857
FXUS63 KGRR 161905
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
305 PM EDT Sun Jun 16 2024

.KEY MESSAGES...

- Thunderstorms Tonight

- Hot, humid with occasional thunder this week

&&

.DISCUSSION...
Issued at 305 PM EDT Sun Jun 16 2024

- Thunderstorms Tonight

The showers and storms across the central and northern zones will
move east this afternoon and should be moving east and out of the
forecast area this evening. Then we watch an MCV moving through
northern Illinois and southern Lake Michigan into our southwest
zones with the potential to bring locally heavy rain, hail and
high winds overnight across the southern half of the forecast
area. SPC marginal risk covers this threat area.

- Hot, humid with occasional thunder this week

The MCV propagates east and out of the forecast area by Monday
morning but is expected to leave behind an outflow boundary across
the southern forecast area that could focus some convection Monday
afternoon. The HREF has apparently picked up on this idea and now
has 70 to 80 percent probabilities of 0.1 QPF in this area on
Monday afternoon. We painted an area of higher POPs there but kept
them in the 25 to 35 percent range given lack of confidence in
the specifics.

The Heat Watch was not changed and it still appears that this
could be a long duration event where the cumulative effect of high
daytime and night time temperatures could necessitate a warning
being issued even without warning criteria being met on any one
day. There remains uncertainty on extent of convection on Tuesday
afternoon, but the the biggest uncertainty remains on Thursday
and Friday as a cold front or convective outflow could modify
temperatures across the northern half of the forecast area.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1210 PM EDT Sun Jun 16 2024

Clusters of thunderstorms and moderate to heavy rain continue to
develop between Muskegon and Milwaukee. Models suggest less
instability available with eastward extent into Michigan, which
would support a slow weakening trend, so not confident enough yet to
include GRR in a TS around 18-19 Z but will do so for MKG.

There has been an increase in the number of short-term models which
capture the mesoscale convective vortex over northern Missouri to
southeast Iowa and thus produce thunderstorms over some of our
aerodromes tonight. If this MCV indeed tracks through northern
Illinois later today and into southwest Lower Michigan with
thunderstorms, storms in our area would be capable of the usual
hazards including IFR visibility with heavy rain, lightning, LLWS or
30-45 knot surface wind gusts and wind shifts. Most likely affected
would be AZO, BTL, and GRR, and MKG. Will go with VCSH for now but
later forecasts may need to narrow down a time frame to add TS at
some point between 02 and 12 Z.

&&

.MARINE...
Issued at 305 PM EDT Sun Jun 16 2024

No changes to the SCA or CFW. Current forecast on track with
southerly winds driving some higher waves from Grand Haven north
with structural and longwave currents making swimming dangerous.

Winds and waves could be locally higher in thunderstorms this
evening south of Muskegon.

Winds and waves will decrease slightly on Monday as the sfc
pressure gradient slackens a little, but the night shift can
evaluate this and determine if small craft and swim conditions
will improve enough for the SCA and CFW to expire at 2 am or have
to be extended.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 2 AM EDT Monday for MIZ037-043-050-
     056.
     Air Quality Alert from midnight tonight to midnight EDT Monday
     night for MIZ037-043-050-056-064-071.
     Excessive Heat Watch from Monday afternoon through Friday
     evening for MIZ043>046-050>052-056>059-064>067-071>074.
MARINE...Small Craft Advisory until 2 AM EDT Monday for LMZ847>849.

&&

$$

DISCUSSION...Ostuno
AVIATION...CAS
MARINE...Ostuno