289
FXUS63 KDTX 301954
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
354 PM EDT Wed Jul 30 2025

.KEY MESSAGES...

- Chance for scattered showers and thunderstorms between roughly 5
PM and 10 PM this evening.

- There is a Marginal Risk of severe weather for areas south of I-94
with isolated damaging winds as the primary threat.

- Steadier rain fills in overnight with a narrow swath of 1 to 2+
inch rainfall totals possible.

- Dry weather, with much cooler and less humid air in place on
Friday, lingering into the weekend.

&&

.DISCUSSION...

Most imminent forecast trend noted on visible satellite/radar is the
development of towering cumulus and radar echoes along the lake
breeze from roughly Mt Clemens to Toledo. So far reflectivity cores
have struggled to grow beyond the freezing level (~13.0 kft agl),
likely encountering a remnant subsidence layer. Mid-level column
winds however will soon back to the SW in advance of a progressive
shortwave, which will moisten this layer and erode what is left of
the capping inversion. This moisture transport has helped fuel the
maintenance of an MCS, which is just now reaching the Lake Michigan
shoreline and adjacent areas to the south. The strongest cells seem
to be focused along the instability gradient, and are likely to
remain south of the Michigan state line through the evening. That
said, moistening profiles should initially lead to deepening
convection along the lake breeze with isolated wind gusts of 40-60
mph not out of the question in any deeper cores. A more likely
threat is heavy rainfall, as these storms are slow-moving and
encountering increasing moisture depths. This first batch of
showers/storms may produce rainfall rates over an inch per hour, but
on a fairly isolated basis along the lake breeze.

A steadier period of rainfall arrives with the remnants of the MCS,
between about 5-8pm tonight. This will be associated with PWAT
values nearing 2.0 inches and warm cloud layer depths to 13.0 kft
agl, although with a rapid loss of instability. Was on the fence
about issuing a Flood Watch for areas along/south of I-94
considering the ambient thermodynamics and strengthening mid-level
frontal zone (850-700mb) as a cold front slowly drops through the
region. Lack of instability however should inhibit rainfall rates
(30-40% chance for 3-hrly rainfall rates of 1") below watch
criteria. Still, basin-wide rainfall totals may approach 2 inches or
more in some spots over a 6-9 hour period. More details are noted in
the Hydrology section.

Showers linger through the morning hours on Thursday, followed by
the influx of strong Canadian high pressure from the north. This
occurs as the high pressure ridge over southern CONUS retrogrades
further into the Desert Southwest. Broad upper level troughing
complements the surface high, which becomes established overhead
through the weekend. This upper level transition plus the clearance
of a strong cold front through the area brings moisture parameters
(i.e. specific humidity, PWAT) from the 95th percentile Thursday to
the 5th percentile by Friday morning, ensuring a dry end to the work
week. H8 temperatures drop into the low teens through the weekend as
well, which combined with the strong subsidence inversion, caps
temperatures in the 70s for most of the weekend. These cooler
temperatures combine with dry conditions and plenty of sunshine for
a mild start to August.

The surface high pressure center slowly drops across the Great Lakes
on Sunday, which shifts winds to the southwest and results in modest
airmass modification to bring highs into the low 80s to start the
work week.

&&

.MARINE...

A frontal boundary draped across the southern Great Lakes slowly
eases southward over the next 24 hours as strong Canadian high
pressure builds over the northern Lakes. A subtle disturbance tracks
along the front tonight, bringing showers and thunderstorms across
southern Lake Huron, Lake St. Clair, and Lake Erie. The pressure
gradient tightens between the inbound high and this system, causing
northeast winds to strengthen to 15 to 20 kt tonight into Thursday.
Gusts to 25 kt and wave action building along the nearshores
prompted a Small Craft Advisory through this period. Waves will be
slow to diminish Friday morning in continued northeast wind, albeit
slightly weaker. The center of high pressure then spreads in from
the north and west late Friday and establishes overhead to support a
period of benign marine weather this weekend.

&&

.HYDROLOGY...

Multiple waves of shower and thunderstorm activity lead to basin
wide rainfall totals of 1 to 2 inches, with locally higher amounts
possible depending on evening activity. First round of convection is
ongoing as thunderstorms develop along the lake breeze. Rainfall
rates in excess of 1 inch per hour will be possible, but on a highly
localized basis where slow-moving and/or training storms occur.
Transition to steady, albeit lighter intensity rainfall is expected
this evening as an elevated frontal zone sets up overhead. Expected
rainfall rates aob 1 inch per hour, do not quite meet Flash Flood
Guidance especially overnight with the transition to a more steady
stratiform rainfall. That said, more localized flooding of urban and
low- lying areas is possible. Further, rises on area rivers and
streams will be possible. Several rivers are exhibiting much above
normal streamflow for late July standards.

&&

.PREV DISCUSSION...
Issued at 108 PM EDT Wed Jul 30 2025

AVIATION...

An isolated thunderstorm remains possible in the PTK to DTW vicinity
this afternoon and evening as convection develops overhead or moves
in from the west. A peak heating dependency suggests a late
afternoon to early evening mention is warranted in the forecast
while surface Td settles from around 70 and the surface front/lake
breeze hybrid also lingers nearby. In the meantime, VFR is a mix of
cumulus with mid and high clouds streaming in from the west. Lake
breeze influence releases this evening and the surface front moves
southward toward the Ohio valley tonight while remaining active at
mid levels across the Midwest and southern Great Lakes, especially
later tonight. Elevated showers become widespread mainly south of
FNT overnight through Thursday morning with a rumble of thunder
possible and rainfall rate restriction generally in the MVFR range.
Moisture added to the shallow boundary layer frontal zone also
brings MVFR ceiling before sunrise over increasingly cool NE wind
through Thursday morning.

For DTW/D21 Convection... A late afternoon to early evening time
window for a thunderstorm is tied to peak heating while humid air
remains over the region. Showers then increase coverage and
intensity later tonight with a rumble of thunder possible. Greater
storm potential continues to to reside across the far southern
portions of the airspace, closer to the instability reservoir,
through tonight into Thursday morning.

THRESHOLD PROBABILITIES...

* Low for thunderstorms this afternoon and again late tonight and
  Thursday morning.

* High for ceiling aob 5000 feet tonight and Thursday morning.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from Thursday morning through Thursday
     evening for MIZ049-055-063.

Lake Huron...Small Craft Advisory from 4 AM to 10 PM EDT Thursday for LHZ421-422.

     Small Craft Advisory from 4 AM Thursday to 4 AM EDT Friday for
     LHZ441>443.

Lake St Clair...None.
Michigan waters of Lake Erie...Small Craft Advisory from 10 PM this evening to 4 AM EDT Friday for
     LEZ444.

&&

$$

DISCUSSION...MV
MARINE.......TF
HYDROLOGY....MV
AVIATION.....BT


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