974
FXUS63 KGRR 211732
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
132 PM EDT Fri Jun 21 2024

.KEY MESSAGES...

- Chance of thunder today, better chance Saturday and Saturday
  night

- Drying out Sunday; Pleasant Monday

- Strong storms possible Tuesday/Tuesday night

- Dry and cooler Wednesday through Friday

&&

.UPDATE...
Issued at 1116 AM EDT Fri Jun 21 2024

The clouds were decreasing around the CWA late this morning and
the instability was building. ML CAPE values were up to 1000 J/kg.
Effective bulk share values were elevated across the northern
zones with a ribbon of 25+ knots. Cams were generating isolated
storms by mid afternoon, which seems reasonable given the setup.
Low level convergence remains weak/neutral through mid afternoon
thus the low level lift may be a limiting factor. The values do
increase inland, mainly east of the CWA by late afternoon so its
possible stronger convection may develop then. We will also be
monitor the convection currently in WI, along with renewed develop
there this afternoon. If this batch of convection holds together,
it may make a run at the Ludington region this evening. Projected
heat index values still support values in the mid to upper 90s for
the advisory area, away from the lakeshore.

&&

.DISCUSSION...
Issued at 338 AM EDT Fri Jun 21 2024

- Chance of thunder today, better chance Saturday and Saturday
  night

The frontal boundary that triggered storms Thursday is now south
of the cwa and pretty nondescript. In it`s wake, we`ll have a
little less instability today, however, precipitable water values
will remain north of 1.75 inches. We probably won`t see much
precipitation this morning. However during the afternoon, the
frontal boundary to the south will begin to move back north and
there may be a 3-6 hr window beginning around 20z in which we see
some convective development over the southeast cwa. There will
likely be a decent lake shadow with any convective development due
to persistent west winds. Gusty winds and torrential downpours are
the main threat with any storms that develop today.

Highs around 90 today and dewpoints in the lower 70s will send
heat indices into the mid 90s again; no changes to the Heat
Advisory.

After a dry night tonight, chances for storms will increase
Saturday from north to south. Showers/storms will likely be
ongoing Saturday morning over northern Lower along the warm
frontal boundary. Once the the attendant low moves east, the cold
front will begin to drop south during the mid to late afternoon.
Showers and thunderstorms will likely develop along the front and
will mostly affect the northern cwa Saturday afternoon. SPC has
placed the entire cwa in a Marginal Risk for severe storms
Saturday/Saturday night. Heavy rain will also be a threat with
PWATs closing in on 2.5 inches.

Will maintain high probabilities (80-90 percent) for showers and
thunderstorms on Saturday night as lead shortwave and sfc cold front
pass through the region. Severe weather and heavy rain threat will
linger in the evening then wane overnight with the passage of the
cold front and departure of the 2"+ PWAT air. The frontal passage
will also bring an end to the persistent heat and humidity.

- Drying out Sunday; Pleasant Monday

On Sunday the primary nrn stream H5 shortwave drops into the nrn
GrtLks Rgn and a cooler/less humid air mass filters in on the heels
of northwest winds. Morning clouds and lingering showers/drizzle
giving way to decreasing clouds in the afternoon. Monday still looks
like a pleasant day as the sfc ridge builds in.

- Strong storms possible Tuesday/Tuesday night

Tuesday could be an active weather day as the warm/humid air makes a
quick return from the southwest and temps push toward 90 again. Fast
northwesterly flow aloft could send an MCS or two our way from
Wisconsin, thriving off cape values possibly in excess of 3000 J/KG.

- Dry and cooler Wednesday through Friday

The push of warmer air is short-lived as the passage of a cold front
on Tuesday night/early Wednesday ushers in another installment
cooler/drier air. Northerly winds on Thursday ahead of an advancing
sfc high sends dew points down into the 40s/50s. Lows in the 50s and
highs in the 70s expected later in the week as high pressure settles
overhead by 12Z Friday.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 127 PM EDT Fri Jun 21 2024

Some showers/thunderstorms possible the next 6 hours before a
quiet stretch should enter the forecast area. eastern areas most
favored for shower/storms this afternoon so included some vcsh
for GRR and LAN. could be a repeat of some fog tonight into early
sunday but confidence too low to include at this time. showers
and breezy conditions begin to work in toward the tail end of the
taf period.

&&

.MARINE...
Issued at 338 AM EDT Fri Jun 21 2024

A weak pressure gradient today will result in low wind and waves.
An approaching cold front Saturday will likely cause wind and
waves to increase and a Small Craft Advisory/Beach Hazards
Statement may be needed by Saturday afternoon.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Heat Advisory until 8 PM EDT Saturday for MIZ056-057-064>067-
     071>074.
MARINE...None.

&&

$$

UPDATE...MJS
DISCUSSION...Meade/04
AVIATION...Maczko
MARINE...04