Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 190357
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1157 PM EDT Mon Sep 18 2017

.AVIATION...

Late evening observations showed mostly low clouds within the
stalled frontal boundary near the Ohio border. Data also showed only
thin cirrus over the remainder of SE Michigan. This sets the stage
for fog development before the front begins to accelerate northward
during the late night through morning. The combination of fog and
advancing low clouds will produce MVFR trending to IFR, and possibly
LIFR north of the cloud edge, as the front progresses across the
terminal corridor. There remains a chance of showers later in the
night as well but coverage will be minimal compared to the
restriction already in place due to clouds and fog. The slow
westward and northward drift of the front is expected to keep the
area under at least MVFR ceiling/fog restriction with scattered
showers through most of the day.

For DTW... Low clouds will continue to solidify along the stalled
front and expand back northward over lower Michigan during the
night. This will ensure coverage of stratus and fog with borderline
MVFR/IFR restriction through the morning with only slow improvement
during the afternoon.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less during the night through late
  afternoon.

&&

.PREV DISCUSSION...
Issued at 957 PM EDT Mon Sep 18 2017

UPDATE...

Cloud cover trends and associated fog potential are being monitored
during the evening for adjustments overnight through Tuesday morning.
Evening satellite imagery indicates low clouds have dissipated along
the western fringe of the stalled frontal zone. This has opened up
some sky over the sharp surface dewpoint gradient roughly along a
line from Port Huron to Howell where some radiational cooling could
result in thicker fog development. This will be complicated by the
stalled front beginning a northward return during the night and
spreading clouds back overhead. Until then, some locally dense fog
will be possible. Temperatures will also continue to cool farther
north through the Tri Cities and Thumb where dewpoint is lower
compared to points south. However, radiational cooling will have more
time to work and boundary layer moisture will be supplemented by
Saginaw Bay and Lake Huron through early Tuesday morning.

The reason the front will make a move back into lower Michigan
during the night is due to the short wave approaching from the mid
Mississippi valley. This system is sustaining a substantial pattern
of showers and thunderstorms over Illinois that indicates the
strength of the wave and the propensity for the frontal zone to
release convection. The ongoing activity is not expected to reach SE
Michigan until late morning or early Tuesday afternoon given the slow
movement of the wave. However, scattered shower development is
possible along the front out ahead of the wave during late night
through early morning.

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


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