450
FXUS63 KDTX 171040
AFDDTX

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

.KEY MESSAGES...

* Slow moving low pressure system drifts over lower Michigan this
  weekend supporting showery and unsettled weather.

* Greatest coverage, consisting of numerous showers and scattered
  thunderstorms, will be focused during the daylight hours each day
  with lesser rain chances overnight.

* Dry conditions return by Monday with below normal temperatures
  through at least the first half of the work week. Breezier
  conditions can also be expected Monday as well.

&&

.AVIATION...

Showers associated with an incoming vorticity center will dot areas
from KPTK north early this morning with additional activity firing
this afternoon with some isolated thunderstorm potential. Ceilings
will jump around a bit this morning, but settle into MVFR mainly and
possible IFR overnight tonight at times. Southwesterly flow will
veer to the west tonight and eventually northwest late tonight into
Sunday morning.

For DTW/D21 Convection...There is another minor chance for
thunderstorms after 19z, but mainly expect SCT-BKN coverage showers.

DTW THRESHOLD PROBABILITIES...

*  Low for thunderstorms this afternoon.

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

&&

.PREV DISCUSSION...
Issued at 308 AM EDT Sat Aug 17 2024

DISCUSSION...

Overnight broken line of showers and isolated embedded thunder
finishes crossing SE MI early this morning as the parent mature,
closed low drifts into lower MI. A brief break in rain chances then
follows for much of this morning as associated vort max swings into
southern Ontario. Lapse rates steepen by late morning as the mid-
level thermal trough axis reaches the area, which in combination with
broader background upper divergence, supports the generation of
scattered showers. Coverage is expected to increase through the
afternoon as daytime heating leads to several hundred J/kg of MLCAPE,
which also offers isolated to scattered thunder potential as well.
Column wind profiles remain weak, largely aob 20kts, precluding any
severe potential from any storm that manages to develop. Loss of
diurnal heating by the latter part of this evening will lead to a
significant reduction in shower coverage with most locales likely
staying dry all night, particularly in the south. Exception to this
is areas along/north of I-69 where widely scattered showers look to
linger due to the closer proximity to the low center/vort max that
will be sliding over central lower MI.

Similar day for Sunday as the low center will have only reached
southern Lake Huron by that point. Greatest persistence and coverage
of showers likely resides over the Thumb given the low positioning
though daytime heating will once again support at least scattered
showers and isolated thunder over the rest of SE MI (if not
greater). Coverage once again decreases Sunday evening with rain
chances completely ending by early Monday morning as low pressure
finally pushes into the Northeast and height rises build back into
southern lower MI.

Upper troughing lingers over the central Great Lakes Monday allowing
northerly flow in the wake of the low to draw a cooler Canadian
airmass south into our area. 850mb temps fall to around 10C, give or
take a degree, limiting high temps to the lower 70s with upper 60s
possible in the Thumb. Improved mixing depths within this cold
advective regime are able to partially tap into the 25-30kt flow
aloft to support a breezier day with gusts generally in the 20-25mph
range across SE MI; though stronger gusts around 30mph will be
possible, especially over the Thumb.

Upper midwestern surface high pressure becomes fully established by
Tuesday bringing weaker, though still north/northeasterly, winds for
the midweek period. Upper trough is slow to depart however resulting
in highs holding below average in the lower to mid 70s for this
timeframe. Moderation in temps back to around normal looks to occur
by late week as upper ridging over the central CONUS gradually
creeps into the Great Lakes.

MARINE...

First of several rounds of showers and thunderstorms is underway at
issuance as the upper low inches eastward this morning. The low is
expected to track directly overhead today, leading to more numerous
coverage of showers and storms especially as daytime heating begins.
A few storms could produce strong downdrafts, supporting localized
potential for thunderstorm gusts to exceed 35 knots. On the broader
scale light southerly prevails for most of the day, shifting to the
north late tonight as the low exits into Ontario. Northwest flow
increases on Sunday as the western flank of the pressure gradient
gains influence. Gusts increase further Sunday night-Monday as a 35
knot jet wraps around the low, which in combination with onshore
flow will likely necessitate Small Craft Advisories early next week.
A quieter pattern returns by mid-week as high pressure builds into
the Great Lakes.

HYDROLOGY...

A broad low pressure system moves very slowly across Lower Michigan
this weekend. This system will support periods of numerous showers
and scattered thunderstorms with each day seeing an uptick in
coverage during the daylight hours. Total additional forecast
rainfall between this morning and Sunday evening will range from a
general 0.25-0.5" south of I-69 to 0.5-1" north. Localized higher
amounts possible dependent on strength and frequency of
thunderstorms. Localized minor flooding of urban, low-lying, or
otherwise flood prone areas remains possible from 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.....DG
DISCUSSION...KDK
MARINE.......MV
HYDROLOGY....KDK


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