414
FXUS63 KGRR 231857
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
257 PM EDT Sun Jun 23 2024

.KEY MESSAGES...

- Two Rounds of Strong to Severe Storms Possible Tuesday

- Dry Mid Week With Rain Returning Friday

&&

.DISCUSSION...
Issued at 257 PM EDT Sun Jun 23 2024

For tonight and Monday, no changes of note. Some model guidance
brings some brief sprinkles to our northwest forecast area and
adjacent waters, but this is low probability and non-impactful
even if it occurs. Otherwise, we are seeing a gradual erosion in
clouds and this should be hastened this evening by nocturnal
stabilization of the boundary layer.

- Two Rounds of Strong to Severe Storms Possible Tuesday

An active long term period is expected across West Michigan. The
first timeframe of concern will be early Tuesday morning as a
typical "ridge rider" MCS pattern sets up. Thunderstorms across
Minnesota and Wisconsin will grow upscale into an MCS. Consensus
among a majority of the 12z CAMS is this arrives still intact with
500+ J/kg of MUCAPE advecting in ahead of it. Some uncertainty
exists in two key areas that will determine whether damaging winds
affect the CWA. First is the extent of low-level stability. Model
soundings show a notable low-level stable layer which may limit to
what extent winds can mix to the surface. Second is uncertainty as
to how much instability advects in ahead of this feature
determining how quickly the MCS dies. The best chance of any wind
impacts if they occur will be along and northwest of a South Haven
to Clare line. Given the progressive nature of the MCS flooding is
not expected with this round.

A second round of severe storms is possible Tuesday afternoon.
However, certainty remains lower as afternoon convective evolution
will depend on evolution of the morning MCS. Model consensus is that
atmospheric recovery does occur behind the MCS with LREF 50th
percentile CAPE values climbing to 1500 J/kg Tuesday evening. In
addition, the morning MCS activity brings the potential of leaving a
remnant outflow boundary which could serve as a convective trigger.
By late Tuesday afternoon into the evening, a cold frontal boundary
moves through the CWA bringing the potential for strong to severe
storms mainly near and south of I96. The main question with this
round is shear. While a low-level jet will be in place, the mid-
level jet will be displaced to the Northwest limiting deep-layer
shear. However, forecast soundings suggest 25-35 knots of effective
shear which would be enough to support a severe weather risk
combined with the ample CAPE. This will need to be monitored over
the next 48 hours as mesoscale trends become more apparent. The
other concern is excessive rainfall. Mean cloud-layer winds look to
be front-parallel which brings a chance of training storms and
locally heavy rain as a result.

- Dry Mid Week With Rain Returning Friday

Wednesday into Thursday will feature dry weather across West
michigan as approaching surface and mid-level ridging cause dry
conditions and at partly cloudy or better skies across West
Michigan. By Friday into Saturday there is a decent signal across
guidance for chances of showers and storms. Chances begin Friday
associated with warm air advection and the lifting of a warm frontal
boundary across the CWA. A cold frontal boundary causes shower and
thunderstorm chances to continue into Saturday as a cold front
passes through.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 121 PM EDT Sun Jun 23 2024

No significant changes with the 18z TAFs. Widespread MVFR
cigs will lift to VFR over the next few hours as better mixing
takes hold. Cigs become indefinite overnight as high pressure
and dry air moves into the area. There are weak signals for a FEW
to SCT stratus deck to develop along the I94 corridor between 9z
and 12z but have omitted this from the TAF given low confidence.
Skies remain mostly sunny into Monday as high pressure continues.
Northwesterly winds of 15G25 knots are expected through the
evening before galling below 10 knots overnight. Winds become more
northerly around 5 knots for Monday.

&&

.MARINE...
Issued at 257 PM EDT Sun Jun 23 2024

We have seen a recent localized uptick in winds and waves at the
Holland buoy and a similar wind bump at the Muskegon Coastal
Marine station. This matches a small pocket of elevated winds
advertised by the 17Z HRRR. This particular model run shows this
pocket peeling quickly southward along the lakeshore as it
becomes more diffuse, so we are therefore still comfortable with
maintaining a 5 PM expiration for our Small Craft Advisory and
accompanying Beach Hazards Statement.


&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 5 PM EDT this afternoon for MIZ037-
     043-050-056-064-071.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for
     LMZ844>849.

&&

$$

DISCUSSION...TJT/Thomas
AVIATION...Thomas
MARINE...TJT