348
FXUS63 KDTX 031727
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
127 PM EDT Wed Jul 3 2024

.KEY MESSAGES...

- Scattered showers exit eastward with a weak cold front this
afternoon. An isolated thunderstorm, capable of gusty wind and
locally heavy rainfall, also remains possible south of the I69
corridor.

- Today`s front ushers in a typical summer air mass for the July 4th
holiday. Partly sunny sky helps lift temperature to highs in the mid
to upper 80s with moderate humidity.

- Showers and thunderstorms spread back across Lower MI Friday as
the next low pressure system sets up in the Midwest.

&&

.AVIATION...

Frontal zone is noted on satellite extending SW-NE from KMGC to KMBS
at TAF issuance. Ceiling observations within the frontal band range
from MVFR to low VFR alongside isolated showers. Larger degree of
solar insolation compared to upstream observations favors the
optimistic VFR forecast locally until the front clears to our east by
00z. Decreasing cloud cover to follow overnight. Modest instability
will struggle to generate deep convection this afternoon with dry
subsident air working in aloft, so convective initiation will be
limited. Still, there is a low (~20%) chance for a pulse- type
thunderstorm to impact one of the Detroit terminal sites between
20z-00z. Southwest winds subside below 7 knots tonight with a 30-60
degree shift to the west-northwest post-front. Just some mid- high
cloud lingers for daytime Thursday along with light west- southwest
winds.

For DTW/D21 Convection...There is a low (~20%) chance of a pulse-
type thunderstorm to impact the airspace between 20z and 00z. Storm
motion will be to the east-northeast at around 30 knots.

DTW THRESHOLD PROBABILITIES...

* Moderate for intervals of 5000 foot or less ceilings before 00z.
  Low tonight and Thursday.

* Low for a pulse thunderstorm between 20z and 00z.

&&

.PREV DISCUSSION...
Issued at 328 AM EDT Wed Jul 3 2024

DISCUSSION...

SE MI is residing within the warm sector of a low pressure lifting
into the northern Great Lakes this morning which has led to
dewpoints creeping back into the 60s. This moisture advection
continues through the afternoon as the theta-e plume draped along
the Mississippi is partially drawn into the region pushing Td`s near
or just above 70F, particularly south of M-59. Rain chances
initially are confined to the Saginaw Valley-Thumb area as decaying
convective remnants currently over northern IL/southern Lake Mi lift
northeast early this morning. An additional window for a couple
(thunder)showers comes late morning-early afternoon for these areas
as the "cold" frontal boundary starts to sag into southern lower MI.
System relative flow has a large parallel component to the SW-NE
oriented front greatly reducing low level convergence along it with
the end result expected to be a fairly sparse coverage of widely
scattered showers. Earlier timing also puts an upper bound on
potential CAPE available (likely around 500 J/kg at best) keeping
any thunder more isolated in nature. These chances shift south and
east through the rest of the afternoon, reaching the Detroit Metro
area after ~19Z. Even with the later/slightly more favorable timing,
CAMs continue to struggle to generate much in the way of convection
in advance of the boundary likely owing to ample cloud cover that
will be in place reducing insolation and subsequent greater
instability. Should frequent/large enough breaks in this cloud occur
however, it wouldn`t take long to fully realize the ~1000 J/kg
MLCAPE potential that the area is residing beneath. There is just
enough bulk shear (25-30kts) that could support an isolated stronger
cell with damaging wind gusts being the potential hazard (again
assuming we see enough breaks to get initiation). Overall chances of
this are low however with better potential focused to our east as
noted in the new Day 1 SPC outlook.

Frontal boundary pushes east of the region late evening, after ~23Z
ending any further rain chances. Only marginally cooler air follows
this front (850mb temps only fall by 1-3C) with the main impact
instead shunted the theta-e plume and associated upper 60-lower 70
dewpoints back into Ohio and Indiana. Short range model solutions
still suggest this frontal boundary holds south enough of the state
border to keep additional convective development likewise confined
south of the CWA as a series of shortwaves ride along it daytime
Thursday. This outcome offers lesser cloud cover and more sun
pushing highs back into the mid to upper 80s, despite the very
modest cooling aloft. Worth noting there is some signal for diffuse
ripples of vorticity embedded within the upper jet that is progged
to be directly over lower MI to spark a brief isolated showers late
afternoon-evening. That said, low confidence in this scenario so
have opted to keep dry forecast (sub 15% PoPs) running in this
update.

Active pattern doesn`t relent for the late week period as another
surface low develops over the north-central Plains before lifting
towards the Straits Friday into Saturday. Respectable moisture
transport accompanies this system as it wraps in lingering Gulf
moisture from the mid-Mississippi pushing PW values over 1.5" and
surface dewpoints near 70f again. Occluding front looks to lift
through SE MI centered around the afternoon hours, which in
combination with aforementioned moisture-rich environment, should
widespread coverage of showers and embedded thunderstorms.

MARINE...

A cold front will move over the Great Lakes this morning and
afternoon which will veer wind direction to the west-southwest by
the afternoon.  There will be a window between 4AM-4PM today where
gust potential will near 25 knots across the Saginaw Bay with the
southwest flow, but will preclude the issuance of a Small Craft
Advisory as model soundings exhibit a stable profile, lowering
confidence of 25 knot gust potential for any 3+ hour window. There
will be a low chance for a shower or storm along the front. High
pressure will fill back in tomorrow, bringing light winds through
the day.

&&

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

$$

AVIATION.....MV
DISCUSSION...KDK
MARINE.......AM


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at www.weather.gov/detroit.