517
FXUS63 KGRR 170006
AFDGRR

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

.KEY MESSAGES...

- Thunderstorms Tonight

- Hot, humid with occasional thunder this week

&&

.UPDATE...
Issued at 802 PM EDT Sun Jun 16 2024

Radar at 00Z has scattered convection moving across Lake
Michigan. These storms are forming on, and being strengthened by,
the outflow from a mesoscale convective vortex (MCV) moving to
the northeast from Illinois. The big question remains how intact
will the storms be as they move from the lake into and across southern
Michigan. Latest CAMS continues to show scattered thunderstorms
moving through Lower Michigan through the first half of the
overnight period from this outflow.

 Another round of showers and storms will form overnight towards
morning coinciding with MCV`s trek to the east. The latest CAMS
have consistently had the potential for heavy rain across the I
96 corridor, especially through Kent and Ionia counties. PWATS
continue to be in the 1 to 2 inch rain and the model soundings
show an almost completely saturated atmosphere early Monday
morning. There is a concern that mid level winds will be weaker
overnight and the MCV will slow down allowing for a longer
duration of showers. There is still some questions as to where
the heaviest rain will fall but given the trends there is the
potential for 1 to 2 inches along and north of the I 94 corridor
and along and south of the I 96 corridor with locally heavier
amounts in and around thunderstorms.

&&

.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 /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 725 PM EDT Sun Jun 16 2024


The first item to watch will be a gust front moving off Lake
Michigan and toward the terminals in the 01z-03z time frame. This
gust front is an outflow boundary from earlier storms across
Northern Illinois. It will cause a wind shift from southerly to
westerly or WSW winds for a short time, potentially gusting to
around 35 kts especially MKG/GRR/AZO. A shower is possible along
this boundary, and perhaps a brief thunderstorm.

After this boundary moves through, some additional showers and
thunderstorms could develop especially closer to 06z. These could
impact just about any of the terminals but those with the higher
likelihood are GRR and LAN. Several runs of high resolution models
have indicated some localized heavy rain rates, thus some reduced
visibilities to IFR may occur in the 06z-12z window, but low
confidence exists at any given terminal.

&&

.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.

&&

$$

UPDATE...Ceru
DISCUSSION...Ostuno
AVIATION...Hoving
MARINE...Ostuno