986
FXUS63 KGRR 150606
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
206 AM EDT Mon Jul 15 2024

.KEY MESSAGES...

- Showers and Thunderstorms this evening and again Monday night

- Diminishing Rain Tuesday then Cooler and Dry to End the Week

&&

.UPDATE...
Issued at 1003 PM EDT Sun Jul 14 2024

 Bottom line up front: A storm system will be moving across Lake
Michigan and through southern lower Michigan, mainly south of the
I 96 corridor, overnight bringing a chance for large hail and
damaging winds. The other concern overnight is the potential for
flooding, especially in low lying regions along and adjacent to
the I 94 corridor in areas that recently received heavy rainfall.

 A remnant MCV or, mesoscale convective vortex, is currently
moving through southern Wisconsin and northern Illinois. This
storm system is expected to move across Lake Michigan around
midnight and then make its way across Southern Lower Michigan.
While the system has shown some signs of weakening, super cells
have formed with the main threat being large hail and damaging
winds. While the Bulk shear is only between 35 to 40 kts, it is
orientated with the bookend vortex which increases potential for
updraft and mesocyclone strength which, coupled with the large
CAPE values, 1500 J/KG in mixed CAPE and 1200 J/KG in DCAPE, will
create an environment for severe storms.

 As this system has moved through WI, there have been reports of
heavy rainfall with reports of 2 inch per hour rates. Considering
the recent rainfall that inundated the region, flood guidance is
around 1.6 inches. So flooding is a concern, especially in low
lying regions and regions near bodies of water. The regions of
most concern is south of I 96, especially along the I 94 corridor
and along and west of US 131.

&&

.DISCUSSION...
Issued at 354 PM EDT Sun Jul 14 2024

- Showers and Thunderstorms this evening and again Monday night

Chose to roughly follow the HREF forecast for POPS tonight since
it seems to be a decent compromise for the wide range of
solutions in the CAMs. The idea is that convection will initiate
in the next few hours across Iowa and southern Wisconsin as an MCV
moves through an area where the airmass fully recovered from last
night`s convection. The storms move east and expand into another
MCS in WNW flow with the southern half or third of the forecast
area with likely POPs and some risk of severe weather, mostly
downburst winds.

These storms should be clearing the eastern forecast area around
06Z followed by a break in the action before the next round
arrives Monday afternoon as heights being to fall and surface cold
front settle south, with an MCS expected to form somewhere
upstream across the Dakotas or Minnesota and dive SE into
Wisconsin, northern Illinois and eventually southwest Lower
Michigan. The general signal in the synoptic pattern has been
there for several days, but the details on the mesoscale have
been quite variable and uncertainty remains. It seems the main
threat will be strong downbursts in a progressive type derecho
regime with the highest chance for severe winds across the
southwest forecast area, late afternoon Monday into Monday night.

- Diminishing Rain Tuesday then Cooler and Dry to End the Week

The cold front continues to get shoved southwards early in the day
Tuesday, then shower chances diminish by the afternoon. Upper
troughing moves in overhead with a weak shortwave presenting low
rain chances Wednesday afternoon across north central lower
Michigan. Surface high pressure builds in from the west Wednesday
evening with dry weather to persist through the end of the week.

Noticeably cooler conditions expected starting Tuesday following the
passage of the cold front Monday. Highs near 80 Tuesday, then into
the 70s through the end of the week. Dewpoints will also fall
resulting in less humid conditions. Overall, expect pleasant and
quiet weather with comfortable temperatures and plenty of sunshine
to end the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 206 AM EDT Mon Jul 15 2024

A round of showers and thunderstorms is moving through the TAF
sites at 06Z. Aviation conditions are a mix of VFR and IFR given
the storms are not widespread. The round of showers and storms
that are over the TAF sites at 06z will gradually work to the east
through 12z. After about 12z, the precipitation should be moving
off to the east into Southeast Lower Michigan. This will leave the
TAF sites in a likely area of MVFR ceilings for much of the
morning. After 18z, we should see a period of VFR through the
afternoon before the next round of storms which will move in after
00z Monday evening. At this point have VCTS in the TAF for Monday
evening.

&&

.MARINE...
Issued at 354 PM EDT Sun Jul 14 2024

Winds and waves will continue to stay below hazardous levels for
mariners and swimmers, except for the potential for some strong
thunderstorms this evening and again Monday night. THese storms
could create locally higher waves and wind gusts over 35 knots.

Some areas of marine fog are also possible through Monday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

UPDATE...Ceru
DISCUSSION...Ostuno/Thielke
AVIATION...Duke
MARINE...Ostuno