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