150
FXUS63 KGRR 071942
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
342 PM EDT Sun Jul 7 2024

.KEY MESSAGES...

- Chances for Showers and Storms Tonight through Monday Night

- Rain Potential Tuesday Night/Wednesday

&&

.DISCUSSION...
Issued at 342 PM EDT Sun Jul 7 2024


- Chances for Showers and Storms Tonight through Monday Night

A weak shortwave trough in the mid levels can clearly be seen this
afternoon in water vapor and radar loops over the Northern Plains
from the Dakotas into Minnesota. This trough moves east while
shearing out and losing amplitude over the Great Lakes tonight
through Monday night. It will be in our area later on Monday into
Monday night. Down at the surface a weak reflection (trough) will
also move through the Lower Peninsula Monday afternoon into Monday
night.

In terms of sensible we will have a chance of showers and
thunderstorms in the forecast all three forecast periods from
Tonight through Monday night. Convection has pressed out over the
lake from WI/IL this afternoon, but as the 12Z HREF suggested it
is weakening. We feel this will largely be eroded by some drier
air and ridging over Lower Michigan.

A more concentrated area of showers and storms will move in
overnight, after midnight, across northwest portions of the
forecast area. This will be associated with a weak 850mb low level
jet of 20-30 knots. We have likely pops in the forecast for a time
overnight (60 pct).

A better chance for showers and storms will come area wide on
Monday afternoon and evening as the surface and upper troughs move
into the area. Monday afternoon`s focus point will likely be the
lake breeze front which will be situated from southwest to
northeast through the CWA. Surface based CAPE will reach the
1000-2000 j/kg range with deep layer shear of 25-35 knots. We are
in the day two general thunder, but we could see an isolated
severe event of wind/hail. Convection will linger into the
evening before gradually fading overnight Monday night.

- Rain potential Tuesday night/Wednesday

What`s left of the tropical depression is forecasted by several
models to move northeast toward the Great Lakes Tuesday night and
Wednesday. The ensembles have been trending farther south with
the surface low. However, the 12z deterministic ECMWF continues to
show a swath of heavy rain falling over the southern half of the
CWA Tuesday night and Wednesday. The global ensembles suggest a
roughly 35 percent chance of greater than 1.5 inches of rain from
this south of I-96 with a 25 percent chance in Grand Rapids. In
addition to the rain from the tropical low, models also show a
couple of short waves rotating through the upper trough. A heavy
rain potential exists, but is less than certain at this point.

Once the low moves east, high pressure will move in with the dry wx
expected Friday/Saturday. Highs will be in the lower to mid 80s
through the period except Wednesday when rain may hold temperatures
in the mid 70s.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 119 PM EDT Sun Jul 7 2024

Scattered to broken diurnal cu has developed across the region but
will dissipate as sunset nears. High clouds from convection to the
west will float across the region overnight. There may be some
shallow fog that develops as well due to recent spotty rains.

Models indicate more storms will develop over Iowa tonight and
move northeast and potentially clip MKG/GRR after 09z. We added a
VCSH comment for that possibility.

&&

.MARINE...
Issued at 342 PM EDT Sun Jul 7 2024

A fairly diffuse surface pressure gradient is in place for much of
the next several days. The GFS is forecasting winds for the most
to be in the 5 to 15 knot range or less through early Wednesday at
the mid lake buoy site. The NAM is a bit stronger tonight (10-20
knots) and stronger yet on Wednesday. The NAM is likely closer to
reality as the Canadian and ECMWF has similar surface pressure
patterns. Bottom line is we are not expected advisory level
concerns (boating or swimming) through Tuesday. Conditions could
be more interesting mid week as the remnants of Beryl approach.

&&

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

&&

$$

DISCUSSION...Duke/04
AVIATION...04
MARINE...Duke