Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 100853
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
353 AM EST Sat Dec 10 2016

.DISCUSSION...

A band of snow showers set up across portions of Washtenaw and Wayne
Counties overnight. The intensity of these snow showers was enough
to coat roadways. There is a mid level short wave now rotating into
Lower Mi. This feature has led to an increase in mid level moisture
across southern Lake Mi and has boosted equilibrium levels. The
result has been an uptick in the lake response. The low level flow
is expected to back a little more southwesterly across nrn
Indiana/far sw Lower Mi this morning. This should cause a temporary
disruption to the organization of the convergence across Se Mi
before better convergence sets up a little farther north (I-96/M 59
corridor). While the mid level short wave impulse departs to the
east this afternoon, model soundings continue to show fairly
respectable inversion heights (around 8k ft). Hi res solutions
suggest lake effect will release off the lake and track across the
nrn Detroit suburbs (possibly up to the 1-69 corridor) this
afternoon. Steep low level lapse rates and lift within the dendritic
growth zone suggests good snowfall rates at times. Minor snow accums
and rapid visibility drops will thus be possible in these snow
showers today. The low level flow will back toward the south this
evening, ending the connection to Lake Mi.

Attention then turns to widespread accumulating snowfall Sunday into
Sunday night. A mid level short wave now rotating into Washington
State will race across the northern Rockies later today and is then
forecast to amplify across the nrn plains/upper MS Valley on Sunday.
Low level troughing in the lee of the Rockies today will establish a
broad region of low-mid level warm air advection across the wrn
Great Lakes tonight. This region of isentropic ascent will spread
across Se Mi during the course of the morning Sunday, leading to
widespread light snow. The approach of the upper wave and increasing
upper level divergence in the entrance region of an upper jet will
increase moist isentropic ascent across the forecast area Sun
afternoon/evening. Within this ascending branch (the developing warm
conveyor) will be strengthening moisture transport. 850mb mixing
ratios are forecast to rise to 3.5 g/kg by Sun evening. Mid level
stability still looks to be relatively high through most of this
evening, resulting in just a weak sfc wave lifting across Lower Mi
Sun night. The persistent forcing and adequate moisture still looks
to produce a long-duration snow event (early Sun morning into Sun
night).

Overall, total forecast snowfall is still in reasonable agreement
with the previous forecast. A trend toward a slightly stronger mid
level wave has warranted a nudge upward in fcst snowfall across the
northern half of the forecast area. With this wave still offshore at
00Z, some sampling issues/initialization errors still suggest a good
deal of uncertainty as to how this energy will phase as it traverses
the Rockies later today. If this system does not amplify as
currently shown by the 00Z suite, the isentropic upglide into the
area will be weaker, resulting in lower snowfall amounts than
currently forecast. If, however, the system deepens more than
expected (or toward the current NAM solution), a stronger and more
persistent region of mid level deformation may set up overhead,
resulting in a stronger moist inflow and higher snow totals. Another
thing complicating this event will be the warming during the course
of it, lead to a steady lowering of snow to liquid ratios. The
changing thermodynamic profile and location of strongest ascent also
suggests the character of the snow (ie flake size) may be highly
variable. Based on the 00Z model suite and taking into consideration
the lowering snow to liquid ratios (especially across the south),
total snowfall of 5 to 9 in. south of I-69 and 4 to 7 in. north
seems reasonable. Given the long duration, it seems more probable
that this event will be a high end winter wx advisory. Since there
is still some fcst uncertainty attm and another model cycle to
better sample this wave, a winter storm watch (as opposed to an
advisory) will be issued for the I-69 counties south Sun and Sun
night.

This system will depart to the east Sun night, with mid level dry
air rapidly overspreading the area early Mon morning, ending
additional snowfall and possibly supporting some drizzle. The medium
range model suite continue to show a deep upper low rotating from
central to eastern Canada next week, driving arctic air into the
Great Lakes. This will support well below normal temps during the
latter half of next week.

&&

.MARINE...

Modest west winds will continue over the Central Great Lakes today
in between weak low pressure over Lake Huron and high pressure over
the Ohio Valley. Gusts look to stay in the 20 to 25 knot range, but
may be stronger in lake-effect snow squalls. These squalls will be
most prevalent over Lake Huron today. Low pressure over Lake Huron
will shift east tonight, allowing winds to relax and coverage of
lake-effect snow showers to decrease and shift more into Canadian
waters. A stronger low pressure system will lift across the region
Sunday into Monday, generating a long-duration snowfall Sunday and
Sunday night. The passage of this system will first provide gusty
south winds late Sunday into Sunday night before winds flip to the
west behind a cold front late Sunday night and early Monday.

&&

.HYDROLOGY...

Low pressure lifting across the area will spread a long-duration
snowfall into Southeast Michigan from very late Saturday night
through late Sunday night. Total snowfall of 4 to 9 inches is
expected at this time, with highest totals south of I-69. Snow
intensity is expected to peak during the afternoon and early
evening, with character of the snow turning more wet and heavy
during the afternoon and evening. Rainfall equivalent of the snow
that will fall is expected to range between one-third and three-
quarters of an inch. Snow will end very early Monday morning.

&&

.PREV DISCUSSION...
Issued at 1221 AM EST Sat Dec 10 2016

AVIATION...

Lake-effect snow showers will continue to stream across the area
tonight. These will mainly affect the Detroit-area terminals, with
brief obstructions to visibility and drops to MVFR possible. The
most solid band looks to remain positioned between DTW and DET
through 07Z. Additional bands are expected to develop after about
10Z and slowly migrate northward through the day as flow off Lake
Michigan turns more to the west-southwest. This will first affect
the Detroit terminals this morning, then PTK and FNT by afternoon.

FOR DTW...Most solid lake-effect band may stay positioned between
DTW and DET through 07Z, while smaller and more cellular showers
affect the terminal directly overnight. Better organization is
expected just before sunrise near DTW, with band or bands shifting
northward through the day as winds turn more west-southwest.
Snowstorm that will impact the area all day Sunday will start to
spread light snow into DTW after 06Z Sunday morning, with heavier
snowfall rates remaining just outside of the TAF period.

//DTW Threshold Probabilities...

* Moderate for ceilings below 5000 feet tonight and Saturday.

* High for snow as precipitation type.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Watch from Sunday morning through late Sunday night
     FOR MIZ060>063-068>070-075-076-082-083.

Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...SC
MARINE.......HLO
HYDROLOGY....HLO
AVIATION.....HLO


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at www.weather.gov/detroit.



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