Expert Forecast Opinion

888
FXUS63 KDTX 210355
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1155 PM EDT Tue May 20 2025

.KEY MESSAGES...

- Multiple rounds of on and off rain showers with occasional periods
of drizzle through Thursday along with below normal temperatures
in the 50s during the day and 40s overnight.

- Warming temperatures over the weekend into early next week.

&&

.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.

&&

.PREV DISCUSSION...
Issued at 352 PM EDT Tue May 20 2025

DISCUSSION...

Mid level frontal forcing downstream of a negatively titled trough
over the northern plains is resulting in expansion of rainfall
across southwest portions of the CWA this afternoon. This rainfall
will be encroaching on a drier airmass as high pressure across the
northern and eastern Great Lakes maintains the dry easterly winds.
The winds are also gusting to around 25 mph in spots this afternoon
with the increased pressure gradient. Soundings show the moistening
of profiles across the south more quickly than the north, so expect
that better rain chances do not reach the Thumb and parts of the Tri-
Cities until after 8 pm given the high dewpoint depressions in the
lower levels. Once better saturation occurs, the latest guidance has
increased QPF amounts for tonight, most notably across the Saginaw
Valley where QPF amounts this evening through tonight may reach to
around a half inch, though if dry air wins out a bit longer these
totals may be a little too high.

A surface low then tracks across southern Michigan border and
meanders across the central and eastern Great Lakes tomorrow morning
into Thursday. This will allow persistent deformation, focused
mainly along and north of I-69, while northerly winds maintain
cooler flow off the lake. Will continue to carry high PoPs with QPF
amounts generally around a quarter inch or less per 6 hours. The
latest trend upwards in precipitation tonight with the continued
light rain tomorrow brings total QPF amounts to near an inch through
8 pm Wednesday across the Saginaw Valley with around a half inch
elsewhere. The main story through Thursday is expect multiple rounds
of on and off rain showers with occasional periods of drizzle
through mid-week along with below normal temperatures. Limited
instability and poor lapse rates will preclude any thunder mention
aside from maybe along the southern Michigan border during this
stretch.

As the low moves into the eastern Great Lakes, deeper northwest flow
will reinforce the colder thermal trough with 850 mb temperatures
forecast to drop below 0C late Thursday into Friday. Afternoon highs
mid to late week will hold mostly in the 50s with overnight lows in
the 40s. Ridging builds out of the plains and over the Great Lakes
over the weekend and into early next week offering an opportunity
for temperatures to gradually recover back into the 60s for
afternoon highs. Main trough finally pushes of the Atlantic coast
leaving limited precipitation chances over the weekend.

MARINE...

A low pressure system centered over Missouri will translate and
broaden eastward tonight, reaching the northern Ohio Valley by
Wednesday. This brings periods of rain and an uptick in winds/waves
for the central Great Lakes. This warrants the continuation of Small
Craft Advisories for all the nearshore zones through Wednesday
morning. East-northeasterly gusts should climb into the upper 20
knot range along the coastal areas this evening with occasional wave
heights in excess of 5 feet. A stronger wind and wave response is
expected over Saginaw Bay, and perhaps western Lake Erie, but gusts
should still hold below gales. Modest pressure gradient maintains 15-
20 knot prevailing winds through Wednesday as the center of the
surface low migrates into the eastern Great Lakes by Wednesday
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 Thursday, and possibly Friday.

HYDROLOGY...

Scattered to numerous rain showers will continue to expand northward
across southeast Michigan into tonight and persist through tomorrow.
Latest trends in regards to total rainfall this afternoon through
Wednesday evening have increased and will approach an inch across
parts of the Saginaw Valley with around a half inch elsewhere. The
rain will occur over a roughly 24 to 36 hour period, so flooding is
not expected and the highest amounts will fall mainly across areas
that continue to be in D1 and D2 drought.

&&

.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 Wednesday for LCZ460.

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

&&

$$

AVIATION.....MR
DISCUSSION...AA
MARINE.......KGK
HYDROLOGY....AA


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