999
FXUS63 KGRR 250559
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
159 AM EDT Tue Jun 25 2024

.KEY MESSAGES...

- Strong to severe storms possible late tonight/Tuesday morning

- Dry Most of Wednesday into Early Friday

- Rain Returns Friday into Saturday

&&

.UPDATE...
Issued at 1000 PM EDT Mon Jun 24 2024

No significant fcst changes are needed this evening. A consensus
of latest CAMs corroborated by recent rgnl radar trends the past
hour or so suggest that the complex of thunderstorms over south
central/se WI will continue to diminish late this evening.

Our primary svr wx threat will come during the early morning hours
Tuesday (around 08Z-13Z) as severe thunderstorms over northern MN
attm congeal into a well developed MCS/squall line that moves se
along the instability gradient.

There is some question as to how much instability will be present
by the time the convection moves into our area. However a
favorable kinematic environment will likely compensate for
diminishing instability and bring potential for damaging wind
gusts particularly across our western/nw fcst area by around
08Z-10Z Tue and lasting through mid morning.

&&

.DISCUSSION...
Issued at 330 PM EDT Mon Jun 24 2024

- Strong to severe storms possible late tonight/Tuesday morning

 Bottom line up front: Storms capable of producing damaging winds
and large hail will be moving through the region tonight through
Tuesday morning. A line of storms will sweep through the region
early Tuesday morning between 5 AM and 10 AM.

Latest CAMS have several rounds moving through. The first should
be a cluster of storms that moves through between midnight to 2
AM timeframe. That cluster has strong enough updrafts with 7 to 8
C/km lapse rates through the mid levels where damaging hail is a
concern. The biggest threat tomorrow will be a long line of
storms that will be extending from an upper level wave moving
through the region overnight. This squall line will feature a
strong pressure gradient with strong low level winds aloft
accompanying this system. There is potentially over 50kts of
winds between 5 to 10kt feet moving onshore after 5 AM. The
question is whether or not it will be able to mix to the sfc. The
strong southerly flow will keep stable air in place along with a
strong inversion that could keep any convection at bay.

Given all of this, the line is not purely thermodynamically
driven but the strong kinematics will assist in downdraft
potential. This line will move through Tuesday morning and exit
the region moving to the southeast early Tuesday afternoon.

There will be another round of storms possible later Tuesday,
however given that the region will be fairly mixed it will lessen
the potential for any storms.


- Dry Most of Wednesday into Early Friday

The long term forecast will be dominated by two key longwave
features. The first being ridging across the southern CONUS and the
second being longwave troughing across northern Canada. This places
lower Michigan in a quasi-zonal longwave pattern with two main
periods of active weather.

Our first active period will continue from Tuesday night as showers
wind down during the morning hours Wednesday with a shortwave
exiting the area. Cannot rule out an isolated rumble of thunder near
JXN given low-level LIs near but confidence is low as instability
ramps up as showers depart. Surface high pressure and mid-level
ridging then build in for the rest of Wednesday and continue into
the start of the day Friday bringing dry weather and highs in the
upper 70s to low 80s.

- Rain Returns Friday into Saturday

By Friday the ridging shifts east with a 30-35 knot SW low-level jet
setting up a warm air advection pattern. As expected in the day 5-6
range, some small differences in timing and placement of key surface
and upper-level features exist, though the overall idea has good
model consensus. Showers and thunderstorms develop potentially as
soon as Friday afternoon in the warm air advection pattern, with the
best chance Friday night into Saturday as a shortwave trough and
associated cold front move through. High pressure returns behind the
departing cold front later Saturday into Sunday returning dry
weather to Mid-Michigan. Highs in the 80s Friday and Saturday fall
into the 70s Sunday behind the cold front.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 158 AM EDT Tue Jun 25 2024

VFR conditions through about 09Z and then a cluster of
thunderstorms will move into central Lower Michigan from the
northwest and bring areas of IFR and MVFR in heavy rain. There
could also be strong downburst winds and some hail with these
storms as they move across western and central Lower Michigan
between about 09Z and 15Z. Areas of LLWS can also be expected even
before the storms

Some MVFR conditions could linger a few hours after the storms
depart but the trend will be for clearing and IFR conditions
during the afternoon.

&&

.MARINE...
Issued at 340 PM EDT Mon Jun 24 2024

Southwest winds will increase tonight behind a warm front and that
will result in wind and waves that will become hazardous to small
craft. High Confidence in several rounds of thunderstorms
developing over the lake after midnight tonight with the
potential for damaging winds and a chance for large hail. Along
with these storms rip currents may persist after thunderstorms
Tuesday morning due to possible water level fluctuations.
Chance in storms Tuesday afternoon into Tuesday evening.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through this evening for MIZ037-043-050-
     056-064-071.
MARINE...Small Craft Advisory until 8 PM EDT this evening for LMZ844>849.

&&

$$

UPDATE...Laurens
DISCUSSION...Ceru
AVIATION...Ostuno/Thomas
MARINE...Ceru