-- Discussion containing changed information from previous version are highlighted. --
192
FXUS63 KGRR 121525
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1125 AM EDT Wed Jun 12 2024

.KEY MESSAGES...

- Showers and storms possible late tonight through Thursday

- Showers/storms ending Thursday evening

- Significant warmup expected over the weekend

&&

.UPDATE...
-- Changed Discussion --
Issued at 1125 AM EDT Wed Jun 12 2024

Earlier batch of showers which actually produced some localized
amounts over 0.25" and a few lightning strikes has mostly
dissipated. Some thicker clouds still remain between AZO and JXN
but these too should exit/decrease soon.

CAMs suggesting a narrow line of showers could re-develop late
this afternoon/early evening along and near a FNT-SBN line related
to a low level convergence boundary, so will have a 20 pop near
LAN/JXN/BTL after 4/5PM.

Newer 12Z data now arriving and while CAMs still not in great
agreement for Thursday a general consensus is that by 00Z Fri/8
PM Thur there will be a scattered to broken line of strong to svr
convection oriented from roughly DSM to DTW. Looks like it may be
a short window/limited area of svr threat for the GRR CWFA before
convection sets up just to our south but will continue to
evaluate.
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 343 AM EDT Wed Jun 12 2024

- Showers and storms possible late tonight through Thursday

The main focus over the next 48 hours will be on the storm potential
for Thursday afternoon and evening.

Before the main chances of showers and storms begin later tonight
and Thursday, we are looking at some small rain chances mainly this
morning. There are a couple of showers and even a storm out over
Lake Michigan early this morning. These are associated with both a
short wave and weakening low level jet that are both moving toward
the area. Both features are weakening, and instability progs show it
weakening also as it approaches. We will have some small chances to
account for this through noon today, before clearing skies will be
the rule and temperatures will be about 10 degrees warmer than
Tuesday with 850 mb temps about 6-8C warmer.

Shower and storm chances will increase once again tonight after
midnight from NW to SE. We will see a low level jet core out ahead
of the next system that will move across WI/U.P./Nrn Lower later
tonight and Thursday morning. The leading edge of it and the
instability gradient will bring scattered to numerous showers and
storms. The highest concentration will be the further north you go,
closer to the better theta e advection. This activity will be
elevated in nature, with little to no threat of severe weather.

It will linger over the area Thursday morning before the low level
jet moves mostly east of the area. The cold front will remain NW of
the area much of the day, dropping in during the afternoon hours.
This normally would be favorable for severe weather. In addition,
there is a good deal of instability (1500-2000 J/kg of CAPE), and
good shear values of 40 knots with mid level winds on the favorable
side. The potential is definitely there, and we agree with the SPC on
their Marginal to Slight Risk.

The severe threat is a conditional one however on if storms can even
develop. There are multiple factors that many of the hi-res CAMs and
synoptic models seem to agree on limiting the afternoon development.
First, is the early convection will likely temper the instability a
bit. Also, the low level jet moving away will not help. The
divergent flow downwind of Lake Michigan with a westerly component
to the wind will not help. Finally, forecast soundings show the
atmosphere capped a few thousand feet agl. The gist of this is that
we will have small pops Thursday afternoon, focused on the SE
portion of the area further away from the lake and with the best
instability.

- Showers/storms ending Thursday evening

Many of the CAMs really don`t show much convective development
Thursday evening. However, the 00z HRRR shows a line of storms
developing around 00z from Greenville southwest to west central IL
along the cold front. If the HRRR is correct, then strong to severe
storms will be possible during the evening, given SBCAPE aoa 2k
j/kg, bulk shear aoa 40kts and surface dewpoints in the mid 60s. The
location of convective development will likely be modulated somewhat
by the degree/coverage of potential convection earlier in the
morning and whether that convection can push the frontal boundary
farther south and out of the cwa during the day prior to any
afternoon convection developing. We`ll continue to monitor the
development of this event.

- Significant warmup expected over the weekend

Once the front moves through Thursday afternoon/evening, high
pressure will nose into the region with a brief push of cooler air;
highs Friday will be 75-80. As the high moves east Saturday, south
flow will develop and an upper ridge will move from the Plains
toward the Great Lakes. Surface temperatures will rise in tandem
with upper heights. Highs Saturday will be around 80, Sunday 85-90,
Monday through Wednesday in the lower to mid 90s. Dewpoints will
reach the 60s Sunday and that`s when we`ll feel the humidity; heat
indices will be in the mid to upper 90s Monday through Wednesday.

Late in the weekend and early next week, the ridge axis will be just
east of the cwa, which means we`ll be susceptible to minor short
waves riding up the west side of the ridge. Chances of rain aren`t
very high during this time, but they aren`t zero either. The west
and northwest parts of the cwa have the best chance at seeing
precipitation.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 717 AM EDT Wed Jun 12 2024

Radar shows a few showers and isolated thunder along the lakeshore
south of I-96. Echos are decreasing a bit as they move east, but
probably have enough energy to affect GRR/AZO/BTL during the next
couple of hours. Otherwise, expect VFR conditions for the rest of
the period.

&&

.MARINE...
Issued at 343 AM EDT Wed Jun 12 2024

Headlines are not likely at this time through the rest of this week
and into the weekend. We are however watching for the possibility of
headlines for late tonight through Thursday time frame.

There is some wind aloft this morning, however it is not extending
down to the water at this time being a warm air advection setup over
the cooler waters. These winds will diminish this morning, before
ramping up later tonight and on Thursday in advance of the next
frontal system. These winds will once again being some warmer winds,
over the more stable waters of Lake Michigan. The models are in good
agreement keeping winds and waves just below criteria. However, a
slight nudge up in trends may necessitate a headline. We will
continue to monitor.

After the Thursday system moves through, high pressure will build in
for a couple of days. This will bring light wind and low waves until
Saturday night/Sunday when winds start to increase once again.

&&

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

&&

$$

UPDATE...Meade
DISCUSSION...NJJ/04
AVIATION...04
MARINE...NJJ