Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 051731
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1231 PM EST Tue Dec 5 2017


.AVIATION...

Strong southwesterly winds continue to be the predominate threat to
aviation as a deepening low pressure system and cold advection
affect the region. Gusts should peak this afternoon as daytime
mixing maximizes, potentially bringing gusts of 35 to 40 knots with
highest gusts expected near MBS closer to the low. Boundary layer
will remain mixed through the overnight keeping winds elevated with
only a slight drop off in gusts from this evening. Gusts will become
renewed on Wednesday as mixing depths yet again increase bringing
down higher wind speeds. Increased mixed layer depth will keep
ceilings primarily VFR although a couple MVFR observations may
linger after 18Z this afternoon as mixing depths are in the process
of increasing. Dry airmass will likely bring a period of SCT sky
cover tonight with BKN returning Wednesday.

For DTW...Strong southwesterly winds will be the main concern
through the taf cycle. Expect gusts to remain at or above 20 knots
through the overnight as well mixed layer remains in place. CIGS
will be VFR as they slowly rise to near 5kft this afternoon. Though
presently in a pocket of SCT sky cover, BKN skies should return
shortly as diurnal mixing peaks.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less this afternoon. Low tonight.

&&

.PREV DISCUSSION...
Issued at 913 AM EST Tue Dec 5 2017

UPDATE...

Just a few cosmetic changes to the forecast which remains in good
shape this morning. Sporadic peak wind gusts near 40 kts were noted
over the last hour from the Saginaw Valley westward toward Chicago
and Milwaukee. Such gusts will become more widespread with peak
heating. Monroe and Lenawee remain on the periphery of the stronger
gradient flow so will continue to monitor potential for now. Wind
chills will fall through the 20s and into the teens by this evening.
Quiet weather otherwise.

PREV DISCUSSION...
Issued at 343 AM EST Tue Dec 5 2017

DISCUSSION...

STRONG WINDS EXPECTED ACROSS MUCH OF SOUTHEAST MICHIGAN TODAY...

NEAR TERM / TODAY THROUGH TONIGHT

As of 340 AM EST...Powerful 982 hPa low pressure system currently
located near western Lake Superior will continue to move
northeastward into central Ontario, further rapidly deepening
throughout the day. Phasing of the northern and southern streams will
allow the low to deepen to sub 970 hPa, with the latest RAP pushing
965 hPa, a quite impressive transition season storm.

Across southeast Michigan, scattered showers continue ahead of an
approaching strong cold front, which will usher in winter-like
temperatures across the region today. Earlier model guidance
indicated a more robust line of showers with even a few embedded
storms, but this scenario has not materialized as a surge in the
cold front has displaced the bulk of the frontogenetical forcing
away from the immediate region. As a consequence, rainfall amounts
for most areas will actually only end up being a tenth of an inch or
less. The cold front is still progged to exit eastern Michigan
roughly 12z, with much drier air filtering in behind the front as
the system`s dry slot moves overhead. Ahead of the front,
temperatures will spike into the mid 50s within the next few hours,
before beginning to plummet after daybreak with falling temperatures
throughout the day. By this evening, there will have been a 20-25
degree temperature swing in the span of roughly 12 hours as
temperatures plummet into the lower 30s. Behind the front, generally
dry weather is expected through at least early this evening.

The big weather story for today will be the strong winds before and
in the wake of the frontal passage. Ahead of the frontal passage,
south/southwesterly winds will commonly gust 30-40 mph, with gusts
exceeding 40 mph possible within showers that are able to tap higher
momentum air aloft. There will be 2 waves of strongest winds today.
The first will occur with the frontal passage itself early this
morning, where a surge in the frontal circulation noted by a kinked
isobaric map and frontal analysis will enhance the frontogenetical
density currents for a quick pop of near 40 mph gusts as winds shift
to the southwest.

A brief reduction in gusts will then be possible shortly after
daybreak due to the initial low capping inversion in the wake of the
front, but gusts of 25-30 mph will still be common due to the tight
gradient flow in place. By around noontime, the second surge in
gusts will begin and persist into the evening hours as strong CAA
and a drying boundary layer profile steepen low-level lapse rates to
nearly superadiabatic and rapidly destabilize the static stability
profile. Boundary layer momentum profile is quite impressive this
afternoon, with enhanced momentum transfer in a mixing depth profile
exceeding 5 kft in many places. This has already been verified by
upstream obs in Iowa/Wisconsin/Illinois frequently gusting 40+
knots. Coupled with the fact that the surface low will end up being
almost 10 hPa deeper than forecasts were indicating just 24 hours
ago, and an enhanced pressure rise field aided by left-exit region
ageostrophic circulation from a potent 160+ southern stream jet
streak moving by overhead, gusts exceeding 45 mph will be likely
today across much of southeast Michigan.

Based on these factors, have issued a Wind Advisory for all of
southeast Michigan excluding Lenawee and Monroe counties, although
these counties may need to be added later this morning after
watching trends in obs. While most gusts will be in the 40-45 mph
range, enough frequent gusts exceeding 45 mph warrant the advisory
issuance, and cannot rule out gusts as high as 55 mph at times in
the Saginaw Valley. Travel will be difficult throughout the day,
especially along north/south oriented highways where a significant
crosswind component will be in play. Unsecured loose outdoor objects
will easily blow around as well, along with possible downed tree
limbs and power lines resulting in sporadic power outages.

Gusts diminish below advisory levels shortly after sunset, but
continue to remain elevated around 25-35 mph. As the surface low
lifts further off to the northeast, the cyclonic flow in its wake
and much colder air moving over the mild lakes will provide the
spark for the onset of a lake effect snow regime. Abundant dry air
in the boundary layer should preclude most activity from affecting
southeast Michigan tonight, but enough saturation in the -10 to -15
C layer across central portions of the forecast area coupled with a
passing secondary shortwave in the cyclonic flow could lead to a
period of potential snow squalls this evening. Confidence is low on
this scenario panning out, but it is a nonzero probability that
should be monitored as the day progresses. If any squalls occur,
gusts easily exceeding 30 mph will be possible along with a quick
coating to a few tenths of snow. Low temperatures will bottom out in
the mid 20s with continued CAA.

SHORT/LONG TERM / WEDNESDAY THROUGH MONDAY

The weather pattern for the next several days will be dominated by a
lake effect snow regime, but southeast Michigan will struggle to see
much in the way of any significant activity, with more of a
southwesterly component in the boundary layer hindering activity
from a more favorable westerly flow regime. Still, periods of
flurries to scattered snow showers will be possible, with generally
little to no snow accumulation. Model guidance continues to hint at
a more substantial band affecting the southern two-thirds of the
area Thursday, but the bulk of accumulations will remain further
west.

Forecast gets a little more muddled heading into the weekend as
passing shortwaves reinforce longwave troughing over the eastern
half of the CONUS, with the potential for a few clipper systems to
affect the region with some light snow. Cold temperatures will
persist through the weekend, with highs in the 20s and 30s. Lows
will generally be in the 20s each night.

MARINE...

Gale warnings remain in effect for all Lake Huron marine areas and
upgraded to storm warning for outer Saginaw Bay and central Lake
Huron. Lake St Clair is upgraded to a gale warning and western Lake
Erie remains at small craft advisory for now. The westerly gales and
gusts to storm force on Lake Huron will be due to a large and strong
low pressure system moving from Lake Superior into northern Ontario
while deepening to 970 mb. Multiple observations during the night
indicated gusts in the upper 40 kt range over southern WI, IA,
northern IL while the system was around 982 mb, and considering the
deepening trend during the rest of the day, an upgrade to storm
warning is warranted for outer Saginaw Bay and central Lake Huron.
Peak gusts in that area will be very close to 50 knots on southwest
direction funneling through Saginaw Bay and into the open waters of
the lake. Wind conditions will drop off from the peak later tonight
but 35-40 knot gales will then continue through Wednesday. Colder
air moving into the region will also promote increasing coverage of
lake effect snow squalls to further hinder marine conditions through
the end of the week.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory until 7 PM EST this evening for MIZ047>049-053>055-
     060>063-068>070-075-076.

Lake Huron...Storm Warning until 7 PM EST this evening for LHZ363-421-441-462.

     Gale Warning from 7 PM this evening to 4 PM EST Wednesday for
     LHZ363-421-441-462.

     Gale Warning until 4 PM EST Wednesday for LHZ361-362-422-442-443-
     463-464.

Lake St Clair...Gale Warning until 10 PM EST this evening for LCZ460.

Michigan waters of Lake Erie...Low Water Advisory until 4 AM EST Wednesday for LEZ444.

     Small Craft Advisory until 4 PM EST Wednesday for LEZ444.

&&

$$

AVIATION.....DRK
UPDATE.......JVC
DISCUSSION...IRL
MARINE.......BT


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