930
FXUS63 KGRR 081955
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
355 PM EDT Mon Jul 8 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

&&

.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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 129 PM EDT Mon Jul 8 2024

Showers have developed along a diurnal heating gradient from near
LAN to southwest of AZO. We`ll keep a VCSH comment in the TAFs for
this. VFR expected through the period. However at MKG/AZO some
shallow fog is possible near sunrise due to recent rainfall.

&&

.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.

&&

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

The remnants of the hurricane are "Berylling" towards southern
Lower Michigan and the current forecast brings a swath of 2 to 3
inches of rain across part of the St. Joe, Kalamazoo and Grand
river basins by Wednesday afternoon. Coordination with the NCRFC
on the effects of the current QPF amounts for flood potential
considering antecedent conditions, which are close to normal for
this time of year, suggests that minor river flooding is
likely, but major flooding is unlikely. We will continue to look
at contingencies should there be significant changes in future
QPF.

Local flooding of tributary creeks and streams and urban/poor
drainage areas seems likely as well, and we may issue an areal
flood watch once the details in the location of the heavy rain is
better determined.

&&

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

&&

$$

DISCUSSION...Ostuno/04
AVIATION...04
MARINE...Ostuno/Thomas
HYDROLOGY...Ostuno