387
FXUS63 KGRR 290741
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
341 AM EDT Sat Jun 29 2024

.KEY MESSAGES...

- Locally Heavy Rain Before Sunrise This Morning

- Low Chance for Showers and Storms This Evening

- Warming Trend early next week

- Risk for Thunderstorms later Tuesday into Wednesday

&&

.DISCUSSION...
Issued at 340 AM EDT Sat Jun 29 2024

- Locally Heavy Rain Before Sunrise This Morning

Scattered to widespread rain continues to move through as an upper
trough descends into the region and a surface low approaches from
the west. Prefrontal surge in moisture as already pushed surface
dewpoints into the mid to upper 60s. Certainly a moist environment
early this morning with PWATS around 2+ inches. As mentioned in the
AFD update last evening, the 00z HRRR painted a 4+ inch narrow swath
of rain right along the I-94 corridor. Fortunately recent runs have
not continued that 4+ inch trend, but nearly all subsequent runs
have advertised the potential for some areas within the I-96 to I-
94 corridors to get up to 2 inches of rainfall early this morning.
The 00z SPC HREF Probability Matched Mean supports this expectation
while painting a few swaths of 1-2 inches across the southern
half of lower Michigan between 2am to 8am this morning.

MRMS 3 hour QPE already highlights areas in Allegan with receiving
a swath of 1 inch rainfall, and with more heavy rain on the way
we will be monitoring for possible flood headlines this morning.

Few embedded thunderstorms are possible in this cluster of showers,
but high Convective Inhibition and low instability early this
morning will put a hamper on severe potential. Most of the showers
should be to our east before CIN erodes and diurnal instability
recovers.

- Low Chance for Showers and Storms This Evening

More isolated shower and thunderstorm activity is anticipated this
afternoon into tonight as a cold front sweeps in from the northwest.
Diurnal instability rebounds into this afternoon while 500mph winds
build overhead supporting deep-layer shear. Noting enough kinematics
to support thunderstorm development with the progression of the cold
front, a few strong to severe storms may develop this afternoon and
evening along the front. Due to the cold front being estimated to
sweep through near to after 00z, enough of a cap may begin to
develop to limit too much activity the further south it progresses,
though there are some suspicious looking soundings that say
otherwise. High-resolution models advertise an isolated
shower/thunderstorm potential which seems reasonable but still worth
monitoring.

- Warming Trend early next week

High pressure at 850 mb will be centered over the northern Great
Lakes region to start the day Monday. Northeast winds over Lower
Michigan will keep the cool air in place on Monday with the low
level thermal trough overhead as shown by the models. Ensemble
temperature graphs reflect this cooler airmass with max
temperature values staying in 70`s. However the 850 mb high
pressure system shifts east of the area Tuesday and warm air
advection develops. The warm air advection then persists through
Tuesday and possibly beyond. Ensemble temperatures plots show a
steady warmup for this period with max temperature values
returning to normal by Wednesday.

- Risk for Thunderstorms later Tuesday into Wednesday

Models are sagging a cold front in our direction later Tuesday and
into Wednesday. There`s some uncertainty if this system will push
through the area or not. They do show the PWAT`s climbing back up
towards 2 inches later Tuesday along with some instability. Given
the moisture, lift and instability, it does look like some storms
will be tracking towards the CWA as that front sags in our
direction later Tuesday. Northwest zones, including the Ludington
area will feature the highest POP`s. Further southeast, given the
uncertainty for the front to push through, we will taper the POPs
off with Jackson featuring the lowest POP`s. The latest ensemble
24 hr qpf values from the GFS and CMC show a decrease in the
values but the do suggests a fairly high probability for
precipitation as most members show some QPF.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 128 AM EDT Sat Jun 29 2024

Based on radar trends, newer model data and lightning plots, I
added more widespread shower and thunderstorm activity to the TAF
forecasts for the remainder of tonight. Convective cells with
occasional lightning strikes were already approaching the KJXN TAF
site. Further west and northwest, the cells crossing Lake MI had a
few lightning strikes in the as well and they were rapidly moving
onshore. Given the very moist airmass moving, it has not take much
of a cell to send VSBY`s down into the IFR category as evidenced
by the brief IFR conditions as KAZO and KBTL in the past hour.
Through the remainder of the night, the atmosphere only becomes
more unstable and the latest model data shows the convection
increasing. However, between 10 and 13z the strongest cells
dissipate from west to east.

IFR is likely to overspread the area as the strongest cells shift
east of the TAF sites. It`s a very moist airmass moving in and
with a cold front approaching from the northwest, that will keep
low level convergence going which will support low ceilings and
even some drizzle. However the low cloud conditions are expected
to lift into the MVFR/VFR range mid to late afternoon. An isolated
shower/storm is possible then, but the coverage at this time
looks to thin to warrant adding them to the TAF forecast.

&&

.MARINE...
Issued at 340 AM EDT Sat Jun 29 2024

A cold front will sweep through this afternoon and evening shifting
winds to the northwest. Waves will quickly build late tonight and
reach near 4 feet after midnight and will increase further into
Sunday morning. Waves will peak in the 4 to 6 foot range Sunday
morning and will gradually diminish into Sunday afternoon. A SCA and
BHS will likely be needed for Sunday morning to early afternoon.
Weather conditions will not be idea for beach weather, with
potential upwelling and temperatures in the 60s.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 AM EDT this morning for LMZ847>849.

&&

$$

UPDATE...
DISCUSSION...MJS/Thielke
AVIATION...MJS
MARINE...Thielke