075
FXUS63 KDTX 201703
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
103 PM EDT Fri Sep 20 2024

.KEY MESSAGES...

- Warm weather continues through this weekend with a slight increase
in humidity each day.

- Scattered showers and storms are possible this afternoon and early
tonight with additional rainfall Sunday night into Monday.

- Lower confidence in rain chances for the middle of next week while
temperatures cool off to seasonable values.

&&

.AVIATION...

Weak south-southeast winds with mid to high clouds around this
afternoon ahead of a weak cold front. Lower VFR clouds will arrive
early this evening, as the weak convergence long the front slowly
crosses southeast Michigan. Enough instability around to support
scattered coverage of showers late this afternoon into the evening,
with isolated thunderstorms not out of the question, but too limited
in potential to highlight attm. Border line MVFR/VFR cigs will be
possible where showers develop. Clouds decrease late tonight behind
the front, which could lead to a period of low stratus or fog by
sunrise Saturday as winds will be nearly calm. Held the visibility
in the tafs in MVFR for now, as confidence is low in dense fog, as
there may be just enough wind around.

For DTW/D21 Convection...Scattered showers around this evening with
a slight chance of thunderstorm as a weak frontal boundary tracks
through. Will continue with the prob30 mention of showers.
Confidence in thunderstorms is too low to include at this time.

 DTW THRESHOLD PROBABILITIES...

* Low for ceilings at or below 5000 ft this evening and tonight.

* Low for thunderstorm chances this evening.

&&

.PREV DISCUSSION...
Issued at 357 AM EDT Fri Sep 20 2024

DISCUSSION...

After 13 consecutive days of dry weather tied to longwave ridging
that encompassed the northeast quadrant of CONUS, the first viable
chance for rainfall commences this afternoon for portions of
Southeast Michigan. A protruding occlusion resulting from a mature
low pressure system located over northern Manitoba supports the
development of a secondary low over Illinois today. Ongoing
convection is expected to drift eastward through the morning hours
with the translation of the trough axis (and weak cold front). Most
deterministic solutions and ensemble members support areas of
measurable rain this afternoon and into the evening hours given some
degree of integrity and staying power of the underlying dynamics.
However, the column is prohibitively subsaturated per the 20.00Z
KDTX RAOB, particularly in the mid-levels. Latest thinking supports
high-based initial showers after 18Z with perhaps some rumbles of
thunder near the Tri-Cities, but SBCAPE is non-existent in a subset
of available NWP data. 20.03Z HRRR does suggest an earlier timing
and fits well with the 20.00Z ARW/NAM3km/FV3 solutions depicting a
stark decay in showers as they approach the Metro area after 22Z
given less favorable ThetaE profiles and declining instability. Made
some adjustments to PoPs to reflect higher confidence in rain for
the northwest part of the CWA and lesser confidence for the
southeast localities.

Southeast winds veer westerly Saturday morning as the composite
frontal boundary heads into southern Ontario. This opens up a brief
period of stability as short wavelength ridging builds across Lower
Michigan. Residual morning clouds burn off by late morning yielding
a similarly warm day in the absence of post-frontal cooling. A more
active upstream pattern materializes Sunday as a shortwave speeds
across The Cascades phases with a southern stream jet streak over
The Dakotas. This effectively kicks the anticyclone into motion and
shears out the feature while the cyclonic wave begins to close off
over northern Ontario.

Corridor of height falls nose into the area with persistent train of
CVA from the high plains into the Great Lakes. Ridge of ThetaE
traces from The Gulf up to the Ontario low with higher potential for
showers and some storms locally Sunday PM and overnight. Timing
uncertainty lends only Chance PoPs Sunday evening before the
northern system`s cold front approaches offering a more substantial
focus for showers and storms. Notable divergence in solutions early
next week with defined post-frontal dryness or a stalled frontal
boundary that generates rain until mid-week. One thing is clear,
temperatures will break from late June readings into more Fall-like
ranges with highs in the 70s.

MARINE...

The long stretch of dry weather and light winds will continue
through the morning hours as high pressure remains in control. The
dry weather will come to an end for portions of the Great Lakes
starting this afternoon and evening as a cold front and upper-level
disturbance aids in the production of scattered to numerous rain
showers with embedded thunderstorms. Thunderstorm activity will be
most likely over northern Lake Huron this afternoon and evening.

Slightly elevated gusts nearing 20 knots will be likely during the
frontal passage across northern Lake Huron, but otherwise, a weak
pressure gradient will maintain lighter winds through the weekend.
Additional rain and thunderstorm chances return Sunday evening
through Monday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...None.
Lake St Clair...None.
Michigan waters of Lake Erie...None.
&&

$$

AVIATION.....SF
DISCUSSION...KGK
MARINE.......AM


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.