Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 251006
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
606 AM EDT Mon Oct 25 2021


.AVIATION...

Variable ceilings when comparing obs between TAF sites this morning.
IFR generally persist at the southern sites into the evening hours
with reinforcing low-level moisture surge. Meanwhile, the duration
for IFR cigs remains shorter at PTK/FNT, and MBS shouldn`t drop
below MVFR. Precipitation is ramping up again with the next wave of
moderate rainfall overspreading from SW to NE. Expect intensity and
coverage to diminish rapidly after 19Z as drier air infiltrates the
mid-levels. Modest pressure gradient remains in place with sustained
winds in the 10-15 knot range (higher for MBS) while strong near-
surface flow ensures energetic northeasterly gusts in the 20-25 knot
range (up to 30 knots for MBS). Rain should taper-off before the end
of the TAF period Tuesday for the Metro terminals as the low exits
east. Some lake-effect showers remain possible over FNT/PTK with the
back edge northerly flow Tuesday morning with lingering MVFR
ceilings.

For DTW...Predominantly IFR ceilings today and tonight. Some
potential to briefly register LIFR, but kept a mention out given the
low confidence in frequency and timing. NE gusts should hold below
30 knots today. MVFR likely to last until midday Tuesday given the
slow progress of building high pressure.

.DTW THRESHOLD PROBABILITIES...

* High for ceiling at or below 5000 feet through 18Z Tuesday.

&&

.PREV DISCUSSION...
Issued at 407 AM EDT Mon Oct 25 2021

DISCUSSION...

A 1001mb surface low is located over west central Illinois as of
06Z. Moderate rain lifted into Southeastern Michigan during the late
evening hours organized primarily by an upright elevated frontal
surface on the 322-328Ke surfaces. Deep, convective origin, thetae
content was then driven up this frontal slope and has since lifted
to north of the I 96 corridor. So a lull in rainfall rates have
developed with signs of dry slotting up right into Lenawee and
Monroe counties. Based on observations, rainfall amounts checked in
well below prior forecasts with around 1.0 inch fallen across
Washtenaw, Wayne, and Lenawee counties. Buyer beware on radar
estimates as the Dual Pol estimates in at closer range have
overestimated amounts by at least 30% and legacy storm totals are
under representing reality by 50%. The most reasonable product to use
is gauge corrected MRMS. So far, social media has been non existent
with any reports so everything appears to be in good shape. Will
likely see slippery traffic conditions with the wet roadways. Will
continue to monitor and address anything with a short fused Flood
Advisory(s) if needed.

For today, models have the warm conveyor lifting into the southern
forecast area including Metro Detroit after 11Z. There has been
signs then that a secondary low pressure center becomes nestled
along the northwest flank of bifurcated flow as the low tracks
across northern Indiana later this morning and emerges near the
western basin of Lake Erie by 18-20Z. Therefore, the potential
exists for a fairly significant low level convergence maximum that
will become positioned directly over Metro Detroit. What has changed
in the models is that the most favorable time period for heaviest
rainfall amounts may now come in the 12-18Z timeframe today. So
plenty of time and atmospheric forcing to come that can get rainfall
amounts to exceed 2.0 inches across portions of Metro Detroit. For
areas across the north, arrival of strong midlevel deformation from
upstream and the beginning of the occlusion process will bring
widespread rainfall to areas north of M 59 corridor from between 15-
03Z.

Did go ahead and issued a Wind Advisory for Bay, Tuscola, Huron,
Sanilac and St Clair Counties 10 AM to Midnight today. Impressive
well mixed thermodynamic environment from the surface to 5.0 kft agl
will develop over Lake Huron in the presence of strong geostrophic
northeasterlies. The potential exists for sustained winds of 30 mph
with wind gusts up to 45 mph at times this afternoon and evening.
The big negative going for today will be a lack of true cold
advection until after Midnight. Thus for messaging purposes, it is
important to stress the strongest winds occuring near the Lake Huron
and Saginaw Bay shorelines. Do think the winds will be much weaker
inland. The better cold advection and more impressive 925-850mb
lapse rates don`t arrive until the winds backs to the north after
06Z for the far eastern Thumb so some uptick but below advisory
levels is anticipated Tuesday morning. Given the lapse rates did
introduce PoPs through Tuesday morning for lake effect rain shower
potential.

Surface ridge axis will then squeeze through all of the forecast
area Tuesday and Tuesday night with deep ridging aloft. Quiet
weather is then expected Tuesday through early Thursday. Interesting
thermal advection pattern is anticipated as the deep ridge axis does
a hard pivot over us increasing residence time of the ridging. The
upper level low is being advertised as less progressive and really
undercutting the Great Lakes region to the south. The net result is
rapid midlevel temperature moderation over the state. Still find it
difficult to believe that temperatures won`t fall into the 30s
Wednesday morning but for now guidance is keeping the coolest
temperatures across western Lower Michigan. Highs Wednesday and
Thursday are expected to be above normal with readings in the middle
50s to lower 60s.

Will state it outright that very low confidence exists in how the
upper level low pressure system will play out during the end of the
weak. A lot of uncertainty exists in how the moisture return will
unfold as an atypical scenario of dry southeast flow is suggested. A
lot of boom or bust potential with regards to all rain or all dry
periods for Friday and Saturday.

MARINE...

Low pressure will continue tracking northeastward across the
northern Ohio Valley today and through tonight bringing an
extended period of wet and unsettled weather. Northeasterly
winds will continue to rapidly strengthen in response to the
tightening pressure gradient. Gale Warnings remain in effect for
most of the Lake Huron open waters and all of the Lake Huron
nearshore waters. Frequent gusts to 40 knots will be possible
especially during the daytime hours and into early tonight. Wave
heights exceeding 10 feet in the Lake Huron nearshore waters along
the Thumb will also be common. Small Craft Advisories are in effect
for Lake St. Clair and western Lake Erie where gusts will be closer
to 30 knots. Northeast flow decreases but remains moderate then
heading into the middle of the week as the low departs off to the
east.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory from 10 AM this morning to 1 AM EDT Tuesday for
     MIZ048-049-054-055-063.

     Lakeshore Flood Advisory until 8 AM EDT Tuesday for MIZ048-049-054-
     055-063.

Lake Huron...Gale Warning until 4 AM EDT Tuesday for LHZ362-363-421-422-441>443-
     462>464.

Lake St Clair...Small Craft Advisory until 10 PM EDT Tuesday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT Tuesday for LEZ444.

&&

$$

AVIATION.....KK
DISCUSSION...CB
MARINE.......IRL


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