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NWS Forecast Discussion

FXUS63 KDTX 232300

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
700 PM EDT Fri Jul 23 2021


Showers will fade as they move into the area early this evening with
another round of light showers possible early Saturday morning as
period of isentropic lift translates through the area. The better
chance for showers/thunderstorms will come Saturday afternoon into
evening as a shortwave and attending surface trough produces an area
of sct-bkn convection. Otherwise, expect any MVFR cigs early this
evening to lift give way to VFR into the overnight hours with
perhaps a period of MVFR again Saturday morning in the wake of
outflow form overnight convection to the north.

For DTW...Other than a passing shower early in the forecast and
another window for showers Saturday morning, the main convective
activity looks to hold off until 22z or so Saturday afternoon.
Ceilings aob 5kft should give way to mid level cigs tonight with
MVFR cigs possible again Saturday as times.


* Medium for ceilings of 5000 ft or less early this evening and
  again Saturday.

* Low for thunderstorms impacting terminal Saturday afternoon.


Issued at 348 PM EDT Fri Jul 23 2021


Scattered showers dot the Lower Peninsula this afternoon as a warm
front continues to nudge in from west Michigan. The main instability
gradient and thunderstorm potential looks to remain to our west
tonight, though some isolated gusty winds or a brief heavy downpour
can`t be ruled out this evening if a storm makes it this far east.
Low-level warm/moist advection will have to contend with modest
height rises that commence this evening as a midlevel ridge folds
directly over the Great Lakes, so any late evening convection is
generally expected to wane overnight. Plenty of clouds linger
tonight just downstream of the warm front which keeps lows very mild
in the mid to upper 60s.

An uptick in shower coverage is expected on Saturday morning as the
midlevel ridge axis departs and opens the door for the warm front to
surge eastward across the forecast area early in the day. There may
be a few rumbles of thunder as well, but the better potential for
thunderstorms holds off until later in the day when better
instability is realized. The air mass that builds in tomorrow will
be warm and muggy with dew points in the 70s and temps rising to the
lower to mid 80s under mostly cloudy skies. Scattered to numerous
showers and storms are then expected through the afternoon and
evening with the severe threat outlined below.

Hi-res guidance has been showing a few windows for potential strong
to severe storm development on Saturday. The first is focused on the
remnant outflow from nocturnal convection over northern Michigan
that sags south during the morning and moves across the Saginaw
Valley and Thumb during the early afternoon. This may coincide with
and/or gain additional support from the warm front moving in from
the southwest. The next round then appears to be driven by
convergence along a prefrontal trough that tracks northwest to
southeast across the entire CWA during the late afternoon to evening
hours. As is typical in a lot of these severe setups, afternoon
destabilization will be conditional on the persistence of morning
showers/storms and cloud cover. It will however not take much
surface heating for ample instability to develop with the quality of
near-surface moisture.

Midlevel lapse rates will be rather paltry but the degree of low-
level moisture will allow for MLCAPE on the order of 1000 to 2000
J/kg to develop given sufficient insolation. Bulk 0-6km wind shear
of 30 to 40 kt will be in place with the higher magnitude located
over the Thumb beneath the stronger belt of midlevel WNW winds.
Slight veering with height and some lengthening of the low-level
shear is seen in hodographs and can`t rule out a supercell/tornado
threat particularly where slight backing of the surface wind occurs
near the warm front, but the main threat appears to be damaging wind
gusts as clusters of storms tend to organize in a semi-linear mode
along the forcing axes. A secondary threat for large hail exists as
well within any vigorous updrafts. Torrential downpours will also be
a concern as a wedge of PWATs nearing 2 inches accompanies the
convection and allows for efficient rainfall production. The main
severe threat appears to taper off after around 10pm, but lingering
instability ahead of the lagging primary "cold front" may lead to
continued convection overnight.

The cold front mentioned above is in quotations because it looks to
be more of a surface trough that will usher in a slightly less humid
air mass early Sunday. In fact, our post-frontal Sunday actually
looks hotter than Saturday as more sunshine allows for better
surface heating. Highs in the upper 80s or around 90. Soundings show
a much drier column overall and the lack of a forcing mechanism
should keep the day precipitation-free apart from some possible
lingering early showers near the stateline. Mainly dry and hot
weather continues into Monday as the Great Lakes reside within the
energetic northwest flow on the periphery of an expansive central
CONUS ridge. Signal is there in mid-range guidance for a stronger
wave to move nearby during the midweek and usher in a slightly
cooler air mass by late week.


Upper-level trough to bring widespread showers and elevated
thunderstorms across northern Lake Huron starting tomorrow morning,
with activity driving through into Central Lake Huron/Saginaw Bay
into the late morning and early afternoon. Ahead and with the
shower/convective activity, expect sustained southerly winds to
creep near 20 knots across northern and central Lake Huron. Outflow
and near surface convergence to then produce scattered to numerous
shower and storm potential across central Lake Huron and all
locations south through the afternoon and evening. Instability will
build through the afternoon which in turn will produce some
localized strong to severe storms. Wind gusts to or in excess of 34
knots will be possible with stronger activity.

General storm activity and severe threat will wane late Saturday
night, with a cold front swinging through on Sunday morning. Passage
of the front will veer wind direction from southerly to westerly
throughout the day on Sunday. A weak pressure gradient to then hold
over the Great Lakes through the midweek period, which will bring
lighter winds to the area.


A storm system will move through the Great Lakes on Saturday with
scattered to numerous showers and thunderstorms expected during the
afternoon and evening. Heavy downpours will be possible in any
thunderstorms with rainfall rates in excess of a half inch per hour
possible in the strongest storms. Basin-average amounts of a quarter
to half inch are forecast for most of SE Michigan but areas that see
multiple rounds of storms may see totals up to or exceeding 1 inch.
The potential for this to occur is highest in the Thumb region
including the Port Huron vicinity. This may lead to localized minor
flooding of low-lying and flood prone areas. The bulk of showers and
thunderstorms are expected to move out late Saturday night.


Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.



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