427
FXUS63 KGRR 120729
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
329 AM EDT Mon May 12 2025

.KEY MESSAGES...

- Warm with scattered showers/storms tonight through Wednesday

- Strong to severe storms possible late Thursday

- Cooler next weekend with a chance of showers

&&

.DISCUSSION...
Issued at 329 AM EDT Mon May 12 2025

- Warm with scattered showers/storms tonight through Wednesday

We will start today off just like the weekend with plenty of
sunshine and mild temperatures. Except today, clouds will be on the
increase this afternoon on the northern periphery of the upper low
that has been affecting the SE U.S. over the weekend. Better low and
mid level moisture will arrive this evening from the south,
providing for increasing chances of rain showers tonight. These
showers look rather light as the dry air in place will eat away at
the incoming pcpn, and as the low will be weakening and moving away
from its moisture source. Thunder does not look likely tonight as
model CAPE progs show that we stay too stable for thunder through
12z Tuesday.

Tuesday will see a transition from the showers along the weakening
occluded front tonight to more of a diurnal afternoon/evening pop up
shower/storm nature that will continue into Wednesday. This
transition occurs as the weakening occluded front moves north of the
area, and we become under more direct influence of the actual upper
low and "cold" pool aloft associated with it. Sfc dew points
approaching 60F combined with sfc temps in the 70s will help touch
off showers/storms. One focus for showers/storms will be a potential
lake breeze off of Lake Michigan. Outflow boundaries from any
individual cells inland will also cause additional development as
shear will be quite weak and cells will be of the pulse variety. The
lack of shear will really limit the potential of strong to severe
storms for Tuesday and Wednesday.

- Strong to severe storms possible late Thursday

We will see a break in most of the convective activity then from
Wednesday night through much of Thursday, with only a small chance
of an isolated shower/storm possible. We will see the upper low pull
away from the area to the east, and short wave ridging will build in
for a period before the next system moves in later Thursday. The
small chance of a shower/storm will remain due to the lingering low
level moisture that hangs on as it never gets swept out.

The Thursday system will be supported by a strong closed upper low
that lifts into the Upper Midwest late in the day. This will bring a
cold front/occluding front toward the area late in the day.
Instability does not build significantly during the heating of the
day. This is likely due to the fact that the cold pool aloft does
not arrive until later. In addition, there is not much of a trigger
earlier even with some instability.

This event is looking more and more likely to be an evening event
for our area. The arrival of the cold pool aloft, a short wave, and
resultant instability developing will all likely fire convection
near the area. Models have been persistent in showing limited deep
layer shear (around 30+ knots), but enough to help the cause for
strong to severe storms. The short wave moving in is likely to try
to form a line ahead of it. Wind looks to be the biggest threat with
dry mid levels leading to DCAPEs of 1000+ J/kg present. Also, can
not rule out a tornado as the triple point will be near the southern
portion of our area. Forecast soundings show some good directional
shear as a result. This will all move out by the early overnight
hours.

- Cooler next weekend with a chance of showers

We will see the dry slot move over the area on Friday in the wake of
the frontal passage. This would support with still mild temperatures
with mainly dry conditions.

There remains some uncertainty with regards to next weekend
depending on the exact track of the upper low. Some model members
are rotating it just north of the area, while others are bringing
this far south with some showers. Too uncertain at this time to say
one way or another, so we will have some small rain chances with
cooler temperatures present.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 132 AM EDT Mon May 12 2025

We anticipate at this time that this forecast period will remain
VFR for its entirety. We will see more of the same as the past
couple of days with mostly clear skies through early this
afternoon. We will see a noticeable difference of more widespread
high clouds stream overhead later this afternoon and especially
this evening with the approach of the system from the south. The
clouds will lower, and a few light sprinkles/showers will be
possible after about 01-02z. We expect that ceilings will remain
above 5k ft agl through 06z tonight. A better chance for rain
showers and lower conditions will become more likely after 06z.

&&

.MARINE...
Issued at 329 AM EDT Mon May 12 2025

No marine wind/wave headlines are likely to be needed through
Wednesday night. The system coming up later today and lingering over
the area will have a weak pressure gradient, and fairly light winds.
The only impact that could result from these weak winds could be
some marine fog with the moist air moving over the cool waters of
Lake Michigan.

The stronger winds that arrive on Thursday will be in advance of the
next stronger storm system that will approach the area. Small Craft
Advisories are looking likely for that time frame, but we still have
some time yet to monitor the trends.

&&

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

&&

$$

DISCUSSION...NJJ
AVIATION...NJJ
MARINE...NJJ