Expert Forecast Opinion

154
FXUS63 KDTX 052308
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
708 PM EDT Sat Jul 5 2025

.KEY MESSAGES...

- Hot and muggy conditions persist into Sunday.

- Showers and thunderstorms move in from the northwest on Sunday with
potential for torrential rainfall and isolated marginally severe
wind gusts.

- Drier, a bit cooler, and less humid with time Monday though
Tuesday.

- Shower and storm potential return mid-late week.

&&

.AVIATION...

Residual high based cumulus across Lower Mi gradually fade this
evening before clouds associated with the broad frontal system to
the west move in toward sunrise. The front spans the area from
northern Ontario to the upper Midwest to the central Plains and is
supporting multiple bands of showers and thunderstorms. This
activity is also expected to dissipate with the loss of daytime
instability before reaching central or southern Lower Mi overnight.
That leaves standard warm sector VFR conditions in place with a
steady SW wind out ahead of the front and along the SE Mi terminal
corridor leading into Sunday morning. A stray elevated shower is
possible within the moisture axis as mid and high clouds move into
the region during the morning, however coverage and intensity
increase considerably during the afternoon. Borderline MVFR/IFR
conditions in numerous thunderstorms occupy a lengthy period of time
in the SE Mi airspace until exiting eastward during Sunday evening.

For DTW/D21 Convection... Thunderstorms are not forecast through
tonight, however potential increases considerably Sunday afternoon
as the Midwest frontal system moves through Lower Mi.

THRESHOLD PROBABILITIES...

* Low for thunderstorms Sunday morning and early afternoon, then
moderate mid afternoon into evening.

* Low for ceiling 5000 ft or less tonight and Sunday morning, then
high Sunday afternoon.

&&

.PREV DISCUSSION...
Issued at 340 PM EDT Sat Jul 5 2025

DISCUSSION...

Return flow from surface high pressure centered near the Mid-
Atlantic helps facilitate another hot and humid day with dewpoints
in the upper 60s to lower 70s amidst southwesterly gradient flow.
Heat indices are now peaking, in the mid 90s for most. Governing
anticyclone aloft gets compressed/sheared eastward as a low
amplitude shortwave trough crosses into the Great Lakes bringing the
next opportunity for widespread shower and thunderstorm activity.
This also nudges a cold frontal boundary into Lower Michigan late
tonight into Sunday which will focus the majority of the upcoming
convective activity.

The progression of the trough axis (and frontal slope) will be
gradual which largely keeps precipitation chances at bay until
midday Sunday. This allows more time for pre-frontal ThetaE
advection to sharply increase specific humidities as 850-700 mb
moisture convergence over the Mid-Mississippi Valley gets
transported along anticyclonically curved trajectories tracing back
to The Gulf. Sunday morning forecast soundings indicate a lot of dry
air aloft marked by 12Z PWAT values around 1.25 inches which
suppresses initial convective potential. However, deep-layer
moisture convergence eventually leads to quick pre-frontal
saturation sending PWATs into the 2.00-2.25 inch range by 21Z. This
should translate into surface-based parcels (SBCAPE of 1250-1750
J/kg) to sustain modest coverage of scattered thunderstorms. Dynamic
support also exists, but with only 30-35 knot flow in the mid-
levels, 0-6 km shear values of 15-20 knots preclude meaningful
severe potential. Main concern will be torrential rainfall and an
isolated severe wind threat due to occasionally excessive water-
loading. Consensus amongst the CAMs indicates a sharp decline in
activity once the front exits to the southeast after 00Z, in
addition to the loss of daytime heating. Expect a mainly dry night,
outside of Metro Detroit, with lingering clouds limiting greater
post-front nocturnal cooling. Automated NBM PoP grids reflect timing
uncertainty in the departure of the boundary, thus Chance (40
percent) PoPs are advertised into Monday morning (see ECMWF/EPS,
CMC/CMCE, GFS/GEFS QPF).

Pattern aloft takes on a more zonal orientation to start the
workweek while cooler northerly surface winds cap highs a few
degrees below normal, in the upper 70s to lower 80s. The cold front
transitions to a stationary boundary, stalling over northern Ohio,
while mainly dry conditions persist locally through Tuesday.
Dewpoints briefly retreat into the lower 60s, perhaps even dipping
into the upper 50s. The front then lifts north as a warm front mid-
week which restores a more unstable airmass over the region. A
shortwave trough passes overhead Wednesday into Thursday while an
upper low closes off over Quebec. This offers renewed potential for
the initiation of showers and thunderstorms. Some early indications
of better shear and steeper mid-level lapse rates could promote
better convective organization, but far too soon to get into any
severe specifics at this time-frame. Daily highs should fall within
a few degrees of climatological normals, more likely to the warm
side.

MARINE...

Low pressure over the far northwest Great Lakes will move across the
northern Lakes tonight and Sunday. This will eventually lead to a
tightening of the local gradient and strengthening southwest winds,
but not until after midnight and into Sunday morning. Strongest
gusts, around 20kts, occur over the central portions of Lake Huron.
Due to shoreline effects, the Saginaw Bay has the potential to
reach/exceed 25kts late tonight and into Sunday morning with waves
approaching 3-4ft. It is a borderline Small Craft but with Small
Craft Advisories already issued, will keep them in place.

A cold front sagging south from the aforementioned low gradually
crosses on Sunday. In advance, scattered to numerous showers and
storms are expected. Severe storms not generally expected however an
isolated strong storm can`t be ruled out. Behind the front mainly
for Sunday night and Monday, northerly winds may increase a bit for
the Saginaw Bay and southern Lake Huron basin with some gusts to
20kts. A few showers/storms look to linger into Monday for the
southern Great Lakes as the front is slow to fully vacate. High
pressure then briefly follows late Monday through Tuesday before
unsettled weather returns midweek.

HYDROLOGY...

Scattered to numerous thunderstorms are forecast Sunday afternoon
and evening amidst hot and humid conditions. Thunderstorms will move
rather slowly and be capable of producing torrential downpours.
Localized areas may experience over an inch of rainfall in less than
an hour which may lead to flooding of urban areas, small streams,
and otherwise poorly drained areas. The main threat for flooding
will be between 3 PM and 9 PM, until the supporting cold front
exits toward the southeast.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...None.

Lake Huron...Small Craft Advisory until 10 AM EDT Sunday for LHZ421-422.

Lake St Clair...None.

Michigan waters of Lake Erie...None.

&&

$$

AVIATION.....BT
DISCUSSION...KGK
MARINE.......RBP
HYDROLOGY....KGK


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