Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 241747
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1247 PM EST WED DEC 24 2014

.AVIATION...

THE SURFACE LOW TRACK FROM ABOUT LIMA TO DTW TO PORT HURON HAS
PROLONGED THE VLIFR RESTRICTION IN THE DTW AREA WHICH IMPROVES
MODESTLY TOWARD MBS...A LITTLE FARTHER FROM THE LOW BUT DEEPER INTO
THE PRECIPITATION SHIELD. THE COMPLEX CEILING AND VISIBILITY PATTERN
WILL IMPROVE AT THE EXPENSE OF STRONG WESTERLY WIND AS THE SYSTEM
EXITS INTO SOUTHERN ONTARIO BY EVENING. COLDER AIR RUSHING IN
THROUGH THE LOW LEVELS WILL THEN EXPEDITE A CHANGE FROM RAIN TO SNOW
MORE RAPIDLY AND A FEW HOURS EARLIER IN THE NIGHT...FROM ABOUT MID
EVENING TO MIDNIGHT ACROSS THE TERMINAL CORRIDOR. AN IFR BURST OF
SNOW IS EXPECTED FROM PTK NORTHWARD AS WESTERLY WIND GUSTS INCREASE
TO 35 KNOTS WITH A FEW HIGHER GUSTS POSSIBLE. THE SNOW WILL TAPER
OFF DURING THE OVERNIGHT HOURS AND THE WIND WILL FOLLOW A DOWNWARD
TREND DURING THE DAYLIGHT HOURS OF CHRISTMAS MORNING.

FOR DTW... THE SLIGHT IMPROVEMENT OF CEILING AND VISIBILITY OBSERVED
EARLIER AT TOL AND TTF HAS REVERSED BACK TOWARD 1/4SM JUST BEFORE
PRESS TIME. THIS WILL REQUIRE ANOTHER EXTENSION OF VLIFR CONDITIONS
AT DTW AS THE SURFACE LOW TRACKS JUST ABOUT OVERHEAD DURING THE
AFTERNOON. WESTERLY WIND GUSTS TO 35 KNOTS WILL MATERIALIZE ON THE
HEELS OF THE SYSTEM AS WILL A CHANGE FROM RAIN TO SNOW...THE
HEAVIEST OF WHICH IS EXPECTED TO OCCUR NORTH OF DTW BUT WITH SOME
MINOR ACCUMULATION STILL POSSIBLE.

//DTW THRESHOLD PROBABILITIES...

* HIGH IN CEILINGS AOB 5000 FEET THROUGHOUT THE TAF PERIOD.

* HIGH IN VISIBILITY BELOW 1/2SM AND CEILING AOB 200 FEET DURING THE
  AFTERNOON.

* HIGH FOR WIND GUSTS TO REACH 35 KNOTS FROM 250 DEGREES TONIGHT.

* HIGH CONFIDENCE IN RAIN CHANGING TO SNOW TOWARD MIDNIGHT.

&&

.PREV DISCUSSION...ISSUED 1028 AM EST WED DEC 24 2014

UPDATE...

INCOMING 12Z SOLUTIONS PERPETUATE ONGOING TREND OF AN EASTWARD
MIGRATION OF TODAY`S RAPIDLY DEEPENING CYCLONE. THE DRIVING
MECHANISM BEHIND THIS SHIFT IS PRESUMABLY THE EASTWARD ADJUSTMENT
OF LARGER SCALE FORCING DUE TO YESTERDAY`S CONVECTION ACROSS THE
SOUTHERN UNITED STATES. AS A RESULT, NEGATIVE TILT OF THE UPPER
TROUGH AND ASSOCIATED SUBSIDENCE OVER THE CWA IS PROGGED TO BE A
BIT DELAYED COMPARED TO PREVIOUS EXPECTATIONS. AS A RESULT, SLIGHTLY
WEAKER AND MORE EASTWARD TRACK MAY ULTIMATELY MAKE FOR A
BORDERLINE WIND ADVISORY, THOUGH SUFFICIENT DYNAMICS MAY YET EXIST
DURING AND JUST AFTER FROPA FOR ADVISED LOCATIONS TO STILL POP A
FEW 45 MPH GUSTS. WILL MONITOR TRENDS THROUGH THE AFTERNOON, BUT
DO NOT PLAN TO MAKE CHANGES TO WIND HEADLINES ATTM AS WINDS WILL
STILL BE QUITE GUSTY REGARDLESS.

POTENTIAL FOR ACCUMULATING SNOWFALL IN THE SAGINAW VALLEY, AND
POTENTIALLY THE THUMB, HAS ALSO INCREASED AS COLD AIR WILL MAKE
FASTER INROADS IN THE LOW LEVELS AND DEEP LAYER ASCENT AUGMENTS
THE COOLING PROCESS. WILL WAIT FOR THE FULL SUITE OF 12Z DATA
BEFORE MAKING ANY HEADLINE DECISIONS FOR THE AFTERNOON PACKAGE, AS
ANY POTENTIAL FOR HIGHER ACCUMS WILL LARGELY HINGE OF TIME OF
CHANGE OVER AND HOW MUCH CAN ACCUMULATE BEFORE SUNSET, BUT A
MORNING UPDATE WILL BE ISSUED MOMENTARILY TO BUMP UP ACCUMS A BIT
ACROSS THE NORTH WITHIN AREA OF HEALTHY DEFORMATION FLOW.

PREV DISCUSSION...ISSUED 425 AM EST WED DEC 24 2014

SHORT TERM...TODAY AND TONIGHT

FIRST THINGS FIRST...THE CURRENT DENSE FOG ADVISORY WILL NEED TO BE
GRADUALLY ADJUSTED EARLY THIS MORNING AS AN INCREASE IN THE COVERAGE
OF SHOWERS LEADS TO A TRANSITION AWAY FROM THE CURRENT WIDESPREAD
ONE QUARTER TO ONE HALF MILE VISIBILITY TO SOMETHING CLOSER TO ONE
OR TWO MILES WITH PATCHY DENSE FOG IN THE ONE QUARTER TO ONE HALF
MILE RANGE BY DAYBREAK. GIVEN TRENDS AS OF 4 AM...WILL CANCEL THE
DENSE FOG ADVISORY AS MOST LOCATIONS ARE NOW AOA 1SM WITH LOWEST
VISIBILITIES OF 1/2 TO 3/4 MILE FROM THE THUMB TO SAGINAW VALLEY.

OTHERWISE...A VERY BUSY PERIOD OF WEATHER IS EXPECTED THROUGH
TONIGHT AS RAPIDLY DEEPENING LOW PRESSURE LIFTS JUST EAST OF DUE
NORTH FROM ITS CURRENT POSITION IN WEST CENTRAL TENNESSEE TO FAR
SOUTHEAST LOWER MICHIGAN THIS AFTERNOON AND ON THROUGH LAKE HURON
TONIGHT. BASED ON AN AVERAGE OF THE 00Z MODEL RUNS...THE CURRENT
LOW PRESSURE OF 1002 MB WILL DEEPEN TO 985 MB AS IT CROSSES
SOUTHEAST MICHIGAN. EVENTUALLY...THIS LOW WILL FALL THROUGH THE
980 MB LEVEL WHILE CROSSING LAKE HURON DURING THE EVENING ON ITS
WAY TO NEAR 960 MBS EAST OF HUDSON BAY UPON REACH FULL MATURITY.

WHILE THE MODEL SPREAD ON THE TRACK OF THIS SYSTEM HAS TIGHTENED
CONSIDERABLY IN THE PAST 24 HOURS...THERE REMAINS ENOUGH OF A
DIFFERENCE TO MAKE A FINAL CALL ON HIGH WIND ADVISORY -VS- WARNING
HEADLINES SOMEWHAT DIFFICULT. IN GENERAL...IT APPEARS THE BEST SURGE
OF SOUTH TO SOUTHWEST FLOW ON THE EAST SIDE OF THIS LOW WILL SHIFT
UP THROUGH THE EASTERN LAKES REGION AND BYPASS THE CWA BY A HAIR.

GIVEN THE FACT THAT THE ECMWF AND CANADIAN GEM...DESPITE HAVING HAD
SOME DIFFICULTY...HAVE BEEN THE MOST CONSISTENT MODELS WITH THE
FORECASTED TRACK OF THIS SYSTEM DURING THE PAST SEVERAL DAYS...WILL
LEAN TOWARD THE CURRENT TRACK RIGHT ALONG THE EASTERN EDGE OF THE
CWA...WHICH IS A BIT FURTHER EAST THAN THE REST OF THE MODELS. THIS
WILL LIMIT THE POTENTIAL FOR WIND GUSTS REACHING WARNING CRITERION
AS WIND GUSTS SHOULD GENERALLY REMAIN IN THE SUB 50 MPH RANGE DURING
THE PEAK PERIOD FROM 00Z-06Z. WITH THE TRACK OVER/JUST EAST OF AREA
LATE AFTERNOON AND EARLY EVENING...THERE WILL MOST LIKELY BE A LULL
IN THE WIND FIELDS BEFORE STRONGER SOUTHWEST FLOW WRAPS IN BEHIND
THE LOW PASSAGE. WILL ISSUE A WIND ADVISORY FOR A PORTION OF THE
AREA...MAINLY FOR THIS EVENING AS STRONGEST WINDS WILL BE DELAYED
SLIGHTLY BY THIS "LULL" WITHIN/NEAR THE LOW PRESSURE CENTER AS IT
PASSES. WILL LEAVE THE SAGINAW VALLEY OUT...AND MAY TRIM OUT A FEW
MORE COUNTIES ON THE WESTERN EDGE OF CWA THAT ARE IN CURRENT WATCH
GIVEN A SLIGHTLY MORE EASTWARD TRACK OF THE LOW.

THIS TRACK ALONG/EAST OF EASTERN CWA WILL ALSO OPEN THE DOOR FOR A
SLIGHTLY QUICKER CHANGEOVER TO SNOW...PARTICULARLY OVER THE SAGINAW
VALLEY. IT APPEARS THE CURRENT FORECAST OF AROUND AN INCH OF SNOW
HAS THIS IN HAND PRETTY WELL AS THE CHANGEOVER WILL STILL OCCUR AS
THE HEAVIEST DEFORMATION PRECIPITATION IS LIFTING NORTH OF THE CWA.
WILL ADJUST TIMING OF CHANGEOVER SLIGHTLY AS WELL AS LOWERING THE
OVERALL TEMPERATURE FORECAST THROUGH THE NORTHWEST HALF THE THE CWA
AS MILDEST AIR REMAINS SOUTHEAST OF AREA DURING THE DAY. THIS WILL
LEAD TO A RATHER TIGHT TEMPERATURE GRADIENT FOR HIGHS TODAY FROM
AROUND 40F WESTERN SAGINAW VALLEY TO MID 50S FAR SOUTHEAST CWA.

LONG TERM...

THE DEEP LOW PRESSURE SYSTEM WILL HAVE BEEN PRETTY MUCH EXITED THE
REGION COME CHRISTMAS DAY...WITH ANY LINGER LOW LEVEL CYCLONIC FLOW
QUICKLY BECOMING ANTI-CYCLONIC AS THE DAY WEARS ON. HOWEVER...WITH
THE STRENGTHENING SUBSIDENCE INVERSION...LOW LEVEL MOISTURE/CLOUDS
WILL LIKELY BECOME TRAPPED. LOW LEVEL LAPSE RATES ARE STEEP DURING
THE MORNING HOURS...GENERATING A BIT OF CAPE TO SUPPORT SLIGHT
CHANCE OF SPRINKLES/VERY LIGHT RAIN/SNOW SHOWERS (ALTHOUGH THE
NAM/GFS SOUNDINGS SHOW NO ICE NUCLEI TO SUPPORT SNOW). SURFACE TEMPS
SHOULD BE JUST ON THE POSITIVE SIDE OF FREEZING...BUT NOT TOTALLY
OUT OF THE QUESTION FOR A TOUCH OF FREEZING DRIZZLE 12-14Z.

WARM ADVECTION THURSDAY NIGHT WILL ONLY SERVE TO STRENGTHEN THE LOW
LEVEL INVERSION BETWEEN 1500-2000 KFT...AND ASSUMING THE CLOUDS HAVE
NOT MIXED OUT BY DAY`S END...SHOULD REMAIN LOCKED IN THROUGH THE
NIGHT...AS CLEARLY SUPPORTED BY NAM SOUNDINGS. HOWEVER...PARALLEL
GFS LOWERS THE INVERSION HEIGHT TO 1000 FEET AND ALLOWS FOR
CLEARING. HAVE LEANED TOWARD THE CLOUDIER SOLUTION...ESPECIALLY
NORTH.

ANOTHER POWERFUL STORM SYSTEM ALONG THE BRITISH COLUMBIA COAST EARLY
THIS MORNING WITH UPPER LEVEL ENERGY AT THE BASE OF THE TROUGH
HEADED TOWARD THE NORTHERN CALIFORNIA COAST LATER TODAY AND INTO THE
FOUR CORNERS REGION ON THURSDAY. THE SPLITTING OF UPPER LEVEL
ENERGY/FLOW IS EXPECTED TO LEAD TO A RATHER TAME FRONTAL PASSAGE ON
SATURDAY...WITH ENOUGH WARM AIR (925 MB TEMPS OF 4 C) IN PLACE TO
JUST SUPPORT A CHANCE OF RAIN SHOWERS AS FRONT MOVES THROUGH.
HOWEVER...THE EURO/CANADIAN SOLUTIONS DO SHOW A RIPPLE RIDING ALONG
THE FRONT...SLIDING THROUGH THE AREA SATURDAY EVENING...WHICH WOULD
BE ABLE TO PROLONG SHOWERS AND SUPPORT A CHANGEOVER TO SNOW SATURDAY
NIGHT...BUT THAT REMAINS TO BE SEEN.

PER 00Z EURO...TURNING MUCH COLDER AS WE END THE YEAR...AS IT
APPEARS ARCTIC AIR (SUB 500 DAM 1000-500 MB THICKNESS) WILL FINALLY
SPILL BACK INTO THE CONUS...SLIDING INTO THE ROCKIES/PLAINS AT
FIRST...AND THEN SPREADING EAST NEXT WEEK.

MARINE...

RAPIDLY DEEPENING LOW PRESSURE WILL TRACK UP FROM THE SOUTH TODAY
BRINGING WITH IT RAIN AND POSSIBLE THUNDERSTORMS. AS THE INTENSE
LOW TRACKS THROUGH LAKE HURON LATE THIS EVENING....GALE FORCE WINDS
ARE EXPECTED TO RAMP WITH GOOD COLD ADVECTION INTO EARLY THURSDAY
MORNING. CONFIDENCE IN GALES OVER THE OPEN WATERS OF LAKE HURON IS
HIGH...AS FEW STRAY GUSTS TO STORM FORCE ARE NOT OUT OF THE
QUESTION. GALE WARNINGS HAVE BEEN ISSUED. A LITTLE CONCERNED ACROSS
LAKE ST CLAIR AND LAKE ERIE...AS THE DURATION OF GALES LOOKS TO BE
SHORT. BECAUSE OF THIS SHORT WINDOW OF GALES...WATER LEVELS ARE ALSO
NOT EXPECTED TO DROP BELOW THE CRITICAL MARK FOR SAFE NAVIGATION.

WINDS WILL DIMINISH DURING THURSDAY AS A RIDGE OF HIGH
PRESSURE BUILDS INTO THE AREA THROUGH THE DAY...WITH LIGHT TO
MODERATE SOUTHWEST WINDS IN STORE FOR FRIDAY AHEAD OF A COLD FRONT
PASSING THROUGH ON SATURDAY.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...WIND ADVISORY FROM 9 PM THIS EVENING TO 5 AM EST THURSDAY FOR
     MIZ049-054-055-062-063.

     WIND ADVISORY FROM 7 PM THIS EVENING TO 3 AM EST THURSDAY FOR
     MIZ069-070-075-076-082-083.

LAKE HURON...GALE WARNING FROM 7 PM THIS EVENING TO 11 AM EST THURSDAY FOR
     LHZ363-421-441>443-462>464.

     GALE WARNING FROM 7 PM THIS EVENING TO 5 AM EST THURSDAY FOR LHZ422.

     GALE WARNING FROM 11 PM THIS EVENING TO 11 AM EST THURSDAY FOR
     LHZ361-362.

LAKE ST CLAIR...GALE WARNING FROM 6 PM THIS EVENING TO 4 AM EST THURSDAY FOR LCZ460.

MICHIGAN WATERS OF LAKE ERIE...LOW WATER ADVISORY FROM 9 PM THIS EVENING TO NOON EST THURSDAY FOR
     LEZ444.

     GALE WARNING FROM 6 PM THIS EVENING TO 4 AM EST THURSDAY FOR LEZ444.

&&

$$

AVIATION.....BT
UPDATE.......JVC
SHORT TERM...DG
LONG TERM....SF
MARINE.......SF


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