-- Discussion containing changed information from previous version are highlighted. --
918
FXUS63 KGRR 131858
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
258 PM EDT Thu Jun 13 2024

.KEY MESSAGES...

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- Low/Medium Confidence on Severe Storms this evening

- Near Record Heat and Bouts of Heavy Rain possible next week
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&&

.DISCUSSION...
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Issued at 257 PM EDT Thu Jun 13 2024

- Low/Medium Confidence on Severe Storms this evening

Current radar trends show a line of strong to severe storms across
the Michigan U.P. along the cold front, while convection is just
starting to initiate across northern Illinois into western Lower
Michigan in an area of low level convergence combined with mid-
upper level ascent in the pre-frontal trough.

We expect that storms will initiate and begin to fill in across
central and southern Lower Michigan in the area of agitated cu
from Ottawa County to Gratiot County and then slowly drift
southeast ahead of the cold front, clearing the southeast forecast
area by 9 pm.

Downdraft potential energy and deep layer shear is sufficient for
a threat of at least isolated damaging winds, which could become
more widespread along an organized line of storms this evening as
the convection moves into a more unstable airmass that has not
been modified by Lake Michigan. That "lake shadow" effect could
inhibit stronger convection across the central forecast area,
decreasing the extent of severe storms there.

The cold front comes through later tonight with fair and cooler
weather for Friday and Friday night.

- Near Record Heat and Bouts of Heavy Rain possible next week

An upper level ridge will be building over the Great Lake region
through the weekend and through mid next week. The biggest concern
through this timeframe will be the building heat. Temperatures
may approach record high temperatures. The hottest day should be
Monday the 17th. Lansing, Kalamazoo and Battle Creek are currently
forecasted to be at or close to records. Grand Rapids current max
temp for Monday the 17th is within two degrees. Current record
for Grand Rapids is 95 (1994). The last time Grand Rapids airport
reached 95 or greater was June 21st, 2022.

This is expected to be the first period of 90 degrees or more for
this year. One of the concerns is the duration of the heat. There
are potentially three to five days in the 90s from Sunday through
Thursday. Lows through this period stay around 70 degrees so
maximum low temperature records are at risk as well from being
broken.

There is the potential through this period, Sunday through
Tuesday, that several Mesoscale Convective Systems (MCS) move
through the Great Lakes. The latest EC/GFS along with ensemble
means are trending toward a dome of High pressure setting up over
the southeast which will force moisture to stream over this high
through the region which could bring a steady stream of moisture
along with the warm air. So while periods of showers will be
possible the heat and humidity could create heat indices near 100.
This could allow for an extended period of Heat Advisory
Criteria.

As far as precipitation goes there is enough spatiotemporal
variability to not have large pops in any one period. However,
latest ensembles show a multi day period of +1 to +2 anomalous
PWATS over Michigan with PWATS upwards of 1.5 inches through this
timeframe. So there could be periods of showers/storms possible
late Sunday and late Monday. Given the hot/humid air mass, and
given the previously mentioned PWATS, heavy rain is a
consideration.
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&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
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Issued at 155 PM EDT Thu Jun 13 2024

Showers and storms still expected this afternoon and evening. Have
increased the onset timing considering the current line moving
over Lake Michigan. Expect them to reach the lakeshore between 19
to 20Z with MKG begin affected first. The line should move through
quickly so expect any showers and storms to be done between 00Z to
03Z.

There is a period of MVFR cigs and vsbys overnight into Friday
morning. Especially between 09Z to 13Z.
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&&

.MARINE...
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Issued at 257 PM EDT Thu Jun 13 2024

Strong thunderstorms this afternoon and evening could bring
locally strong winds to 50 knots, hail and higher waves.
Otherwise, southwest winds to 20 knots this afternoon will
decrease overnight. Then winds will go north behind a cold front
on Friday to 20 knots and some choppy waves around Big and Little
Sable Points.
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&&

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

&&

$$

DISCUSSION...Ostuno/Ceru
AVIATION...Ceru
MARINE...Ostuno