289
FXUS63 KGRR 210610
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
210 AM EDT Fri Jun 21 2024

.KEY MESSAGES...

- Risk for Severe Storms through this afternoon

- Heat continues

- One or more rounds of Strong/Severe Storms possible Saturday
  into Saturday night

&&

.UPDATE...
Issued at 927 PM EDT Thu Jun 20 2024

Radar indicates dwindling precipitation with low prospects for
redevelopment overnight, so will lower PoPs this update. The
only other concern of note will be the possible development of
patchy fog overnight especially where the heavier precipitation
had occurred. This would include the I-96 corridor from Grand
Rapids to Lansing as well as areas from Rockford to Belding.

&&

.DISCUSSION...
Issued at 311 PM EDT Thu Jun 20 2024

- Risk for Severe Storms through this afternoon

SPC`s meso page shows ML CAPE values around 1500 J/kg extending
from near Grand Rapids to Lansing and down into the Jackson
region. Low level convergence is enhanced in this region and as a
result scattered storms have developed. These storms have produced
outflow boundaries some of which have collided. Effective bulk
shear was up to 25 knots and some organization has occurred with
the cells. High res models suggest the cells will gradually build
east so the risk for a few severe storms remains possible into the
late afternoon hours.

- Heat continues into Saturday

The low level thermal ridge remains over the southern and central
zones into Saturday. At the same time there will be plenty of
moisture in this warm air mass with PWAT values hovering in the
1.75 to 2.00 inches. Progged heat index values are shown to
continue to top out in the 93 to 97 degree range during this
timeframe. As a result we will keep the head advisory going.

- One or more rounds of Strong/Severe Storms possible Saturday
  into Saturday night

Parameters are coming together to warrant a greater risk for
severe weather during this time frame. Overall the models are
generally showing a stronger mid level wave digging into the area
in a pattern of stronger mid to upper level flow. A morning low
level jet could set off an MCS, especially over northern parts of
the CWA. A second low level jet arrives in the evening. Ensemble
surface based CAPE values from the models are showing at least
1000 J/kg in the afternoon/evening. With the potential for
stronger wind fields, favorable dynamics and instability, we may
end up with organized storms with stronger/organized wind gusts
being a main hazard. With several rounds of heavier rain possible,
values could be excessive.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 210 AM EDT Fri Jun 21 2024

Areas of MVFR/IFR in fog/stratus expected through 12Z with a
small chance of local LIFR vsbys as well, mainly at JXN and LAN.
Conditions lifting toward VFR by afternoon, although a few
t-storms possible in the afternoon at LAN and JXN. Once again the
strongest cells may contain torrential rain, gusty winds and hail
so if they directly impact a terminal there could be brief IFR or
lower conditions and possibly a wind gust over 30 kts. Convection
ending by 00Z this evening.

&&

.MARINE...
Issued at 311 PM EDT Thu Jun 20 2024

Overall, outside of any storms the winds and waves remain low into
Friday. On Saturday its looking like the winds and waves will be
building with a high potential for hazardous swim and boating
conditions arriving by evening.

&&

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

&&

$$

UPDATE...TJT
DISCUSSION...MJS
AVIATION...Meade
MARINE...MJS