633
FXUS63 KDTX 190924
AFDDTX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
424 AM EST THU DEC 19 2024
KEY MESSAGES
- FLURRIES OR A STRAY SNOW SHOWER THIS EVENING GIVE WAY TO PARTIAL
CLEARING LATE TONIGHT.
- THE NEXT LOW PRESSURE SYSTEM MOVES INTO THE GREAT LAKES LATE
THURSDAY WITH A ROUND OF SNOW EXPECTED TO LAST THROUGH FRIDAY. SNOW
COVERED ROADS ARE LIKELY TO AFFECT THE FRIDAY MORNING COMMUTE.
- A GENERAL 1 TO 3 INCH SNOW ACCUMULATION IS EXPECTED ACROSS SE
MICHIGAN DURING THE THURSDAY NIGHT AND FRIDAY TIME PERIOD.
- LOCALLY HIGHER TOTALS ARE POSSIBLE DUE TO LAKE EFFECT BANDS IN THE
THUMB, ESPECIALLY FRIDAY INTO FRIDAY NIGHT.
- MUCH COLDER AIR MOVES IN FOR THE WEEKEND LIMITING DAYTIME
TEMPERATURES TO THE 20S WITH SINGLE DIGIT READINGS POSSIBLE AT
NIGHT.
DISCUSSION
SPRAWLING HIGH PRESSURE DRIFTS OVERHEAD THIS MORNING, ALTHOUGH FOR
NOW NORTHERLY FLOW ON ITS EASTERN FLANK CONTINUES TO ACTIVATE LAKE
MICHIGAN SNOW SHOWERS THAT ARE JUST MAKING IT INTO THE SAGINAW
VALLEY AND THUMB REGIONS. VERY WEAK RADAR RETURNS (< 15 DBZ) ARE AN
INDICATION OF FLURRIES (AS OBSERVED AT THE WHITE LAKE OFFICE) AND/OR
LIGHT FREEZING DRIZZLE. FORECAST SOUNDINGS DO MAINTAIN A SATURATED
BOUNDARY LAYER THROUGH MOST OF TODAY (BEFORE THE ARRIVAL OF SYNOPTIC
PRECIPITATION) WITH TEMPERATURES AT THE TOP OF THE BL AROUND -10 C
(QUESTIONABLE FOR ICE NUCLEATION). THAT SAID, UPPER LEVELS WILL
SATURATE THROUGHOUT THE DAY WITHIN A MORE SUITABLE THERMAL RANGE FOR
ICE NUCLEATION. THE ADDED ICE CRYSTAL SEEDING FROM ALOFT FAVORS
PERIODS OF FLURRIES TODAY RATHER THAN DRIZZLE.
ALL EYES ARE ON THE UPSTREAM CLIPPER SYSTEM THAT HAS BEEN TRACKING
THROUGH THE DAKOTAS THIS MORNING AND WILL EVENTUALLY REACH SE
MICHIGAN THIS EVENING. EARLY INDICATIONS OF THIS SYSTEM WILL BE A
THICKENING CLOUD CANOPY, ALTHOUGH THE 00Z DTX SOUNDING FROM LAST
NIGHT SHOWS A SUBSTANTIAL DRY SUBSIDENCE INVERSION. THIS WILL BE A
MAJOR INHIBITOR TO SNOW POTENTIAL EARLY ON FOR THIS SYSTEM. UPSTREAM
SATELLITE/RADAR IMAGERY SHOWS AN EXPANSIVE REGION OF WARM ADVECTION
AHEAD OF THIS SYSTEM, WITH A SUBTLE RIPPLE IN THE H7-H6 MB HEIGHT
FIELD AND LEFT EXIT REGION DYNAMICS PIVOTING TOWARD SE MICHIGAN
TONIGHT AS THE PARENT TROUGH AMPLIFIES AND NEGATIVELY TILTS. THE
LEADING "ARM" OF ISENTROPIC ASCENT WILL SURGE INTO THE NORTHERN
PORTIONS OF THE CWA (NORTH OF I-69), BUT WILL STRUGGLE TO MAKE MUCH
OF AN IMPACT FURTHER SOUTH DUE TO A REINFORCED DRY SLOT BELOW 10.0
KFT. SO FOR THE EARLY EVENING, THE SAGINAW VALLEY AND THUMB WILL SEE
ACCUMULATING SNOWFALL WHILE THE REMAINING SOUTHERN ZONES WILL SEE
LIGHT SNOW SHOWERS AT BEST.
PRIME TIME FOR MORE WIDESPREAD LIGHT SNOWFALL ACCUMULATIONS WILL BE
BETWEEN 00Z-12Z (7PM-7AM) FRIDAY AS THE AMPLIFYING PARENT WAVE
DRIVES VORTICITY ADVECTION INTO THE AREA AND THE SURFACE LOW SLIDES
JUST SOUTH OF THE STATE LINE. A FAIRLY LONG DURATION OF MODEST,
BROAD ASCENT WILL LEAD TO SNOWFALL RATES OF AROUND 0.1-0.2" PER
HOUR, EQUATING TO SNOWFALL TOTALS OF 1 TO 3 INCHES BY FRIDAY
AFTERNOON WHICH IS QUITE TYPICAL FOR CLIPPER SYSTEMS.
DURATION/MAGNITUDE FALLS JUST SHORT OF WINTER WEATHER ADVISORY
CRITERIA, ALTHOUGH ONE MAY BE ISSUED IN SUBSEQUENT FORECAST UPDATES
IF PEAK SNOWFALL RATES CENTER AROUND THE MORNING COMMUTE FRIDAY. ONE
OF THE MAIN FACTORS LIMITING RATES/ACCUMULATION IS A LACK OF
MEANINGFUL MOISTURE ADVECTION AS EVIDENT IN DRY POCKETS ON FORECAST
SOUNDINGS. WILL SAY THAT THE MOISTURE CONTENT ALOFT WITHIN THE ICE
NUCLEATION ZONE IS MUCH MORE ROBUST THAN THE LOWER LEVELS, SO
SEEDING SHOULD HELP KEEP PRECIPITATION TYPE AS SNOW RATHER THAN
FREEZING DRIZZLE.
ONE OF THE MORE UNCERTAIN ASPECTS OF THIS SYSTEM CONTINUES TO BE THE
LAKE EFFECT RESPONSE ALONG THE LAKE HURON SHORELINE FRIDAY AFTERNOON-
EVENING. ALL OF THE THERMODYNAMIC FACTORS WILL BE IN PLACE FOR
HIGHER SNOWFALL RATES AND EFFICIENT MICROPHYSICS, INCLUDING A
SATURATED DGZ AND UNSTABLE BOUNDARY LAYER. THE QUESTION HOWEVER IS
WHETHER THE BAND WILL CLIP INLAND LOCATIONS AND IF SO, FOR WHAT
DURATION. BOTH HREF PROBABILITIES AND LOCAL GUIDANCE INDICATE A 30-
40% CHANCE FOR SNOWFALL ACCUMULATIONS AROUND THE THUMB TO EXCEED 4
INCHES (ADVISORY THRESHOLDS) WITH THE TIPPING POINT BEING IF THE
LAKE EFFECT BAND IS ONSHORE.
ANY LAKE EFFECT ACROSS THE THUMB SHOULD COME TO AN END SATURDAY
MORNING AS A COLDER AND DRIER ARCTIC AIRMASS BUILDS IN FROM CANADA.
DAYTIME HIGHS IN THE 20S AND OVERNIGHT LOWS IN THE SINGLE DIGITS TO
LOW TEENS ARE EXPECTED, BUT WITH MINIMAL WIND WIND CHILLS STAY
WITHIN A FEW DEGREES OF THE TEMPERATURE. THE DRIER AIR WILL ACTUALLY
AFFORD PERIODS OF SUN THROUGHOUT THE WEEKEND. LOOKING FORWARD TO THE
HOLIDAY WEEK, THERE IS QUITE A BIT OF ENSEMBLE SPREAD WHEN IT COMES
TO HOW QUICKLY THE ARCTIC AIR RETREATS. THE ENS IS COLDER, THE GEFS
IS WARMER, AND THE GEPS HAS A LOT OF VARIANCE IN ITS SOLUTION SPACE.
WHILE THERE IS GOOD AGREEMENT IN THE ENSEMBLE ENVELOPE FOR
MEASURABLE PRECIPITATION TO OCCUR CHRISTMAS EVE, THE SPREAD IN
TEMPERATURES INTERSECTS THE FREEZING MARK SO THERE ARE BOTH RAIN AND
SNOW SOLUTIONS FOR EARLY NEXT WEEK.
MARINE
WINDS QUICKLY DECREASE THIS MORNING AS HIGH PRESSURE BRIEFLY SLIDES
ACROSS THE CENTRAL GREAT LAKES. MODEST EASTERLY FLOW (15-20KT) WILL
DEVELOP BY EVENING IN ADVANCE OF A CLIPPER SYSTEM DROPPING THROUGH
THE UPPER MIDWEST. THIS SYSTEM ARRIVES OVER OUR REGION LATE TODAY
THROUGH THE FIRST HALF FRIDAY BRINGING WIDESPREAD SNOW SHOWERS.
WINDS VEER TO NORTHERLY LATTER HALF OF FRIDAY ON THE BACKSIDE OF THE
LOW THOUGH WINDS ARE STILL ONLY EXPECTED TO REACH 20-25KTS,
STRONGEST IN THE BAY AND NEAR THE THUMB. THAT SAID, ONSHORE FLOW
WILL REQUIRE ANOTHER SMALL CRAFT ADVISORY FOR THE THUMB/BAY
NEARSHORE WATERS. A SECOND HIGH THEN BUILDS OVER THE REGION THIS
WEEKEND.
PREV DISCUSSION
ISSUED AT 1229 AM EST THU DEC 19 2024
AVIATION
WIDESPREAD STRATUS FEATURING A CLOUD BASE AT 2500 TO 3500 FT WILL
MARK CONDITIONS INTO THE MID AFTERNOON HOURS. WINDS DURING THIS TIME
EMERGING FROM THE SOUTHEAST. INITIAL BAND OF LIGHT SNOW ARRIVING IN
ADVANCE OF AN APPROACHING CLIPPER ARRIVES FOR THE LATE AFTERNOON AND
EVENING PERIOD. GREATER LIKELIHOOD FOR THIS TO IMPACT FNT/MBS WITH
SOME REDUCTION IN CLOUD BASE AND VISIBILITY A POSSIBILITY. A MORE
WIDESPREAD SNOWFALL ACCOMPANIES THE CLIPPER OVERNIGHT AND EARLY
FRIDAY, LEADING TO EXPANSIVE IFR CIGS/VSBY WITH SOME INTERVALS OF
LIFR A POSSIBILITY WITHING BRIEF HIGHER SNOWFALL RATES. LIGHT SNOW
WILL TRANSITION TO A LOWER COVERAGE OS SNOW SHOWERS BY FRIDAY
AFTERNOON AS WINDS VEER TO NORTHEAST BEHIND THE SYSTEM.
FOR DTW...INITIAL PERIOD OF SNOW MOST LIKELY HOLDS JUST NORTH OF THE
TERMINAL EARLY THIS EVENING. ONSET OF ACCUMULATING SNOW WITH
ACCOMPANYING REDUCTION OF VISIBILITY EXPECTED WITHIN THE 06Z-08Z
WINDOW. SNOW PERSIST THROUGH THE MORNING HOURS FRIDAY, WITH AN
ACCUMULATION OF 1 TO 2 INCHES MOST PROBABLE.
DTW THRESHOLD PROBABILITIES...
* HIGH FOR CEILING AT OR BELOW 5000 FEET THROUGH FRIDAY.
* HIGH FOR PRECIPITATION TYPE OF SNOW TONIGHT AND FRIDAY.
DTX WATCHES/WARNINGS/ADVISORIES
MI...NONE.
LAKE HURON...SMALL CRAFT ADVISORY UNTIL 7 AM EST THIS MORNING FOR LHZ421-441-442.
LAKE ST CLAIR...NONE.
MICHIGAN WATERS OF LAKE ERIE...NONE.
DISCUSSION...MV
MARINE.......KDK
AVIATION.....CB
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