Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 250259
AFDDTX

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

.UPDATE...

AS THE LOW CONTINUES TO PULL AWAY FROM SOUTHEAST MICHIGAN THE BEST
WIND GUSTS WE CAN GENERATE AT THE METAR SITES ARE GUSTS TO 30 TO
35 KNOTS. WITH THE BEST DESCENT BEHIND THE COLD FRONT OFF TO THE
EAST THIS WILL FALL SHORT OF THE 39 KNOTS NEEDED FOR A WIND
ADVISORY...THEREFORE WILL CANCEL THE WIND ADVISORY TONIGHT FOR
SOUTHEAST MICHIGAN.

&&

.AVIATION...ISSUED 620 PM EST WED DEC 24 2014

LOW PRESSURE CONTINUES TO TRACK THROUGH THE CENTRAL GREAT LAKES THIS
EVENING. IFR CONDITIONS SHOULD EVENTUALLY IMPROVE TO MVFR AT THE
EXPENSE OF STRONG WESTERLY WINDS AS THE SYSTEM EXITS INTO SOUTHERN
ONTARIO. COLDER AIR RUSHING IN THROUGH THE LOW LEVELS WILL THEN
EXPEDITE A CHANGE FROM RAIN TO SNOW...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...CIGS WILL BOUNCE FROM IFR TO MVFR BEFORE SETTLING TO MVFR
LATE THIS EVENING. 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 BELOW 5000 FEET THROUGHOUT THE TAF PERIOD.

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

* HIGH CONFIDENCE IN RAIN CHANGING TO SNOW TOWARD MIDNIGHT.

&&

.PREV DISCUSSION...ISSUED 918 PM EST WED DEC 24 2014

UPDATE...

A QUICK BURST OF SNOW HAS TRACKED THROUGH LIVINGSTON COUNTY. WITH
ADDITIONAL SNOW UPSTREAM UPGRADED AND INCLUDED THIS COUNTY IN THE
WINTER WEATHER ADVISORY AS OBSERVATIONS AND SPOTTER REPORTS HAVE
INDICATED WET HEAVEY ACCUMULATING SNOW. ALTHOUGH WINDS STILL HAVE
NOT APPROACHED ADVISORY CRITERIA THINKING IS WINDS COULD PICK UP
AS THE LOW DEEPENS AND HEADS OFF TO THE EAST...TIGHTENING UP THE
PRESSURE GRADIENT.

PREV DISCUSSION...ISSUED 822 PM EST WED DEC 24 2014

UPDATE...

GUSTY WINDS ARE DEVELOPING BEHIND A DEPARTING LOW PRESSURE EXITING
THE CENTRAL GREAT LAKES AS COLD AIR FILTERING IN HAS STARTED TO
CHANGE THE RAIN OVER TO SNOW IN PORTIONS OF THE SAGINAW VALLEY.
WHILE THE LOW HAS TRACKED A BIT FARTHER EAST THAN PREVIOUSLY
EXPECTED...THERE IS NOT ENOUGH OF A WHOLESALE CHANGE IN THE
FORECAST AT THIS POINT TO BEGIN TAKING HEADLINES DOWN. WILL
CONTINUE TO MONITOR TRENDS CLOSELY AS A LARGE AREA OF SNOW OVER
THE GRAND RAPIDS AREA TRACKS NORTHEAST THROUGH THE WINTER WEATHER
ADVISORY AREA AND WINDS INCREASE ACROSS THE WIND ADVISORY
COUNTIES.

PREV DISCUSSION...ISSUED 328 PM EST WED DEC 24 2014

SHORT TERM...THIS EVENING THROUGH TONIGHT

THE CHRISTMAS EVE CYCLONE WILL FINALLY SHOW ITS HAND THIS EVENING AS
IT UNFOLDS OVER THE GREAT LAKES REGION. CONSOLIDATION OF UPPER
ENERGY IS FORECAST TO OCCUR SLIGHTLY NORTH AND EAST OF PREVIOUS
MODELED PROJECTIONS, LIKELY DUE TO IMPACTS OF CONVECTION OVER THE
SOUTHERN UNITED STATES. THIS HAS LED THE 12Z SUITE TO DELAY
DEEPENING OF THE CYCLONE AND MOVE IT A BIT EAST. AS A RESULT, THE
PRESSURE GRADIENT MAXIMUM THAT WILL FORM ON THE BACK SIDE OF THE LOW
WILL ONLY MANAGE TO SCRAPE THE EASTERN CWA AND WILL INSTEAD
PRIMARILY IMPACT ONTARIO. DESCENT WITHIN THE FRONTAL ZONE AND
TRAILING SUBSIDENCE/CAA MAY YET BE SUFFICIENT TO SQUEEZE OUT A FEW
45 MPH GUSTS WITHIN THE AREA ADVISED FOR WIND, BUT GENERALLY EXPECT
GUSTS TO 35 TO 40 MPH...STILL GUSTY ENOUGH TO WARRANT CONTINUING
WITH THE MARGINAL ADVISORY FOR NOW. STRONGEST WINDS ARE STILL ON
TRACK TO IMPACT THE AREA 00-04Z.

SECOND FACET OF THIS SYSTEM IS SNOW POTENTIAL. FROM A THERMODYNAMIC
STANDPOINT, THE EVOLUTION WILL BE ENTIRELY DIFFERENT THAN MODELED
OVER THE LAST SEVERAL DAYS. PREVIOUS RUNS HAVE INDICATED A
PRONOUNCED WARM AXIS WRAPPING AROUND THE LOW AND WELL INTO THE CWA,
DELAYING THE CHANGEOVER TO SNOW FOR ALMOST EVERYONE AND ALLOWING
HIGHS TO REACH WELL INTO THE 50S OVER MUCH OF THE CWA. TODAY,
HOWEVER, IT IS EVIDENT THAT HIGHS WILL STRUGGLE INTO UPPER 40S FOR
MOST LOCATIONS AND SHORT TERM PROJECTIONS INDICATE H85 TEMPS FALLING
BELOW 0C AREAWIDE BY 03Z.

FROM A SYNOPTIC/MESOSCALE STANDPOINT, COLD AIR SUPPORTIVE OF SNOW
WILL STILL BE SLOW TO INFILTRATE THE BOUNDARY LAYER OVER SOUTHEAST
MICHIGAN. HOWEVER, NAM12 CROSS-SECTIONS SHOW AN INCREDIBLY LOW
STATIC STABILITY PROFILE ACROSS THE CWA THAT WILL BE VERY RECEPTIVE
TO ANY FORCING AT ALL, AND FORCING WILL BE PRESENT IN ABUNDANCE.
FRONTOGENETIC LEFT EXIT REGION DYNAMICS, DESPITE BEING LARGELY
DISPLACED TO THE SOUTHEAST, WILL BE SUFFICIENT OVER THE AREA TO
ALLOW FOR AGGRESSIVE TILTING OF ISENTROPES IN THE 850-400MB LAYER.
THE RESULTANT DEEP FGEN RESPONSE ZONE WILL SUPPORT A BAND OF
ENHANCED ASCENT WITHIN AN ALREADY ROBUST DEFORMATION ZONE. IN FACT,
CROSS-SECTIONS BORDER ON BEING CONVECTIVELY UNSTABLE AT THE NAM12
GRID SCALE NORTHWEST OF THE GLACIAL RIDGE. EVEN IN THE CASE THAT
UPRIGHT CONVECTION DOES NOT DEVELOP, GEOSTROPHIC ANGULAR MOMENTUM
PROFILES SUGGEST CSI COULD EVEN EVOLVE IN ITS PLACE. IT HARDLY SEEMS
TO MATTER, AS ONE WAY OR ANOTHER ASCENT WILL BE INTENSE WITHIN A
POTENTIALLY NARROW BAND BETWEEN ROUGHLY SAGINAW AND FLINT. IT IS
DOUBTFUL, AND CERTAINLY PLAYING WITH FIRE SO TO SPEAK, TO EXPECT
MARGINAL BOUNDARY LAYER TEMPERATURES TO CONTINUE TO SUPPORT RAIN
WITHIN SUCH AN ENVIRONMENT. AS SUCH, EXPECT THAT RATES COULD BE
INTENSE AS WELL WITH THE POTENTIAL FOR 2-3" IN A 1-2 HOUR TIME
WINDOW WITHIN THE ADVISED AREA. THIS WOULD BE SUFFICIENT TO DISRUPT
TRAVEL AND CONFIDENCE IS HIGH ENOUGH TO WARRANT ISSUANCE OF A WINTER
WX ADVISORY FOR THE NORTHWESTERN PORTION OF THE CWA ATTM. WOULD NOT
BE TERRIBLY SURPRISED TO SEE A NARROW BAND OF MODERATE TO HEAVY SNOW
SET UP WITHIN THE ADVISORY AREA AND BE FLANKED ON EITHER SIDE BY 34
DEGREE RAIN. AT THIS TIME, HOWEVER, WILL HEDGE TOWARD A CHANGE TO
ALL SNOW IN THE NORTHWEST PORTION OF THE ADVISORY AND RAIN/SNOW MIX
SOUTHEAST. OBSERVATIONS WILL BE MONITORED THROUGH THE EVENING.

LONG TERM...CHRISTMAS THROUGH NEXT WEDNESDAY

THE STORM SYSTEM IMPACTING THE REGION TODAY WILL RAPIDLY LIFT ACROSS
QUEBEC ON CHRISTMAS. GUSTY WINDS WILL PERSIST THROUGH THE EARLY PART
OF THE DAY. LIMITED MIXING DEPTHS SHOULD KEEP THE PEAK GUSTS BELOW
30 MPH. AMPLE LOW LEVEL MOISTURE AMIDST A DEEPENING LOW LEVEL
INVERSION WILL SUSTAIN A HEALTHY STRATO CU FIELD DURING THE DAY.
THERE IS NOT A SUBSTANTIAL AMOUNT OF COLD AIR WRAPPING AROUND THIS
LOW PRESSURE SYSTEM. SO EVEN WITH THE POTENTIAL FOR SOME SNOW COVER
ACROSS THE NRN HALF OF THE FORECAST AREA...DAYTIME TEMPS SHOULD
REMAIN LARGELY ABOVE FREEZING. LOW LEVEL WARM AIR ADVECTION WILL
ALREADY GET UNDERWAY TOMORROW AFTERNOON INTO THE EVENING IN RESPONSE
TO BUILDING MID LEVEL HEIGHTS AND STRENGTHENING LOW LEVEL SW FLOW.
MODEL SOUNDINGS ARE AT LEAST SUGGESTIVE OF ENOUGH LOW LEVEL DURING TO
OFFER A POTENTIAL FOR SOME CLEARING SKIES THURS NIGHT. THIS MAY
OFFER JUST ENOUGH DECOUPLING IN THE BOUNDARY LAYER TO SOME SUBTLE
COOLING DESPITE THE LOW LEVEL WARM AIR ADVECTION.

THE BUILDING HEIGHTS ACROSS THE GREAT LAKES REGION WILL CONTINUE
INTO FRIDAY IN RESPONSE TO LONG WAVE TROUGH AMPLIFICATION ACROSS THE
CENTRAL ROCKIES. THIS WILL SUPPORT A MODEST WARM UP HEADING INTO THE
FIRST HALF OF THE WEEKEND. THE MEDIUM RANGE MODEL SUITE ARE IN
REASONABLE AGREEMENT IN SHOWING A FAST MOVING SHORTER WAVELENGTH
IMPULSE EJECTING OUT OF THE BASE OF THE LONG WAVE TROUGH AND
ROTATING ACROSS THE NORTHERN GREAT LAKES SAT INTO SAT NIGHT.
ALTHOUGH THE WAVE IS FORECAST TO BECOME HIGHLY SHEARED AS IT ROTATES
INTO THE REGION...ENHANCED UPPER JET FORCING WILL INVOKE A HEALTHY
DEEP LAYER FRONTOGENETICAL RESPONSE OVER SE MI SAT AFTN INTO SAT
NIGHT. THERMAL PROFILES WILL REMAIN WARM ENOUGH TO SUPPORT ALL RAIN
THROUGH MOST OF THIS EVENT...WITH A POSSIBLE CHANGE OVER TO SNOW SAT
NIGHT AS THE FORCING EXITS TO THE EAST.

A PATTERN SHIFT TOWARD STRONGER RIDGING OFF THE WEST COAST OF NOAM
NEXT WEAK WILL SUPPORT A TRANSITION TOWARD MORE TROUGHING OVER
EASTERN CANADA AND THE NORTHERN TIER OF THE US. THIS WILL OFFER AN
OPPORTUNITY FOR BETTER INFILTRATION OF COLDER AIR INTO THE GREAT
LAKES REGION. SO COLDER TEMPS IN COMPARISON TO THE LAST FEW WEEKS
ARE EXPECTED TO TAKE HOLD ACROSS THE REGION.

MARINE...

LOW PRESSURE OVER NORTHWEST OHIO WILL STRENGTHEN AS IT TRACKS INTO
FAR SOUTHERN LAKE HURON THIS EVENING. THE LOW WILL CONTINUE TO
DEEPEN AS IT LIFTS NORTHEAST INTO QUEBEC OVERNIGHT. THE RESULT WILL
BE A BACKING OF THE WINDS FROM NORTH TO WEST ACROSS LAKE HURON
TONIGHT...WITH THE INTENSIFICATION OF THE LOW LEADING TO GALE FORCE
WIND GUSTS.

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...WINTER WEATHER ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR MIZ047>049-
     053-054-060-061-068.

LAKE HURON...GALE WARNING UNTIL 11 AM EST THURSDAY FOR LHZ363-421-441>443-
     462>464.

     GALE WARNING UNTIL 11 AM EST THURSDAY FOR LHZ361-362.

     GALE WARNING UNTIL 5 AM EST THURSDAY FOR LHZ422.

LAKE ST CLAIR...GALE WARNING UNTIL 4 AM EST THURSDAY FOR LCZ460.

MICHIGAN WATERS OF LAKE ERIE...GALE WARNING UNTIL 4 AM EST THURSDAY FOR LEZ444.

     LOW WATER ADVISORY UNTIL NOON EST THURSDAY FOR LEZ444.

&&

$$

UPDATE.......KURIMSKI
AVIATION.....KURIMSKI
SHORT TERM...JVC
LONG TERM....SC
MARINE.......SC


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



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