-- Discussion containing changed information from previous version are highlighted. --
850
FXUS63 KGRR 122338
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
738 PM EDT Wed Jun 12 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

&&

.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 /00Z TAFS THROUGH 00Z FRIDAY/...
-- Changed Discussion --
Issued at 738 PM EDT Wed Jun 12 2024

Ongoing cirrus across West Michigan are providing high VFR cigs
across the area at present. Cigs fall to around 5kft early
Thursday morning as a dying complex of showers and storms
approaches the area from Minnesota/Wisconsin. Have removed
showers from the I94 TAF sites as latest trends suggest showers
remain north, and kept VCSH in for the I96 TAF sites as coverage
will be more scattered than across Central Lower. Isolated thunder
is possible along with brief windows of MVFR visbys but
confidence is to low to include in the TAF. Will monitor upstream
trends and amend as needed. Southwest winds of 10-15 knots gusting
to 25 knots are expected from early morning to mid-afternoon
Thursday.

A second round of showers and storms is possible after 18z
Thursday, however significant uncertainty in timing and coverage of
showers and storms exists. Given this, have elected to go with
PROB30 for SHRA for TAF sites given low confidence in shower/thunder
impacts to individual terminals. If a storm impacts a terminal
Thursday evening gusts exceeding 35 knots cannot be ruled out.
VFR expected except for locally MVFR or lower possible if
thunderstorms occur Thursday 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.

&&

$$

DISCUSSION...Meade/Laurens
AVIATION...Thomas
MARINE...Meade