682  
FXUS63 KDTX 111211  
AFDDTX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI  
711 AM EST SUN DEC 11 2016  
   
UPDATE
 
 
SNOW HAS BECOME WIDESPREAD ACROSS THE FORECAST AREA. FAIRLY HIGH  
LIQUID TO SNOW RATIOS AT THIS TIME HAVE LED TO A VERY DRY AND FLUFFY  
SNOWFALL, WITH MORNING AMOUNTS OF 1 TO 2 INCHES ALREADY. GIVEN THE  
EXPECTATION FOR SOME COMPACTION DURING THE COURSE OF THE DAY,  
TOTAL FORECAST SNOWFALL AMOUNTS FOR THE EVENT WILL REMAIN  
UNCHANGED ATTM. A FORECAST UPDATE WILL BE ISSUED SIMPLY TO REMOVE  
THE EARLY MORNING WORDING.  
 

 
   
PREV DISCUSSION
 
 
ISSUED AT 556 AM EST SUN DEC 11 2016  
 
AVIATION...  
 
SNOW OVER SOUTHEAST MICHIGAN WILL BECOME MORE INTENSE BY AFTERNOON,  
WITH ALL SITES EXPECTED TO DROP TO LIFR VISIBILITIES FOR MUCH OF THE  
AFTERNOON AND EVENING. SNOWFALL RATES WILL LIKELY REACH ONE-INCH PER  
HOUR AT THE DETROIT-AREA TERMINALS DURING THIS TIME. SOME  
IMPROVEMENT IN VISIBILITY IS EXPECTED DURING THE LATE EVENING, AS  
SNOWFALL INTENSITY BEGINS TO WANE. SNOW SHOULD EXIT BY 08Z. TOTAL  
ACCUMULATIONS AT THE TERMINALS ARE EXPECTED TO RANGE BETWEEN 5 AND  
10 INCHES, WITH HEAVIEST AMOUNTS THE FURTHER SOUTH YOU GO.  
 
FOR DTW...IFR CONDITIONS ARE EXPECTED TO PERSIST THROUGH THE MORNING  
AS INTENSIFYING SNOWFALL KEEPS VISIBILITIES BELOW 2 MILES. LIFR  
CONDITIONS ARE EXPECTED DURING THE PEAK OF THE SNOWSTORM DURING THE  
AFTERNOON AND EVENING. IMPROVEMENT SHOULD OCCUR AFTER 05Z, WITH SNOW  
TAPERING OFF BY 08Z. TOTAL ACCUMULATION AT DTW IS EXPECTED TO RANGE  
BETWEEN 7 AND 10 INCHES.  
 
//DTW THRESHOLD PROBABILITIES...  
 
* HIGH FOR CEILINGS BELOW 5000 FEET THROUGH THE TAF PERIOD  
 
* MODERATE FOR VISIBILITY BELOW 1/2 MILE IN SNOW 20-02Z.  
 
* HIGH FOR SNOW AS PRECIPITATION TYPE THROUGH THE TAF PERIOD  
 
PREV DISCUSSION...  
ISSUED AT 405 AM EST SUN DEC 11 2016  
 
DISCUSSION...  
 
AS OF 330 AM, LIGHT SNOW STRETCHES FROM ERN SD ALL THE WAY ACROSS  
SRN LOWER MI. THIS SNOW IS OCCURRING WITHIN A BROAD REGION OF MOIST  
ISENTROPIC ASCENT ALONG THE 290-300K SURFACES. THIS RIBBON OF ASCENT  
WILL BE PERSISTENT THROUGH THE DURATION OF THE MORNING, WHILE THE  
MID LEVEL FRONT AND ASSOCIATED STRONGER FRONTAL FORCING SLOWLY LIFTS  
TOWARD THE NORTH. THE OVERALL WEAK NATURE OF THE FORCING THIS  
MORNING, WITH THE REGION OF LIFT NOT ALWAYS CONDUCIVE TO GOOD  
SNOWFLAKE SIZE, WILL LIMIT ACCUMULATIONS TO AROUND AN INCH (MAYBE  
TWO) BY EARLY AFTERNOON.  
 
CONDITIONS ARE THEN EXPECTED TO BECOME MUCH MORE CONDUCIVE TO HEAVY  
SNOWFALL DURING COURSE OF THE AFTERNOON. THE MID LEVEL WAVE NOW  
LOCATED OVER THE MONTANA/NORTH DAKOTA BORDER WILL TRACK ACROSS MN  
INTO NRN WI/WRN UPPER MI THIS EVENING, SPREADING MID LEVEL HEIGHT  
FALLS DOWNSTREAM ACROSS LOWER MI. IN RESPONSE, SFC LOW PRESSURE WILL  
DEVELOP ACROSS THE WRN GREAT LAKES THIS AFTERNOON, THEN LIFT ACROSS  
NORTHERN MI TONIGHT. THE KEY FEATURE OF INTEREST IS THE RAPID  
DEVELOPMENT OF AN UPPER JET MAX DOWNSTREAM ACROSS SE CANADA THIS  
AFTERNOON. UPPER DIVERGENCE WILL RAPIDLY INCREASE OVER LOWER MI,  
WITHIN THE ENTRANCE REGION OF THIS JET. THE CORRESPONDING  
AGEOSTROPHIC RESPONSE WILL LEAD TO AN INCREASE IN THE ISENTROPIC  
ASCENT FROM THE NRN OHIO VALLEY INTO SRN LOWER MI. THE 00Z MODEL  
SUITE INDICATE REDEVELOPMENT OF THE MID LEVEL FRONT/DEFORMATION  
REGION ACROSS METRO DETROIT THIS EVENING. MOIST ASCENDING INFLOW  
INTO THE DEFORMATION WILL SUPPORT AN UPTICK IN SNOWFALL INTENSITY IN  
THE 18 TO 21Z TIME FRAME, WITH PEAK FORCING NOW EXPECTED FROM 21Z TO  
03Z (DURING THE EVENING HOURS). MODEL CROSS SECTIONS DEPICT SOME  
DECREASING INSTABILITY ALOFT ATOP THE STRONG INFLOW. MOISTURE  
TRANSPORT STILL LOOKS QUITE GOOD AS THE BULK OF THE 00Z MODEL SUITE  
INDICATE MIXING RATIOS OF 3 TO 4 G/KG IN THE 850-750MB LAYER LIFTING  
ACROSS THE OH/MI STATE LINE THIS EVENING.  
 
ALWAYS THE MOST CHALLENGING ASPECT IN WINTER SYSTEMS IS WHERE THE  
MESO SCALE FORCING WILL SUPPORT THE MORE INTENSE SNOWFALL. THIS  
SYSTEM IS NO DIFFERENT. THE BULK OF THE 00Z SUITE, INCLUDING THE  
HRRR AND ARW, SUGGEST THE MID LEVEL DEFORMATION WILL SET UP  
SOMEWHERE ACROSS METRO DETROIT/ANN ARBOR UP INTO PORT HURON LATER  
TODAY. STORM TOTAL QPF AMONG THE 00Z SUITE GENERALLY RANGE ANYWHERE  
FROM 7 TENTHS UP TO AN INCH. PREVIOUS RUNS OF THE ECMWF HAVE BEEN ON  
THE LOWER END WITH RESPECT TO TOTAL LIQUID QPF. THE NOTED UPTICK IN  
THE 00Z RUN LENDS SUPPORT TO THE REST OF THE MODEL SUITE. MODEL  
CROSS SECTIONS INDICATE THE DEPTH OF THE LIFT WILL SUPPORT BETTER  
SNOWFLAKE SIZE DURING THE AFTERNOON AND EVENING. OF SOME CONCERN IS  
AN ISOTHERMAL LAYER IN THE -5 TO -10C RANGE AT TIMES DURING THIS  
EVENING, WHICH WOULD SUPPORT GOOD SNOWFALL AGGREGATION.  
 
TAKING THESE FACTORS INTO CONSIDERATION AND WITH SNOW TO LIQUID  
RATIOS FROM METRO DETROIT SOUTH EXPECTED TO SLOWLY DROP FROM AROUND  
15:1 DOWN TO 8:1 BY LATE TONIGHT, TOTAL SNOWFALL OF 6 TO 10 INCHES  
LOOK POSSIBLE SOUTH OF A HOWELL TO PORT HURON LINE. IT IS VERY  
POSSIBLE THAT THE BULK OF THIS COULD FALL IN A 6 HOUR WINDOW  
BETWEEN 4 PM AND 10 PM. THEREFORE, THE ADVISORY FOR THESE LOCALS HAS  
BEEN UPGRADED TO A WARNING. FARTHER NORTH, FORCING IS EXPECTED TO BE  
WEAKER. HOWEVER, LIQUID TO SNOW RATIOS WILL BE HIGHER WHICH WILL  
STILL SUPPORT ADVISORY TYPE ACCUMS. BASED ON THE LATEST GUIDANCE,  
FCST SNOW AMOUNTS ACROSS THE SAGINAW VALLEY AND THUMB WILL BE  
TAPERED TO 4 TO 6 INCHES, WITH 6 TO 8 HOLDING ALONG THE I-69  
CORRIDOR FROM LANSING TO LAPEER. ONE OF THE MAIN CONCERNS DURING  
THIS EVENT WILL BE WHERE THE MORE PERSISTENT MID LEVEL DEFORMATION  
AND PIVOT POINT SET UP. A SLIGHT DEVIATION FROM THE CURRENT FORECAST  
THOUGHTS WILL REQUIRE SOME ADJUSTMENTS TO FCST SNOWFALL AMOUNTS  
DURING THE DAY.  
 
A PUNCH OF MID LEVEL DRY AIR ARRIVING OVERNIGHT WILL RESULT IN AN  
ABRUPT END TO ACCUMULATING SNOWFALL PRIOR TO THE MON MORNING RUSH  
HOUR. THE DEPARTURE OF THE MID LEVEL WAVE TO THE EAST OF THE GREAT  
LAKES ON MON WILL SUPPORT PROLONGED SUBSIDENCE. LOW LEVEL COLD AIR  
ADVECTION ON MON WILL BE FAIRLY WEAK. THIS AND WITH STEEP LOW LEVEL  
LAPSE RATES, TEMPS SHOULD HOLD IN THE 30S ON MONDAY. A DEEP UPPER  
LOW IS STILL FORECAST TO ROTATE ACROSS CNTL AND ERN CANADA NEXT  
WEEK, DRIVING ARCTIC AIR INTO THE GREAT LAKES BY MIDWEEK AND  
RESULTING IN MUCH BELOW NORMAL TEMPERATURES.  
 
MARINE...  
 
WINDS WILL TURN SOUTHERLY TODAY, BECOMING A LITTLE GUSTY DURING THE  
AFTERNOON AND EVENING, IN ADVANCE OF LOW PRESSURE MOVING THROUGH THE  
GREAT LAKES. THE SOUTHERLY WINDS WILL ALLOW WAVE HEIGHTS TO BUILD TO  
ABOUT 4 FEET ALONG THE NEARSHORE WATERS OF SOUTHERN LAKE HURON FOR  
SEVERAL HOURS THIS EVENING BETWEEN PORT SANILAC TO PORT AUSTIN. THE  
LOW WILL ALSO SPREAD STEADY SNOWFALL ACROSS THE AREA THROUGH MUCH OF  
TONIGHT, WITH POOR VISIBILITIES BELOW ONE MILE EXPECTED THROUGH MUCH  
OF THE AFTERNOON AND EVENING. WINDS WILL THEN SHIFT WESTERLY AND  
GUST UP TO 30 KNOTS EARLY MONDAY MORNING BEHIND A COLD FRONT, WITH  
ANOTHER REINFORCING SHOT OF COLD AIR EARLY ON TUESDAY MAINTAINING  
MODERATE SPEEDS. ARCTIC AIR SPILLING INTO THE REGION WILL THEN BRING  
THE THREAT FOR FREEZING SPRAY, RENEWED LAKE-EFFECT SNOW SQUALLS, AND  
POTENTIALLY GALE FORCE WIND GUSTS OVER LAKE HURON TUESDAY NIGHT  
THROUGH THURSDAY.  
 
HYDROLOGY...  
 
LOW PRESSURE LIFTING ACROSS THE AREA WILL SPREAD A LONG-DURATION  
SNOWFALL INTO SOUTHEAST MICHIGAN TODAY THROUGH LATE TONIGHT. STORM  
TOTAL SNOWFALL IS EXPECTED TO REACH BETWEEN 6 AND 10 INCHES ALONG  
AND SOUTH OF THE M-59 CORRIDOR. TOTALS NORTH OF THERE WILL REACH 4  
TO 8 INCHES, WITH AMOUNTS SHOWING A DECREASING TREND AS YOU MOVE  
FROM SOUTH TO NORTH. SNOW INTENSITY IS EXPECTED TO PEAK DURING THE  
AFTERNOON AND EARLY EVENING. RAINFALL EQUIVALENT OF THE SNOW THAT  
WILL FALL IS EXPECTED TO RANGE BETWEEN ONE-THIRD AND THREE-QUARTERS  
OF AN INCH. SNOW WILL TAPER OFF AFTER 1 AM, WITH ONLY A FEW LIGHT  
SNOW SHOWERS EXPECTED BY SUNRISE.  
 

 
   
DTX WATCHES/WARNINGS/ADVISORIES
 
 
MI...WINTER WEATHER ADVISORY UNTIL 4 AM EST MONDAY FOR MIZ047>049-  
053>055-060>062.  
 
WINTER STORM WARNING UNTIL 4 AM EST MONDAY FOR MIZ063-068>070-075-  
076-082-083.  
 
LAKE HURON...NONE.  
LAKE ST CLAIR...NONE.  
MICHIGAN WATERS OF LAKE ERIE...NONE.  

 
 

 
 
UPDATE.......SC  
AVIATION.....HLO  
DISCUSSION...SC  
MARINE.......HLO  
HYDROLOGY....HLO  
 
YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE  
AT WWW.WEATHER.GOV/DETROIT.  
 
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