FXUS63 KDTX 250220
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
920 PM EST THU DEC 24 2009

.UPDATE...

INTERMITTENT POCKETS OF STRONGER WARM AIR ADVECTION/ISENTROPIC 
ASCENT LEADING TO A FEW PATCHES OF LIGHT PRECIPITATION FROM TIME TO 
TIME THIS EVENING...ESPECIALLY INTO THE WESTERN SAGINAW VALLEY.  
HOWEVER A SOLID DRY LAYER WITHIN THE 925-700 MB LEVEL HOLDING TOUGH 
WITH THE FORCED ASCENT PROVING TOO BROAD/DISORGANIZED THIS FAR
EAST TO PROVIDE BETTER SATURATION. THIS WILL REMAIN THE CASE
THROUGH MUCH OF THE NIGHT...WITH PRECIPITATION COVERAGE REMAINING
ISOLATED AT BEST AND VERY LIGHT IN INTENSITY. THE PRESENCE OF A
SUFFICIENT MELTING LAYER WILL ENSURE THIS FALLS IN THE LIQUID FORM
(OCCASIONAL SLEET MIXED IN WITH EVAPORATIVE COOLING/SUBFREEZING WET
BULB TEMPS)...LEAVING A PTYPE OF -RA VERSUS -FZRA DEPENDENT ON
THE NEAR SURFACE TEMPERATURES.

A MORE ORGANIZED BAND OF PRECIPITATION STILL EXPECTED TO EXPAND 
FROM SOUTH TO NORTH ACROSS THE AREA WITHIN THE 11-15Z WINDOW
FOCUSED ALONG THE LEADING EDGE OF A GOOD 850-700 MB THETA-E
GRADIENT/MOISTURE SURGE AND IN ADVANCE OF THE NORTHWARD ADVANCING
OCCLUDED LOW LEVEL FRONTAL ZONE. LATEST OBSERVATIONS ALREADY
INDICATING GRADUAL WARMING IN THE LOWEST LEVELS ACROSS THE FAR
SOUTH. THE DEGREE OF EVAPORATIVE COOLING/PRECIPITATION INTENSITY
ASSOCIATED WITH THIS BAND WILL LIKELY BE INSUFFICIENT TO COOL THE
COLUMN ENOUGH TO FLIP THE SURFACE TEMPERATURES BACK BELOW FREEZING
BEFORE THE WARM AIR ADVECTION STRENGTHENS BY 14-16Z. THEREFORE
WILL LEAVE A CHANCE MENTION FOR FREEZING RAIN ACROSS THE SOUTHERN
TIER...BUT WILL DROP THE ADVISORY FOR LENAWEE/MONROE/WAYNE
COUNTIES WITH THIS UPDATE.

A NARROW 1-2 HOUR WINDOW WILL EXIST DURING THIS TIME WHERE SURFACE 
TEMPERATURES HOVER AT OR JUST BELOW FREEZING TOWARD THE NORTH...
BEFORE THE RAPID WARMING ABOVE THE BOUNDARY LAYER MIXES DOWN AND 
PUSHES SURFACE TEMPERATURES ABOVE FREEZING.  ICE ACCUMULATIONS WILL 
REMAIN MINOR...BUT CERTAINLY THE POTENTIAL REMAINS FOR A LIGHT GLAZE 
TO OCCUR ON SOME UNTREATED...SECONDARY AND ELEVATED SURFACES.
THEREFORE THE ADVISORY WILL REMAIN IN PLACE...WITH A MINOR EDIT TO
REFINE THE ENDING TIME OF THE HEADLINE /BEFORE NOON/.

&&

.PREV DISCUSSION...ISSUED 701 PM EST THU DEC 24 2009 

AVIATION...

LOW LEVEL SATURATION CONTINUES TO BE CONFINED WITHIN A NARROW LAYER 
/800-1400 FT/ WITH A DEEP DRY LAYER THEN EXTENDING 6-9K FT ABOVE 
THIS LAYER.  THIS WILL ENSURE SOME PESKY IFR/LOW MVFR STRATUS 
LINGERS OVERNIGHT...WHILE MAINTAINING DRY CONDITIONS INTO THE EARLY 
MORNING HOURS.  DEEPER MOISTURE WILL LIFT NORTHEAST AND ACROSS THE 
AREA BETWEEN 12-15Z.  A NARROW WINDOW EXISTS FOR A BRIEF PERIOD OF 
FREEZING RAIN DURING THIS TIME AS SURFACE TEMPERATURES INITIALLY 
HOVER AROUND FREEZING.  THIS CONTINUES TO WARRANT A MENTION FROM PTK 
NORTHWARD. FOR THE DETROIT AREA...DURATION WILL BE TOO LIMITED /IF 
AT ALL/ TO MAINTAIN A PREVAILING MENTION OF FREEZING RAIN AT THESE 
TERMINALS. LIGHT RAIN WILL CONTINUE INTO THE AFTERNOON WITH
CEILINGS PROGRESSIVELY LOWERING TOWARD IFR THROUGH THE DAY. WINDS
WILL INCREASE OUT OF THE EAST-SOUTHEAST WITH MIXING SUFFICIENT TO
YIELD GUSTS TO 25 KNOTS DURING THE DAYTIME.  

PREV DISCUSSION...ISSUED 357 PM EST THU DEC 24 2009 

SHORT TERM...THROUGH 8 AM TOMORROW

UPPER LOW SPINNING OVER TEXAS WILL LIFT NORTHEAST TONIGHT...AND 
MERGE WITH THE LARGER BUT WEAKER UPPER SYSTEM OVER THE NORTHERN 
PLAINS. AS THIS OCCURS...SURFACE LOW PRESSURE CENTER OVER ARKANSAS 
WILL DEEPEN AS IT LIFTS UP INTO NORTHERN MISSOURI AND SOUTHERN IOWA 
BY 8 AM CHRISTMAS MORNING. THIS WILL SPREAD A MIX OF PRECIPITATION 
INTO SOUTHEAST MICHIGAN...WITH MOST OF THE ACTION WAITING UNTIL 
AFTER 3 AM.

FIRST BAND OF PRECIPITATION TRIED TO LIFT NORTHWARD ACROSS SOUTHEAST 
MICHIGAN THIS AFTERNOON IN ASSOCIATION WITH AN INITIAL PUSH OF 
THETA-E ADVECTION AND ISENTROPIC LIFT RIDING ALONG AN OLD FRONTAL 
BOUNDARY. NO MEASURABLE PRECIPITATION HAS BEEN REPORT THUS FAR IN 
ASSOCIATION WITH THIS BAND...WITH ASOS AND OTHER GROUND REPORTS 
INDICATING ONLY A FEW SNOW FLURRIES OR FREEZING RAIN SPRINKLES. THIS 
WAS BELIEVED TO BE DUE TO A GOOD AMOUNT OF VERY DRY AIR IN THE 
925-700 MB LAYER THAT WAS INDICATED BY THE 12Z DTX AND APX 
SOUNDINGS. GFS AND NAM BOTH SHOW THIS FEED OF DRY AIR WILL REMAIN IN 
PLACE THROUGH MOST OF THE OVERNIGHT HOURS...BEFORE A BIG PUSH OF 
THETA-E AND ISENTROPIC ASCENT OCCUR AFTER ABOUT 08Z. UP UNTIL 
THEN...MODELS ARE FAIRLY NEUTRAL WITH REGARD TO BOTH OF THESE 
PARAMETERS...AND WITH THE DRY LAYER IN PLACE ALOFT...HAVE KEPT POPS 
LIMITED TO LOW CHANCES UNTIL AFTER 08Z. THIS IDEA IS LENDED STRONG 
SUPPORT BY THE HIRES LOCAL WRF REFLECTIVITY OUTPUT FROM 15Z...WHICH 
SHOWS ONLY AN OCCASIONAL LIGHT SHOWER BRUSHING THE FAR WESTERN PART 
OF THE FORECAST AREA. FORECAST SOUNDINGS ARE MAINLY SUPPORTIVE OF 
FREEZING RAIN OVERNIGHT...BUT WILL ALSO CARRY SLEET MENTION UNTIL 
THE COLUMN MOISTENS AS WET BULB EFFECTS COULD CUT IN TO THE WARM 
LAYER.

AFTER 08Z (3AM)...EXPECT PRECIPITATION TO INCREASE AS THE OCCLUSION 
THROWN OFF THE SYSTEM TO OUR WEST BEGINS TO LIFT INTO SOUTHEAST 
MICHIGAN. MUCH STRONGER ISENTROPIC ASCENT AND BIG PUSH OF LOW/MID 
LEVEL THETA-E WILL QUICKLY WORK TO OVERCOME THE DRY LAYER...ALLOWING 
FOR AN INITIAL MIX OF SLEET AND FREEZING RAIN UNTIL THE COLUMN 
SATURATES...THEN ALL FREEZING RAIN. CANNOT RULE OUT AN EARLIER 
CHANGEOVER TO RAIN BEFORE 8 AM ALONG THE OHIO BORDER HOWEVER. NAM 
SOUNDINGS FOR TOLEDO SHOW ELEVATED WARM LAYER REACHING A MAX TEMP 
AROUND 10 C...AND CONCERNED THE 800 FOOT DEPTH OF FREEZING TEMPS MAY 
NOT BE ENOUGH TO SUPERCOOL THE WATER DROPLETS BEFORE THEY FALL TO 
THE SURFACE.

HAVE WENT AHEAD AND ISSUED A WINTER WEATHER ADVISORY FOR THE ENTIRE 
FORECAST AREA TO COVER LIGHT ICING THAT IS EXPECTED LATE TONIGHT AND 
EARLY TOMORROW...BEFORE PRECIPITATION CHANGES OVER TO ALL RAIN. 
ADVISORY IS BROKEN UP INTO 3 CHUNKS...FALLING IN A 3 AM TO 1 PM 
WINDOW AS AREA OF ICING LIFTS FROM SOUTH TO NORTH.

LONG TERM...CHRISTMAS DAY THROUGH NEXT THURSDAY

MIXED PRECIPITATION WILL QUICKLY CHANGE OVER TO RAIN CHRISTMAS 
MORNING AS STRONG WARM AIR ADVECTION INCREASES 850 MB TEMPERATURES 
TO THE +4 TO +6 CELSIUS RANGE. WITH THE LACK OF ICE CRYSTALS GIVEN A 
DEEP WARM LAYER BETWEEN 2K TO 7K FT AND TEMPERATURES IN THIS LAYER 
HITTING +8 CELSIUS ALL LIQUID IS EXPECTED TO HIT THE GROUND...WITH 
FREEZING RAIN EXPECTED UNTIL SURFACE TEMPERATURES RISE ABOVE 
FREEZING FROM SOUTH TO NORTH. THE FREEZING RAIN COULD LEAD TO 
CONTINUED HAZARDOUS DRIVING CONDITIONS DURING THE MORNING 
HOURS...THEREFORE WILL KEEP THE WINTER WEATHER ADVISORY THROUGH THE 
MORNING HOURS. WITH COPIUS AMOUNTS OF MOISTURE...MIXING RATIOS OF 
5-6 G/KG...AND ISENTROPIC LIFT ALONG STRONG WARM THETA-E 
ADVECTION...POPS WILL BE AT 100 PERCENT THROUGH THE EARLY AFTERNOON 
HOURS.

THE WARMER AIR ADVECTING INTO THE AREA SHOULD LEAD TO WARMER 
AFTERNOON TEMPERATURES...AS ALL OF THE MOS NUMBERS SUGGEST. THE 
WARMER DAYTIME HIGHS AND LATER ARRIVAL OF THE COLD AIR WILL MEAN 
PRECIPITATION TYPE WILL REMAIN AS ALL RAIN THROUGH THE AFTERNOON ON 
CHRISTMAS. ALONG WITH THE WARM AIR...A SIGNIFICANT DRY SLOT OF AIR 
WILL MOVE IN AFTER 18Z AT AND ABOVE 700 MB...FURTHER LIMITING ANY 
POTENTIAL FOR ICE CRYSTAL SEEDING FROM LAYERS FURTHER UP IN THE 
COLUMN. THIS DRY AIR WILL ALSO MEAN MUCH LOWER RAIN CHANCES DURING 
THE LATE AFTERNOON HOURS AS THE MOISTURE AND FORCING HEAD OFF TO THE 
NORTHEAST.

COLD AIR ADVECTS IN FROM THE SOUTH FRIDAY NIGHT AS THE CHANGEOVER TO 
SNOW BEGINS. THE LINGERING WARM AIR WILL MEAN A SLOWER CHANGEOVER 
THAN PREVIOUSLY FORECAST...WITH ALL RAIN LINGERING ACROSS THE 
DETROIT METRO AREA INTO THE EVENING HOURS. THE CHANGEOVER SHOULD BE 
COMPLETE SOMETIME AFTER MIDNIGHT...HOWEVER THE MOISTURE WILL BE VERY 
LIMITED WITH THE MOISTURE DEPTH ONLY EXTENDING TO 850 MB. THIS LACK 
OF MOISTURE AND FORCING IN GENERAL WILL MEAN FAIRLY LOW PRECIP 
CHANCES ONCE THE CHANGEOVER OCCURS WITH THE LOW STILL SITTING ACROSS 
IOWA.

DESPITE A PREVIOUSLY DRY FORECAST...SATURDAY IS LOOKING LIKE A 
BETTER CHANCE FOR SNOW THAN PREVIOUSLY THOUGHT WITH SOME MODEST 
ISENTROPIC LIFT AND 700-850 MB FRONTOGENESIS. WHILE THE MOISTURE 
AVAILABLE WILL BE MEAGER IN THIS LAYER AT 1-1.5 G/KG...IT SHOULD BE 
ENOUGH TO WARRANT CHANCY POPS...WITH HIGHER NUMBERS OUT TO THE WEST 
CLOSER TO THE APPROACHING SURFACE AND UPPER LEVEL LOW PRESSURE 
SYSTEMS.

THE LOW WILL CONTINUE TO MOVE THROUGH THE CENTRAL GREAT LAKES ON 
SUNDAY...WITH A LINGERING TROUGH BEHIND THE LOW LINGERING INTO 
MONDAY. THIS SLOWLY DEPARTING LOW WILL BRING A CONTINUED CHANCE FOR 
SNOW SHOWERS SUNDAY AND MONDAY AS SEVERAL SHORTWAVES PASS THROUGH 
THE CENTRAL GREAT LAKES IN THE NORTHWEST FLOW. HIGH PRESSURE WILL 
THEN BUILD IN BEHIND THE DEPARTING LOW TUESDAY AND 
WEDNESDAY...PROVIDING A DRY PERIOD OF WEATHER BEFORE ANOTHER LOW 
PRESSURE SYSTEM DROPS DOWN OUT OF CANADA LATE IN THE WEEK. THIS LOW 
SHOULD BE COLD ENOUGH FOR ALL SNOW...HOWEVER THE FAST MOVING NATURE 
AND MEAGER MOISTURE ASSOCIATED WITH THIS CLIPPER SHOULD MUTE 
SNOWFALL AMOUNTS AS IS PASSES THROUGH SOUTHERN CANADA.

MARINE...

STRONG EASTERLY WINDS WILL DEVELOP ACROSS THE CENTRAL GREAT LAKES AS 
A STRONG LOW PRESSURE SYSTEM TRACKS NORTHWARD THROUGH THE CENTRAL 
PLAINS TONIGHT AND LIFTS INTO THE MIDWEST TOMORROW. GALE FORCE WIND 
GUSTS ARE EXPECTED TO OCCUR BY LATE TONIGHT ACROSS THE NORTHERN HALF 
OF LAKE HURON...WITH GUSTS TO 30 KNOTS EXPECTED ACROSS THE REMAINDER 
OF THE LAKE. WINDS ARE EXPECTED TO DECREASE LATE FRIDAY NIGHT AS THE 
LOW PRESSURE SYSTEM STALLS AND BEGINS TO WEAKEN.

IN ADDITION TO THE WINDS...PRECIPITATION WILL OCCUR ACROSS THE 
CENTRAL GREAT LAKES AS THIS SYSTEM LIFTS TO OUR WEST. A WINTRY MIX 
OF FREEZING RAIN AND SLEET WILL OCCUR TONIGHT AND FRIDAY MORNING 
BEFORE CHANGING OVER TO ALL RAIN IN THE AFTERNOON.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...WINTER WEATHER ADVISORY...MIZ063-MIZ068-MIZ069-MIZ070-MIZ075...FROM 
     6 AM FRIDAY TO 10 AM FRIDAY.

     WINTER WEATHER ADVISORY...MIZ047-MIZ048-MIZ049-MIZ053-MIZ054-MIZ055-
     MIZ060-MIZ061-MIZ062...FROM 6 AM FRIDAY TO NOON FRIDAY.

LAKE HURON...
     GALE WARNING...LHZ361-LHZ362...FROM 4 AM FRIDAY TO 4 AM SATURDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS FROM PORT AUSTIN TO PORT 
     HURON INCLUDING OUTER SAGINAW BAY...UNTIL 4 AM SATURDAY.

LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.

&&

$$

UPDATE.......MR
AVIATION.....MR 
SHORT TERM...HLO 
LONG TERM....KURIMSKI 
MARINE.......HLO

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