Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 031804
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
204 PM EDT Mon Aug 3 2020

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 328 AM EDT Mon Aug 3 2020

Scattered showers and a few thunderstorms can be expected today and
Tuesday. Then high pressure builds in for the rest of the week with
fair weather and comfortable temperatures.

&&

.UPDATE...
Issued at 1236 PM EDT Mon Aug 3 2020

A thermal trough aloft is trying to push in from the north and
west this afternoon which will increase instability. The thermal
trough is associated with an upper trough that is moving in as
well. August sunshine into a cooling airmass aloft will result in
MUCAPE values rising to around 1000 J/KG per the RAP model. This
type of instability is already in place across the southern half
of the forecast area due to a higher low level moisture content
there (surface dew points in the lower 60s). We expect scattered
showers and isolated thunderstorms to continue to develop this
afternoon, likely peaking in coverage and intensity around max
heating in the 400pm to 600pm time frame. Cannot rule out some
small hail in the strongest activity during this time frame. The
activity will be diurnally driven so a gradual fade to the showers
and storms will occur during the evening hours. Even though there
is a semblance of a lake breeze developing showers are even
possible at the lakeshore as the flow in the cloud bearing layer
(1-3km) over the lake is from the west and northwest. Highs today
will be below normals for this time of year in the 70s.

&&

.DISCUSSION...(Today through next Sunday)
Issued at 328 AM EDT Mon Aug 3 2020

...Scattered Showers and a few Thunderstorms through Tuesday...

A sharp upper trough axis extends south from a closed 500 mb low
centered north of Lake Huron with vort lobes rotating through
across Lower Michigan today into Tuesday. Lapse rates are steep
enough for some thunder, especially during the diurnal enhancement
in the afternoon.

...Stretch of fine summer weather into the weekend...

Drier weather arrives by Wednesday as heights rise slightly and
surface high pressure builds in. Looks like fine summer weather
into the weekend with cool temperatures and low humidity as dew
points will be in the 40s and 50s. Heights continue to rise with
moderating temperatures over the weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 204 PM EDT Mon Aug 3 2020

An upper trough axis continues to pivot through the Great Lakes
region this afternoon into early Tuesday, resulting in persistent
cloud cover, especially north of I-96, as well as a few showers.
These showers and a few storms will last into much of the
afternoon and evening, and will be more prominent for southern
and eastern TAF sites, such as AZO, BTL, JXN, and LAN. IFR
ceilings will also be possible into the early morning hours
Tuesday, primarily for southeastern TAF sites.

&&

.MARINE...
Issued at 1236 PM EDT Mon Aug 3 2020

North flow to slightly northeast flow will continue this
afternoon and tonight over Lake Michigan. Waves during this period
will peak this evening up near Big and Little Sable Points and
down towards South Haven.

Latest wave model runs though are indicating higher waves even
this afternoon/evening in our middle zones (Holland/Grand
Haven/Muskegon) and again on Tuesday afternoon in all areas. We
will take a look as we move through the afternoon but we may need
to expand our SCA and BHS headlines to all zones all the way
through Tuesday evening (possible midnight Tuesday night).

Waterspouts look to be possible today through Tuesday night with
the upper trough overhead and plenty of over water instability. A
healthy shortwave plows across Northern Lake Michigan. We will be
on the southern periphery, but likely close enough for deeper
showers worth of producing waterspouts in this regime.


&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through Tuesday evening for MIZ064-071.

     Beach Hazards Statement until 11 PM EDT this evening for MIZ037-
     043.

LM...Small Craft Advisory until 11 PM EDT this evening for LMZ848-849.

     Small Craft Advisory until 11 PM EDT Tuesday for LMZ844-845.

&&

$$

UPDATE...Duke
SYNOPSIS...Ostuno
DISCUSSION...Ostuno
AVIATION...Thielke
MARINE...Duke


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