801
FXUS63 KGRR 151911
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
311 PM EDT Thu Aug 15 2024

.KEY MESSAGES...

- Risk for thunderstorms into Friday night

- Chances for scattered showers/storms through the weekend

- Cooler than normal much of next week



&&

.DISCUSSION...
Issued at 245 PM EDT Thu Aug 15 2024

- Risk for thunderstorms into Friday night

As the low pressure system tracks into the region from the west
through Friday, it will bring with it moisture, lift and
instability. SPC HREF 4 hr thunder probabilities don`t really
increase significantly until after 08z tonight. They remain high
until mid morning when they then decrease steadily. So it looks
like a break in the storm activity is expected late morning into
part of the afternoon. We do destabilize in the afternoon with
surface based CAPE values likely to reach 1000 J/kg away from Lake
MI. Forcing is shown to be weak then. So if storms do develop
then, then should remain isolated/scattered during the mid to
late afternoon hours. The main mid level wave/vort max arrives
from west in the 00z-04z Sat period. There`s a signal in the
models for a cluster of storms to accompany this feature. The SPC
HREF 4h thunder probabilities increase once again. LPMM 24 hr qpf
values show small swaths of values over 2 inches 00z Fri to 00z
Sat so locally excessive rain may occur. We will feature
relatively high POPs trough the period as a result.


- Chances for scattered showers/storms through the weekend

Vertically stacked and slow-moving area of low pressure will be
centered over Lower Michigan on Saturday and southwest Ontario on
Sunday. Scattered showers or thunderstorm cells will tend to be
reinvigorated with daytime heating, with greater precip chances
inland and away from Lake Michigan. CAPE will be more modest than
Friday, and shear will still be limited, but 20-25 knot winds in the
lower atmosphere not far off the surface may allow for localized
gusts over 30 mph to mix down with some showers. Though high
temperatures will be cooler than normal, dew points will still be on
the muggier side.

- Cooler than normal much of next week

A tendency for synoptic-scale ridging over the Rocky Mtn front range
and troughing over the eastern Great Lakes early-mid next week will
provide a flow of drier and cooler-than-normal air as high pressure
builds in the Midwest. Good confidence among the ECMWF and GFS
ensembles for highs mostly in the mid 70s. A small minority of
members give rain chances Tuesday night or Wednesday depending on
strength and location of shortwave troughs moving over the ridge.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 158 PM EDT Thu Aug 15 2024

Fair confidence regarding flight category changes due to ceilings
as MVFR should be slowly working in across the area from northwest
to southeast later this evening and overnight. IFR is also
possible between 09 and 14 Z mainly north of MKG-GRR and also
south of AZO-BTL.

Less confidence in narrowing down the chance of thunderstorms.
First, light rain (mostly VFR) is slowly spreading in for this
afternoon. That will transition to progressively more convective,
intermittent/temporary and more moderate intensity rain showers as
instability increases this evening through early Friday morning.
Decided to remove VCTS for this evening (around 00Z to 04Z) since
most models now show too little instability through the colder
reaches of the clouds, though a flash of lightning somewhere in
southwest Michigan can`t be ruled out. After 04Z, instability is
expected to increase gradually, eventually supporting scattered
thunderstorms for a few hours affecting most TAF sites after about
08 to 11 Z.

&&

.MARINE...
Issued at 245 PM EDT Thu Aug 15 2024

The winds and waves will remain elevated through much of tonight
as the pressure gradient tightens up as a result of the
approaching low pressure system We did slightly expand the
hazardous beach/boat risk this afternoon based on the latest model
and observational trends. On and off hazardous conditions are
predicted into the weekend so we will adjust headlines as we get
closer to those periods.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 2 AM EDT Friday for MIZ037-043-050-
     056.
MARINE...Small Craft Advisory until 2 AM EDT Friday for LMZ846>849.

&&

$$

DISCUSSION...MJS/CAS
AVIATION...CAS
MARINE...MJS