487
FXUS63 KGRR 201147
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
747 AM EDT Thu Jun 20 2024

.KEY MESSAGES...

- Continued hot/humid with scattered storms

- Very warm and windy Saturday; Heat Advisory extended

- Showers and Thunderstorms Saturday night

- Dry and somewhat more comfortable Sunday through Tuesday

&&

.DISCUSSION...
Issued at 340 AM EDT Thu Jun 20 2024

- Continued hot/humid with scattered storms

Radar shows scattered storms moving east of US-131 and north of
I-96. These formed southwest of GRR earlier in an area where low
level lapse rates were maxed around 5.5 c/kg and LI`s were around
-5c. These storms will move out of the cwa by 12z.

Surface obs indicate a weak cold front is located near M-46 and
moving south. Convergence near this boundary coupled with various
weak outflow boundaries will continue to produce convection today.
Additionally, northeast winds behind the frontal boundary will
converge with lake breeze boundaries that develop behind the wave
of low pressure that the GFS/ECMWF have for a couple of days
showed that moves ENE along the frontal boundary and that will
likely result in more showers/storms today. PWATs are progd close
to 2 inches again this afternoon, so a heavy rain threat exists.
Indeed, local probability matched mean from the 00z HREF shows the
potential for 1-2 inch rainfalls in the heavier cells that
develop. The rain loading could also result in some stronger wind
gusts.

South of the frontal boundary, heat and humidity will remain with
heat indices 95-97 degrees.

Temperatures will fall to around 60 tonight over the northern cwa
that will be influenced by the northeast winds from Lake Huron.
Elsewhere lows will fall into the upper 60s. Generally dry
conditions are expected tonight.

Friday will see highs 85 to 90 with heat indices 94-96 degrees.
The weak frontal boundary from today will move north Friday
generating more scattered convection. Given PWATs aoa 2 inches,
a heavy rain threat will exist.

- Very warm and windy Saturday; Heat Advisory extended

Breezy warm sector will overspread Southwest Michigan on Saturday as
a warm front lifts into nrn Lwr MI with a cold front situated to our
west from near MSP to DSM. Southwest wind gusts of 30-35 mph
possible in the afternoon once deeper mixing gets going within
tightening, cyclonic sfc pressure gradient. Much of the day should
be dry, at least near and south of I-96, with highs in the lower 90s
away from Lk MI. The heat advisory over srn sections has been
extended into Saturday to account for this final surge of hot air.
Highest pops will be up near U.S. 10, closer to the sfc warm front.

- Showers and thunderstorms Saturday night

A shortwave preceding the primary H5 trough approaching from the
northwest comes through on Saturday night along with the sfc cold
front. While deep layer shear values around 35 kts would support the
possibility of strong to severe storms, it`s questionable how much
instability will be available given the late-night frontal passage.
Either way will have high pops (60-90 percent) during this time.

- Dry and somewhat more comfortable Sunday through Tuesday

We eventually dry out on Sunday behind the cold front with temps and
dew points falling to more comfortable levels. A surface high moves
in for Monday and that may keep things dry into Tuesday as well.
However pops ramp up again by midweek as the next cold front and H5
trough drop in from the northwest. An even cooler air mass looks to
follow that front by around Thursday of next week with dew pts
dropping into the 40s.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 747 AM EDT Thu Jun 20 2024

VFR expected for most of today/this evening although brief
periods of MVFR/IFR are possible if and when any of the widely
scattered showers and storms impact a terminal. The most likely
timeframe for thunderstorms is between roughly 20Z and 02Z, some
of which may contain torrential downpours and gusty winds. MKG
should be excluded from the thunder threat later today due to a
stable onshore/lake breeze. Scattered showers and storms weakening
and ending after sunset then the main concern will become areas
of fog and stratus developing overnight, especially where rain
occurred.


&&

.MARINE...
Issued at 340 AM EDT Thu Jun 20 2024

High pressure over Lake Superior will produce a very weak pressure
gradient during the next couple of days resulting in low wave
heights. Southwest winds will increase Saturday ahead of the next
cold front and may pose a threat to boaters and beachgoers.

&&

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

&&

$$

DISCUSSION...Meade/04
AVIATION...Meade
MARINE...04