Expert Forecast Opinion

519
FXUS63 KDTX 210845
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
445 AM EDT Mon Jul 21 2025

.KEY MESSAGES...

- Warming trend through mid-week.

- Hot and humid Wednesday and Thursday. The heat index may reach 100
degrees on Thursday.

- Widespread chances for showers and thunderstorms late Thursday
through Friday.

&&

.AVIATION...

High pressure centered over northern Lake Huron today will lead to
mostly clear skies and light (under 10 knots) northeast winds right
into tonight. Just enough low level moisture around to allow for
passing scattered 4-5 kft clouds, particularly during the early
afternoon.

For DTW/D21 Convection...Thunderstorms are not expected.

THRESHOLD PROBABILITIES...

* Low for cig aoa 5000 feet around mid day.

&&

.PREV DISCUSSION...
Issued at 345 AM EDT Mon Jul 21 2025

DISCUSSION...

The center of a surface high pressure moves across Lake Huron to
southern Ontario today bringing stable conditions locally and result
in a dry forecast with a few scattered daytime cumulus. Humidity
will be comfortable today with dewpoints in the 50s. Temperatures
will be near to slightly below the average today with mid 70s to low
80s for most locations. The track of the high pressure will lead to
an northeasterly onshore flow and keep things cooler for those along
the shoreline zones. Look for afternoon highs to climb only to around
70s. Very few clouds tonight will lead to overnight lows dipping
into the 50s.

The surface high pressure slides east Tuesday leading to a more
southerly component to the surface flow. This will mark the start of
warming trend with Tuesday high temperatures forecast into the lower
80s for most of the CWA. The main culprit of increasing heat and
humidity through mid-week will be an amplifying mid-upper ridge
across the Mississippi River Valley into the Great Lakes. The
increasing south-southwest flow in the low levels will shove 850 mb
temperatures of 20C or greater into Michigan for Wednesday and
Thursday. The increase in humidity will be characterized by surface
dewpoints into the 60s for Wednesday and then into the 70s for
Thursday. Latest guidance brings greater confidence in a dry forecast
for Wednesday as shortwave energy and associated convection riding
the northern portion of the ridge are directed across northern
Michigan. Thus, the outgoing forecast will be dry for Wednesday. As
of now, Wednesday heat indices hold below 100 degrees locally, though
it will be possible to see these values met to our west as the
hotter airmass makes inroads into western Michigan. Heat and humidity
peak Thursday with current forecast offering heat indices topping
100 degrees, which would require heat headlines.

Better chances for rain enter the forecast Thursday evening into
Thursday night as troughing across Canada with embedded shortwaves
help flatten the ridge. The later timing also adds some confidence
to 100 degree heat index forecast verifying Thursday. The available
moisture and instability will support a chance of showers and
thunderstorms through the overnight period into Friday. Precipitable
Water values of 1.75-2.00 inches spread into central Michigan,
including the Saginaw Valley, during this time. Thus, any showers
and/or thunderstorms would be very efficient rainfall producers. The
associated frontal zone providing a focus for thunderstorms will
drop south and cool conditions off and at least below the 90s
Friday. However, warm and muggy conditions will remain as quasi-
zonal flow sets up that will usher in shortwave features that take
advantage of unstable airmass and keep weather unsettled for Friday
into Saturday.

&

MARINE...

High pressure building across the northern Great Lakes maintains dry
conditions and light to moderate northeast wind today. Wave heights
are projected to remain near 4 feet along the Lake Huron nearshore
waters this morning and a Small Craft Advisory is in effect. Waves
will gradually diminish through the late morning and early
afternoon. The high progresses into the eastern Great Lakes by
Tuesday, shifting light prevailing wind from the southeast. A warm
front lifts into the northern Great Lakes Wednesday into Thursday,
bringing the next chances for thunderstorms mainly across northern
Lake Huron.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...Small Craft Advisory until 8 AM EDT this morning for LHZ441>443.

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

$$

AVIATION.....SF
DISCUSSION...AA
MARINE.......TF


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