-- Discussion containing changed information from previous version are highlighted. --
619
FXUS63 KGRR 130547
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
147 AM EDT Thu Jun 13 2024

.KEY MESSAGES...

- Showers and storms possible late tonight through Thursday

- Much warmer late this weekend and early next week

- Very low chance for a shower/storm early next week

&&

.UPDATE...
Issued at 1024 PM EDT Wed Jun 12 2024

Based on some of the latest model runs, especially the HRRR, we
did lower the POPs for tonight. Any ML CAPE remains well west of
Lake MI at this time and PWAT`s indicate that it`s fairly dry
over MI right now. Each of these limits our convective potential
tonight. But a low level jet is shown to move in later tonight and
that could advect moisture and instability in quickly. Thus there
is some uncertainty regarding how that convection will evolve
through the remainder of the night. The upstream convection will
have an impact on the winds over Lake MI. This in turn will impact
the waves. Given the uncertainty, we held off on the SCA and
Beach Hazards for Thursday, but close monitoring will be needed
and it`s possible we will start off Thursday with headlines for
boaters and swimmers.

&&

.DISCUSSION...
Issued at 255 PM EDT Wed Jun 12 2024

- Showers and storms possible late tonight through Thursday

Shortwave sliding east from MN brings a chance of convection later
tonight with guidance consensus generally keying on the area
north and west of GRR. This is where higher PWATs are expected to
move in and where the low level jet could be more of a player.
However showers/storms are possible just about anywhere overnight
as mid level theta-e advection and steeper mid level lapse rates
near 7 C/KM push in from the west.

Still a low confidence/conditional forecast for Thursday regarding
the severe weather threat. CAM solutions have been all over the
place, both with the coverage of any early day convection as well
as the evolution/coverage of things later in the day. The general
theme though looks to be for morning shortwave/WAA activity to
depart by Noon, followed by a lull, then scattered to broken
development along the southward advancing sfc cold front after 3
or 4 PM. A more congealed line then looks to take shape just to
our south by 8-9 PM. So our window of severe weather threat looks
to be relatively short/limited, maybe 4-6 hours and focused
mainly near/south of I-96.

Even though SB Capes of 2000-2500 J/KG and deep layer shear
greater than 35 kts are progged later Thursday, one big potential
limiting factor is if the low level flow goes westerly at 10-20
kts in the afternoon as suggesting in some of guidance. The stable
flow off Lk MI and lack of sfc convergence would then limit storm
potential/coverage in the GRR CWFA.

Pops decreasing from north to south later Thursday night as drier
air arrives behind the front on the heels of developing northerly
flow.

- Much warmer late this weekend and early next week

Southerly flow will develop on the back side of the departing ridge
this weekend and result in a gradual moderating trend of
temperatures. High temps will reach near 80 degrees Saturday and
well into the 80s Sunday as stronger southerly flow waa develops.

Upper level height rises and moderating h8 temps to around 20 C
early next week and a consensus of latest ensemble guidance
numbers suggest high temps will reach into the lower 90s by
then. It is not unusual for our area to have our first or second
ninety degree high temp reading of summer around this time of year.

- Very low chance for a shower/storm early next week

The building heat and humidity early next week will bring
limited potential for isolated showers and storms. One potential
triggering mechanism for convective initiation to occur will be
several shortwaves that will move through.

However there really isn`t a stronger forcing mechanism for
convective initiation to occur. The vast majority of the
early to midweek time frame will be dry and quite warm and humid.
So we will only carry very low pops for a shower or storm early
to midweek at this time.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
-- Changed Discussion --
Issued at 136 AM EDT Thu Jun 13 2024

Scattered showers and isolated thunderstorms have developed along
the lake shore and are slowly moving east. MKG will have the best
chance of seeing precipitation within the next couple of hours.
Farther south, an outflow boundary has pushed ahead of the
precipitation which usually means we`ll see some decay in that
precipitation over the next hour or so. As the low level jet
impinges on the region, an uptick in moisture and instability will
mean that we could see continued convective development.
Otherwise, VFR conditions expected.

Models are not in good agreement with convective trends this
afternoon. However, given the moisture and instability that`ll be
in place over the region coupled with the arrival of a weak cold
front, we should see showers/storms develop after 20z and be east
of the terminals by mid evening.
-- End Changed Discussion --

&&

.MARINE...
Issued at 255 PM EDT Wed Jun 12 2024

Conditions late tonight into Thursday may flirt with Beach Hazard
Statement/Small Craft Advisory criteria, specifically between
Holland and Whitehall. Increasing south-southwest flow late
tonight and early Thursday, aimed into this concave segment of
shoreline, should push wave heights to at least 2 to 4 feet and
perhaps higher. For now am thinking Moderate swim risk at Pere
Marquette and Grand Haven State Park although the possibility for
High risk (3-5 ft waves) does exists and marine headlines could
be needed in later updates.

The sfc cold front sags southward down Lk MI Thursday
afternoon and evening and any of the higher morning surf caused
by overnight low level jet should gradually subside with arrival
of looser pressure gradient near the front. However scattered
storms along the front, possibly strong to severe, could present a
marine hazard at times.

Will have to also monitor the (low) possibility of a period of
increased north-northwest winds Friday afternoon/evening which
could result in about a 4-6 hour window of rough/near advisory
conditions mainly south of Whitehall. This is for an advancing sfc
high set-up, which is typically a sneaky marine event in our
nearshore area during the summer months.

High pressure settles overhead on Saturday which will result in
lighter winds/winds however some potential exists for stronger
southerly flow to develop on Sunday.

&&

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

&&

$$

UPDATE...MJS
DISCUSSION...Meade/Laurens
AVIATION...04
MARINE...Meade