Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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196
FXUS63 KDTX 141446 AAA
AFDDTX

Area Forecast Discussion...UPDATED
National Weather Service Detroit/Pontiac MI
946 AM EST Thu Dec 14 2017

.UPDATE...
As of 945 AM EST...Chilly start to the morning across southeast
Michigan, as recent snowfall and fresh snowpack allowed temperatures
to fall well into the single digits for most areas outside of Detroit
metro and Monroe County, with a few low temperature readings even
below zero in the Saginaw Valley. For today, little change to the
ongoing forecast except to tweak based on latest trends in
observations. A dry day is forecast for most, with the exception of
being a few stray lake effect snow showers possible along the
nearshore of Lake Huron in the Thumb. Little accumulation is expected
if these bands drift onshore (less than 1 inch), but with offshore
flow in place, current bands over southern Lake Huron depicted on the
Exeter, Ontario radar should remain out over the open water,
gradually shifting eastward with time. High temperatures will be
15-20 degrees below normal, only in the upper teens for most areas.

&&

.PREV DISCUSSION...
Issued at 524 AM EST Thu Dec 14 2017

AVIATION...

Remaining MVFR/lower VFR stratus/strato-cu cigs will wrap southeast
of all terminals during the first few hours of the forecast period.
Approach of next shortwave in Arctic flow will begin to spread cigs
back into area in the 00z-03z time frame, especially for KMBS/KFNT
with trajectory off of Lake Michigan in developing SW flow. Flurries
may even occur at these sites before 12z, but better chance comes
after 12z Friday.

For DTW...Last of the cigs in the 2500-4500 foot area (and any
flurries) will shift out of terminal by 14z-15z. VFR for the rest of
the day as flow backs from NW to WSW. A return of sub-5kft cigs will
be possible in the 03z-05z time frame.

//DTW THRESHOLD PROBABILITIES...

* Medium confidence in cigs aob 5000 ft agl through 14Z and again
  after 03z tonight.

PREV DISCUSSION...
Issued at 305 AM EST Thu Dec 14 2017

DISCUSSION...

The strong clipper, which set record daily snowfall records at
Detroit and Flint yesterday (6.3/7.9 inches respectively), already
over/near State College Pennsylvania (as of 8Z), with subsidence and
drying taking place over southeast Michigan this morning. Still
contending with Lake Effect snow showers over the eastern Thumb
Region, but this activity should be moving offshore shortly after
12z, as winds shift to the northwest, and surface ridge builds into
the region today. With the fresh snow cover, even with a good deal
of sunshine, expecting highs only in the upper teens to around 20 C,
as mixing depths come up short of 2 kft.

Deep PV anomaly dropping south from the Arctic circle will move over
Ontario by Friday morning, with the extension/trough axis sliding
though the Central Great Lakes during Friday. With this forcing and
associated surface trough, snow showers are expected with about 1.75
G/KG of specific humidity at 850 mb to work with. Even after the
trough passage, westerly flow off Lake Michigan and cold advection
will result in steep low level lapse rates as we head into the
afternoon/early evening. Deep layer of supersaturation with respect
to ice, 25 to 50 J/KG of surface based Cape embedded within the DGZ,
and winds around 35 knots at the top of the boundary layer will
support possible snow squalls. Even outside of any squalls, probably
looking at gusts of 25 to 30 mph with the deeper mixing. Enhanced
blowing snow may be a concern due to the powdery snow left over from
yesterday`s snow storm. Temperatures falling into the lower 20s
within the heavier activity assuring roads becoming snow covered
and/or icy, along with the brief near white-out conditions possible.
Expecting many locations to receive a quick inch or two during the
Friday-Friday evening period.

Pretty good chance of additional snow Friday Night Into Saturday, as
as low level frontal boundary hangs around the I-94 corridor, with
mid level fgen flare-up farther north as moisture advection takes
place from the Midwest. 850 mb specific humidity progged to climb to
around 2.5 G/KG (per 00z EURO). Seems like another narrow swath of
inch or two is possible as the frontal boundary lingers into
Saturday, but uncertainty on the exact position, with waning FGEN on
Saturday as a flat upper level ridge builds in.

Dampening upper level wave ejecting out of the Baja California area
looks to be tracking through the Ohio Valley on Sunday, caught up in
the southwest flow of the upper level trough situated over the
Rockies. With the system sliding off to the east along or just south
of the Michigan border, our warm up looks to be tempered, but
temperatures still likely climbing above the freezing mark. None-the-
less, some type of light wintry mix remains possible in the Sunday-
Monday time frame.

MARINE...

Northeast winds gusting into the 25 to 30 knot range will steadily
ease and back to the northwest this morning. In response, waves in
the 5 to 7 foot range will gradually subside and shift east away
form the nearshore waters. Small Craft Advisories will continue into
the afternoon due to the waves. The next clipper disturbance will
arrive on Friday, bringing a renewed threat of snow squalls to Lake
Huron. While winds will likely gust into the 25 to 30 knot range,
both in the southwest and northwest flow ahead of and behind this
system, widespread gale conditions are not anticipated. Wind will
ease into Saturday as Arctic high pressure brushes by to the north.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 4 PM EST this afternoon for LHZ421-
     441>443.

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

$$

UPDATE.......IRL
AVIATION.....DG
DISCUSSION...SF
MARINE.......DG


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