Expert Forecast Opinion

614
FXUS63 KDTX 041003
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
603 AM EDT Fri Jul 4 2025

.KEY MESSAGES...

- Hot and muggy conditions through this weekend with highs in the
upper 80s today and lower 90s Saturday into Sunday.

- 20 to 40% chance for showers and thunderstorms this afternoon
outside of the Thumb, then dry on Saturday.

- 40 to 50% chance for showers and thunderstorms late in the day on
Sunday.

&&

.AVIATION...

Weak frontal boundary now just west of the area will gradually lift
across the region through the day. This may provide a focus for a
widely scattered coverage of showers and thunderstorms this
afternoon and evening as instability grows with daytime heating.
Confidence in occurrence remains quite low attm. Otherwise, VFR
conditions this taf period with a sct/bkn coverage of high based
diurnal cu. Modest early day winds from the southeast, becoming
southerly this afternoon.

For DTW/D21 Convection... There is a low chance of thunderstorms as
warmer and more humid air returns to the DTW vicinity in the
afternoon.

THRESHOLD PROBABILITIES...

* Moderate for ceiling 5000 ft or less this afternoon and evening.

* Low for thunderstorms this afternoon and evening.

&&

.PREV DISCUSSION...
Issued at 357 AM EDT Fri Jul 4 2025

DISCUSSION...

An amplified upper ridge is beginning to traverse into the Great
Lakes as a closed upper low takes shape over northern New England.
This pattern shift is marked in the low levels by a stalled frontal
boundary that lifts north as a warm front today. The front is
currently draped along a corridor from western Lake Erie to north of
Lansing with dew points in the lower to mid 50s to the north and mid
60s to the south. Convection has been active overnight along the
front in WI, driven by a shortwave riding the inbound crest of the
ridge. This feature will be the focus of convective chances locally
today, with some uncertainty yet as to how the upstream
convection/MCV evolves in the next 6 to 12 hours.

Overall flavor of 00z CAMs suggest activity wanes by mid morning as
it crosses Lake Michigan, but the instability gradient in place
along the boundary may be sufficient to sustain some of the
convection into the local area. Regardless, convergence along the
front with a subtle boost aloft from the passing wave this afternoon
may be sufficient for scattered showers and storms to develop
overhead despite the background ridge building in. Highest precip
chances of 30 to 40% will be for areas outside of the Thumb which
likely stays dry today. Relatively warm mid levels will put a speed
bump in the instability profile with HREF ensemble mean SBCAPE
generally in the 500 to 1000 J/kg range. The veering 250 mb jet
brings an atypical NW to SE storm motion but with effective shear in
the 25 to 30 kt range supportive of loosely organized multicell
cluster mode. The severe threat is low but a few 40+ mph gusts will
be possible in the strongest cells.

Heat and humidity build in today through the rest of the holiday
weekend with highs in the upper 80s today south of the front and
then into the lower 90s Saturday and Sunday. Dew points well into
the 60s will boost heat index into the mid 90s each day with some
upper 90s likely in the Metro Detroit area Sat/Sun. Lows Sunday
morning only dip into the 70s. Stable conditions beneath the ridge
will keep the forecast dry Saturday but then a cold front brings the
next chance for showers and storms on Sunday. Forcing is
unimpressive with this front given a disorganized PV feature and
slow, gradual height falls from the positively tilted trough. This
is reflected at the surface by several dry solutions despite ample
instability ahead of the front. 40 to 50% PoPs are warranted and
supported by ensemble solution space but may be shifted downward if
subsequent runs continue to lean toward a drier fropa. Low amplitude
zonal flow sets up through next week to offer a typical summertime
pattern with highs in the 80s and relatively lower humidity. High
pressure to start the week gives way to rain chances by midweek as a
disturbance works across the Ohio Valley.

MARINE...

The now stalled cold front over the far southern Great Lakes
gradually lifts back north today shifting winds to southerly.
Influence of high pressure lingers however, maintaining a weak
gradient a generally light winds throughout the day. Low pressure
ejecting out of the northern Plains leads to a tightening of the
gradient and strengthening southwest winds Saturday. Strongest
gusts, around 20kts, are expected over the central portions of Lake
Huron. Due to shoreline effects, the Saginaw Bay has the potential
to reach/exceed 25kts which could warrant the need for a Small Craft
Advisory. This also increases advection of a hot, humid airmass back
over the central Great Lakes fueling storm chances for the latter
half of the weekend. Aforementioned low slides across the northern
Great Lakes late Saturday-Sunday while dragging a cold front towards
the area. This front slowly sags through the region late Saturday
night through Sunday bringing the next chances for numerous showers
and thunderstorms. These storm chances likely persist into daytime
Monday for the southern Great Lakes as the front is slow to fully
vacate. High pressure then briefly follows late Monday through
Tuesday.

&&

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

$$

AVIATION.....MR
DISCUSSION...TF
MARINE.......KDK


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