Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 182308
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
708 PM EDT Mon Sep 18 2017

.AVIATION...

A frontal boundary stalled just to the east and south of SE Michigan
will lead to a wide range of aviation conditions tonight before the
boundary begins to drift back over lower Michigan overnight through
Tuesday. VFR clear in the MBS area will hold during the evening
while the western fringe of the cloud band continues to brush the
FNT region. The main challenge here will be timing fog formation
under ideal radiational cooling conditions before clouds arrive late
tonight and Tuesday morning. PTK southward through the DTW corridor
will be affected considerably more with borderline MVFR/IFR ceiling
while fog also remains possible considering the cool temperatures
and high dewpoint gradient across the region. This is also where the
best chance will be for some redevelopment of showers late tonight.
The slow westward and northward drift of the front is expected to
keep the terminal corridor under at least MVFR ceiling/fog
restriction with scattered showers through most of Tuesday.

For DTW... The front stalled nearby to the east and south will begin
to move back northward during the night. This will ensure continued
coverage of stratus and borderline MVFR/IFR restriction tonight and
much of Tuesday along with a chance of showers. There also remains
some potential for fog as surface moisture remains high near the
boundary. The forecast will maintain a prob30 for showers late
tonight into early Tuesday morning.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less tonight through Tuesday.

&&

.PREV DISCUSSION...
Issued at 317 PM EDT Mon Sep 18 2017

DISCUSSION...

Its been a slow process to scour out the low level moisture/clouds
today, as the drier air in the low level just slowly bleeds into
southeast Michigan. Fortunately, with the elevated temperatures to
start the day, maxes have still been able to rise into the 70s for
the most part. Surface high pressure to move over northern Lower
Michigan tonight, and have elected to add patchy fog (mainly northern
areas) to the forecast as surface dew pts still running on the high
side, and light northeast flow off lake huron does not give a whole
lot of confidence in continued drying out/lowering of the dew pts.
Still, wild card will be if there is sufficient moisture in the
2000-4000 foot layer to generate clouds, which would mitigate fog
threat, in addition to potential high clouds as upper level wave
tracks into Western Ohio Valley overnight. This upper wave trending
to slow down and potentially cutoff over the central sections of Ohio
Valley tomorrow (see GFS), but more than likely the 700-500 MB
circulation/low slips just north of Michigan border (see 12z Euro).
Thus, surface ridging/low level anti-cyclonic flow looks to be
weakening over Lower Michigan, with inverted surface trough and
deeper moisture backing in along the southeast Michigan/Canadian
border. This trough should provide the focus for showers, especially
across far eastern and southern areas, along with the moisture
advection/isentropic ascent. In fact, a few showers or light rain
could develop/arrive late tonight across the south if deeper moisture
does not push much farther east late this afternoon/evening.
Instability tomorrow looks marginal at best, and was tempted to
remove thunder mention all together as mid level lapse rates are
weak, but Euro still indicates showalter index at or slightly below
zero. Clouds, scattered showers, and light easterly flow expected to
hold maxes in the 70s once again.

Upper level ridge axis building over the Central Great Lakes on
Wednesday should suppress/cap any possible diurnal instability
buildup, but not totally out of the question a shower or two goes
up, mainly over the Thumb Region on the outer fringe of the ridge
and in the vicinity of the warm front.  925 MB in the 20-22 C range
suggesting highs climbing back into the lower 80s.

No major changes to the extended forecast... A broad surface high
pressure system situated over the eastern U.S. and an expansive
upper-level ridge aloft will support hot and dry conditions across
Michigan Thursday through the weekend. Went more aggressive compared
to the in-house blend with daytime highs as 850 mb temperatures
averaging 16 - 18 C aloft and generally clear skies will help
support highs in the mid to upper 80s. The next possible chance to
see more seasonal temperatures will be next week during the mid-week
period as Hurricane Maria moves off into the northern Atlantic,
allowing upper-level flow to turn more zonal across the Great Lakes.

MARINE...

Surface winds will veer toward the northeast tonight into Tuesday as
the center of surface high pressure slides into Quebec. A slight
uptick in the gradient will sustain northeast winds generally in the
10 to 20 knot range on Tuesday. Persistent high pressure is forecast
to take hold across the eastern Great Lakes during the latter half
of the weak, supporting generally light and variable winds across
the region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....BT
DISCUSSION...SF/AM
MARINE.......CB


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