Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210509
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1209 AM EST Wed Nov 21 2018

.AVIATION...

Arctic frontal boundary on pace to track across the southeast
Michigan airspace during the early-mid morning hours. The frontal
passage will prompt a reduction of ceiling height into MVFR, with
the potential for snow showers to briefly increase in coverage.
Window for snow showers now appears limited to a 3-4 hour period at
any one location given the narrow forcing aligned along the
advancing front. Accumulation minimized by both the lack of duration
and intensity - a couple hundredths at most. Firm pre-frontal
southwest wind overnight will turn northwesterly with the frontal
passage Wednesday morning. The drier post-frontal northwest
trajectory will favor improving conditions during the latter half of
Wednesday, cigs settling into VFR before scattering late in the day.

For DTW...Window for light snow showers/flurries centered 11z-15z.
Forecast holding at MVFR restrictions given expected limited
intensity. Little to no accumulation expected. Prevailing southwest
winds continue overnight, with cold frontal passage then bringing a
wind shift to northwest around 15z.

.DTW THRESHOLD PROBABILITIES...

* Medium for ceiling 5000 ft or less overnight, then high Wednesday
  morning.

* High for ptype as snow Wednesday morning.

&&

.PREV DISCUSSION...
Issued at 341 PM EST Tue Nov 20 2018

DISCUSSION...

Weak corridor of isentropic ascent supporting light snow shower
activity over central Wisconsin and southern Minnesota this
afternoon well in advance of a surface cold front extending from
northern MN into North Dakota this afternoon. The front will be
driven southward through the CWA overnight as digging jet energy
fosters an expanding corridor of height falls over the northeast
quadrant of the country and eastern Canada through Thanksgiving. At
the same time, upstream longwave amplification will strengthen
developing mid-level confluence helping to force strong Arctic high
pressure into the Central and Lower Great Lakes Wednesday through
Thursday. Plume of mid-level dry air associated with this developing
subsidence corridor will catch up to, and pass, the surface front
late tonight effectively stripping out mid-level moisture within the
DGZ during fropa. Any potential for accumulation, even just a couple
tenths, will therefore be dependent on surface trough entrainment of
the Lake Michigan plume. Southwest fetch veering to westerly during
this time should be sufficient to produce a healthy coverage of very
light SN worth high chc and low-end likely pop, but confidence is
high that accumulation will be limited to under one half inch.

Anticyclonic flow associated with the aforementioned Arctic high
will build in quite aggressively through Wednesday as 850mb temps
fall to -15C and approach -20C in the Thumb. Temperatures will
either hover near early-morning highs around 30F or slowly fall
during the day. Boundary layer clouds are forecast to scour out by
Wed evening as winds begin to diminish under clear skies. Radiating
potential will be far from optimal as decoupling may take most of
the night with high pressure centered well north and west. The lake
aggregate will further ensure an elevated gradient in the Thumb.
Even so, magnitude of the cold will easily support lows in the
teens, with the expectation for expanding high clouds taking the
edge off by Thanksgiving morning. Attm, support guidance that
reflects a west-east temperature gradient, reflecting a good
handling of these radiative forcings. Current forecast lows ranging
from 10 in Bay County to the upper teens in the urban heat island is
a good start. Cold airmass firmly in place for Thanksgiving day with
highs firmly in the 20s in most areas to go along with light wind.
Temps hold steady or warm Thursday night as warm advection begins to
emerge in advance a storm that will track to the northwest this
weekend.

Upper level ridging over the region will continue on Friday, keeping
the CWA dry.  Changes quickly come back Friday night as troughing
and embedded system move across the Northern Plains.  This system
will bring the next chances for rain and some snow on Saturday
before moving off to the northeast by Sunday.  There may be some
snow showers early Saturday morning at the onset as temps look to be
around freezing before temps warm slightly and precip switches over
to rain.  SE Michigan will then get a short dry period to end the
weekend, before more active weather comes back into play with a
system that is currently forecasted to move up from the southwest.

MARINE...

The Gale Watch is upgraded to a warning for tonight and Wednesday
across the open waters of Lake Huron. Small Craft Advisories are
extended through Wednesday night. A cold front exiting the central
Great Lakes this morning is leaving sub-gale northerly wind and high
waves ongoing across Lake Huron. This will be followed by a quick
progression of high pressure and backing of the wind to southwest
during the afternoon. Southwest wind then increases to near 30 knots
this evening as the next low pressure system reaches eastern Lake
Superior. This system sweeps a stronger cold front across the region
tonight which brings a 40 kt northwest gale to the open waters with
waves exceeding 15 feet through Wednesday morning. Marine conditions
remain unsettled Wednesday night as high pressure builds into the
central Great Lakes. The high moves quickly across Lake Huron during
Thanksgiving leaving increasing south wind to finish the week.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning until 4 PM EST this afternoon for LHZ362-363-462-463.

     Small Craft Advisory until 4 AM EST Thursday for LHZ421-441>443.

     Gale Warning until 10 AM EST this morning for LHZ361.

     Small Craft Advisory until 10 AM EST this morning for LHZ422.

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

$$

AVIATION.....MR
DISCUSSION...JVC/SP
MARINE.......CB

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at www.weather.gov/detroit.



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