391
FXUS63 KDTX 151957
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
357 PM EDT Thu Aug 15 2024

.KEY MESSAGES...

* Showers increase coverage this evening becoming widespread with a
  rumble of thunder possible late tonight through Friday morning.

* A short break occurs late morning and early Friday afternoon
  followed by another round of thunderstorms from mid afternoon
  through 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.

&&

.DISCUSSION...

A well organized but slow moving low pressure system tracks through
the Midwest tonight while spreading showers a few thunderstorms
across Lower Mi into Friday morning. This occurs along and ahead of
a warm front that is still west and south of SE Mi in the morning.
The warm front does make some northward progress by afternoon before
occlusion allowing a warm sector instability profile a chance to
build in during peak afternoon heating. Mid afternoon to mid Friday
evening then holds as the primary time window for severe
thunderstorm potential. The Day 2 Marginal Risk remains in place in
the afternoon update.

Early to mid afternoon observations support model depictions of the
mid level system becoming closed over the surface low center in
northern MN by this evening. The mid level moisture axis extends
from the Gulf coast northward and becomes sharply defined into the
eastern flank of the MN system. Veered low to mid level wind then
sets up an effective pattern of moisture transport that is already
underway around the 700 mb level but which reloads and deepens into
Lower Mi tonight. This is the primary forcing for showers and
possibly a rumble of thunder, although with a minimal amount of
elevated instability indicated in model data. HREF mean surface
based CAPE is zero through 15Z Friday, as expected with the front to
our south, and MUCAPE holds below 1000 J/kg as an elevated
instability indication. This matches up with some old school
deterministic 850 mb LI right around zero within the Theta-E ridge.

Progression of the surface occlusion draws the warm front up into
central Lower Mi Friday afternoon. The warm sector then briefly
covers the area with good timing for a mid afternoon boost in
surface based instability. Consensus of deterministic and HREF CAPE
rises into the 1500-2000 J/kg range by 21Z but is strongly dependent
on adequate recovery from morning rain activity of generous coverage
and duration. There does appear to be a respectable decreasing cloud
trend possible during early afternoon judging by mid level model RH
fields suggesting the forecast be open to at least low end recovery
of instability. The surface frontal features then become a good
focus for convergence just as difluent flow crosses into Lower Mi
ahead of the mid level circulation. The limiting factor for severe
intensity storms may end up being the low end wind profile that
barely reaches 30 knots through 500 mb during late afternoon into
Friday evening. Mixed pulse/multicell storm mode in an otherwise
moist vertical profile favors locally heavy rainfall with a rain
loaded borderline severe wind gust as the primary hazards.

The mature closed low pressure system continues to occlude and cut
off Friday night. A mid level dry slot is projected across model
solutions which brings a few hours of dry weather later Friday
night. Moisture wraps around the center of the mid level low by
Saturday morning setting up a textbook closed low/cold core shower
pattern. Coverage is modulated by the diurnal cycle and a
thunderstorm is also possible during peak heating Saturday
afternoon. The stalled system sets up a repeat shower/storm
performance for Sunday and temperatures hover a few degrees below
normal throughout the weekend.

&&

.MARINE...

Leading wave of showers has already started to impact inland
portions of lower Michigan at issuance, and will gradually expand
over the waterways this evening-overnight as elevated portions of a
warm front pivot into the Great Lakes. Not seeing much instability
with this first band of precipitation, limiting widespread thunder
chances until daytime Friday. Wind gusts approach 25 knots overnight-
Friday morning, but frequent gusts and wave heights fall short of
advisory thresholds. Several additional rounds of showers and
thunderstorms will be possible Friday and Friday night ahead of a
slow-moving upper low. There is a chance that storms Friday
afternoon-evening could be strong, producing localized wind gusts in
excess of 35 knots along with lightning and heavy rain. Shower and
thunderstorm chances last through the weekend as the low drifts
slowly overhead, eventually shifting southerly winds to the north by
early Sunday morning. Onshore flow and another uptick in wind speeds
are then possible early next week in the wake of the low.

&&

.HYDROLOGY...

Showers increase coverage tonight with a rumble of thunder possible
late tonight and Friday morning. Storms become likely again from mid
afternoon into early Friday evening, and this is when flooding has
greater potential as storms will be capable of locally heavy
rainfall. A broad low pressure system then stalls over Lower
Michigan Friday night through the weekend. Total Rainfall over the 3
day stretch is expected to be near 1 inch, but any strong
thunderstorms could produce enhanced totals around 2 inches.
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.

&&

.PREV DISCUSSION...
Issued at 155 PM EDT Thu Aug 15 2024

AVIATION...

VFR conditions will prevail as coverage of high clouds increase into
the evening. The increasing mid level moisture will also bring
increasing rain chances this evening and tonight while some degree
of low level dry air holds. Expecting conditions to remain
predominately VFR with FEW-SCT cloud bases below 5,000 feet, though
any heavier shower may bring a brief period below VFR. Covered the
more likely time frames for pockets of rainfall with TEMPO groups
between 00Z and 09Z with potential for a break in activity early
tomorrow morning before an uptick in shower activity occurs again
around 12Z and beyond. Isolated thunderstorms will be possible, but
low probability precludes mention at this time.

For DTW/D21 Convection...Low probability for an isolated
thunderstorm late tonight and Friday morning. Friday afternoon will
hold a slightly better chance for thunder.

DTW THRESHOLD PROBABILITIES...

*  Low for thunderstorms late tonight and Friday.


&&

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

$$

DISCUSSION...BT
MARINE.......MV
HYDROLOGY....BT
AVIATION.....AA


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