590
FXUS63 KGRR 171107
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
707 AM EDT Wed Jun 17 2026
.KEY MESSAGES...
- Chance of Severe Weather and Flooding in Southern Michigan
- Quieter Late Week Then Rain and Storms Possible Sunday
&&
.DISCUSSION...
Issued at 401 AM EDT Wed Jun 17 2026
- Chance of Severe Weather and Flooding in Southern Michigan
Anomalously deep low pressure cyclone for June, featuring two
pressure minima at or below 990 mb over Minnesota and southern
Lake Michigan, associated with a developing coupled upper-level
jet structure supporting significant upper-level divergence, will
provide dynamic weather to the Midwest today. A several hour
period of rain and thunderstorms between early-mid afternoon and
late evening will bring threats of strong winds and flooding.
Ahead of the low, a 60 to 70 knot 850 mb jet nosing into southern
lower Michigan late in the day will help supply moisture for a
large area of moderate to heavy rain. Initially rain-cooled air at
the surface over Lower Michigan will keep much of the severe and
tornado threat over Illinois and Indiana. However there are signs
of a weaker warm frontal boundary advecting into southwest lower
Michigan under destabilizing midlevels during the evening.
Vertical wind shear in the low and mid levels will become extreme
for a time this evening especially in southern Michigan between 5
and 9 PM, including sfc-1 km shear values around 50 knots in the
vicinity of an anticipated convective complex embedded within the
rain.
While surface parcels may not be outright convectively unstable
in the vicinity and north of I-94 this evening, a strong to severe
wind threat may still manifest in portions of the area. Some runs
of the HRRR and other HREF member models develop narrow bands of
wake-low enhanced southeast winds at times on the northwest edge
of the precipitation shield. Additionally, some runs of the HRRR
are producing small scale signatures of Gravity Wave Associated
Convection (GWAC) within one of the cells in southern Michigan
amid the complex, which if this does indeed occur, may entail an
increased hail threat despite meager elevated instability, and a
temporary wind direction reversal from southeast to northwest as
the stable air in the lower levels is lifted and dropped.
As far as precipitation, broadly 1 to 2 inches of rain is
supported in the larger mesoscale picture. However, depending on
the evolution of the smaller mesoscale convective features, a
narrower swath of 2 to 4, perhaps isolated 5 inches of rain may
fall in several hours. This is shown in the HREF`s local
probability matched mean QPF. A flood watch has been issued to
cover this threat of flooding particularly for low-lying and urban
areas.
- Quieter Late Week Then Rain and Storms Possible Sunday
Thursday and Friday look cool and breezy with just very light and
scattered rain showers possible at times through Saturday. The
next shortwave trough ejecting from the western CONUS may develop
another surface low over the Plains to Midwest late in the
weekend. Depending on the track, another batch of heavy rain could
move in later on Sunday. There is still a wide distribution of
QPF amid the ensembles due to differences in the low`s track, so
confidence in heavy rain or thunderstorms is not high for now.
&&
.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 707 AM EDT Wed Jun 17 2026
Associated with a strong low pressure system for June, rain is
expected to move in from the west between 16 and 19 Z, gradually
becoming heavier with worsening visibility and ceilings between 19
and 21 Z. Enough instability for thunderstorms embedded within
the rain will likely be present starting between 20 and 22 Z,
persisting for a few hours, then followed by a few more hours of
prob30 thunderstorms as the predominant rain ends and showers
become the mode. LLWS and gusty surface winds over 30 knots are
possible during the hours of most intense rainfall.
&&
.MARINE...
Issued at 452 AM EDT Wed Jun 17 2026
Small craft advisory is in effect beginning this afternoon as
southeast winds strengthen with gusts to 30 knots under rain and
thunderstorms through this evening. Gale force wind gusts from
the southeast could occur in the vicinity of some showers/storms
as indicated by high resolution models. Tonight into Thursday,
west winds will build hazardous waves, and a beach hazard
statement will be needed.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 4 PM EDT this afternoon through late tonight
for MIZ064>067-071>074.
MARINE...Small Craft Advisory from 2 PM this afternoon to 4 PM EDT
Thursday for LMZ844>849.
&&
$$
DISCUSSION...CAS
AVIATION...CAS
MARINE...CAS
NWS GRR Office Area Forecast Discussion
