593
FXUS63 KDTX 210751
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
351 AM EDT Wed May 21 2025

.KEY MESSAGES...

- Damp and chilly today with rain showers and drizzle.

- Temperatures remain well below normal through Friday with highs in
the 50s and lows in the 40s.

- Daytime temps rebound slightly into the 60s this weekend with
drier conditions likely.

&&

.DISCUSSION...

A stagnant upper air pattern takes shape today through the end of
the week, with growing likelihood for the cool weather to persist
even into early next week as persistent longwave troughing dominates
the governing pattern. The process kicks off as a 500mb shortwave
tracking into the northern Ohio Valley and its parent closed low
over the eastern Dakotas shear out through the day. The attendant
surface low center elongates just south of the Ohio stateline,
sustaining moist and cool easterly flow across SE MI within its
northern periphery. A band of showers along the 700mb moisture axis
works northward early this morning before a transition to mostly
drizzle into midday. Additional showers then fill in later today as
the next wave of moisture and deformation slide up the frontal slope.

Good depth of moisture and PWAT around 1 inch will support
occasional moderate rates, but poor lapse rates will generally limit
prospects for thunderstorms and heavier rainfall. Will maintain a
slight chance for thunder south of a line from Adrian to Port Huron
where the 850mb warm sector nudging in brings slightly negative
showalter indices this afternoon. The steadiest rain looks to set up
near the Saginaw Valley where the elevated deformation axis pivots
in place. Rainfall totals for most areas will range between one and
three quarters of an inch, but some locations may exceed 1 inch. The
latest HREF offers a 30 to 50% probability for over 1 inch in the
Saginaw Valley, and 10% or less elsewhere. All in all it will be a
raw day, particularly in the Thumb and Tri-Cities where a biting 20
to 30 mph northeast wind off Lake Huron keeps temps in the lower 50s
amidst the damp conditions. Elsewhere, wind will be weaker but highs
will still struggle to reach the mid to upper 50s. These temps will
be some 20 degrees below normal and may challenge the daily low max
record at Flint (52 in 1986).

Additional light showers and drizzle work across the area tonight
into Thursday as the surface low stalls over the Ontario peninsula.
Wind direction backs to northwest during the day tomorrow and
becomes breezy to around 25 mph for the whole forecast area. As the
low gradually releases east Thursday night, it pulls in 850mb temps
around 0C along with some drier air that may allow for some breaks
in the stratus by Friday. Despite better surface heating, temps will
still be limited well below normal in this air mass. An upper low
over northeast Ontario retrogrades across the Great Lakes during
this period with additional scattered showers possible. This low
then merges with the prior low over New England and stalls there
through the weekend. This sets up continued northwest flow to
perpetuate the thermal troughing. However, prospects for some more
sun and air mass modification through the weekend should promote a
slight warming trend back into the 60s.

&&

.MARINE...

A low pressure system reaches the northern Ohio Valley today. This
sustains periods of rain and an uptick in winds/waves for the
central Great Lakes. Small Craft Advisories drop-off for Lake St.
Clair and western Lake Erie later this morning while continuing into
Thursday for the Lake Huron nearshore zones. Northeasterly gusts
should climb into the upper 20 knot range along central Lake Huron
and Saginaw Bay this afternoon with occasional wave heights in
excess of 5 feet. Modest pressure gradient maintains 15-20 knot
prevailing winds as the center of the surface low migrates into the
eastern Great Lakes by this evening, backing winds toward the north.
The system will be slow to fully exit the vicinity which keeps
additional opportunities for Small Craft Advisory conditions
into Thursday night.

&&

.HYDROLOGY...

Most areas have received between a quarter and a half inch of
rainfall over the past 24 hours. Numerous rain showers will continue
to spread across the state today into tonight. From this
morning through Thursday morning, additional rainfall amounts will
range between one and three quarters of an inch, with localized
areas possible to reach 1 inch. There is a 30 to 50 percent
probability to exceed 1 inch in the Saginaw Valley. Given the long
duration of rainfall and predominantly light rainfall rates,
flooding is not anticipated apart from nuisance ponding where more
moderate rainfall occurs.

&&

.PREV DISCUSSION...
Issued at 1155 PM EDT Tue May 20 2025

AVIATION...

Cool and moist easterly flow will hold firm through Wednesday, north
of a low pressure system slowly tracking across the northern Ohio
Valley. The low level environment will steady saturate during the
early morning period, supporting an increase in coverage of stratus
featurning a cloud base at low MVFR to IFR. This will occur within
additional pockets of light rain. A high coverage of IFR stratus
expected to persist with periodic shower development throughout
Wednesday. Easterly winds overnight, backing to northeasterly with
time by late Wednesday. Some gustiness to the wind possible at MBS
given trajectory off Saginaw Bay.

For DTW/D21 Convection...best thunderstorm chances remain south of
the Michigan state line.

THRESHOLD PROBABILITIES...

* High for ceiling 5000 feet or less tonight and Wednesday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...Small Craft Advisory until 10 PM EDT Thursday for LHZ421-422-
     441>443.

Lake St Clair...Small Craft Advisory until 10 AM EDT this morning for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 10 AM EDT this morning for LEZ444.

&&

$$

DISCUSSION...TF
MARINE.......KGK
HYDROLOGY....TF
AVIATION.....MR


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at www.weather.gov/detroit.