Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 242350
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
650 PM EST Fri Nov 24 2017

.AVIATION...

VFR will remain in place under scattered to broken high clouds and
gusty wind will diminish during the evening. Surface wind decoupling
will set up LLWS prior to the frontal passage mainly in the DTW area
before the low level jet exits eastward. Surface gradient wind in
the 10-15 knot range will produce about 35 knots of shear at base of
the old boundary layer inversion/top of the shear layer. The
approach of the cold front will then provide a small window for
light rain but mostly virga on radar as dry air over lower Michigan
erodes the frontal moisture observed over Wisconsin during early
evening. The lakes could help MVFR ceiling expand along and behind
the front as it passes through SE Michigan first at MBS during the
overnight hours and then exiting the region around mid Saturday
morning. MVFR ceiling is expected to hold on through the morning and
then lift into a broken VFR ceiling with northwest wind gusting near
20 knots as cooler air moves into the region during the afternoon.

For DTW... VFR under scattered to broken high clouds will give way
to ceiling below 5000 ft overnight as a cold front approaches the
region. MVFR ceiling will follow the frontal passage Saturday
morning. Southwest wind around 10 knots will shift northwest and
increase to gusts near 20 knots by Saturday afternoon.

//DTW THRESHOLD PROBABILITIES...

* Low for ceiling 5000 ft or less prior to 09z, medium after 09z
  and then high Saturday morning.

&&

.PREV DISCUSSION...
Issued at 335 PM EST Fri Nov 24 2017

DISCUSSION...

NEAR TERM / TODAY THROUGH TONIGHT

As of 335 PM EST...Rather mild, and breezy, post-Thanksgiving day
continues across the region with southeast Michigan sitting well
within the warm sector of a deep (986 hPa) low pressure system
tracking across west-central Ontario. Gusty southwest flow
occasionally 20-30 mph at times, coupled with strong warm air
advection in the 925-850 hPa layer (temps approaching 10C) has
allowed temperatures to spike into the lower and mid 50s for most
areas; 5-10 degrees above average for late November and a respite
from the recent cooler weather. The low pressure system across
western Ontario will continue to track off well to the north heading
into this evening, with continued dry weather outside of a few light
sprinkles over Lake Huron. Patchy mid and high clouds will continue
across areas mainly north of I-69 into the evening, with mostly sunny
skies for locations to the south.

Heading into tonight, the low pressure system`s cold front will move
southeast across the region accompanied by a shortwave impulse,
noted by a brief dip in the northern stream PV reservoir. A
continued dry atmospheric column, coupled with the frontal passage
being displaced far from the parent low, will result in a mainly dry
frontal passage overnight (although with a marked increase in cloud
coverage), with latest projections progging the front to be pushing
well into Lake Erie by daybreak. Cannot totally rule out some brief
isolated showers with the front as there will be some modest
isentropic forcing immediately along the front and favorable right-
entrance region dynamics to wring out the little moisture available.
Best chance for any precip (in the form of rain with surface and low-
level temps well above freezing) looks to be in the 03z-09z
timeframe accompanying the frontal passage. Low temperatures will
generally be in the lower 40s across the region tonight.

SHORT TERM / SATURDAY THROUGH MONDAY NIGHT

The flow will shift to the west/northwest behind the frontal passage
heading into Saturday, and continue to remain breezy at times as a
tight pressure gradient lingers over the region in between the
departing low pressure system to the northeast and building high
pressure across the Plains. As the aforementioned shortwave impulse
swings through, it will bring a secondary cold frontal passage by
the afternoon hours, reinforcing a shot of cold air advection as 925
hPa temps crater back to the -2 to -4C range and 850 hPa temps less
than -5C. Lingering cyclonic flow aloft will lead to lake-enhanced
cloud cover for much of the day with mostly cloudy stratus. Enough
trapped low-level moisture will lead to brief periods of isolated
sprinkles/flurries throughout the course of the day, with snow
chances increasing during the afternoon as temperatures fall into
the 30s. High temperatures will rise little from their lows tonight,
peaking in the low to mid 40s by the late morning hours before
gradually falling during the afternoon.

High pressure over the Plains will slide southeastward during the
day Sunday, with surface ridging and increasing heights aloft. Flow
will become southwesterly, but not as gusty as the Friday or
Saturday with a slightly weaker pressure gradient. Dry weather will
prevail and continue into Monday with ridge amplification
continuing. With limited low-level temperature advection, Sunday and
Monday will see highs around normal for this time of year, ranging
from the low to mid 40s.

LONG TERM / TUESDAY THROUGH THURSDAY

Partly cloudy skies and warm southwest flow ahead of the low
pressure system moving through northern Ontario will help warm
Tuesday into the upper 50s with 60+ highs possible for the southern
counties, some 10-15 degrees above average for late November. This
system will pull a cold front through SE Michigan late Tuesday, but
with most of the forcing located to the north, only a slight chance
of showers expected for our area. High pressure to build back in on
Wednesday, with post-frontal high temps more seasonal in the 40s.

Shortwave energy from separated southern stream wave is progged to
arrive on Wednesday evening, bringing a slight chance for some rain
and possibly snow mixed in for some areas overnight. Better chance
for widespread precip comes on Thursday as mid/upper trough arrives
and low pressure develops over the Great Lakes. Rain showers are
looking most likely Thursday, with a transition to snow showers
possible for the early morning on Friday. Cooler air will fill in
behind this system for the first part of next weekend.

MARINE...

Gusty southwest winds will persist through early tonight in advance
of a cold front.  Gusts to gales will exist during this time from
Saginaw Bay into the central and southern lake Huron open waters.
Winds turn northwesterly and remain brisk in the wake of this front
late tonight and Saturday.  This will bring the potential for
another small window of gusts to near gales over the north half of
lake Huron Saturday afternoon and evening.  Winds then briefly ease
Saturday night and early Sunday under passing high pressure. Renewed
strengthening of southwest winds in the wake of this high expected
by late Sunday, leading to another period of gusts to at least 25-30
knots.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.

Lake Huron...Gale Warning until midnight EST tonight for LHZ362-363-421-422-
     462>464.

     Small Craft Advisory until 4 AM EST Saturday for LHZ441>443.

Lake St Clair...Small Craft Advisory until 4 AM EST Saturday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 AM EST Saturday for LEZ444.

&&

$$

AVIATION.....BT
NEAR/SHORT TERM...IRL
LONG TERM...TF
MARINE.......MR


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