Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 181628
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1128 AM EST THU DEC 18 2014

.UPDATE...

AREAS OF PATCHY FREEZING DRIZZLE WILL CONTINUE TO IMPACT SOUTHEAST
MICHIGAN THROUGH EARLY AFTERNOON. CURRENT RADAR IMAGERY AND LOCAL
OBSERVATIONS ARE SHOWING FREEZING DRIZZLE FILTERING INTO
SOUTHEAST MICHIGAN. PLENTY OF MOISTURE IN THE BOUNDARY LAYER AND
SURFACE TEMPERATURES HOVERING NEAR FREEZING WILL RESULT IN
FREEZING DRIZZLE CONTINUING INTO THE AFTERNOON HOURS. LOCATIONS
MOST LIKELY TO SEE FREEZING DRIZZLE WILL BE BETWEEN THE I 69 AND I
94 CORRIDOR. MOTORISTS SHOULD BE ADVISED IF TRAVELING IN SOUTHEAST
MICHIGAN AS LIGHT ICING ON ROADWAYS COULD LEAD TO SLIPPERY
CONDITIONS MAINLY ON BRIDGES...OVERPASSES...AND EXIT RAMPS. DRIER
AIR WILL BEGIN TO FILTER INTO THE REGION BY LATE
AFTERNOON...CAUSING FREEZING DRIZZLE TO COME TO AN END.

&&

.AVIATION...ISSUED 653 AM EST THU DEC 18 2014

//DISCUSSION...

A WEAKENING SURFACE TROUGH OF LOW PRESSURE WILL SETTLE SLOWLY OVER
SE MICHIGAN DURING THE DAY AND MAINTAIN MVFR CEILING AT ALL TERMINAL
SITES. A FEW POCKETS OF SNOW GRAINS OR VERY LIGHT DRIZZLE/FZDZ WILL
BE POSSIBLE BUT VERY TRANSIENT DURING THE MORNING. THIS WILL BE
MONITORED FOR POTENTIAL UPDATES IF ANY OBSERVATIONS SUGGEST A
VISIBILITY RESTRICTION...WHICH WOULD INDICATE MORE SUBSTANTIAL
ACTIVITY. OTHERWISE...MVFR STRATUS WILL LINGER THROUGH MIDNIGHT
FOLLOWED BY DRY AIR AND LESS CLOUDS TOWARD SUNRISE AS THE TROUGH
WEAKENS AND THE WIND TURNS MORE FROM THE NORTH.

FOR DTW... MVFR CEILING WILL BE FIRMLY IN PLACE THROUGH THE DAY WITH
PRECIPITATION LIMITED TO VERY LIGHT AND BRIEF SNOW GRAINS OR
DRIZZLE/FZDZ DURING THE MORNING. NO OPERATIONAL IMPACT IS EXPECTED
AND THE POTENTIAL WILL DIMINISH AS THE TROUGH WEAKENS DURING THE
MORNING. THIS WILL NOT BE CARRIED IN THE FORECAST BUT WILL BE
MONITORED FOR SIGNS OF INCREASED ACTIVITY. MVFR CLOUDS WILL LINGER
THROUGH THE NIGHT AND THEN DIMINISH FROM NORTH TO SOUTH AFTER
SUNRISE FRIDAY.

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CEILING BELOW 5000 FEET TODAY THROUGH TONIGHT.

&&

.PREV DISCUSSION...ISSUED 347 AM EST THU DEC 18 2014

SHORT TERM... TODAY AND TONIGHT

A BRIEF PERIOD OF FREEZING DRIZZLE AND SNOW GRAINS OBSERVED HERE
LOCALLY TO MATCH UP WITH RADAR AND LIGHT VISIBILITY RESTRICTION IN A
FEW THUMB LOCATIONS TO CARRY A MENTION IN THE FORECAST DURING THE
EARLY MORNING. EXPECT THE WEAKENING LOW LEVEL WIND FIELD AND
CORRESPONDING LOSS OF SHEAR AT THE INVERSION LAYER WILL CAUSE THE
PATTERN TO DISSIPATE BY MID MORNING. THE REMAINING BOUNDARY LAYER
MOISTURE WILL KEEP CLOUDS LOCKED IN OVER LOWER MICHIGAN FOR THE REST
OF THE DAY AND THE WIND WILL DIMINISH COMPARED TO YESTERDAY. THE
LIGHT WIND WILL MAKE IT EASIER TO TAKE TEMPERATURES IN THE 20S TO
START THE DAY WHICH WILL THEN STRUGGLE TO MAKE THE MID 30S DURING
THE AFTERNOON. THE CLOUDS WILL BE SUPPORTED MAINLY BY BOUNDARY LAYER
CONVERGENCE WITHIN THE WEAK SURFACE PRESSURE TROUGH THAT WILL SETTLE
SLOWLY FROM NORTHERN LOWER INTO SOUTHERN LOWER MICHIGAN DURING THE
DAY. 00Z OBSERVED AND MODEL SOUNDINGS ALSO SHOW A WELL DEFINED
SUBSIDENCE INVERSION BASED AT ABOUT 900 MB THAT WILL PREVENT MIXING
OF BOUNDARY LAYER MOISTURE AND ANY RESULTING CLOUD EROSION. THE
INVERSION WILL BE MAINTAINED THROUGH THE DAY BY CONFLUENT FLOW ALOFT
DOWNSTREAM OF THE BLOCKING UPPER LEVEL RIDGE EXTENDING FROM HUDSON
BAY INTO THE UPPER MIDWEST.

DURING THE LATE AFTERNOON AND TONIGHT...THE POTENTIAL FOR LAKE
EFFECT SNOW IN THE THUMB IS KEPT IN MIND AS LOW LEVEL FLOW VEERS
TOWARD THE NORTH. EARLY MORNING OBSERVATIONS FROM THE CANADIAN RADAR
IN EXETER INDICATE THE SURFACE TROUGH FOCUSING SOME MODEST LAKE
EFFECT SNOW SHOWERS INTO SOUTHERN ONTARIO. THIS MATCHES UP WITH
MODEL SOUNDINGS THAT SHOW CONVECTIVE DEPTH UP TO ABOUT 850 MB WITH
DELTA THETA-E NEAR 4K OR LI ABOUT -1C. THERE IS ALSO SOME SEEDING OF
THE DGZ FROM MID LEVEL MOISTURE THAT WILL BE STRIPPING AWAY DURING
THE MORNING. THIS SHOULD RESULT IN A DIMINISHING TREND BEFORE THE
WIND STARTS TO VEER. THEN...BY TONIGHT...DRY AIR COMING DOWN FROM
NORTH OF LAKE HURON WILL MAINTAIN A DRY DGZ AND FURTHER SUPPRESS
CONVECTIVE DEPTH TO 900 MB/3000 FEET OR LESS WHILE INSTABILITY
WITHIN THE LAYER DECREASES. THESE CONDITIONS WILL NOT SUPPORT ANY
MEANINGFUL SNOW SHOWERS AND MAY NOT EVEN BE GOOD ENOUGH FOR CLOUDS
OVERNIGHT THROUGH SUNRISE FRIDAY.

LONG TERM...FRIDAY THROUGH WEDNESDAY

HIGH PRESSURE WILL BE IN CONTROL OF THE CENTRAL GREAT LAKES LATE IN
THE WEEK AND THROUGH THE WEEKEND.  TEMPERATURES WILL BE RIGHT AROUND
NORMAL FOR THIS TIME OF YEAR WITH HIGHS IN THE LOW TO MID 30S AND
LOWS IN THE LOW TO MID 20S.  THE MAIN CONCERN DURING THIS PERIOD
WILL BE CLOUDS ON FRIDAY AS PREVIOUS MODEL RUNS INDICATED LINGERING
STRATUS DURING THE DAY WITH LOW LEVEL MOISTURE TRAPPED UNDERNEATH AN
INVERSION.  HOWEVER RECENT MODEL SOUNDINGS INDICATE A DECENT PUSH OF
LOW LEVEL DRY AIR MAKING ITS WAY FROM THE LAKE SUPERIOR REGION ON
FRIDAY...CLEARING OUT THE CLOUDS AND LINGERING LOW LEVEL MOISTURE
ACROSS SOUTHEAST MICHIGAN. GIVEN THE MODEL AGREEMENT AND RECENT
MODEL TRENDS WILL BE MORE OPTIMISTIC WITH CLOUD COVER DURING THE DAY
ON FRIDAY.  A WEAK MID LEVEL SHORTWAVE TRACKING THROUGH THE SOUTHERN
GREAT LAKES SATURDAY AND SATURDAY NIGHT IS EXPECTED TO BE NOTED ONLY
BY THE CLOUD COVER IN THE MID LEVELS.

THE MAIN SYSTEM OF NOTE IN THE EXTENDED IS A STRONG LOW PRESSURE
SYSTEM THAT COMES TOGETHER OVER THE GREAT LAKES REGION DURING THE
EARLY TO MIDDLE PART OF NEXT WEEK.  THE FIRST PART OF THIS SYSTEM
WILL BE NOTED BY WARMING TEMPERATURES ON MONDAY AS A CLIPPER SYSTEM
DEVELOPS OVER THE NORTHERN PLAINS.  THIS SYSTEM WILL THEN STALL AND
RAPIDLY UNDERGO CYCLOGENSIS OVER THE WESTERN GREAT LAKES ON
TUESDAY AS IT GETS REINFORCEMENTS FROM A VERY STRONG 140 KNOT
UPPER LEVEL JET AS THE MID LEVEL TROUGH BECOMES CLOSED OFF AS
HEIGHTS FALL. THE AREA LOOKS TO BE IN THE WARM SECTOR DURING THE
FIRST PART OF THIS EVENT ON TUESDAY...WITH MAINLY RAIN EXPECTED. A
STRONG PUSH OF COLD AIR TUESDAY NIGHT WILL CHANGEOVER THE RAIN TO
ALL SNOW BY CHRISTMAS EVE.

THE MAIN QUESTION THEN BECOMES HOW MUCH WRAP AROUND MOISTURE
WILL BE AVAILABLE FOR SNOWFALL ONCE THE MAIN SYSTEM AND MOISTURE
HEAD EAST AS THE SYSTEM DEEPENS FURTHER ACROSS THE MIDDLE ATLANTIC
STATES.  AT THIS POINT IT APPEARS MEAGER AMOUNTS OF MOISTURE WILL
BRING A FEW TENTHS OF AN INCH OF SNOWFALL ON CHRISTMAS EVE BEHIND
THE COLD FRONT...HOWEVER ADDITIONAL MOISTURE FROM THE GREAT LAKES
COULD CERTAINLY BOOST VALUES SOME IF LAKE EFFECT BANDS BECOME
VIGOROUS.  AT THIS POINT IT APPEARS CONFIDENCE IN THE SYSTEM PLAYING
OUT IN THIS GENERAL SENSE IS MEDIUM TO HIGH GIVEN THE MODEL
AGREEMENT...HOWEVER THERE IS QUITE A BIT OF UNCERTAINTY REGARDING
SNOWFALL AMOUNTS AS THE GFS HAS A MUCH BIGGER PUSH OF COLD AIR WHICH
IS NOT SURPRISING AS IT PHASES THE SYSTEMS QUICKER AND DEEPENS THE
LOW SOONER THAN THE ECMWF.  FURTHER RUNS WILL BE NEEDED BEFORE
WORKING OUT THE DETAILS REGARDING THIS SYSTEM AT THE TAIL END OF THE
EXTENDED.

MARINE...

HIGH PRESSURE WILL BUILD INTO THE AREA TODAY...BRINGING LIGHTER
WINDS TO THE MARINE AREAS.  WINDS AND WAVES HAVE FALLEN BELOW SMALL
CRAFT ADVISORY CRITERIA...THEREFORE THE HEADLINES WILL BE ALLOWED TO
EXPIRE AS ANTICIPATED.  THE HIGH WILL PROVIDE LIGHT WINDS AND WAVES
THROUGH THE WEEKEND.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MICHIGAN WATERS OF LAKE ERIE...NONE.
&&

$$

UPDATE.......RK
AVIATION.....BT
SHORT TERM...BT
LONG TERM....KURIMSKI
MARINE.......KURIMSKI


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