227
FXUS63 KGRR 040750
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
350 AM EDT Thu Jul 4 2024

.KEY MESSAGES...

- Warm Independence Day With Slight Chance of a Shower

- Scattered Thunderstorms Friday, Low Confidence of Severe Potential

- Showers Possible Saturday, Unsettled Pattern Continues Next Week

&&

.DISCUSSION...
Issued at 350 AM EDT Thu Jul 4 2024

- Warm Independence Day With Slight Chance of a Shower

Temperatures are expected warm up into the mid 80s today with light
winds. The cloud cover forecast for late morning through afternoon
has become a little more pessimistic, as there may be several hours
when it is mostly cloudy. Also feel compelled to broadbrush a slight
chance of showers everywhere south of Whitehall to Mount Pleasant
for the afternoon and evening.

A midlevel shortwave trough, augmented by overnight storms in
Kansas, is likely to spread cirrus and altostratus over Lower
Michigan during the late morning and afternoon. This cloud deck will
likely contain ice and may precipitate as virga or sprinkles, but
would also serve as ice nuclei feeders to the diurnal cumulus that
might develop underneath. Whether the models are overdeveloping
cumulus underneath the midlevel clouds remains to be seen, but zones
of surface convergence may be a focus for some development, and also
reasonably steep lapse rates suggested in the models above the LCL
up to the altostratus cloud deck would allow cumulus to attain
decent depth.

The number of convection-allowing models among the 00Z suite
producing scattered light rain showers between afternoon and dusk,
perhaps even an isolated downpour, seemed too great to ignore. On
the plus side, the chance of lightning does appear to be rather
small given the CAPE and better updraft strength would be in the
lower/warmer part of the cloud. You shouldn`t be rained out of your
holiday outdoor activities unless you are really unlucky.

- Scattered Thunderstorms Friday, Low Confidence of Severe Potential

A round or two of scattered showers/storms is likely between Friday
morning and afternoon, but uncertainty in the evolution of the
responsible low-pressure system makes it difficult to ascertain if
there is much threat of severe weather.

A broad synoptic-scale trough over the northern Plains today should
deepen a little as it progresses east, closing into a 500 mb low
over Minnesota tonight. By 8 AM Friday, a surface to 850 mb low
should be located somewhere over Wisconsin with a quasi-stationary
or warm front extending east across Lower Michigan. Models differ
greatly in the position and strength of the surface low Friday
morning, and the strength of the pressure field near the frontal
triple point over Lower Michigan by mid day. The surface to 850 mb
wind fields around this low also vary greatly among each model,
resulting in substantial differences in vertical shear
profiles/hodographs, some of which would support severe weather,
including a tornado, and some of which would not. This will have to
be monitored closely, though, given the reasonable magnitude of 0-6
km bulk shear, 0-3 km CAPE, and total MLCAPE advertised by most
models.

- Showers Possible Saturday, Unsettled Pattern Continues Next Week

As we enter this holiday weekend there remains quite a bit of
uncertainty headed in the mid to long range pattern. The upper
level jet stream should remain over the region Saturday through
Sunday. This should provide enough dynamics that, coupled with
mid to lowlevel moisture and with the instability brought by a
positively tilted trough, should allow for a chance of showers
Saturday. Any showers should be scattered and light QPF associated
given lack of deep moisture.

  The operational models and ensembles diverge on a signal into
early next week, however there remains chances for showers and
storms especially Monday into Tuesday. There remains a low
confidence in timing. Max Temperatures look to remain below normal
through the upcoming week. This should make the week quite
comfortable, depending on humidity this week.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 202 AM EDT Thu Jul 4 2024

VFR will persist through the forecast period as high pressure
rolls into the region. There are signals that MKG may see fog
early Thursday morning but there is low confidence that visibility
will be greatly impacted. There is low confidence in a slight
chance for showers this afternoon at KGRR. As such have put VCSH
into the TAF but any showers are a low probability and shouldn`t
reduce cigs/vsbys.

&&

.MARINE...
Issued at 350 AM EDT Thu Jul 4 2024

Light winds and waves 1 foot or less for Independence Day. On
Friday, scattered thunderstorms could cross Lake Michigan between
morning and afternoon. Somewhat stronger winds could build up waves
and perhaps make the swim risk moderate, though there is more
variability than typical among the models regarding the exact wind
speeds and direction through the day.

&&

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

&&

$$

DISCUSSION...CAS/Ceru
AVIATION...Ceru
MARINE...CAS