National Weather Service United States Department of Commerce


311
FXUS63 KDTX 082309
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
709 PM EDT Wed Jul 8 2020

.AVIATION...

A rapid decline in instability as nocturnal cooling strengthens with
time will limit the potential for additional thunderstorm
development going forward early tonight. Lingering high clouds for
the overnight period. Steady destabilization of the resident hot and
humid environment will again present a window from mid afternoon
into the evening hours for possible thunderstorms Thursday. Greater
potential may again tend to focus along the Detroit corridor as
inbound lake breeze activity engages this environment.

DTW THRESHOLD PROBABILITIES...

* Low for thunderstorms Thursday afternoon and evening.

&&

.PREV DISCUSSION...
Issued at 322 PM EDT Wed Jul 8 2020

DISCUSSION...

Several convergent boundaries are noted across Se Mi on latest vis
satellite imagery, the strongest of which have resulted from the
Lake St Clair/Erie marine layers. The convergence and modest daytime
instability (0-1km ML Cape around 1500 J/kg) have allowed scattered
convection this afternoon. Much like yesterday, the convection has
produced some fairly good outflow boundaries, which will only enhance
boundary layer convergence. Thus scattered showers/thunderstorms
will persist into the early evening hours before the loss of daytime
heating and/or the boundary layer becomes outflow dominated. Steep
low level lapse rates is again providing some decent downdraft
potential despite the modest CAPE and weak shear. So isolated
stronger thunderstorms with gusty winds remain possible. With forcing
anchored along stationary lake breeze boundaries, localized heavy
rain and flooding remain of concern.

The mid level ridge will slide east of Se Mi tonight into Thurs,
allowing light southerly return flow to take hold across Se Mi. This
will sustain another muggy night as most locations will not see min
temps drop below 70. Diurnal mixing With 850mb temps forecast to
hold around +20C will yield afternoon highs in the low to mid 90s on
Thurs. The light southerly flow will sustain some weak boundary
layer moisture advection through the day. This should keep sfc
dewpoints in the mid 60s/near 70, supporting afternoon heat indices
of 95 to 100 degrees. The current heat advisory will remain in tact.
There is a chance the mid shift will have to expand it to include
other counties. Modest daytime instability with a weak cap and the
presence of sfc boundaries will again support a chance of afternoon
convection.

A long wave trough is forecast to move into the western Great Lakes
Thurs night. Model solutions are actually in reasonable agreement
that upscale convective growth across Nebraska tonight will initiate
a convectively induced short wave impulse, forecast to advance into
srn WI/nrn Il Thurs night/Friday morning. The latest NAM and ARW
actually suggest this will be a fairly compact wave as it rotates
across srn Mi on Friday. Low level moisture advection ahead of this
feature with modest mid level instability will support a chance of
convection Thurs night. The combination of large scale ascent and
diurnal heating will warrant high pops for showers and thunderstorms
Fri and Fri evening. The approach of the wave will support a little
better wind sheer in comparison to this past week, but it will still
be on the weak side (under 25 knots). Severe potential on Friday
will hinge not only on the timing of the more widespread convection
but also the impacts that early day cloud cover/showers may have on
instability later in the day.

Some amplification of the longer wavelength trough will take hold
across the eastern Great Lakes into the Mid Atlantic over the
weekend, driving a little cooler airmass into Lower Mi. There has
been some variation within the model solution space with respect to
timing/amplitude of short wave impulses advancing into this longer
wavelength trough. This adds some degree of uncertainty as to weekend
convective potential. The GFS/NAM are on the stronger side of the
spectrum and indicate fairly good convective chances during the
latter half of the weekend. At this stage of the forecast, chance
pops will suffice. Forecast building of the subtropical ridge back
into the MS/OH valleys by the end of the forecast period continues
to suggest a return of hot weather by the middle of next week.

MARINE...

An overall weak gradient across the region tonight into Thurs night
will maintain light and variable winds across the lakes region. The
better thunderstorm chances this evening into Thursday will remain
primarily over land areas where instability will be a little
greater. Weak low pressure is forecast to move across Lower Mi
Friday. This system will bring a much better chance for
thunderstorms to the marine areas. This system will also result in
light south-southeast winds across the marine areas Friday with
winds backing toward the northwest by Saturday as the low pressure
system moves east of the region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Heat Advisory until midnight EDT Thursday night for MIZ053-061-062-
068>070-075-076-083.

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

$$

AVIATION.....MR
DISCUSSION...SC
MARINE.......SC

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