801
FXUS63 KDTX 121939
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
339 PM EDT Fri Jul 12 2024

.KEY MESSAGES...

- Widely scattered showers and storms will continue across Southeast
Michigan through the early evening. These storms are not expected to
be severe but may produce isolated gusty winds and small hail.

- There will be several windows for thunderstorms Sunday through
Tuesday. Severe weather and heavy rainfall will be possible with
these systems but with relatively low confidence on timing and
location.

- A period of hot and humid conditions is forecasted for Sunday and
Monday with temperatures reaching near 90 and heat index in the mid
90s.

- Cooler, drier weather will move in Wednesday through the rest of
the week.

&&

.DISCUSSION...

The main lobe of PV advection associated with a compact mid-level
wave is departing to the east this afternoon, leaving generally weak
forcing for additional showers and thunderstorms into the early
evening. Any additional convection will be driven primarily by
smaller scale processes along the lake breeze and any other surface
heating boundaries and will continue to carry lower confidence on
placement and timing. Based on the current satellite view, the
agitated cumulus across the northern half of the CWA will be the
favored region in the short term. Will hold on to the chance (~30%)
PoP through peak heating with expectation for convection to wind
down after sunset. Severe weather is not likely given MLCAPE on the
order of 1000 J/kg or less and 0-6km bulk shear around 25 to 30 kt
per latest SPC mesoscale analysis, but gusty winds to 40 mph and
small hail will be possible in any stronger cores. Quiet conditions
expected tonight with enough boundary layer moisture to support some
fog development early tomorrow morning given nearly calm surface
winds.

Broad surface high pressure sticks around into Saturday as a mid-
level shortwave ridge induces deep layer subsidence overhead. This
favors sunny skies (apart from a diurnal cu field) and a cap to
limit convective chances, so will keep the forecast dry. The ridge
axis will hold just to our east, so burgeoning west to southwest
flow will be able to advect in a slightly warmer air mass. With the
boost from diabatic heating, the result will be high temps in the
mid to upper 80s. Heat index will likely approach 90 in the
afternoon as dew points creep into the upper 60s.

Sunday continues to present a good chance of more active weather as
the ridge moves out and more energetic northwest flow begins to
settle in. This pattern shift causes a warm front to move into the
region Sunday morning with the overall setup suggesting a much
warmer, muggier, and more unstable air mass should build in through
the day. Timing of convection in this regime will carry inherent
uncertainty dependent on upstream convective trends that cascade
along the instability gradient, but latest guidance including hi-res
runs continues to hone in on thunderstorms developing over northern
Minnesota Saturday evening and tracking southeast into Lower
Michigan Sunday morning. Various scenarios exist regarding timing
and subsequent placement of the CAPE gradient for this window: 1)
Faster progression of the MCS would likely outpace the instability
resulting in a line of decaying storms/showers early Sunday morning.
See 12z HRRR/ARW/NSSL. 2) Slower progression of the MCS allows CAPE
to build in ahead of it which presents a higher severe threat mid-
late morning. See 12z NAM3km/FV3.

Considerable uncertainty follows for the rest of Sunday`s precip and
temperature forecast given the moving parts with the morning
convection and subsequent recovery of instability/forcing. Overall
flavor of 12z guidance does hint at a relatively lower chance of
later day storms which is plausible considering any robust morning
MCS would likely leave a wake subsidence bubble. With 850mb temps
reaching the upper teens C, potential is there for high temps around
90. Meanwhile, dew points exceed 70 degrees in virtually all NBM
members so oppressive humidity looks likely to settle in.

Lower forecast confidence exists on Monday as well as we continue to
sit on the eastern extent of the Plains/Corn Belt warm and humid air
mass beneath an active northwest upper jet pattern. Specifics will
again be dictated by evolution of upstream convection but a chance
(30-50%) chance of showers and thunderstorms continues to be
warranted given the unstable environment with ripples of PV energy
traversing through the flow. 850mb temps do take a step up to 20 C
on Monday which supports highs in the lower 90s with continued high
humidity.

A potent shortwave tracks across the Canadian Prairie on Monday
before swinging across the northern Great Lakes early Tuesday. This
sends a cold front across the region and commences a period of
deeper troughing that allows relief from the heat and humidity
Wednesday through the late week. Tuesday still looks to be another
hot, muggy, and stormy day before the cold front clears the area.
Highs in the 70s to lower 80s and mainly dry conditions follow as
high pressure settles in.

&&

.MARINE...

High pressure area centering near the eastern Great Lakes today
maintains light winds, generally out of the southeast. Isolated to
widely scattered thunderstorms have developed over Lower Michigan
this afternoon with shear vectors favoring some propagation into
Lake Huron through this evening. Locally higher winds/waves are
possible along with pockets of hail in excess of 0.75 inches with
any stronger storms. The ridge migrates further east Saturday while
a secondary, and more diffuse, area of high pressure lifts toward
the Tri-State area offering continued weak gradient flow and gusts
AOB 15 knots. Storm-free conditions are forecast Saturday. Marginal
Small Craft Advisory conditions will be possible Sunday afternoon as
flow shifts southwesterly over Saginaw Bay and peak gusts approach
25 knots. Broader coverage of showers/storms to follow Sunday and
Monday with the arrival of a warm front and approaching upper level
trough from the west. Stronger storms are again possible both days
which could pose a heightened risk for locally higher winds/waves
and some hail.

&&

.HYDROLOGY...

Several thunderstorm complexes may move through the region Sunday
through Tuesday as a hot and humid air mass builds in. The first
system is becoming increasingly likely to move through Sunday
morning and may produce heavy rainfall rates of over 1 inch per hour
for a brief period. Confidence is low on specific timing and
location at this time but given recent heavy rainfall, areas of
minor flooding will be possible.

&&

.PREV DISCUSSION...
Issued at 1239 PM EDT Fri Jul 12 2024

AVIATION...

A compact mid level circulation centered over the KMBS region around
noon will exit to the northeast across Lake Huron this afternoon.
Ongoing diurnal destabilization will support weak to moderate
instability this afternoon, mainly from FNT north. Model soundings
do however suggest some degree of warming in the mid levels in the
wake of this short wave, most notably across metro Detroit. Model
soundings in the vicinity of KFNT and KMBS this afternoon still look
to be relatively uncapped. While this may result in scattered to
isolated late day convection, the chances at the terminals are low
and will therefore be left out of the TAF.A weak gradient and mainly
clear skies tonight with ample boundary layer moisture will be
supportive of radiational fog development overnight/early Sat
morning.

For DTW/D21 Convection...Latest model soundings show a little better
mid level warming during the course of the afternoon across metro
Detroit. Developing east-southeast winds are also expected to push
the marine layer inland. This and early day cloud cover should limit
instability. Therefore late afternoon/evening convection across the
metro airspace should be rather isolated.

DTW THRESHOLD PROBABILITIES...

* Low for thunderstorms this afternoon and evening.


&&

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

$$

DISCUSSION...TF
MARINE.......KGK
HYDROLOGY....TF
AVIATION.....SC


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