891
FXUS63 KGRR 090557
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
157 AM EDT Tue Jul 9 2024

.KEY MESSAGES...

- Thunderstorms this Evening, with isolated hail/wind threat

- High confidence in heavy rain, medium confidence in location for
  late Tuesday night into Wednesday

- Warmer with a chance of weekend showers

&&

.UPDATE...
Issued at 1010 PM EDT Mon Jul 8 2024

A few showers and thunderstorms may continue for the next few
hours before diminishing, with the main threat south of I-96.
Other that some cloud to ground lightning and small hail, no
significant hazards are expected with these storms.

The remnants of Beryl will be headed toward Michigan over the next
couple days. Model guidance continues to fine tune the track, but
it is looking very likely that most of the region will be impacted
by this system as early as Tuesday afternoon or evening, and
lasting into Wednesday. Very high precipitable water in
association with this tropical system will help produce some areas
of heavy rainfall, especially near and south of I-96 based on the
latest guidance. If the region can destabilize Tuesday afternoon
ahead of the main rain shield associated with Beryl, thunderstorms
may develop even before the main system arrives. Convective
allowing models are showing widespread 1"-3" of rain across the
region with some pockets of 3"-6" possible. These types of amounts
could fall under persistent heavy rain bands, especially along the
pivot point just north of Beryl`s center. These types of amounts
could lead to various forms of flooding especially near and south
of I-96. Some locally significant urban and poor drainage
flooding, including some impacts to travel due to water covered
roads as well as high water from flooded creeks and streams (but
probably not the major main stem rivers it appears, aside from
minor flooding in spots), are all possible later Tuesday into
Wednesday. Continue monitoring the latest forecasts.

&&

.DISCUSSION...
Issued at 355 PM EDT Mon Jul 8 2024

- Thunderstorms this Evening, with isolated hail/wind threat

Surface trough combined with late afternoon/early evening max in
diurnal instability is resulting in some updrafts current going
up across the southwest forecast area and conditions seem
favorable for this activity to spread east into this evening with
some potential for isolated cores capable of producing downbursts
and hail. The storms should diminish with decreasing sfc-based
instability later this evening.

- High confidence in heavy rain, medium confidence in location for
  late Tuesday night into Wednesday

The slug of tropical moisture associated with the remnants of
Beryl will be making its presence felt in at least the southern
half of Lower Michigan by late Tuesday night and continuing into
Wednesday afternoon.

We are concerned with the potential for high rainfall rates as
baroclinic processes associated with a tropical cyclone
transitioning to extratropical have been known to result in
extreme rain rates/amounts. The good news here is that the storm
will not be slow moving. Even so, a wide swath of 2 to 3 inches of
rain with locally 4 to 5 inches is squarely in the forecast
envelope of QPF amounts. See the Hydro section of this discussion
for details on possible impacts.

Forecast uncertainty on where the northern edge of the heavy rain
axis continues to be high, and it is possible the I-96 corridor
gets into the 2 inch plus amounts. The higher confidence for those
amounts remains further south, along the I-94 corridor.

- Warmer with a chance of weekend showers

The modified tropical low will be east of the cwa by Wednesday
night. High pressure will begin to build in to the region at the
surface, but an upper trough will remain overhead. Thus we`ll likely
see some diurnal cu as lapse rates will steepen with the cooler air
aloft. Isolated showers will be possible too both Thursday afternoon
and Friday afternoon. However, a stronger short wave is progd to
rotate through the trough over the weekend and that will boost our
rainfall chances to 40-50 percent. A cold front may try to make a
run at Lower Michigan next Monday which would also enhance our
chances for storms.

It`s going to feel steamy again as the temperatures will slowly
climb from around 80 Thursday to near 90 by Monday. Concurrently,
dewpoints will rise through the 60s to around 70 and so humidity
levels will feel very high.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 157 AM EDT Tue Jul 9 2024

A fair amount going on during this TAF period. We are starting off
mainly VFR, but there is some rain hanging on at JXN at 06Z. That
rain should be over fairly quick, but some light fog (MVFR) will
develop across the south at AZO, BTL and JXN. The potential is
there for JXN to dip lower, but left that out for now. We will
be VFR by mid morning at all sites.

The next focus is the threat for showers and thunderstorms across
the south at AZO, BTL and JXN this afternoon in the 18Z to 00Z
time frame. These showers and storms will be focused along I-94.
Occasional MVFR and IFR is possible for short periods with showers
and storms. This evening after 00Z we shift gears into rain
spreading in with the tropical moisture associated with the
remnants of Beryl. Visibilities and ceilings sink after 00z with
MVFR spreading into most TAF sites (MKG is the exception) between
00Z and 06Z this evening. IFR conditions surge in just beyond this
TAF period between 06Z and 12Z on Wednesday.

&&

.MARINE...
Issued at 355 PM EDT Mon Jul 8 2024

Winds and waves are expected to remain relatively light through
Tuesday then offshore winds could pick up on Tuesday night and
Wednesday as low pressure, the former Hurricane Beryl, move
through. A blend of the NBM and NAM12 was used which keeps winds
below SCA, but this will have to be looked at for trends in wind
the next couple days.

&&

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

&&

$$

UPDATE...Hoving
DISCUSSION...Ostuno/04
AVIATION...Duke
MARINE...Ostuno/Thomas