156
FXUS63 KGRR 160553
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
153 AM EDT Tue Jul 16 2024

.KEY MESSAGES...

- Severe weather risk tonight

- High pressure builds in for Thursday and Friday

&&

.UPDATE...
Issued at 1042 PM EDT Mon Jul 15 2024

Warm advection "wing" of strong thunderstorms with heavy rain and
small hail has formed across southern Lake Michigan into far
southwest Lower Michigan and these storms will bring a threat of
flooding and isolated downbursts to Van Buren and Kalamazoo
Counties through midnight. Then the squall line across northern
Illinois will move east and track across the I-94 corridor, with
downbursts and QLCS tornado threat.

The POPs and weather grids were updated to raise POPs south of
I-96 and lower POPs to the north.

&&

.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 /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 153 AM EDT Tue Jul 16 2024

Showers and thunderstorms producing occasional MVFR/IFR
conditions and briefly gusty winds before exiting to the east by
09Z or so. Numerous MVFR cigs forming in the wake of the exiting
convection and lingering through much of the morning then cigs
lifting to VFR levels toward/after 18Z. Additional widely
scattered showers and possibly an isolated t-storm possible with
the cold frontal passage later today/this evening, with best risk
near LAN/JXN, otherwise VFR continuing into tonight. Winds
shifting westerly this morning, becoming gusty to 20 kts this
afternoon, the diminishing after 00Z this evening while turning
northwest.


&&

.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 until 4 AM EDT early this morning for MIZ050-056>059-
     064>067-071>074.
MARINE...None.

&&

$$

UPDATE...Ostuno/Ceru
DISCUSSION...MJS
AVIATION...Meade
MARINE...MJS
HYDROLOGY...CAS