442
FXUS63 KGRR 160121
AFDGRR

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

.KEY MESSAGES...

- Chances of showers and thunderstorms through the weekend

- Cooler than normal much of next week

&&

.UPDATE...
Issued at 921 PM EDT Thu Aug 15 2024

Widespread coverage of showers associated with lead shortwave,
elevated warm front, and incoming axis of higher PWATs continues
to slowly march across the area. There has been no thunder at all
with this activity as the leading edge of any respectable MUCape
values is still well to our west. The risk for a tstm does
increase late tonight as RAP MUCapes of 500-1000 J/KG arrive and
the next shortwave currently over Iowa/srn MN approaches. Only
change to the fcst was to drop the mention of thunder in the
grids until after 06Z and that was done earlier this evening.


&&

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

- Chances of showers and thunderstorms through the weekend

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.

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 /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 723 PM EDT Thu Aug 15 2024

No significant changes to the going forecast. A band of showers
crosses through West Michigan through the evening with more
scattered showers behind it. Instability builds in early Friday
morning bringing thunder chances with a cluster of showers moving
through in the 08z-12z range. Have VCTS in the TAFs given the more
scattered nature of thunderstorms. Isolated thunder is possible
earlier but confidence is too low for TAF inclusion. MVFR and
lower visibilities are possible in any thunderstorms. A few hours
of MVFR cigs are likely Friday morning, longest at MKG.
Uncertainty exists in how long MVFR cigs continue into the morning
as it will depend in part on how much rainfall occurs. Even worst
case scenario, any MVFR cigs lift to VFR by the afternoon. An
isolated shower or storm is possible in the afternoon but coverage
is to low to warrant TAF inclusion at this time. The next
organized complex of storms holds off until after 00z Friday.

&&

.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.

&&

$$

UPDATE...Meade
DISCUSSION...MJS/CAS
AVIATION...Thomas
MARINE...MJS