729
FXUS63 KGRR 201020
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
620 AM EDT Mon Jul 20 2026
.KEY MESSAGES...
- Thunderstorms Tonight With Damaging Winds Possible
- Dry and Cooler Mid to Late Week
&&
.DISCUSSION...
Issued at 237 AM EDT Mon Jul 20 2026
- Thunderstorms Tonight With Damaging Winds Possible
Primary concern with this forecast package is for the potential of
severe thunderstorms tonight. A cold front looks to trigger storms
across the upper midwest this afternoon which progresses eastward
Monday evening. Questions remain as to the strength of any
convection, with timing favored to be late this evening into the
early overnight hours.
Evaluation of forecast profiles suggests the potential for an MCS on
the nose of a 40 kt LLJ, with strong low-level and deep layer shear.
This lends itself to a primary threat of damaging winds, with large
hail in any discrete convection given steep mid-level lapse rates.
Instability looks to be sufficient as well (albeit waning with
eastward extent), so even with uncertainties as to convective
strength, the environment warrants the Slight (Level 2/5) risk in
place across most of the CWA. The threat ends Tuesday morning as the
cold front exits the CWA.
- Dry and Cooler Mid to Late Week
Tuesday through late week will feature a relatively stable pattern
with persistent upper-level troughing and surface ridging across the
area. Combined with the dry airmass, dry conditons are favored at
this point. Look for highs in the 70s mid-week climbing back into
the low 80s by the weekend.
&&
.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 620 AM EDT Mon Jul 20 2026
MVFR conditions due to smoke (IFR at GRR) will continue for the
next few hours before improving late morning as mixing increases.
Gusty winds to around 20 knots then are expected through the
evening. Showers and storms over Wisconsin then arrive after 00z
tonight with MVFR and lower conditions and gusty winds possible.
Given low confidence in coverage and strength of storms, have
maintained PROB30s with this package. Post storms, low ceilings
are then forecast to develop with MVFR conditions likely.
&&
.MARINE...
Issued at 237 AM EDT Mon Jul 20 2026
Winds and waves increase this morning ahead of a cold front with 3-6
foot waves building by this afternoon. Winds veer northwesterly
Tuesday behind a cold front with waves to 5-8 feet. Winds of 20-30
knots are expected with a 40% chance of gale force gusts Tuesday
evening. Hazardous winds and waves continue into Wednesday evening.
Will hoist a Small Craft Advisory and Beach Hazards Statement for
this multi day hazardous wind and wave event. In addition,
thunderstorms, the strongest of which may have winds in excess of 34
knots, are possible this evening across the nearshore and open
waters.
&&
.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 from 11 AM EDT this morning through
Wednesday evening for MIZ037-043-050-056-064-071.
MARINE...Small Craft Advisory from 11 AM this morning to 11 PM EDT
Wednesday for LMZ844>849.
&&
$$
DISCUSSION...Thomas
AVIATION...Thomas
MARINE...Thomas
NWS GRR Office Area Forecast Discussion
