FXUS63 KGRR 102247

Area Forecast Discussion
National Weather Service Grand Rapids MI
647 PM EDT Mon Aug 10 2020


Issued at 302 PM EDT Mon Aug 10 2020

- Damaging Winds Possible This Evening

- Quieter Weather Until the Weekend


.DISCUSSION...(This evening through next Monday)
Issued at 302 PM EDT Mon Aug 10 2020

-- Damaging Winds Possible This Evening --

An interesting day for sure, with two SPC category changes in our
area because of an upstream derecho moving into southern WI and
northern IL. Severe thunderstorms are exiting Iowa and in the
process of moving into southern WI/northern IL as of this writing,
producing widespread damaging wind gusts of 70-100 mph. Reports of
extensive tree damage, flipped trailers, and roofs blown off have
been received. These storms are thriving off MLCAPE of 3500 J/kg
and 30-40 kts of effective shear. While our deep layer shear is
not as high, our region is under the influence of MLCAPE values
around 3000 J/kg this evening and about 30 knots of effective
shear. While the 850-300 mb mean storm motion is oriented WSW-ENE,
the 1000-500 mb thickness pattern (and CAM sim ref) suggests a
southeastward jaunt once the line gets to metro Chicago or
southern Lake Michigan. We are not 100% certain about this
propagation, however, and it is possible that areas further north
between I-94 and I-96 could be on the northern cusp of this line
of storms.

This will be a close call for us but the Enhanced Risk area does
outline the locations most likely to get some damaging winds that
could easily knock down trees and power lines, but could also
cause roof and structural damage. The area of greatest concern
would be from South Haven east toward Kalamazoo/Battle Creek where
damaging winds and loss of power are a distinct possibility,
especially from 5pm-9pm. East of there, storms should be in a
weakening mode. The higher-end damage may be more pronounced
across northern Illinois stretching over to northern IN. Still,
all interests particularly near and south of I-94 need to pay
close attention tonight and have a way to receive alerts and seek
sturdy structure. This is particularly true for campers and those
stationed along the lakeshore, who are more vulnerable.

Speaking of the lakeshore, there is a chance that this line of
storms produces a meteotsunami. These form as strong or severe
thunderstorms cross the lake at a high rate of speed and produce
pressure jumps, partially dependent on the bathymetry of the lake
as well. We did receive a report of a pressure jump of 6 mb in 10
minutes in Cedar Rapids, IA, which is sufficient for meteotsunami
development over Lake Michigan. The greatest risk for one
developing would be somewhere in the 5pm-7pm time frame,
especially near or south of the South Haven area. An abrupt
fall and rise in water level (minutes apart) would be the hazard.
You do not want to be on the beach or in the water when this

-- Quieter Weather Until This Weekend --

We should be in a quiet pattern until this weekend, when a cold
front may be approaching by late in the period and warm, humid air
ahead of it may help produce some scattered showers and


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 644 PM EDT Mon Aug 10 2020

Thunderstorms will be ongoing at the beginning of the period.
Current trends indicate the back edge of the pcpn should be at MKG
by 02z or so. After that clouds should lift, but some MVFR/IFR fog
is likely to develop prior to sunrise for a few hours.


Issued at 302 PM EDT Mon Aug 10 2020

Marine interests should consider seeking port early this evening
before a severe line of thunderstorms moves in between 5pm-7pm,
especially south of Holland. These storms may produce winds in
excess of 50 knots, and a meteotsunami (abrupt fall/rise of water
level) is possible. A Coastal Flood Warning and Beach Hazards
Statement will be issued for Van Buren County, with a Coastal
Flood Advisory and Beach Hazards Statement for Allegan County.


MI...Beach Hazards Statement through Tuesday morning for MIZ064-071.

     Lakeshore Flood Advisory until 10 AM EDT Tuesday for MIZ064.

     Lakeshore Flood Warning until 10 AM EDT Tuesday for MIZ071.

LM...Small Craft Advisory until 10 AM EDT Tuesday for LMZ844-845.




NWS GRR Office Area Forecast Discussion