789
FXUS63 KDTX 041958
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
358 PM EDT Thu Jul 4 2024

.KEY MESSAGES...

- Patchy light rain or sprinkles remain possible this afternoon and
evening, mainly toward the Ohio border, while a heavier shower is
possible toward the Tri Cities and northern Thumb.

- A Low pressure system in the Midwest brings a surge of showers and
thunderstorms across all of Lower Michigan Friday. There is a
Marginal Risk of severe thunderstorms in the afternoon and evening.

- The low pressure system lingers over the Great Lakes Saturday
keeping a chance of showers in the forecast.

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

&&

.DISCUSSION...

Upstream convective complexes changed the mesoscale environment
considerably over the last 24 hours as activity grew upscale into a
well organized MCV now affecting Lower MI this afternoon and
evening. The mid level circulation is weakening with time but
remains capable of light showers across the area through early
evening. Dry air below 700 mb, shown in the 12Z DTX sounding and in
hi-res model soundings, is proving to be an effective obstacle for
greater coverage reaching the ground but is expected to be overcome
in pockets until the mid level MCV circulation moves into Ontario by
mid evening. Hourly mesoanalyses reflect the stabilizing influence
of the high cloud cover limiting MLCAPE to sub 500 J/kg farther
north toward the hybrid surface front/trough/lake breeze boundary
lingering in northern to central Lower MI. An isolated heavier
shower materialized there as of forecast issuance suggesting a
rumble of thunder is possible through late afternoon and early
evening peak heating.

A weak mid level short wave ridge follows passage of the evening MCV
circulation leading to a few hours of dry weather with some decrease
in cloud cover late tonight. A quick transition into the next low
pressure system then begins as the leading edge of mid level
moisture approaches Lower MI Friday morning. Model trends over the
last few forecast cycles indicate slower timing and much less
coverage of showers on the leading edge of weaker/less effective
moisture transport. While this salvages the morning mostly free of
rain in SE MI, it does open the door to greater potential for severe
thunderstorms in the afternoon. The upgraded outlook is mostly out
of respect for the seasonably strong forcing brought by the mid
level wave and upper level jet, and for the potential of surface
based instability capitalizing on the lack of morning activity. It
would not take much to enable overachievement of MUCAPE from sub
1000 J/kg HREF projections into the 1000-1500 J/kg range combined
with the inbound wind profile producing bulk shear in the 40-50 kt
range. The other moving part that may be a negative factor is
occlusion of the surface system which could pinch the instability
axis southward before or during the afternoon to early evening storm
window. As usual, timing will then be everything in the
determination of severe storm outcomes.

The mid level dry slot is projected with good model consensus to
punch through southern Lower MI Friday evening while still shrinking
the deformation pattern from SW to NE into northern Lower MI. This
occurs as the mid level circulation deamplifies rapidly leaving just
a stray shower possible and entry level chance POPs maintained for
the bulk of Saturday. The in-between mid level ridge then remains on
schedule for Saturday night bringing weak high pressure and a chance
to close out the holiday weekend with warm and dry weather through
Sunday.

&&

.MARINE...

A weak trough settles across the central Great Lakes this evening
with isolated showers possible mainly near the shoreline areas.
Otherwise, weak winds and dry conditions prevail through early
Friday. Low pressure arriving from the Midwest on Friday then brings
a more active pattern with scattered to numerous showers with
embedded thunderstorms expected after noon. Isolated storms may
become severe with wind gusts in excess of 35 knots and hail. The
low begins to move out Friday night with winds veering to the
northwest, but lingering troughing holds through Saturday with
additional showers possible. A broad ridge of high pressure then
fills in on Sunday with mainly dry conditions and continued light
winds. The next cold front moves across 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 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 0.5 inch until activity diminishes
and exits eastward toward evening. Steady motion of the system
mitigates any serious concerns with flooding potential limited to
ponding of water on roads and in areas prone to collecting standing
water.

&&

.PREV DISCUSSION...
Issued at 209 PM EDT Thu Jul 4 2024

AVIATION...

Area of showers lifting into far southern Lower Michigan will need
to overcome an abundance of dry air in low-levels before rainfall
has any chance at reaching the surface. Forecast soundings continue
to advertise large dewpoint depressions below the 5 kft AGL level
which suggests an initial virga response. Overall convective picture
appears rather muted for this afternoon/evening, therefore only went
with a TEMPO for showers as lighting should be sparse in areal
coverage and frequency. Initial lake breezes are evident on radar
with limited inland penetration and a lack of parcel activation as
diurnal heating struggles beneath the cloud canopy. Upstream
observational trends, in conjunction with latest CAMs, favor column
moistening and subsequent rainfall lifting northeast by 20Z with
generally restriction-free ceilings and visibilities. Convective
trends will need to be monitored for any necessary amendments
related to updraft overachievement and potential isolated storm
development through the early evening hours once a vicinity trough
moves through the Lower Peninsula over the next several hours.
Conditional on evening rainfall/moistening and nearly SKC conditions,
some fog is possible early Friday morning, thus TEMPO-ed in a brief
MVFR mention. Dry start in-store Friday with increasing VFR clouds.

For DTW/D21 Convection...Showers and perhaps rumble of thunder
possible this afternoon and evening. Low confidence in thunderstorm
development from the current line of showers and with additional
activity from an inbound trough, therefore opted for a -SHRA
mention. Better chances for storms expected Friday afternoon/evening
with scattered activity. Did include a PROB30 for tomorrow.

DTW THRESHOLD PROBABILITIES...

* Low for thunderstorms today from 20-00Z, then medium for Friday
  from 22-00Z.


&&

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

$$

DISCUSSION...BT
MARINE.......TF
HYDROLOGY....BT
AVIATION.....KGK


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