-- Discussion containing changed information from previous version are highlighted. --
643
FXUS63 KGRR 051436
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1036 AM EDT Wed Jun 5 2024

.KEY MESSAGES...

- Several Rounds of Showers/Storms Today and Thursday

- Cooler With Some Showers for the Weekend

&&

.UPDATE...
-- Changed Discussion --
Issued at 1023 AM EDT Wed Jun 5 2024

No real changes to the going forecast. We`re still expecting
another round of storms to develop early this afternoon ahead of
the cold front. Mesoanalysis shows 1500 j/kg SBCAPE in place
across the western half of the cwa and this is progd to increase
to 2k j/k farther east this afternoon. Precipitable water values
approaching 2 inches will result in a heavy rain threat if some of
the echos begin to train.

Wind remains the main threat this afternoon with hail a secondary
concern. As the left exit region of the upper jet impinges on the
cwa this afternoon, we`ll see storms quickly develop. Shear values
around 35 kts will enable a few of the storms to organize and
become strong to severe. The cold front should be east of the cwa
by 00z, ending the severe threat.

We also issued a Small Craft Advisory and Beach Hazards Statement for
the entire nearshore waters from midnight tonight through mid
afternoon Friday. It`s worth noting that north of Whitehall today,
the southwest wind flow will result in waves up to 4 feet for a
few hours before the front moves through.
-- End Changed Discussion --

&&

.DISCUSSION...
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.

- Cooler with some showers for the weekend

The models are in decent agreement in keeping unseasonably deep mid
level low over the Great Lakes Region this weekend.  This feature
alone will act to keep condition on the cooler than normal side.
Model ensemble max temps do show a fair amount of  spread in the
values for Friday into Sunday, but overall they remain below normal
and we will keep it that way.  As for the precipitation, a digging
upper level wave arrives at some point Saturday into Sunday. This
feature will generate some added lift and model trends are
suggesting we may see an increased potential for some showers when
this feature arrives. However this feature may be lacking moisture
with the Gulf moisture cut off, so for now we will feature POPs no
higher than chance.  With a cold pool aloft, the afternoon heating
may enhance the precipitation potential, mainly Saturday into Sunday.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 653 AM EDT Wed Jun 5 2024

Presently an cluster of showers is moving across West Michigan with
brief drops to MVFR visbys possible. Have removed TSRA from the
TAFs before 15z based on latest guidance and radar trends that
suggest the southern portion of the complex will mostly dissipate
before reaching LAN. MVFR cigs spread into the area this morning
as increasing low- level moisture takes hold, becoming IFR for a
time at MKG. After 15z, a broken line of storms develops and moves
eastward through the afternoon as the atmosphere remains
unstable. Given continued confidence in coverage and convergence
on timing, have added a TEMPO group to highlight the best window
for storms at each terminal. Any stronger storms could feature
gusts over 40 knots, with the best chance at LAN/JXN/BTL.
Confidence in these conditions is to low to include in the TAF.
MVFR will be prevailing this afternoon with brief drops to IFR
possible in any heavier thunderstorms.

Additional showers and storms may develop behind the main line,
however coverage is too sparse to go with more than VCSH. Most
sites will see a break in convection. Cigs also improve to VFR
behind the line. Finally, additional scattered showers are
expected to develop after 04z near MKG and spread eastward as they
fall apart through 10z. Have included VCSH at AZO/BTL/MKG/GRR
where confidence is higher in showers, and a PROB30 group at
LAN/JXN as conditions will depend on showers holding together.
Isolated thunderstorms also cannot be ruled out tonight.

&&

.MARINE...
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.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
MI...Beach Hazards Statement from late tonight through Friday
     afternoon for MIZ037-043-050-056-064-071.
MARINE...Small Craft Advisory from midnight tonight to 4 PM EDT Friday
     for LMZ844>849.
-- End Changed Discussion --

&&

$$

UPDATE...04
DISCUSSION...MJS/Thomas
AVIATION...Thomas
MARINE...Thomas