921
FXUS63 KGRR 151943
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
343 PM EDT Mon Jul 15 2024

.KEY MESSAGES...

- Severe weather risk tonight

- High pressure builds in for Thursday and Friday


&&

.DISCUSSION...
Issued at 308 PM EDT Mon Jul 15 2024

- Severe weather risk tonight

The atmosphere continues destabilize ahead of the approaching mid
to upper level wave in the upper MS Valley. Surface based CAPE
values ranged 2500 J/kg here in MI to near 7000 J/kg in portions
of IA and IL. Effective bulk shear values were maximized upstream
in WI, closer to the wave and just north of the instability
corridor. VWP`s from upstream radars still show 50 to 55 500 mb
winds, supporting the stronger deep layer shear. The 12z KABR
sounding showed a deep EML, which are commonly seen with higher
end severe weather events. Convection should develop in IA in the
next few hours, which is supported by the latest high res models.
The initial convection should evolve into bowing linear complex
as it tracks east southeast. Damaging winds are the main risk with
the complex here in MI.  As the low level jet ahead of it
strengthens this evening, some of the deeper instability should
advect up into Lower MI supporting the complex to impact southern
parts of Lower MI. How far north the strongest wind reach into the
state is somewhat uncertain but near and south of I-96 is still
looking like the region of greatest risk for high winds. The
period of 02z to 07z seems to be the timeframe of greatest risk.

- High pressure builds in for Thursday and Friday

Much more stable conditions arrive Thursday and into Friday as a
large Canadian high pressure system builds in. The steady cool air
advection ahead of it will lead to lower than normal temperatures.
Subsidence associated with this system will act to dry out that
atmosphere and lead to limited cloud cover.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 147 PM EDT Mon Jul 15 2024

Leftover moisture evaporating off the ground is creating a few to
broken layer of cumulus around 2500 feet early this afternoon.
This is expected to gradually mix out with drier air over the next
several hours, and it should be mainly VFR until thunderstorms
arrive later this evening.

The 12Z HREF member models arrived with a fair amount of agreement
on the time of arrival of the thunderstorms. Confidence in TS with
heavy rain and sudden wind gusts 40 or 50 knots is highest around
AZO and BTL. Confidence is a little less but still medium for TS
at MKG, GRR, LAN, and JXN. After the storms, MVFR ceilings are
likely through Tuesday morning with gradual improvement during
afternoon.

&&

.MARINE...
Issued at 308 PM EDT Mon Jul 15 2024

A line of storm is likely to track through the marine zones
tonight. Commonly these systems stir up the winds and waves. The
impacts may linger into Tuesday. We increased the waves to show
generally 2 to 4 footers, and it would not take too much increase
in the winds to push those wave values to 4 feet or higher. No
headlines at this time but close monitoring will be needed as BHS
and SCA`s may be need for later tonight into Tuesday.

We will have to monitor the advancing high pressure system for
Wednesday into Thursday which can sometimes lead to hazardous
beach and boating conditions.

&&

.HYDROLOGY...
Issued at 308 PM EDT Mon Jul 15 2024

Not too dissimilar from what happened last night, models are
consistently suggesting a swath or two of 1-to-2.5 inch rain amounts
falling very quickly with storms tonight. The heavy rain from the
remnants of tropical cyclone Beryl last week, plus areas of heavy
rain last night, make it increasingly likely that any given
location`s soil in southern Michigan will not be as capable of
absorbing the rain it normally could.

Any locations which receive 2 inches of rain tonight can flood roads
in low-lying spots, even up to car roofs in some cases (such as 28th
& Division in Grand Rapids). Small creeks and streams would be the
first to rise. Buck Creek in Grandville, Sycamore Creek in Holt, and
the Grand River at Jackson are some of our forecast points that
particularly vulnerable, but really any river`s headwaters could
flood quickly. The main-stem rivers including at Lansing, Ionia,
Marshall, and Comstock/Kalamazoo, could see a new push toward higher
crests in the coming days, but only if a large portion of their
upriver drainage basins receive substantial rain.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 9 PM EDT this evening through late tonight for
     MIZ050-056>059-064>067-071>074.
MARINE...None.

&&

$$

DISCUSSION...MJS
AVIATION...CAS
MARINE...MJS
HYDROLOGY...CAS