640
FXUS63 KGRR 222341
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
741 PM EDT Sat Jun 22 2024

.KEY MESSAGES...

- Strong/severe storms this evening into tonight

- Round of storms possible for late Monday night into early
  Tuesday

&&

.UPDATE...
Issued at 1137 AM EDT Sat Jun 22 2024

Based on the latest wave model forecasts, we expanded the small
craft advisory and beach hazards statement one county/zone further
north.

One area of heavy rain was tracking through the Pentwater
to Ludington region. It appears that fairly widespread amounts of
around an inch may accompany this initial batch. Additional heavy
rain is likely this afternoon and evening with the storms in
southern WI tracking east northeast. Off of SPC`s meso analysis
page, PWAT values were around 2 inches in this region with really
impressive 925-850 mb moisture transport going on. 12z soundings
from GRB and APX support an efficient warm rain event along with
warm topped convection with the MPL well under the trop. Rainfall
rates could easily top an inch per hour. Thus we will maintain the
flood watch.

As for the severe potential, low level convergence was maximized
closer to the warm front over the northern zones with divergence
flow further south. Instability was building throughout the area
with ML CAPE values up to 500 J/kg and bulk effective shear values
over 30 knots. An area of stronger 0-1 km shear vectors was noted
west of Pentwater and moving northeast. The flow remains
relatively divergent for much of the CWA for a few more hours so
we may struggle to develop widespread convection. However by mid
afternoon and more so late afternoon we could start to see an
increase in the low level convergence which would support newer
storm development. We will also need to monitor the trends of the
storms in southern WI as when the try to track into the CWA, they
could generate enough low level lift to persist. Given the
favorable instability and shear at that time, we could end up with
an organized cluster of storms rolling through. Until then, it
appears the northern parts of the region is the favored location
for strongest storms given stronger 0-1 km bulk shear values
through mid afternoon.

&&

.DISCUSSION...
Issued at 234 PM EDT Sat Jun 22 2024

- Strong/severe storms this evening into tonight

The combination of decent ML CAPE values of 500 to 1000 J/kg, 100
to 125 J/kg of 0 to 3 km ML CAPE and increased surface convergence
along a warm front has produced a favorable setup for some storms
with rotating updrafts over northeast parts of the CWA. The low
level convergence is progged to weaken over the next hour or two
so that may limit the coverage of storms. By 23z, the storms in
WI may be working into western parts of the CWA. Overall the low
level convergence is still shown to be weak but high res models
are showing another area of storms arriving in the 01z to 03z
window. This is when the mid level jet is pushing in from the
west. As a result these storms do have a potential for enhanced
wind gusts if they do indeed form. Its not until 12z Sunday that
the cold front moves in so another round of storms will be
possible then.

- Round of storms possible for late Monday night into early
  Tuesday

Models are showing a mid level wave dropping down from the
northwest during this time. MU CAPE values are over 1000 J/kg with
favorable deep layer shear values. Thus a potential exists for a
round of thunderstorms during this period. Given the combination
of shear and instability the convection could become organized.
Tuesday afternoon, MU CAPE values top 2000 J/kg and favorable deep
layer shear is shown to exist but the models are also showing a
cap. For now we will maintain a low risk for storms then.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 741 PM EDT Sat Jun 22 2024

Thunderstorms crossing Lake Michigan should affect MKG shortly
after the starting valid time of 23/00Z for this latest TAF
package. Farther south, we could be looking at much more of a
delay at the other terminals, where I did try to refine timing
with a brief period of -TSRA for each site.

As noted before, wind intensity associated with convection
remains uncertain and have therefore downplayed this for now, but
may have to add gusts in the 30-40kt range depending on initial
observations on this side of the lake.

Expect LLWS to manifest at MKG especially after thunderstorms that
are currently over Lake MI pass east of the terminal. On the
backside of the storms, IFR cloud cover will likely spread in as
the winds shift around to the west northwest. Only gradual
improvement to MVFR is expected Sunday.


&&

.MARINE...
Issued at 234 PM EDT Sat Jun 22 2024

Southwest winds out ahead of the main wave may gust over 20 knots
at times this afternoon but stronger winds will be possible
tonight into Sunday as the wave moves in. Thus we will maintain
the marine an beach hazard headlines for tonight into Sunday
morning.

&&

.HYDROLOGY...
Issued at 234 PM EDT Sat Jun 22 2024

The PWAT values are up around 2 inches and area soundings are
showing low lcl`s along with thin deep CAPE, especially for
northwest zones. Favorable 925 to 850 mb moisture transport exists
through southern WI and into our northwest zones including the
Pentwater to Ludington region.  The deep wave moving in from the
west will encounter an unstable air mass here in MI for this
afternoon and into tonight leading to showers and storms with
heavy rain. Already there has been 1.5 to 2.5 inches for parts of
Mason, Oceana and Lake counties. High res models show swaths of
heavier rain still to come with locally over another 1.5 inches
looking likely. Given the risk for additional heavy rain in
combination with the rainfall which has already fallen, there
could be some flooding from these cells so we will keep the flood
watch going. For the larger rives, they are relatively low.
Projected rainfall amounts keep the forecast points under action
stage at this time.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch until 6 AM EDT Sunday for MIZ037>040-043>046-050>052.
     Beach Hazards Statement through Sunday morning for MIZ050-056-
     064-071.
MARINE...Small Craft Advisory until noon EDT Sunday for LMZ844>847.

&&

$$

UPDATE...MJS
DISCUSSION...MJS
AVIATION...TJT
MARINE...MJS
HYDROLOGY...MJS