IN THE 12-14Z PERIOD.  THIS 
WILL RESULT IN 1-2 INCHES OF ACCUMULATION BEFORE THE SNOWFALL 
INTENSITY AND COVERAGE QUICKLY DIMINISHES AFTER 15Z.  

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CEILINGS FALLING BELOW 5000 FT AFTER 12Z
  FRIDAY MORNING.

* HIGH CONFIDENCE IN SNOW AS PRECIPITATION TYPE FRIDAY MORNING.

&&

.PREV DISCUSSION...ISSUED 409 PM EST THU FEB 21 2013 

SHORT TERM...TONIGHT

THE BULK OF THE SHORT TERM PERIOD WILL BE CHARACTERIZED BY HIGH 
AMPLITUDE RIDGING THAT HAS DEVELOPED BETWEEN YESTERDAY'S DEPARTING 
LOW NOW OFF THE ATLANTIC COAST AND THE DEEPENING LOW OVER THE 
CENTRAL PLAINS. LONGWAVE FEEDBACK WITHIN THIS HIGH AMPLITUDE PATTERN 
WILL ENSURE ONLY A SLOW EASTWARD PROGRESSION OF THE UPSTREAM CYCLONE 
DURING THE OVERNIGHT HOURS. AS A RESULT, DEEP LAYER SUBSIDENCE AND 
HIGH PRESSURE WILL REMAIN IN CONTROL THROUGH AT LEAST 06Z WITH 
NOTHING MORE THAN HIGH CLOUDS ADVECTING ACROSS THE RIDGE AXIS AND A 
PARTIAL COVERAGE OF STRATUS IN THE LOW-LEVELS THROUGH THE EVENING 
UNTIL EASTERLY FLOW BUILDS IN MORE AGGRESSIVELY AFTER 00Z. THE 12Z 
PROGS ARE IN STRONG AGREEMENT THAT MODERATE STRENGTH ISENTROPIC 
ASCENT WILL DEVELOP AND WILL BE ONGOING AFTER 06Z THIS EVENING IN 
RESPONSE TO THE APPROACH OF A PV ANOMALY THAT WILL DEEPEN ALONG THE 
NORTHERN FRINGE OF THE 150KT JET OVER THE SOUTHERN STATES. 
COINCIDENT WITH THE APPROACH OF THIS WAVE WILL BE A RAMPING UP OF 
BROAD SCALE ASCENT ALONG WITH DEVELOPING STRONG LEFT EXIT FORCING 
AND ASSOCIATED FGEN LIFTING INTO SOUTHEAST MICHIGAN BY AROUND 09Z. 
GIVEN THE LOW STATIC STABILITY ENVIRONMENT INDICATED ON 12Z NAM 
CROSS-SECTIONS, CONFIDENCE IS HIGH THAT LIGHT TO MODERATE SNOWFALL 
WILL BE SPREADING INTO THE CWA FROM THE SOUTHWEST TO NORTHEAST 
DURING THIS TIME. WITH TOP-DOWN SATURATION STILL SOMEWHAT OF A 
CONCERN, PRECIP STILL APPEARS LIKELY TO BE CONFINED TO LOCATIONS 
SOUTH OF M59 BY 12Z IN SPITE OF THE RATHER AGGRESSIVE QPF OFFERED BY 
THE EC/UK/GEM.

LONG TERM...

THE WINTER SYSTEM THAT HAS BEEN DRONED ABOUT FOR THE ENTIRETY OF THE 
PAST WEEK WILL BE ONGOING ACROSS THE SOUTHERN 1/2 TO 2/3RDS OF THE 
CWA AT 12Z FRIDAY. LEFT EXIT REGION DYNAMICS OF A VERY WELL 
DEVELOPED UPPER LEVEL JET STREAK WILL INITIALLY SUPPORT THE MAIN 
CLOSED LOW ANOMALY OVER PORTIONS OF MID MISSISSIPPI RIVER VALLEY 
HEADING INTO THE START OF THE PERIOD. THE NOSE OF THE UPPER LEVEL
JET WILL EXTEND RAPIDLY EASTWARD...ORPHANING THE CLOSED LOW
CIRCULATION TO THE WEST. HOWEVER...WHAT WILL HAPPEN IS THAT A
PORTION OF THE U/L PV ANOMALY WILL FRACTURE AND SPIRAL THROUGH
SOUTHEASTERN MICHIGAN BETWEEN 12-18Z. THIS PV SPLINTER IS A SIGNAL
OF THE VERY STRONG PRESSURE GRADIENT AT THE 305K SURFACE. THE BEST
WAA/FRONTAL FORCING WILL BE RATHER HIGH IN THE COLUMN...AT UPWARDS
OF 600MB. THE MAIN JET AXIS SOUTH OF THE AREA SHOULD KEEP THE
TRUE WARM SECTOR TO THE SOUTH. SOME DIFFERENCES EXIST...WITH THE
ECMWF SNEAKING SOME RELATIVELY WARMER LOW LEVEL AIR INTO AND
NORTHWARD THROUGH THE DETROIT METRO AREA. THE MAIN POINT TO DRIVE
HOME IS THAT MOST TO ALL OF THE MEANINGFUL PRECIPITATION RATES
WILL OCCUR WHEN THE THERMAL PROFILES ARE SUPPORTIVE OF SNOW.
300-305K ISENTROPIC ASCENT IS STREAMLINED WITH DEEP SATURATION
ALONG THE MAIN MOISTURE AXIS UPWARDS OF 30 KFT. STEEP NEAR DRY
ADIABATIC LAPSE RATES WILL EXIST 300-500MB WHICH IF YOU ADD IT
ALL UP...SUGGESTS THERE WILL BE GOOD PRECIPITATION RATES TOMORROW
IN A ZONE THAT WILL LIFT NORTHWARD DURING THE MORNING. FAVORABLE
THERMODYNAMICS ARE THERE...FRONTAL FORCING SET AT/JUST BELOW -14C
WITH AN ISOTHERMAL AGGREGATION LAYER (-4C) BETWEEN 3-9 KFT AGL.
THERE IS A BIG NEGATIVE HOWEVER...THE NEGATIVE FOR AMOUNTS IS THAT
DURATION WILL NOT BE LONG AT ALL...AN HOUR OR TWO FOR ANY ONE
AREA. THIS WILL LIMIT SNOWFALL ACCUMULATIONS TO AROUND AN INCH
SOUTH OF M 59...1 TO 2 POSSIBLE FROM M 59 NORTHWARD THROUGH THE
THUMB...AND 1 TO 3 INCHES FOR THE SAGINAW VALLEY.

A WELL DEVELOPED AND RIP ROARING DRY SLOT WILL THEN LIFT THROUGH THE 
ENTIRETY OF THE CWA PRIOR TO THE NOON HOUR. A TRANSITION TO DRIZZLE 
WITH A WINTRY MIX LOOKS GOOD AS THERE SHOULD BE A LOSS IN ICE 
NUCLEATION. BASICALLY KEPT TO THE INHERITED WEATHER WITH NO QUALMS 
WITH THE GRIDS. FROM THIS VANTAGE POINT...NOT EXPECTING THE FREEZING 
DRIZZLE TO THE BE THE MOST IMPACTFUL WX OF THIS EVENT GIVEN THAT 
FREEZING DRIZZLE WILL BE FALLING INTO THE FRESH SNOW/OR WET
TREATED ROADWAYS. WILL OFFER LATER FORECAST SHIFTS THE FREEDOM TO
MONITOR ADDITIONAL OBSERVATIONAL DATA.

THE GFS ARRIVED WITH SIGNIFICANT DIFFERENCES IN HANDLING THE 
SHORTWAVE ENERGY FOR EARLY NEXT WEEK. ONSET TIMING OF PRECIPITATION IS 
DIFFERENT BY SOME 24 HOURS NOW. LITTLE TO NO CHANGES HERE OUTSIDE OF 
THE OFFERED GUIDANCE.

MARINE...

A STRONG EAST OR SOUTH-SOUTHEAST BREEZE WILL DEVELOP LATE TONIGHT 
INTO FRIDAY AS LOW PRESSURE SYSTEM LIFTS OUT OF THE CENTRAL PLAINS. 
ALTHOUGH SUSTAINED WINDS WILL REACH 25 KNOTS OR HIGHER...STABILITY 
IS EXPECTED TO LIMIT WIND GUST POTENTIAL TO AROUND 30 KNOTS. 
SIGNIFICANT WAVE HEIGHTS SHOULD HAVE NO PROBLEM BUILDING TO 7 TO 9 
FEET OVER THE OPEN WATERS OF CENTRAL LAKE HURON AND NEAR WEST-FACING 
SHORES WHERE THE LENGTH OF THE EASTERLY WIND FETCH WILL BE 
MAXIMIZED. THE APPROACH OF THE CENTER OF THE WEAKENING LOW WILL THEN 
BRING COMPARATIVELY TRANQUIL CONDITIONS AND SIGNIFICANT WAVE HEIGHTS 
UNDER 4 FEET FOR THE REST OF THE WEEKEND.

&&

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

&&

$$

AVIATION.....MR
SHORT TERM...99
LONG TERM....CB
MARINE.......99

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