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663
FXUS63 KGRR 050717
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
317 AM EDT Wed Jun 5 2024

.KEY MESSAGES...

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- Several Rounds of Showers/Storms Today and Thursday

- Cooler With Some Showers for the Weekend
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&&

.DISCUSSION...
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Issued at 314 AM EDT Wed Jun 5 2024

- Several Rounds of Showers/Storms Today and Thursday

The first round of showers and storms is associated with a vorticity
maximum visible on Satellite near Chicago that will move into lower
Michigan in the next few hours. Rain will arrive by 4 am across
the southwest part of the CWA. That lifts northeast with the wave
providing morning showers, and some embedded thunderstorms given
elevated instability. Strong MLCIN and no shear means these storms
will be garden variety thunderstorms.

Attention then turns to the second round of storms associated with a
shortwave driving a cold-frontal boundary through the CWA today.
The question is how much does instability redevelop behind the
morning storms given cloud cover and marginal mid-level lapse
rates. However, consensus does suggest the area from Lansing to
Marshall and southeast (including Jackson) may develop around 1000
J/kg of CAPE by late morning and combined with 25-40 knots of
0-3km shear supports a chance of a few strong to severe storms.
Primary concern would be for water- loaded downdrafts given PWATS
climbing north of 1.5 inches, and isolated hail to around quarter
size can`t be ruled out in any severe storms. The other concern is
for locally heavy rainfall and flooding. Cloud-layer winds
suggest northeasterly storm motion along the front with high PWATS
and a deep warm cloud layer. This suggests storms will be
efficient rain producers, and there is a potential for several
storms to train over an area. If training occurs, localized areas
could see 1-2+ inches of rainfall, a scenario supported by high-
resolution guidance. These thoughts align well with the current
SPC and WPC marginal risks.

Round two ends by 22z-00z as the front exits the area. Round
three comes after midnight tonight as another shortwave/vorticity
maxima is driven across the CWA triggering scattered showers.
MUCAPE around 500 J/kg supports an isolated thunder threat with
these showers. Then the main trough and associated cold pool move
overhead Thursday with 500 mb temperatures falling to -15C to
-20C. This should be enough to trigger isolated to scattered
diurnal instability showers. The best chance for this will be
across Central Lower Michigan where the heart of the 500mb cold
pool resides. Highs fall from near 80 today to the low 70s
Thursday.
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.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 133 AM EDT Wed Jun 5 2024

It`s looking like a stormy pattern for the TAFs through the
period. Convection upstream around Nrn IL into NW IN it headed
northeast towards the TAF sites. While some decrease in the
convection may occur, it`s looking more likely that TS will be
observed prior to 12z. This initial batch of convection should
push east of TAF sites by mid morning, however the atmosphere
remains unstable and high res models are regenerating new
convection by midday. There is some uncertainty on the timing and
overall coverage of this next round of storms but trends are
supporting greater coverage. So the TAF sites will feature another
period of storm with VCTS in that window. While another break in
the convection is likely by mid afternoon, the cold front will
still be off to the west, and will be headed east. The atmosphere
will remain unstable until this feature pushes through, not until
toward the end of the period, 06z Thu. It`s possible that another
batch of convection could roll in ahead of the front. Any stronger
storm through the period could contain gusts over 30 knots.

Considerable moisture will be lifting northward into the TAF
sites today. As a result, ceilings will be lowering, and MVFR
levels look likely by daybreak with a potential for IFR.
Visibilities could lower briefly down into the IFR range if a
heavier cell tracks over a TAF site.

&&

.MARINE...
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Issued at 314 AM EDT Wed Jun 5 2024

A round of showers and embedded thunderstorms is moving across the
lake this morning, with additional showers and storms later today
ahead of a cold front. Locally higher winds and waves are
possible in any thunderstorms. This cold front causes our current
southerly winds to become westerly. Behind the front (after 06z
Thursday), winds become gusty increasing to around 25 knots. A
Small Craft Advisory and Beach Hazards Statement will likely be
needed for these winds and their associated waves, however will
defer decisions to the day shift given we are around 24 hours out.
Additional headlines may be needed for the next few days after as
cold air keeps winds and waves elevated.
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&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

DISCUSSION...MJS/Thomas
AVIATION...MJS
MARINE...Thomas