415
FXUS63 KGRR 291615
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1215 PM 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

&&

.UPDATE...
Issued at 1035 AM EDT Sat Jun 29 2024

As expected, widespread morning rain has moved off to the east,
leaving us in cloudy but drier conditions. A few breaks of sun are
trying to develop, which is a trend that should continue as the
day unfolds. The big question remains how much shower/storm
redevelopment happens this afternoon. 12z HRRR and NAMNest are
both trying to generate a broken line of storms by 18z that
primarily affects the far southeast corner of our counties (i.e.
Jackson area), but confidence in this redevelopment is still
limited. However, extremely humid air continues to move into the
area this morning, so any breaks of sun will allow temps (and
instability) to begin climbing quickly.

&&

.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 /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1215 PM EDT Sat Jun 29 2024

MVFR conditions at all the terminals early this afternoon will
slowly improve to VFR late this afternoon and evening. There is
potential for some showers and perhaps isolated convection to
develop in the vicinity of the I-94 corridor from KBTL eastward
through mid afternoon.

At this time we will have VCSH in the forecast through mid
afternoon at KBTL and KJXN as there is not enough potential for
storms to warrant inclusion in any of the forecasts. However these
will be updated if and when any storms develop early this
afternoon.

Isolated showers and perhaps an isolated storm may develop this
evening and could potentially briefly affect any of the terminals.
However for now we kept this out of the terminal forecasts for
this evening given quite low probability of occurrence. We expect
a mix of MVFR to VFR conditions overnight into Sunday morning with
MVFR due to low clouds.

&&

.MARINE...
Issued at 1035 AM EDT Sat Jun 29 2024

The only change this morning is that we`ve gone ahead and issued
the Small Craft and Beach Hazards statements for the upcoming
hazardous conditions. The worst conditions are expected Sunday
morning, but overall conditions will be hazardous for most of
tonight and most of the day Sunday. All zones will be impacted.

Previous Discussion: 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...Beach Hazards Statement from 10 PM EDT this evening through
     Sunday evening for MIZ037-043-050-056-064-071.
MARINE...Small Craft Advisory from 10 PM this evening to 10 PM EDT Sunday
     for LMZ844>849.

&&

$$

UPDATE...AMD
DISCUSSION...
AVIATION...Laurens
MARINE...AMD