157
FXUS63 KGRR 151120
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
720 AM EDT Mon Jun 15 2026
.KEY MESSAGES...
- Thunderstorms Likely Tuesday, Marginal Risk for Severe
- Soaking Rain and Possible Severe Storms Wednesday Afternoon-Eve
&&
.DISCUSSION...
Issued at 350 AM EDT Mon Jun 15 2026
- Thunderstorms Likely Tuesday, Marginal Risk for Severe
Not much change from previous thinking as shear and sufficient
instability increase during peak heating ahead of a strengthening
surface low over Wisconsin on Tuesday. Despite the cooler-than-
normal temperatures and dew points in the 50s, lapse rates from
the low to mid levels support deep convection with CAPE around
1000 J/kg and a marginal threat for severe wind and hail depending
on how much destabilization and organization can occur.
The kinematics appear rather messy with models developing
multiple convective cells and potentially more than one round of
convection between mid day and evening, but one can find spots
where 0-1 and 0-3 km bulk shear approaches 30 knots with either
straight or somewhat curved hodographs, allowing for some updrafts
to become mesocyclonic with a nonzero tornado threat. Some narrow
swaths of rain totals greater than an inch are also possible.
- Soaking Rain and Possible Severe Storms Wednesday Afternoon-Eve
Definitely an unusual June pattern with a large upper low parked
over south-central Canada, and a 100 knot upper level jet
extending from the Pacific Northwest through the Great Lakes into
the Northeast, with robust Clipper-like shortwaves rapidly
progressing through the flow. The MSLP of the surface low within
the ECMWF and GFS dips below 990 mb as it marches through the
Upper Midwest on Wednesday, which is 4 to 5 standard deviations
below normal for this time of year.
There remain some differences in the track of the low regarding
latitude and also timing, however, the chance of rain Wed
afternoon-evening is very high, with the middle 50 percent of
ensemble members providing widespread 0.5 to 1 inch of rainfall.
Instability does appear to be elevated for the most part, though
the surface in southern Michigan may not stay entirely stable
especially if a northern track verifies. Agree with SPC`s take on
a slight risk of severe weather for I-94 and south given a the
extreme amount of low to mid level wind shear for the time of
year. A hail and/or wind threat may occur even with a somewhat
stable surface if gravity wave associated convection develops
within this unbalanced upper- level flow regime as the 00Z NAM
pressure fields might suggest.
&&
.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 720 AM EDT Mon Jun 15 2026
VFR is expected with a patch of clouds at or above 20,000 feet,
and daytime cumulus development above 5,000 feet. Winds will be
gusty from the west-southwest during the day.
&&
.MARINE...
Issued at 350 AM EDT Mon Jun 15 2026
A moderate swim risk may develop this morning with 15 knot WSW
winds. A high swim risk and small craft advisory conditions are
likely on Tuesday, and again Wednesday night into Thursday.
Thunderstorms may also be present over Lake Michigan on Tuesday
and Wednesday.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...CAS
AVIATION...CAS
MARINE...CAS
NWS GRR Office Area Forecast Discussion
