Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 081501
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1001 AM EST Fri Dec 8 2017

.UPDATE...

Clouds and flurries/snow showers have been increasing across Central
Michigan within increasing low level moisture advection under low
level southwest flow. Relatively low level inversions heights, under
4k ft, will keep the intensity of these snow showers light. Some
higher res solutions are however suggesting a potential enhancement
off Saginaw Bay hitting portions of Huron County with a little
heavier snow bursts this afternoon. The low level flow is forecast to
veer slightly more westerly this afternoon, likely pushing the lake
clouds and snow showers toward the I-69 corridor. The flow will then
back a little more southwesterly this evening, focusing the Lake Mi
plume back toward the Saginaw Valley/northern thumb. An update will
be issued mainly to reduce afternoon/evening pops south of the I-69
corridor. Given the very shallow convective cloud depths, any
accumulations this afternoon/evening should be minimal.

&&

.PREV DISCUSSION...
Issued at 544 AM EST Fri Dec 8 2017

AVIATION...

Modest warming and drying of the low levels this morning with
southwest flow has dissipated most of the low clouds. Low clouds
(3000-6000 Feet) should return to MBS this afternoon, and gradually
work southeast through evening hours. Light snow then spreading in
after midnight from west to east as low pressure moves into southern
Lower Michigan. Southwest winds ahead of the low today look to be in
the 10 to 15 knot range, with gusts at or above 20 knots. Winds
dropping at or below 10 knots tonight as the low arrives.

For DTW...Today looks to be mostly clear (below 12 kft) for much of
the day before mid clouds spread in and thicken up this evening.
Light snow developing late tonight, and continuing Tuesday morning.
Snowfall accumulations around 2 inches expected before activity
tapers off Saturday afternoon.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings at or below 5000 feet after 9Z Saturday and
  continuing during Saturday.

* High confidence in precip type being snow.

PREV DISCUSSION...
Issued at 320 AM EST Fri Dec 8 2017

Discussion...

A rather strong PV anomaly associated with sharply digging shortwave
which has crossed the north slopes of Alaska to its current position
over central Canada will provide the first widespread light snowfall
for southeast lower Michigan late tonight into Saturday. Low level
pressure forecast with this system is rather interesting (as well as
tricky) and seems modulated by latent heat from the Great Lakes.

The initial focus for cyclogenesis occurs over and along the eastern
shore of Lake Michigan as this shortwave digs into the region. The
surface reflection of this system then almost leap frogs to southern
Lake Huron on Saturday, or at the very least elongates into that
area along/south of rather strong baroclinic zone that is actually
left behind by slightly weaker, albeit geographically larger, PV
anomaly which will brush the northern Great Lakes over the next 12
hours or so. Either way, Hires models all indicate this enhanced
pressure drop over the warmer Great Lake waters with the transition
from Lake Michigan to Lake Huron occurring as the shortwave itself
reaches its furthest southern position and pivot, in a neutral tilt
configuration, eastward from the central to eastern Great Lakes.

What this suggests for the area would be slightly better or more
persistent low level forcing (and hence better light snow potential)
over the northwest or northern half of the forecast area with a
pretty sharp cutoff towards only an inch or so of snow over the far
southern forecast area. Will make minor adjustments to the snowfall
forecast to better capture this scenario. This will put most areas
in the 1 to 3 inch snow range for the 12 hour period from roughly 4
am tonight to 4 pm Saturday afternoon. Given these forecast amounts
and the relatively long time frame, do not expect conditions to
necessitate an advisory for most locations.

There is one caveat to this as Hires models all suggest a period of
lake effect/lake enhanced snow shower/squall potential from the
Thumb back west to near the Saginaw Valley on Saturday afternoon
assoicated with the thermally enhanced surface low pressure over
southern Lake Huron. This may very well provide a several hour
period of higher snow rates and bring local totals into the 3 to 4
inch range. Given the difficult to forecast mesoscale influences,
this question will likely end up being the biggest forecast headache
for this winter system.

This shortwave pivots east rather quickly late Saturday/Saturday
night, leaving the area in a relatively dry (and cold) weather for
the last half of the weekend. However, the overall pattern remains
on the active side as a series of clipper type shortwaves sweep
through the central Great Lakes in sharp northwest flow regime that
remains in place with the highly amplified eastern NOAM longwave
upper trough. The first of these, which is forecast with reasonable
agreement amongst the medium range models, suggests another period
of light, but widespread measurable snow, in the Monday/Monday eve
time frame. While timing/placement differences between the models
increase into mid next week, the possibility of another similar
clipper system seems pretty reasonable in the Wednesday or Thursday
period.

The chilly temperature regime that has overspread the region within
the developing upper trough will persist and intensify over the next
5 or 6 days, particularly within cold air advection on Tuesday in
the wake of aforementioned clipper. By this time, the daytime highs
in the upper 20s to lower 30s will be replaced widespread lower/mid
20s with overnight lows well down into the teens (and probably in
some cases the single digits by Wednesday morning).

MARINE...

Southwest gales over central sections of Lake Huron this morning,
with gusts around 30 knots over the rest of Lake Huron. Winds then
decreasing in the afternoon into tonight, as low pressure tracks
into southern Lower Michigan Saturday Morning. Winds will
subsequently shift to the north on Saturday, and increasing back
into the 30 to 35 knot range over southern Lake Huron Saturday
evening, resulting in waves of 5 to 10 feet. Another Gale warning
may be needed for this period, before winds decrease and shift to
the west by Sunday morning. Winds look to become even lighter as we
head into Monday, dropping at or below 15 knots as high pressure
tracks through the area.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning until 4 PM EST this afternoon for LHZ363-421-422-441-
     462.

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

$$

UPDATE.......SC
AVIATION.....DG
DISCUSSION...DG
MARINE.......SF


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