FXUS63 KDTX 241646
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1146 AM EST THU DEC 24 2009

.UPDATE...

AMENDMENT WAS MADE TO THE FORECAST THIS AFTERNOON/EARLY EVENING TO ADD
A SLIGHT CHANCE FOR MIXED FROZEN PRECIPITATION TO THE FORECAST.
CHANCES ARE WEST OF A LINE FROM DETROIT TO FLINT TO SAGINAW. RADAR
IMAGERY FROM NORTHEAST INDIANA AND NORTHWEST OHIO SHOWS SOME
RETURNS SHOWING UP...COINCIDING WITH INITIAL SURGE OF MID/LOW
LEVEL THETA-E ADVECTION. VERY LITTLE OF THIS SEEMS TO BE HITTING
THE GROUND GIVING MID-LEVEL DRY AIR IN PLACE ALOFT...AND POPS ARE
LIMITED TO ONLY 20 PERCENT. UPSTREAM OBSERVATIONS OF
PRECIPITATION SHOW MAINLY FREEZING RAIN...HOWEVER TAKING INTO
ACCOUNT ANY WET-BULB COOLING AS THAT DRY LAYER TRIES TO
SATURATE...THINK WE WOULD ALSO SEE SOME SLEET OR SNOW UP HERE.
AMOUNTS WILL BE VERY LIGHT IF ANY.

FORECAST FOR TONIGHT/CHRISTMAS DAY HAS NOT CHANGED AT THIS POINT
WHILE NEW DATA AND UPSTREAM TRENDS ARE ASSESSED. 

&&

.PREV DISCUSSION...ISSUED 645 AM EST THU DEC 24 2009 

AVIATION...

SOME IFR CEILING HAS DEVELOPED IN THE STRATUS FROM MBS DOWN TO ABOUT 
PTK WITH MVFR IN THE DETROIT AREA. THE EDGE OF THE CLOUDS WAS 
OBSERVED AT THE OHIO BORDER TO ABOUT LANSING TO BIG RAPIDS AT PRESS 
TIME. FROM HERE, EXPECT THE IFR TO GRADUALLY LIFT AND THE 
SOUTHERN/WESTERN EDGE OF THE CLOUDS TO MOVE NORTHWARD/EASTWARD 
DURING THE MORNING. DRY BOUNDARY LAYER AIR HAS HELD UP OVER OHIO AND 
IT WILL FOSTER THE CLEARING TREND AS FLOW ABOVE THE BOUNDARY LAYER 
VEERS TO SOUTHEAST. VFR CONDITIONS ARE THEN EXPECTED BY EARLY 
AFTERNOON THROUGH MOST OF TONIGHT.

ATTENTION THEN TURNS TO THE ONSET OF MIXED PRECIPITATION LATE 
TONIGHT. DUE TO THE DRY AIR IN THE LOW LEVELS, CONFIDENCE IS ON THE 
HIGH SIDE THAT ONSET OF WINTRY MIX WILL BE AFTER 09Z AND PERHAPS 
CLOSER TO 12Z. ANY MIX OF SNOW AND SLEET WILL BE VERY BRIEF AND 
QUICKLY REPLACED BY A 2 OR 3 HOUR PERIOD OF LIGHT FREEZING RAIN
AS WARMER AIR SURGES INTO THE REGION IN THE MID LEVELS. IT IS
EXPECTED THAT SURFACE TEMPERATURES WILL BE NEAR 30F RATHER THAN IN
THE MID TO UPPER 20S, SO THE DURATION AND INTENSITY OF ICING IS
EXPECTED TO BE LIGHT BEFORE READINGS RISE ABOVE FREEZING BY MID TO
LATE CHRISTMAS MORNING. AS TEMPERATURES RISE, PRECIPITATION WILL
CHANGE TO ALL RAIN BUT CEILING WILL FALL TOWARD IFR.

PREV DISCUSSION...ISSUED 417 AM EST THU DEC 24 2009 

SHORT TERM...TODAY

CONSIDERING WHAT IS GOING ON TO OUR WEST TODAY, SORTING OUT CLOUD 
COVER VARIATIONS WILL BE A WELCOME TASK. THERE WILL BE SOME RETURNS 
ON RADAR FROM OVER SW LOWER MICHIGAN BUT THAT IS AS CLOSE AS THE 
PRECIPITATION WILL GET TO OUR AREA THROUGH THE DAY. IN FACT, SOME 
FILTERED SUNSHINE WILL BE POSSIBLE AT TIMES, ESPECIALLY NEAR THE 
OHIO BORDER, WHICH WILL ALLOW TEMPERATURES TO MAKE A RUN AT THE 
FREEZING MARK FOR HIGHS ONCE AGAIN.

MODEL ANALYSIS FIELDS HAVE SUCCESSFULLY CAPTURED THE DRY AIR 
OBSERVED IN THE LOW LEVELS FROM THE CENTRAL APPALACHIANS INTO SE 
MICHIGAN, WHICH WILL PLAY A STRONG ROLE IN HOW CONDITIONS UNFOLD FOR 
THE REST OF TODAY. EARLY MORNING SATELLITE IMAGERY SUGGESTS THE 00Z 
DTX SOUNDING IS STILL REPRESENTATIVE OF CONDITIONS IN THE LOW 
LEVELS, THAT BEING A SUBSTANTIAL DRY LAYER BETWEEN ABOUT 900 MB AND 
600 MB. THIS DRY LAYER WILL BE MAINTAINED BY A NEAR CONSTANT SE FLOW 
FROM CENTRAL OHIO AND WILL EFFECTIVELY EAT INTO ANY MOISTURE 
TRANSPORT ALOFT AND PREVENT PRECIPITATION FROM REACHING THE GROUND 
THROUGH THE DAY. ALSO, BECAUSE THE UPPER LEVEL SYSTEM WILL BE 
RELOADING WITH THE STRONG CIRCULATION OVER TEXAS, SOME BACKING OF 
THE FLOW IS EXPECTED ALOFT WHICH WILL HELP THIN SOME OF THE HIGH 
CLOUDS AT TIMES DURING THE AFTERNOON. COMBINE THIS WITH THE TENDENCY 
FOR THE LOW CLOUDS TO SCOUR OUT NEAR THE OHIO BORDER AND THE RESULT 
IS SOME FILTERED SUNSHINE OVER ABOUT THE SOUTH HALF OF SE MICHIGAN, 
AT LEAST FOR WHILE BEFORE THINGS START TO FILL IN AGAIN TOWARD 
EVENING.

LONG TERM...TONIGHT THROUGH NEXT WEDNESDAY

THERE ARE A NUMBER OF DIFFICULT DECISION POINTS TO WADE THROUGH WITH 
THE UPCOMING PRECIPITATION EVENT...THE MAJORITY OF WHICH COME VERY 
LATE IN THE TONIGHT PERIOD.  

THE FIRST ITEM OF CONSIDERABLE INTEREST IS THE TIMING OF ONSET FOR 
THE PRECIPITATION. BOTH THE 00Z GFS/NAM STUCK TO THEIR RESPECTIVE 
CONSISTENT SELVES...WITH THE GFS CONSIDERABLY FASTER THAN THE NAM 
(BY AS MUCH AS 12 TO 15 HRS IN THE FAR NORTHWESTERN CWA). WHILE IT 
HAS BEEN DRONED ABOUT AD NAUSEAM...IT SIMPLY COMES DOWN TO TIMING OF 
LOW LEVEL SATURATION UNDER AN ESTABLISHED DRY EASTERLY ANTICYCLONIC 
FEED. ULTIMATELY...THIS SHOULD BEGIN TO OCCUR IN EARNEST SOMETIME IN 
THE 8-11Z TIMEFRAME...WHICH IS IN LINE WITH A COMPROMISE OF THE 
NAM/ECMWF AND TO SOME EXTENT THE GFS. WILL OUTRIGHT SAY HAVE SOME 
PRETTY STRONG RESERVATIONS AGAINST THE GFS TIMING AS A WHOLE. THAT 
IS LARGELY IN PART THE GFS ITSELF REMAINS SUBSATURATED A BETTER PART 
OF THE NIGHT BETWEEN 2-8 KFT. ALL OF THE MODELS AGREE A DRY 850MB 
AIRMASS...OVER TN/VA/NC THIS PAST EVENING...WILL SNEAK NORTHWARD IN 
ADVANCE OF THE LOW TO MIDLEVEL OCCLUSION. A CHECK OF THE RECENT 
RAOBS (KRNK/KOHX) CONFIRMS THIS DRY AIRMASS WITH 850MB DEWPOINT 
DEPRESSIONS IN THE 6-8C RANGE.

NOW ONTO PRECIPITATION TYPE. A PERUSAL OF THE 00Z MODEL SUITE AGAIN 
SUGGESTS THAT NEAR SURFACE TEMPERATURES/WET BULB TEMPERATURES WILL 
NOT SURPRISINGLY DEPEND UPON THE AMOUNT OF SATURATION/LONGEVITY OF 
SOUTHEASTERLY DRY FEED. WHILE THE GFS HAS BEEN CONSISTENTLY COOLER 
WITH THE EARLY PORTION OF THIS STORM SYSTEM...THE LATEST RUN 
ACTUALLY CHECKED IN SLIGHTLY WARMER (0C OR GREATER) THAN THE NAM IN 
REGARDS TO SURFACE T/TW. THIS JUST AFFIRMS THE MODELS HAVE LOCKED 
ONTO THE 0C ISOTHERM WHICH KEEPS ALL PRECIPITATION IN PLAY. THE 
NEGATIVES AGAINST A LONG DURATION FREEZING RAIN EVENT ARE AGAIN:  
TEMPERATURES SHOULD ONLY DIP INTO THE 30-31F RANGE THIS EVENING...A 
REASONABLY DEEP 900-800MB WARM LAYER THAT IS 4-6C...NOT A CLASSIC 
OVERRUNNING SCENARIO IN THAT THE SOUTHEASTERLY FLOW WILL EVENTUALLY 
TURN INTO WAA AS THE HIGHER LATENT HEAT CONTENT ARRIVES. 
BOTTOMLINE...EXPECTATIONS ARE FOR A TWO HOUR FREEZING RAIN WINDOW 
THAT WILL BEGIN ACROSS THE SOUTHWEST AROUND 8Z AND END ACROSS THE 
NORTHEAST BY 15Z. THE ICING MAY PRESENT SOME ISSUES EARLY TOMORROW 
MORNING...WITH AMOUNTS OF UP TO .2 INCHES POSSIBLE. GIVEN THE VERY 
LATE PERIOD 2/PERIOD 3 TIMING...HAVE DEFERRED TO THE DAY SHIFT TO 
MAKE HEADLINE DECISIONS. BY MID MORNING PRECIPITATION IS EXPECTED TO 
BE A STRAIGHT RAIN FOR ALL AREAS.

SUBSIDENCE IN THE WAKE OF THIS FRONTAL FEATURE SHOULD DRAMATICALLY 
SHUTDOWN PRECIPITATION FRIDAY AFTERNOON. A WELL DEFINED COLD POOL 
WILL ARRIVE FRIDAY EVENING CHANGING PRECIPITATION BACK TO AN ALL 
SNOW. ANOTHER...BUT MUCH WEAKER...TROWAL IS EXPECTED TO DEVELOP AS 
THE NORTHERN STREAM PV ANOMALY BECOMES THE DOMINANT PLAYER IN THE 
MID MISSISSIPPI STORM SYSTEM. THE INFANCY OF THE NEW TROWAL DYNAMICS 
SHOULD TAKE PLACE DIRECTLY OVER THE CWA AND AFFECT MAINLY THE THUMB 
REGION. SNOWFALL ACCUMULATIONS FRIDAY NIGHT OF AN INCH OR LESS WILL 
BE POSSIBLE FOR THAT AREA. THE ADVERTISED DRYSLOT WILL REIGN ON 
SATURDAY BRINGING NOTHING MORE THAN FLURRIES OR SOME LIGHT SNOW 
SHOWER ACTIVITY. CONFIDENCE FOR THE REMAINDER OF THE WEEKEND GOES 
DOWNHILL QUICKLY AS IT BECOMES A FUTILE ATTEMPT AT TIMING THE PARENT 
ANOMALY AND THE INDIVIDUAL SHORTWAVES/VORTICITY LOBES. SNOW SHOWERS 
APPEAR A GOOD BET WITH STEEP LAPSE RATES UP TO 8000 KFT BUT ANY 
SIGNIFICANT ACCUMULATIONS SEEM UNLIKELY. TEMPERATURES WILL REMAIN 
FAIRLY STATIC WITH DAYTIME VALUES IN THE UPPER 20S TO NEAR 30...AND 
NIGHTTIME LOWS IN THE TEENS TO LOWER 20S.

MARINE...

FAVORABLE MARINE CONDITIONS WILL CONTINUE THROUGH TODAY AS HIGH 
PRESSURE REMAINS STRETCHED OVER THE EASTERN GREAT LAKES. WIND 
CONDITIONS WILL THEN DETERIORATE TONIGHT THROUGH CHRISTMAS DAY AS 
THE LARGE AND STRONG LOW PRESSURE SYSTEM CONTINUES TO DEVELOP OVER 
THE MIDWEST. THE INCREASED PRESSURE GRADIENT OVER THE CENTRAL GREAT 
LAKES WILL FORCE EASTERLY GALES ON THE NORTH HALF OF LAKE HURON WITH 
FREQUENT GUSTS TO 30 KNOTS OVER THE SOUTH HALF LIMITED BY INCREASED 
STABILITY OF WARM AIR OVER THE COLDER LAKE WATER. A GALE WARNING AND 
SMALL CRAFT ADVISORY HAS BEEN HOISTED ACCORDINGLY FOR THOSE AREAS 
THROUGH FRIDAY NIGHT. THE WARMER AIR MOVING IN ALOFT OVER THE COLDER 
NEAR SURFACE AIR WILL CAUSE MIXED PRECIPITATION AND COULD LEAD TO 
ICING OF DECK SURFACES.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.

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...FROM 7 PM THURSDAY TO 4 AM 
     SATURDAY.

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

&&

$$

UPDATE.......HLO
AVIATION.....BT
SHORT TERM...BT
LONG TERM....CB
MARINE.......BT

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