670
FXUS63 KGRR 041554
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1154 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

&&

.UPDATE...
Issued at 1154 AM EDT Thu Jul 4 2024

Surface high pressure is located across the Great Lakes region as
we approach midday. Aloft, a mid level shortwave is moving toward
our area from Illinois. Rain showers precede this shortwave over
Illinois and Indiana. The current thought is the going forecast
has things covered well as only very small chances for spotty
showers exist. The 12Z HREF continues to show some isolated rain
shower activity this afternoon and early evening, but there is not
a consensus within the members of the HREF on precipitation
occurring. HRRR model soundings this afternoon show some mid
level moisture with very little to almost zero CAPE. Below the mid
level moisture, rather dry air exists down to the surface. The
dry air will work on any precipitation moving our direction and
will likely keep us precipitation free. So, we expect partly sunny
to mostly cloudy skies with some virga or perhaps an isolated
light shower or a few sprinkles. We think most areas will remain
dry. High temperatures will be in the 80s with light winds. All
said, a forecast for the fourth that is pretty typical for
Southwest Lower Michigan.

&&

.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 broad brush 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 low level 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 /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 747 AM EDT Thu Jul 4 2024

VFR will persist through the forecast period. There is low
confidence in a slight chance for showers this afternoon. Have
continued with VCSH into the TAF but any showers are a low
probability and shouldn`t reduce cigs/vsbys. Showers and storms
will move into the region overnight into Friday with periods of
showers moving into MKG after 11Z. Not expecting reductions at
this time.

&&

.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.

&&

$$

UPDATE...Duke
DISCUSSION...CAS/Ceru
AVIATION...Ceru
MARINE...CAS