102
FXUS63 KGRR 031846
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
246 PM EDT Fri Jul 3 2026
.KEY MESSAGES...
- Severe Thunderstorm Threat Today into Tonight
- Scattered Thunderstorms Possible This Weekend
&&
.DISCUSSION...
Issued at 246 PM EDT Fri Jul 3 2026
- Severe Thunderstorm Threat Today into Tonight
This morning`s CAMs have been offering the strongest signal I have
seen yet this week for developing convection this afternoon amid a
destabilizing atmosphere in our CWA (other than the signal they
had for US-10 and north storms on Wednesday). Despite the majority
of CAMS struggling to initialize and maintain the northern
Illinois convection, most ended up eventually developing either
scattered convection or a loosely organized line of storms with a
severe wind threat anyway (albeit modeled storms arriving later
than reality). It is likely that the storms currently over Lake
Michigan will be today`s primary threat, followed by a lull in its
wake in the evening, then perhaps a secondary development of
scattered storms later this evening or overnight.
For this afternoon, concern is for destabilization occuring near and
south of I-96 as increasing cumulus field is seen developing.
Downdraft CAPE, as has been the case this week, is substantial at
1000-1200 J/kg. Deep layer shear is not great, but wind shear 0-3 km
is marginally supportive for a wind-driven QLCS at 25 knots, perhaps
could assist the development of mesovorts along the line which would
enhance wind damage threat in narrower corridors (tornado threat is
low but "gustnadoes" may instead be the mode).
- Scattered Thunderstorms Possible This Weekend
While some uncertainty remains in the details, scattered
thunderstorms with an isolated gusty wind threat remain favored
for parts of the day and evening Saturday and Sunday. A deepening
upper level trough over the Great Lakes this weekend will maintain
instability and a couple embedded shortwaves would provide added
lift to weaken any cap. This weekend`s storms will be less of a
"ring of fire" progressive MCS setup, and more of a pop up, hit or
miss, especially inland away from Lake Michigan during the
afternoon Saturday.
&&
.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 128 PM EDT Fri Jul 3 2026
A complex forecast is underway associated with the timing of
thunderstorms today. A line of storms across Illinois is ongoing
with additional development out ahead of it. Tracking this line of
storms brings it to MKG/BIV around 20z and progressing east with
time, which the RRFS/RRFS-MPAS seem to be the only guidance
handling this well. Additional development is possible ahead of
this line which we are already seeing upstream. Have progressed to
TEMPO TSRA for AZO/BTL/JXN where the best environment is ahead of
this line and PROB30s for MKG/GRR/LAN have been maintained.
Amendments will be made as needed. Storms will be capable of winds
in excess of 40 knots, as well as MVFR and lower conditions.
Depending on how the afternoon storms evolve, one or more
additional rounds of storms are possible after 00z, however
confidence is low. Other than that there are some signals for fog
to develop near and north of MKG overnight. Southwest winds to 15
knots will continue through the afternoon before winds become
light and variable overnight.
&&
.MARINE...
Issued at 239 PM EDT Fri Jul 3 2026
Severe thunderstorms crossing southern Lake Michigan this
afternoon with gusts 50 knots. Additional scattered storms are
possible late this evening or overnight. Waves and swim risk
expected to be fairly low this weekend in the absence of storms.
A couple scattered storms could develop over the Lake this
weekend.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Heat Advisory until 8 PM EDT this evening for MIZ046-051-052-
056>059-064>067-071>074.
MARINE...None.
&&
$$
DISCUSSION...CAS
AVIATION...Thomas
MARINE...CAS
NWS GRR Office Area Forecast Discussion
