146
FXUS63 KDTX 170405
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1205 AM EDT Sat Aug 17 2024

.KEY MESSAGES...

*  Isolated to scattered thunderstorms possible this afternoon and
   early evening. This activity will be capable of heavy rainfall
   and an isolated storm could produce damaging wind.

* Broad coverage of low pressure stalls across Lower Michigan this
  weekend. The system supports showery and unsettled weather with
  temperatures on the cool side of normal through Sunday.

&&

.AVIATION...

Organized convergence associated with parent vorticity maximum will
result in Wisconsin thunderstorm activity to push into the Saginaw
Valley and northern Thumb region tonight. Tail of the vorticity
forcing with the best low-mid thetae advection will also result in
chances for showers and thunderstorms at the Detroit terminals
between 08-11Z. Relatively quiet conditions are expected across much
of the area for a brief time Saturday morning before a longer
duration of shower and possibly some scattered thunderstorms
Saturday afternoon. Winds will trend southerly today to westerly
Saturday.

For DTW/D21 Convection...Chance for thunderstorms exists 08-11Z
Saturday morning. Included a prevailing for showers after 19Z
Saturday.

DTW THRESHOLD PROBABILITIES...

*   Medium for thunderstorms early this morning. Increasing
    potential Saturday afternoon but low confidence.

*  High for cigs at or below 5000 ft this tonight and Saturday.

&&

.PREV DISCUSSION...
Issued at 1046 PM EDT Fri Aug 16 2024

UPDATE...

Updated the gridded forecast to increase PoPs overnight (60-70%
range) out of respect for the line of showers and embedded
thunderstorms now pushing onshore of western Lower Michigan. Model
signal exists for well organized 925-850mb convergence tracking
through the northern cwa north of I 69 after 09Z associated with the
parent vorticity max. The best 900-700mb thetae advection response
appears set for areas south of I 69 including metro Detroit for the
same time period. Mesoanalysis and forecast soundings suggest
MUCAPES of over 1000 J/kg is likely tonight which will support
thunderstorm activity. A skinny CAPE profile and a forecasted stable
layer between 2.5 and 4.0 kft agl will limit the strong or severe
thunderstorm threat. Southeast Michigan has a General Thunderstorm
designation in the SWODY1 outlook.

PREV DISCUSSION...
Issued at 344 PM EDT Fri Aug 16 2024

DISCUSSION...

An occluded frontal system ahead of a mid level closed low will
progress across Michigan through the remainder of the day. The early
day showers with isolated embedded thunderstorms has worked to
stabilize the atmosphere early this afternoon bringing a period of
dry conditions and some sunshine. The big question coming into today
was how much instability we were able to build back into southeast
Michigan for the mid-late afternoon and early evening period.
Earlier day satellite showed an area of clearing across southwest
Michigan in the relatively small warm sector zone ahead of an
advancing weak cold front and an area of sustained convection that
developed near the triple point in west central Michigan just off the
shoreline around 15-16Z. So far, the west central Michigan activity
has been weakening and isolated showers near the I-69/I-94
intersection have struggled to gain any strength thus far. If an
isolated strong to severe thunderstorm weather threat is to
materialize, it will likely be within the 20Z to 01Z window and be
focused more across the southern portions of the CWA in the better
warm sector air and instability (~500-1000 J/kg MLCAPE per RAP).
Other environmental conditions are not all that impressive though.
Left exit region upper jet dynamics help a little with larger scale
ascent, but lower level flow will be fairly weak with 850-700 mb
winds of largely less than 20 knots. This yields effective shear
values of around 20 knots and mid-level lapse rates will be weak at
5.5 C/km per SPC Mesoanalysis. Also of note are that many of the
latest CAMs are offering limited convection during the best
destabilization time. All that said, a Marginal Risk remains in place
today for isolated strong to possibly severe thunderstorms that can
develop and maintain strength across Lower Michigan within the higher
humidity airmass. Strong downburst winds will be the main threat
along with brief heavy downpours supported by dewpoints in the upper
60s to low 70s.

Any evening convection wanes with the loss of daytime heating
leading to a brief period of dry conditions through about 06Z when
the stacked low pressure system moves into central Lower Michigan
offering renewed scattered showers and isolated thunder. Highest
PoPs focused more towards the Saginaw Valley closer to the surface
low pressure. The stacked occluded low will linger over the region
this weekend bringing continued rainfall. Saturday will see a
diurnal uptick in shower and isolated thunderstorm activity, but
even less shear expected over the weekend will preclude any mention
of organized thunderstorms. Isolated to scattered precipitation
lingers Saturday night as the low slowly moves east into Ontario.
Northwest flow then takes hold for Sunday, but shortwaves rotating
around the low will maintain precipitation chances through Sunday.
Diurnal heating Sunday afternoon will again bring potential for the
diurnal increase in coverage of showers and general thunderstorms.
Average rainfall totals over the course of this afternoon through
Sunday will be up to 1.00-1.50 inches with the highest totals
focused north of I-69.

Gusty conditions pick up Sunday night and Monday as the northwest to
north flow ushers in cooler air aloft and mixing depths tap into 25-
30 knots of flow. Wind gusts during this time will be largely
between 20-25 mph, but gusts to around 30 mph will be possible
across the Thumb. High pressure will build into the Midwest bringing
dry conditions by Monday with daytime highs on the cooler side of
normal in the low to mid 70s. Similar weather conditions with mainly
weaker winds will carry into the mid-week period as the high
pressure tracks across the Great Lakes.

MARINE...

Ongoing band of showers will continue to impact Lake St. Clair and
the majority of Lake Huron for the next couple of hours. Behind this
band is a corridor of clearing, although new showers and storms have
started popping up near the warm front in SW lower Michigan. So far
these cells have struggled to hold together for very long, but with
clearing skies and increasing instability there is still a chance
(30-40%) that a stronger storm may impact the waters after about 00z
(8pm EDT) tonight. Stacked low pressure system responsible for
today`s unsettled weather will remain in control through the
weekend, maintaining shower and thunderstorm chances for both
Saturday and Sunday. The surface low passes through early Sunday
morning to shift winds from southerly to northerly. Onshore wind
direction and a belt of enhanced low level winds will increase winds
and wave heights toward Small Craft Advisory thresholds Sunday night
and Monday. Quiet and dry weather is then anticipated for middle of
next week.

HYDROLOGY...

A humid airmass will support locally heavy rainfall with any shower
or thunderstorm that is able to develop and maintain strength across
Lower Michigan this afternoon and early evening. Activity is
trending towards being very isolated, but any storm that can
maintain any strength could be capable of a quick half inch to an
inch of rainfall.

A broad stacked low pressure system moves very slowly across Lower
Michigan during the weekend. This system will support several
periods of numerous showers and scattered thunderstorms with each
day seeing an uptick in coverage during the daylight hours. Total
rainfall from this afternoon through Sunday evening is expected to
be 1-1.5 inches on average and focused mainly north of I-69.
Localized totals between 2-3 inches possible dependent on strength
and frequency of thunderstorms. Localized minor flooding of urban,
low-lying, and otherwise flood prone areas remains possible in the
strong thunderstorms, but the off and on nature of the rainfall is
not expected to cause significant flooding.

&&

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

$$

AVIATION.....CB
UPDATE.......CB
DISCUSSION...AA
MARINE.......MV
HYDROLOGY....AA


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at www.weather.gov/detroit.