Expert Forecast Opinion

126
FXUS63 KDTX 301102
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
702 AM EDT Wed Sep 30 2026

.KEY MESSAGES...

- Light rain chances gradually rise from west to east today.

- Muggier Thursday as the coverage and intensity of rainfall
increases; locally heavy rain is possible.

- Turning cooler and drier over the weekend.

&&

.AVIATION...

High cloud observed this morning will gradually lower from west to
east this afternoon-evening as a tropical moisture axis progresses
and brings rain showers into the airspace. VFR is expected to
persist through early afternoon across all of SE Michigan before
cigs begin to lower first at MBS and then the rest of the airspace.
MBS will see the greatest coverage of rain and lowest ceilings by
tonight, where IFR my just graze the terminal. Highest probabilities
of IFR however are centered over northern Michigan. Elsewhere,
cigs/vsbys fluctuate between low VFR and MVFR as waves of rain
attempt to saturate a persistent sub-cloud layer. The most likely
timeframe for restrictions will be after 00z tonight as a warm front
lifts into the airspace and organizes a band of showers and possibly
a few embedded thunderstorms overhead. Limited instability keeps
thunder chances low however. Moisture holds steady over the region
into Thursday morning to keep sub-5.0 kft agl cloud around to end
the TAF period.

D21/DTW Convection...Low potential for thunderstorms forecast this
evening and overnight.

.DTW THRESHOLD PROBABILITIES...

* Low for ceilings at or below 5,000 feet through 18z. Medium 18z
through the end of the TAF period.

* Low for thunderstorms this evening and overnight.

&&

.PREV DISCUSSION...
Issued at 404 AM EDT Wed Sep 30 2026

DISCUSSION...

The blocked/stable synoptic pattern over the Great Lakes has begun
to break down early this morning. Prior ridging continues to flatten
and fold into northeastern CONUS, trailing the early week coastal
system`s departure. Meanwhile, much longer wavelength disturbances
upstream are tracking zonally across central Canada and southern
CONUS. Southeast Michigan will be positioned between these two waves
through the rest of the week, eventually leading to periods of
rainfall. At present, leading geopotential height falls have
overspread Lower Michigan, marked by a thickening canopy of cirrus
aloft. The southern stream wave/closed-low absorbed what remains of
eastern Pacific Hurricane Polo leading to the advection of an
equatorial airmass from the Four Corners region up to James Bay.

The distribution of saturation is currently skewed toward the upper
levels as top-down moistening ramps up over the next 12 hours. Given
that the column has been abnormally dry since last Thursday (see
KDTX RAOBs since 24.12Z), it will take time to prime the low levels
and achieve adequate saturation depths before rainfall becomes more
prevalent. Latest mosaic radar imagery shows a sharp cutoff to the
rain edge, currently confined to areas west of a Chicago to Gaylord
line. The swath of rainfall should migrate eastward throughout the
day as the theta-e ridge rolls southeast by this afternoon as
embedded shortwave energy gets deflected away from the higher
amplitude northern stream wave. The majority of 00Z/06Z solutions
depict some degree of measurable precipitation for most areas by 00Z
this evening. The lowest confidence in this initial round of diurnal
showers continues for locations along/south of I-94 where saturation
processes are the most delayed.

The axis of persistent tropical moisture advection drifts over Lower
Michigan Thursday amidst a shifting deformation zone and reinforcing
CVA. This unfolds as the southern stream low starts to open/phase
with the deeper continental wave to the north. Expect longer and
more continuous periods of rainfall as Thursday progresses with
notably muggier conditions. Surface dewpoints easily climb into the
mid 60s, if not upper 60s, while 850 mb temperatures rise into the
upper teens (Celsius). PWATs respond accordingly, climbing above
1.75 inches for some areas, which raises the prospect for localized
heavy rainfall (see Hydrology Discussion). From a stability
perspective, the thunder threat appears quite low given the warm
advection scheme, before an approaching cold front extending from
the Ontario surface reflection slowly tracks through the region.

Aggressive post-frontal system-relative isentropic downglide leads
to a quick conclusion to showers, before the midday hours Friday. N
northwest flow arrives downstream of 1027 mb high pressure over the
Upper Midwest. Seasonably cool and dry though the weekend.

MARINE...

South to southwest winds will slowly increase ahead of the next
cold front, which arrives and stalls over southern Lower Michigan on
Thursday. With warm advection leading to more stable profiles
and showers moving through during this timeframe, wind gusts are
expected to top out only around 25 knots. With the front hung up
into Thursday night, the post frontal cold advection now looks
slower; thus, the potential for northwest wind gusts reaching or
exceeding 25 knots looks to be late in the day on Friday as 850
MB temps lower into the mid single digits by Friday.

Another large area of high pressure building in on Friday night and
Saturday will promote light winds for the first half of the weekend.
A stronger low tracking through the northern Great Lakes for the
second half of the weekend brings another opportunity for winds to
reach or exceed 25 knots over Lake Huron.

HYDROLOGY...

The transport of tropical moisture into southern Michigan and an
approaching cold front will lead to expanding rainfall today into
Friday. The heaviest rain is forecast to occur between Thursday
afternoon and Friday morning. There is medium confidence in total
rainfall between 0.5 and 1.5 inches. A band of more persistent rain
is likely to become established across the area, with positioning
currently favored north of I-69. Within the model probability space,
there is a chance for rainfall to exceed 2 inches in areas which
experience the more persistent rainfall, suggesting at least a
chance of minor flooding.

&&

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

$$

AVIATION.....MV
DISCUSSION...KGK
MARINE.......SF
HYDROLOGY....SC/KGK


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