544
FXUS63 KDTX 051816
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
216 PM EDT Fri Jul 5 2024

.KEY MESSAGES...

- A Low pressure system arrives over lower Michigan this afternoon
  and evening. This will bring scattered to numerous showers with
  embedded thunderstorms, with highest coverage favored over the
  Tri- Cities and Thumb.

- There is a Marginal Risk of severe thunderstorms this afternoon
  and evening for all of SE MI.

- The low pressure system lingers over the Great Lakes tomorrow
  keeping a low chance (25%) of showers and thunderstorms in the
  forecast.

- The holiday weekend closes out with dry weather Sunday under
  partly sunny sky.

&&

.AVIATION...

Ample amount of shear and CAPE exist to sustain scattered
thunderstorms this afternoon and late evening. Several storms have
already developed INVOF MBS along a convergence boundary while
thunderless showers crop up over the Metro Detroit terminals.
Expectation is that most thunderstorm activity will hold off until
late this evening for FNT south, thus the later TEMPO group
adjustments. Clouds are largely expected to remain high-based, but a
brief BKN high-MVFR ceiling could arise early this afternoon. Light
to modest southwest winds drop-off overnight with some HZ
reintroduced Saturday morning. Potential exists for low-MVFR
ceilings/visibilities overnight, but not confident enough for
inclusion for the 18Z TAF cycle. Borderline MVFR ceilings should
settle in Saturday morning with stratus favored amidst the cooler
post-frontal airmass.

For DTW/D21 Convection...Scattered showers are expected this
afternoon along a pre-frontal trough with storms more likely to
develop this evening ahead of an inbound cold front. Will highlight
the earlier period with -SHRA then target the late evening (01-04Z)
period for thunder. Storms will generally be single cellular or
multicellular with SW-NE storm motion.

DTW THRESHOLD PROBABILITIES...

* Low for thunderstorms between 18Z and 22Z today, then medium
  between 01Z and 04Z this evening.

* Low for ceilings at or below 5000 ft this afternoon then medium
  late tonight into Saturday.

&&

.PREV DISCUSSION...
Issued at 931 AM EDT Fri Jul 5 2024

UPDATE...

Ongoing cluster of showers and scattered thunderstorms extending
from central Lower Mi into northern Lake Mi is tied to a mid level
short wave. An axis of positive mid level vorticity extends from this
short wave south across southern Lake Mi. It is this region of
positive mid level vorticity that will initiate and/or sustain
convection as it moves east across Se Mi this afternoon. Diurnal
destabilization will have some degree of limitation as a result of
areas of mid/high clouds. Current hi res solutions suggest ML CAPE
values ranging anywhere from a a few hundred J/kg across the south to
around 1k J/kg across the Saginaw Valley and thumb region with
afternoon temps in the low 80s and sfc dewpoints in the mid/upper
60s. The 12Z DTX sounding and latest NAM/RAP soundings have some
indication of a shallow mid level stable layer (strongest in the
south). The combination of large scale ascent and diurnal
destabilization should erode this weak cap later today. Steep mid
level lapse rates this afternoon and effective shear values around 35
knots will support some stronger updrafts, capable of producing
locally strong wind gusts (up to 60 MPH) and large hail (up to one
inch in diameter). These factors will continue to warrant an
isolated (marginal) risk of severe thunderstorms today.

While this initial region of ascent will advance across Se Mi mid to
late afternoon, a secondary short wave pivoting around the longer
wavelength trough will rotate across central Lower Mi this evening.
This may allow some convective redevelopment across the Saginaw
Valley and thumb region during the course of the evening, predicated
on the amount of residual instability.

PREV DISCUSSION...
Issued at 337 AM EDT Fri Jul 5 2024

DISCUSSION...

1008 mb low pressure system over southern Wisconsin will expand and
move into lower Michigan through this afternoon and evening. Prior
to the arrival of low pressure, moisture transport magnitude will
increase this morning and afternoon under favorable southwest flow,
increasing PW values to around 1.50 inches and h850 dew points aoa
12C. Isolated to scattered rain showers will initially be possible
through the late morning and early afternoon hours once daytime
heating starts and instability builds. Expansion to numerous showers
with embedded thunderstorms then turns increasingly likely through
the late afternoon and evening hours as the low and associated
triple point arrival over Michigan, coinciding with peak diurnal
heating, where modest CAPE values of 500-800 J/kg will be in place.
Latest model guidance places the low center around the Tri- Cities or
just north with the warm front extending into the Thumb by the
afternoon/early evening. This is where the bulk of the activity will
likely commence and thus where PoPs are greatest (70%). Expansion of
a cold front moves through after 21Z through late tonight, where
increasing shower and storm activity will then be possible for the
remainder of SE MI.

There will be plenty upstream mid and upper-level clouds filtering
in through the day which will not be a favorable factor for
instability. 0-6/1-6km shear around 30-35 knots will help support
some organized convection. The hodographs are quite impressive by 21Z
and are nearly straight through northern lower and portions of the
Tri-Cities and Thumb, with a bulk of strong shear noted in the 6-12
km layer. EL in modeled soundings fall within the 6-12 km layer so a
portion of this shear will not be realized with convection, but
overall it shows strong speed shear through the mid to upper-levels.
Pulse, multicell clusters and/or mini-lines with possible bowing
segments will all be in the realm of potential storm modes noting the
above. Hail to an inch (lower than average freezing levels, straight
hodographs) and/or gusts to 60 mph (wet microbursts) will be the
primary severe threats. SWODY1 highlights a marginal risk for severe
weather across SE MI today, with 2PM - 10 PM as the primary threat
window, noting the modest instability as an inhibiting factor.

Some isolated residual showers will be possible during the overnight
period as a trailing upper-level trough axis swings over the state.
Otherwise, passage of the low pressure system will have pushed the
cold front over SE MI, which will bring a little relief from the
heat as highs peak in the upper 70s tomorrow. A lower end chance
(25%) for a shower or storm will hold through the afternoon and
evening as a weaker shortwave pulses overhead while very modest
instability builds (mainly north of M59). A major inhibiting factor
to organized shower and storms activity will be a building mid-level
cap around h600 that increases in strength through the afternoon.

An brief shortwave ridge with an expanding surface high pressure
system will fill in over Michigan on Sunday limiting precipitation
as highs rise back into the low to mid 80s. A series of upper-level
waves will expand through the Plains through the early week period
with trough amplification commencing into the western portion of the
Midwest. This will draw in warm air downstream across SE MI, with
h850 temperatures peaking around 18C by Monday evening supporting
highs into the mid to upper 80s. Additionally this will mark
increasing chances for showers and thunderstorms by Monday, initially
along a leading shortwave, with increasing potential for showers and
storms on Tuesday as the trough axis moves over the state.

MARINE...

Relatively quiet conditions to start the day as a weak ridge passes
over the region this morning. This is followed by the arrival of a
low pressure system which sends a warm/occluded front eastward
across the central Great Lakes during the daylight hours. Scattered
showers and storms will accompany this system, and isolated storms
may become severe with gusts in excess of 35 knots and hail
possible during the afternoon and evening. The cold front moves
through this evening as the low slowly departs to the east which
will bring an end to much of the convection tonight. Lingering
showers may continue into parts of Saturday as weak troughing holds
over the region. A broad ridge of high pressure then fills in on
Sunday with mainly dry conditions and continued light winds. The
next frontal boundary moves into the Great Lakes on Monday, bringing
another period of unsettled weather to start the week.

HYDROLOGY...

Low pressure moving into the Midwest swings a frontal system through
Lower Michigan during Friday while supporting scattered to numerous
showers and scattered thunderstorms across the area. Locally heavy
rainfall could briefly reach rates of 1 inch per hour while basin
average totals come in closer to 0.25 to 0.75 inch until activity
diminishes and exits eastward toward evening. Highly localized totals
between 1- 2 inches will be possible with repeated thunderstorms,
which will be most likely across the Tri-Cities and/or Thumb. Steady
motion of the system location and location of the higher end rainfall
totals mitigate any serious considerations of flooding, with threats
limited to ponding of water on roads and in areas prone to
collecting standing water.

&&

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

$$

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


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