899
FXUS63 KGRR 201857
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
257 PM EDT Mon Jul 20 2026
.KEY MESSAGES...
- Storms Tonight
- Upper Troughing Continues through next week
&&
.DISCUSSION...
Issued at 258 PM EDT Mon Jul 20 2026
- Storms Tonight
A vigorous upper shortwave trough is phasing with troughing over
eastern Canada, while a deepening surface low north of Lake
Superior drives a cold front into a moist and unstable airmass
across Wisconsin. The convective allowing models show a line of
storms initiating in that area this afternoon then moving
southeast across Lake Michigan and into western Lower Michigan
this evening. They also show a general weakening trend as
instability decreases this evening, although shear profiles remain
impressive.
The main threat appears to be downburst winds associated with
bowing line segments along with isolated large hail in the
strongest, most persistent updrafts. The threat should be ending
Tuesday morning as the cold front pushes through.
- Upper Troughing Continues through next week
The heat and humidity of last week looks to be gone for a while
with all the long range models showing a persistent pattern of
ridging across the SW CONUS and troughing across the NE CONUS.
This will keep the Great Lakes region in a northwest flow regime
for the next week at least, with occasional cold front passages
that could focus convection. The next one appears to be next
weekend. Overall, a cooler and drier than normal weather regime
for the rest of the month is expected.
&&
.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 153 PM EDT Mon Jul 20 2026
A few areas of MVFR smoke visibility remain but gradual
improvement has been occurring. Otherwise, cloud bases are
expected to be at or above 4,000 feet this afternoon with
southwest winds. Thunderstorms from Wisconsin are expected to
cross Lake Michigan into Western Lower Michigan late this
evening. Models differ on how far east the storms will extend
into Lower Michigan tonight. For now, will continue prob30
thunderstorms especially between 01 and 08 Z. Heavy rain with IFR
visibility, and stronger wind gusts over 35 knots or LLWS are
favored to be associated with storms if they do occur.
&&
.MARINE...
Issued at 258 PM EDT Mon Jul 20 2026
SCA continues through Wednesday evening. We may need a Gale
Warning for strong north winds Tuesday night as the pressure
gradient tightens between the departing low and the advancing
high. Conditions will quickly improve on Thursday as the high
builds in.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Air Quality Alert until midnight EDT tonight for MIZ037>040-
043>046-050>052-056>059-064>067-071>074.
Beach Hazards Statement through Wednesday evening for MIZ037-043-
050-056-064-071.
MARINE...Small Craft Advisory until 11 PM EDT Wednesday for LMZ844>849.
&&
$$
DISCUSSION...Ostuno
AVIATION...CAS
MARINE...Ostuno
NWS GRR Office Area Forecast Discussion
