NWS Forecast Discussion

FXUS63 KDTX 240802

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
402 AM EDT Sat Jul 24 2021


Main focus with this forecast package will be the threat for severe
weather, mainly during the afternoon and evening. Damaging winds and
locally heavy rainfall will be the primary threats with large hail
up to 1 inch also possible. While probability is low, an isolated
tornado or two cannot be ruled out with activity today.

A warm front currently lifting through Michigan will advect a very
warm and humid airmass bringing more than enough moisture to fuel
strong to severe thunderstorms. Dewpoints in the wake of this warm
front will rise into the 70s while PWAT values are progged to climb
to between 1.75" to 2.00". This past evening`s 00Z RAOB already
showed high moisture in place with a PWAT of 1.66". Several forcing
mechanisms at play today will result in a few windows for showers
and thunderstorms to develop. Some scattered activity will be
possible through this morning associated with isentropic ascent along
the warm frontal boundary, but the first main window for strong to
severe thunderstorms will be related to ongoing nocturnal convection
driving through northern lower Michigan. This convection may layout
an outflow boundary across the Saginaw Valley and northern Thumb as
bulk of activity looks to stay just north. This will coincide with
the better synoptic forcing along the progressing warm front and
shortwave trough. There should be enough instability to support
thunderstorms, but lack of shear early in the day leads to mainly a
wind and locally heavy rainfall threat as convection may struggle to
become better organized.

The main window for severe weather will start in the early/mid
afternoon and carry through the evening forced by a prefrontal and
the mid level shortwave digging into Lake Michigan by late this
afternoon. Severe potential will be dependent on how early day
convection and cloud cover may inhibit max destabilization, as is
the case in these types of situations. However, the conditional
threat remains good as the better shear will arrive at this time
with increased flow around the base of the shortwave trough leading
to better organized convection. The rich low level moisture under
diurnal heating should still lead to moderate to locally strong
buoyancy/destabilization yielding MLCAPE values of 1000-2000 J/kg.
The mid level lapse rates will be mostly below 6 C/km, though much
steeper low level lapse rates develop in the afternoon leading to a
greater threat for damaging winds. Hail still remains possible if any
updrafts can become organized. Cannot rule out an isolated tornado
threat early in the convective evolution ahead of the shortwave as
forecast soundings suggest some discrete supercell potential while
low level shear increases with backing of surface winds near the warm
front into this evening. Locally heavy rainfall can be expected with
any activity throughout the day given the expected high degree of

Early isolated to scattered convection posing a severe threat early
in the afternoon should evolve into the evening hours as more of a
scattered to broken line segments with clusters of storms advancing
along the frontal zone/prefrontal trough. Storm motion will be from
northwest to southeast at around 30 mph. The main time frame for
severe weather will be from 3 pm to about midnight tonight with
northern portions of the CWA being impacted in the earlier portion of
this time frame and southern portions towards the later.

Once the main prefrontal trough pushes east of southeast Michigan,
some lingering activity may remain as the lagging frontal boundary
eventually arrives. No strong cold air intrusion is expected in the
wake of this frontal passage, but it will help lower the humidity
across the area. As clouds begin to clear out for Sunday,
temperatures are expected to climb into the upper 80s to 90 degrees.
Dry conditions carry into Monday with surface high pressure in
place. Ridging builds across the Rockies putting southeast Michigan
on the northeast end of the heat dome and under persistent northwest
flow aloft. Various shortwaves passing within this flow will bring
the next chance for precipitation on Tuesday as well some cooler
temperatures by mid to late next week.



Upper-level trough to bring widespread showers and elevated
thunderstorms across northern Lake Huron this morning,
with activity driving through into Central Lake Huron/Saginaw Bay
into the late morning and early afternoon. Sustained southerly flow
will ramp up across northern and central Lake Huron through the mid
and late morning hours, increasing to or slightly above 20 knots.
Outflow and low-level convergence to then produce scattered
to numerous showers and storms mainly for the Saginaw Bay, southern
Lake Huron, and locations south this 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 the stronger storms.

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 today with
scattered to numerous showers and thunderstorms expected during the
afternoon and evening. High degree of moisture will arrive lift into
southeast Michigan with PWAT values increasing to or near 2.00".
Heavy downpours will thus 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 flooding of low-
lying and flood prone areas. The bulk of showers and thunderstorms
are expected to move out late tonight.


Issued at 1157 PM EDT Fri Jul 23 2021


Scattered showers will be possible Saturday morning within area of
isentropic lift in advance of approaching upper level disturbance.
Better shower/thunderstorm chances will then arrive in the late
afternoon and evening along a surface trough associated with the
upper wave. Generally expect VFR to occasional MVFR (with shras or
tsras) ceilings.

For DTW...Scattered showers will develop early Saturday morning with
some MVFR cigs possible. Additional showers/thunderstorms are then
expected mainly Saturday evening (with just isolated activity during
the afternoon).


* Medium for ceilings of 5000 ft Saturday morning, low Saturday

* Low for thunderstorms impacting terminal Saturday morning,
  moderate by Saturday evening.


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



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at www.weather.gov/detroit.

NWS DTX Office Area Forecast Discussion