255
FXUS63 KGRR 212348
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
748 PM EDT Tue May 21 2024

.KEY MESSAGES...

- Severe Weather Potential Tonight

- Dry for Wednesday and Thursday; Rain Chances Return Friday

- Rain Chances Linger Through the Holiday Weekend

&&

.DISCUSSION...
Issued at 350 PM EDT Tue May 21 2024

- Severe Weather Potential Tonight

Not expecting more than scattered showers and thunderstorms
through this evening before a squall line moves across Lake
Michigan from Wisconsin and reaches the western zones around
midnight.

Flat shortwave ridging at mid-levels is apparently suppressing
afternoon convection today, but heights will begin to fall as
a negatively-tilted shortwave trough closes off over Minnesota
accompanied by sfc cyclone tracking into Canada with a cold front
advancing into Lower Michigan overnight.

An analysis of model soundings and other model-generated severe
weather probability guidance suggests the greatest chance for
severe weather tonight will be across the western forecast area
near the Lake Michigan shore. A line of storms in western Iowa at
330 PM EDT is progged to expand and move east, crossing Lake
Michigan. Main threat for our nearshore waters and western
forecast area is for downburst winds but transient QLCS
mesocyclone circulations cannot be ruled out, resulting in
enhanced swaths of higher winds and even a quick tornado.

Model soundings suggest instability decreases as the storms move
east and some of the model guidance show a corresponding weakening
of the line of storms before they reach Grand Rapids.

The front becomes more parallel to the environmental winds on
Wednesday slowing and stalling near or south of the forecast area
with lingering showers possible near JXN during Wednesday.

- Dry for Wednesday and Thursday; Rain Chances Return Friday

Good forecast certainty remains for a dry Wednesday and Thursday and
this will be accompanied by much more seasonal highs in the 70s.
Humidity will be substantially less oppressive too with dewpoints
dropping into the 50s and then 40s by Thursday.

GOES satellite water vapor channels show an upper PV maximum
currently diving southward into the Pacific Northwest region. This
feature will eventually emerge as a cyclonically breaking upper wave
over the Upper Mississippi valley region Friday afternoon/evening.
Strong upper divergence with this feature will support a solid
coverage of showers and storms pushing eastward into our forecast
area Friday night, although the best QG forcing should remain well
northwest of our area. This combined with nocturnal stabilization
should limit the threat for severe thunderstorms.

- Rain Chances Linger Through the Holiday Weekend

Unfortunately, forecast details for Memorial Day weekend remain
quite uncertain. A brief lull in precipitation for at least part of
Saturday seems probable, but that`s about as definitive as I want to
get right now. Starting Saturday afternoon, PoPs for the rest of the
holiday weekend look to be in the neighborhood of 20 percent, which
is right around climatology. High temperatures will be quite close
to climatology as well in the neighborhood of low 70s.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 748 PM EDT Tue May 21 2024

This set of terminal forecasts features a fairly significant
change from previous thinking with regards to thunderstorms
later tonight. This change is dropping thunder wording altogether
at all of the sites except KMKG, where we went with a VCTS.
Convection is not as widespread or strong as thought earlier. The
latest model trends give us increasing confidence that the
convection out to our west may just clip the NW portion of our
area. It will be a close call at KMKG, thus the reason with VCTS
there.

The biggest impact will likely be some MVFR cloud cover that will
move in near the cold front late tonight and first thing Wednesday
morning. A few showers will be possible with the actual front,
then just clouds. The clouds will then lift and break up Wednesday
afternoon.

Winds will increase late this evening from the SSW with gusts up
to 25 to 30 knots. The winds will generally stay up most of the
rest of the forecast period, becoming more from the SW on
Wednesday.

&&

.MARINE...
Issued at 350 PM EDT Tue May 21 2024

No changes to the Small Craft Advisory. In addition to strong
southwest winds from the tightening pressure gradient, there could
be gusts over 50 knots tonight as a squall line moves across Lake
Michigan, arriving in the nearshore area around midnight.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 8 PM EDT Wednesday for LMZ844>849.

&&

$$

DISCUSSION...Ostuno/TJT
AVIATION...NJJ
MARINE...Ostuno/Thielke