Expert Forecast Opinion

529
FXUS63 KDTX 171958 CCA
AFDDTX

Area Forecast Discussion...CORRECTED
National Weather Service Detroit/Pontiac MI
358 PM EDT Sun Aug 17 2025

.KEY MESSAGES...

- Breezy NE wind brings cooler and less humid air into SE Michigan
that is more noticeable by tonight.

- Dry weather Monday and Monday evening, then scattered showers and
storms become possible early Tuesday through Tuesday night. Locally
heavy rainfall is the primary concern with this activity.

- Seasonable temperatures and dry weather move in and hold Wednesday
through Friday.

&&

.DISCUSSION...

Dry northeast wind from Canada brings a taste of late summer to the
area today into Monday as high pressure builds in just north of the
Great Lakes. Afternoon visible satellite showcases a typical
northerly cold advection pattern complete with cellular
stratocumulus field. Steadily decreasing humidity and clearing skies
carry through the evening with gusty winds along the Lake Huron
shoreline gradually easing a bit after dark. The drier air that
settles in through tonight is characterized by sfc dew points in the
50s and PWAT below 1.00". A cool and almost crisp morning is
followed by a modest increase in humidity tomorrow as low-level flow
turns more southeast and advects the Lake Erie moisture plume into
the area. Plenty of stability offered by mid-level ridging will
ensure a dry and comfortable day, but can`t rule out a stray lake
effect shower in Monroe County along the moisture gradient. Highs
range from the mid to upper 70s in the Thumb to lower 80s farther
inland.

The ridge begins to flatten Monday night as a series of shortwaves
pass through the Upper Midwest into the northern Great Lakes. A sfc-
850mb low emerges over the northern Plains tomorrow and tracks into
the central Great Lakes on Tuesday. This system engages the theta-e
gradient stalled over the western Great Lakes and sends it east into
the area Tuesday morning. Model guidance favors one or more MCSs to
develop along the gradient upstream over WI/IL Monday evening with
low probability for some of this convection, likely decaying as it
outpaces available instability, to reach the Saginaw Valley early
Tuesday morning. The most likely scenario is that resident ridging
holds the gradient at bay until late morning, then the inbound warm
front provides a trigger for scattered convection through the rest
of the day as the low works through the area. The LLJ is not
particularly strong but should be sufficient to send PWAT up to
around 1.75 to 2.00", supportive of heavier showers and storms.
However, mid-level lapse rates of 6 C/km limits CAPE to below 1000
J/kg which would be a limiting factor for robust convection. Will
need to monitor development of any upstream MCV that would be
capable of locally increasing shear and instability/moisture
advection not currently captured in model data, but for now the
severe threat appears low. Convective potential gradually wanes
through the night as the cold front passes through.

A resurgence of cooler, drier Canadian air ensues on Wednesday
behind the departing system. There will be a slight chance (15%) for
additional showers as the moisture axis takes time to vacate. The
dry air mass has staying power through the end of the work week as a
large subsidence shield spreads over the Great Lakes in response to
Hurricane Erin passing through the western Atlantic. Dry weather
with light winds, relatively low humidity, and seasonable temps can
be expected. The next cold front arrives next weekend with the next
opportunity for showers and storms.

&&

.MARINE...

Northeast winds gusting around 25 knots over southern Lake Huron
this afternoon, as strong high pressure (1028 MB) is centered over
Northwest Ontario. Small craft advisories continue into tonight as
large waves will be slow to subside. Winds become easterly on Monday
as high pressure retreats and a warm front lifts in from the Western
Great Lakes, allowing waves to diminish further. However, channeling
of the winds over northern third of Lake Huron/near the Straights
looks to support wind gusts of 25 knots to possibly briefly near 30
knots.

Surface low pressure tracking through northern Lower Michigan on
Tuesday looks to be triggering showers and thunderstorms, especially
over northern Lake Huron where southeast winds look to be at least
25 knots, with the potential to reach around 30 knots.

Northeast winds to redevelop over all of Lake Huron Tuesday night as
the low tracks to the east, and another round of small craft
advisories are likely as winds look to gust around 25 knots once
again on Wednesday over southern Lake Huron. Surface ridge tracking
through the Central Great Lakes will allow for light winds on
Thursday and Friday however.

&&

.PREV DISCUSSION...
Issued at 119 PM EDT Sun Aug 17 2025

AVIATION...

Shallow cold advection in the wake of a cold front has led to
prevailing MVFR to low VFR ceilings this afternoon across Southeast
Michigan. Building surface high pressure will continue to north
winds of 10 to 20 knots throughout the afternoon. Loss of daytime
heating should result in dissipation of boundary layer cloud this
evening. VFR cumulus is anticipated again by late morning/afternoon
Monday.

For DTW/D21 Convection...Thunderstorm activity is not anticipated
this period.

THRESHOLD PROBABILITIES...

* High for ceilings at or below 5000 feet through approximately 20z-
22z.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 11 PM EDT this evening for MIZ048-049-
055-063.

Lake Huron...Small Craft Advisory until noon EDT Monday for LHZ421-441.

Small Craft Advisory until midnight EDT tonight for LHZ422.

Small Craft Advisory until 8 AM EDT Monday for LHZ442-443.

Lake St Clair...None.
Michigan waters of Lake Erie...None.
&&

$$

DISCUSSION...TF
MARINE.......SF
AVIATION.....CB


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