Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 130353
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1153 PM EDT Tue Sep 12 2017

.AVIATION...

VFR in the form of cirrus bands will gradually thicken and lower
during the night as the remnants of Irma attempt to spread northward
into low level dry air already over lower Michigan. Coverage of
showers over Ohio diminished considerably in this environment and
will remain too sparse and light to include in the terminal
forecasts. Low level flow veering toward the south will carry in
moisture during the afternoon but rain activity may still be south
of the Ohio border by then. Borderline VFR/MVFR ceiling is possible
over the DTW corridor during the afternoon as low level moisture
increases and before boundary layer mixing lifts the cloud base. The
northern fringes of more concentrated showers could brush the Ohio
border up to about DTW during Wednesday evening.

For DTW... Light easterly wind will veer toward the south through
the morning. Otherwise, high clouds will thicken and lower and any
radar returns indicated will be mainly virga before low level
moisture increases during afternoon. Light showers will be possible
later in the day into Wednesday evening but coverage is expected to
be too low to mention for now.

//DTW THRESHOLD PROBABILITIES...

* Medium for ceiling 5000 feet or less from about sunrise through
  late afternoon.

&&

.PREV DISCUSSION...
Issued at 300 PM EDT Tue Sep 12 2017

DISCUSSION...

Very dry airmass in place, as 12Z DTX checked in with PW value of
0.40 inches, with a 700 MB dew pt depression of 32 C and 850 dew pt
depression of 25 C. Very large leftover circulation of Irma will
transport a narrowing moisture axis (5 C dew pt at 700 MB) into
southeast Michigan Wednesday morning, with could be sufficient to
generate a few light showers/sprinkles, with waves of isentropic
ascent propagating north. Showalter index is seen dipping to around
zero, but probably just enough warm air around 600 MB to prevent any
thunderstorm activity. Surface dew pts progged to slip above 60
degrees tomorrow, and holding near 60 on Thursday. Circulation to
track through the Ohio Valley Wednesday night, exiting East on
Thursday with pronounced upper level ridging becoming re-established
over Lower Michigan by Thursday night, setting us up for continued
warm days and cool nights into the Weekend. With the higher dew pts
and clearing Thursday night, may need to add fog to the forecast as
we get get closer.

A drying trend will develop across the region from Friday into the
weekend as ridging builds in the upper levels and resulting surface
high pressure takes over. Light southerly winds will help bring
daily high temperatures to the upper 70s to lower 80s through the
weekend. Lows will hover in the upper 50s to lower 60s. Next chance
for showers will be late Sunday into early Monday as a low pressure
system passes to the north of the Great Lakes and pulls a cold front
through the area. High pressure will return following the front and
temperatures for the start of the week will be cooler with highs in
the lower to mid 70s.

MARINE...

A ridge of surface high pressure, interrupted only briefly by the
weak remnant low that was once Irma, will remain in place over the
central Great Lakes throughout much of the week. The surface wind
direction will veer to the south and remain generally at 10 knot or
less through the end of the work week. 12z DTX sounding shows a very
dry airmass encompassing the area under this ridge of high pressure.
So, even as some of the moisture from Irma lifts into the area in
the Wednesday to Thursday time frame, only scattered light showers
are anticipated.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
DISCUSSION...SF/TF
MARINE.......DG


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