-- Discussion containing changed information from previous version are highlighted. --
573
FXUS63 KGRR 261728
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
128 PM EDT Sun May 26 2024

.KEY MESSAGES...

- Showers and Thunderstorms Arrive Late Afternoon

- A Few Showers Possible Monday

- Considerable Clouds With Showers Monday Night Through Tuesday
  night

- Drier With A Warming Trend Wednesday Through Friday

- Rain Returns To The Forecast For Next Weekend

&&

.UPDATE...
Issued at 1033 AM EDT Sun May 26 2024

With high confidence for waves to be 4 feet or higher on Memorial
Day, we went ahead and issued a Small Craft Advisory and Beach
Hazards Statement. The NSH, SRF, ZFP and HWO were updated to
reflect these changes.

As for the severe weather threat, not much has changed. The cloud
cover is thickening up. We do build/advect in some instability
this afternoon, enough to generate some storms but overall the
projected amount looks limited with MU CAPE values likely staying
under 1000 J/kg through 04z. Bulk deep layer shear is forecast to
increase later this afternoon up to around 35 to 45 knots which is
sufficient for organized convection but it will have limited
amounts of instability to work with. Later tonight though, the
13z HRRR is showing a several hour window for CAPE values to
possibly exceed 1000 J/kg which could lead to stronger convection
as the cold front swings through. This is when the deep layer
shear drops off, which will be a limiting factor for organized
convection.   We did increase the POPs for later tonight as a
result.

&&

.DISCUSSION...
Issued at 306 AM EDT Sun May 26 2024

- Showers and Thunderstorms Arrive Late Afternoon

A Marginal Risk for severe weather remains near and south of I-96
today and tonight, with a Slight Risk south of I-94. Overall there
are two periods of interest. One will be from late afternoon into
early evening with the arrival of a surface warm front, bringing a
risk for thunderstorms that have primarily a hail threat. The other
period is later at night out ahead of a surface cold front where
there is a limited opportunity for a few storms to pop, which could
contain a locally damaging wind gust and some hail.

The right entrance region of an upper jet moves in later today. At
the mid levels, a 500 mb shortwave trough is forecast to move into
WI between 18z-00z. An 850 mb low is set to track across southern WI
and central/northern Lake Michigan late this evening and tonight. A
surface low is shown by the ECMWF to be near Milwaukee by 00z. The
RAP13 is very similar. Using the HREF, the surface warm front is
forecast to move through the CWA in the 4pm-8pm time frame from SW
to NE. Broad consensus exists amongst the CAMs that a band of
showers and thunderstorms will be associated with this front. SBCAPE
values of 500-1000 J/kg at most are forecast, but 0-3 km shear looks
poor with generally 20 kts or so expected, and 0-6 km shear a bit
better at around 35 kts. The risk for severe storms appears low with
this line, with hail being the main threat.

After the warm front lifts, there will be a limited opportunity for
storm redevelopment in the warm sector just ahead of a surface cold
front. The problem with this scenario is that the window for surface
based instability regeneration is small, and the timing of the cold
front passage (in the 04z-08z Monday time frame) is not favorable
for severe weather. Some of the CAMs show no instability being
regenerated, and little to no convection reforming. This is a real
possibility. The HRRR probably has the most (~1000 J/kg), which
could help yield a few strong to severe thunderstorms given
growing 0-3 km shear values around 35 kts. Overall, the risk for
severe winds/hail looks low tonight but if any storms can form in
the warm sector, some locally damaging gusts would be the main
threat. While the risk for a weak tornado is not zero, there is
not enough high resolution model agreement on the airmass
destabilization and degree of curvature in the lowest 3 km of the
hodograph, per sample soundings across the region, to warrant much
of a threat to the region at this time.

- A Few Showers Possible Memorial Day

The upper wave and surface low pull away toward northern Lower MI
and the U.P. on Monday. A few leftover showers are possible
primarily north of M-20. The pressure gradient force will help
deliver wind gusts around 30 mph especially Monday afternoon. In
addition, given low level cold advection, high temperatures will
noticeably dip into the low to mid 60s, making for a rather cool
day.

- Considerable Clouds With Showers Monday Night Through Tuesday night

An upper trough will be situated over the Great Lakes region from
Monday night through Tuesday night. Shortwaves will be working
through the upper trough. This setup typically features a fair
amount of clouds, especially diurnally driven clouds. Showers are
likely as well with the maximum coverage occurring during afternoons
and evenings when instability is maximized. This increase can even
be seen in global model precipitation output. We have thunder in the
forecast as well. MUCAPE values increase to around 1,000 j/kg on
Tuesday afternoon. Not out of the realm of possibilities to see some
small hail on Tuesday given colder temperatures aloft with the
trough. Any rainfall should be on the lighter side during this time
frame as the deeper lower tropospheric moisture will have been
shunted east and south with the cold frontal passage tonight.

- Drier With A Warming Trend Wednesday Through Friday

Surface and upper ridging will work through the Great Lakes region
mid to late week which will insure that we see dry weather. Thermal
trough will be overhead on Wednesday when mid afternoon 850mb
temperatures bottom out around +4C. Wednesday will likely be a bit
cooler of a day given the thermal trough and the fact we may see a
fair amount of diurnal cumulus as we see some cyclonic curvature
trying to hang on before the ridging moves in. All in all a quiet
weather period mid to late week. Temperatures should warm a bit each
day with 60s on Wednesday, warming to near 70 for Thursday and into
the 70s for Friday.

- Rain Returns To The Forecast For Next Weekend

There is some disparity in the forecast for Saturday as the ECMWF
moves a cold front into Wisconsin during the day and into our area
Saturday night. The GFS is slower with this progression. At this
point we have low pops in the forecast for some showers (20-30 pct).
At this time range a tough call as to which model has a better
handle with the pattern trending more zonal. Feel our low pops are
warranted. Another warm day regardless, with highs in the 70s
again.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
-- Changed Discussion --
Issued at 128 PM EDT Sun May 26 2024

We will see initially VFR conditions at the beginning of this
forecast period drop down to likely IFR conditions in showers and
storms by early this evening. This will come via the line of
showers and storms that is just starting to enter the extreme SW
corner of Michigan as of 1730z. We expect that ceilings will not
initially be low with some residual drier air in the lower levels.
We will see some heavier showers and storms bring down the
visibilities to IFR. We have included a tempo group for the first
hour or two of storms to account for the gusty winds as the line
moves in.

The line of showers and storms will lift NE through the area this
evening. We should see the widespread rain move out, with some
scattered mainly showers after that. Lower ceilings are expected
to follow the back edge of the band of rain within an hour or two.
MVFR, and eventually IFR ceilings will spread across the region
with a few spotty showers continuing. The only noticeable change
for the end of the forecast period will be winds switching to
become from the SW, and becoming a bit gusty with gusts to 25 to
28 knots.
-- End Changed Discussion --

&&

.MARINE...
Issued at 306 AM EDT Sun May 26 2024

Easterly winds today will ensure the highest wave action stays
from open lake toward the Wisconsin lakeshore. However, given the
passage of a cold front very late tonight into early Monday, winds
swing back to the west and WNW and increase in magnitude. The
combination of wind and wave action will likely warrant a SCA for
most if not all zones on Monday, with winds of 15-25 kts (possibly
as high as 30 kts) and waves building into the 3-5 ft range or
even a bit higher.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from Monday morning through Monday
     evening for MIZ037-043-050-056-064-071.
MARINE...Small Craft Advisory from 8 AM to 8 PM EDT Monday for LMZ844>849.

&&

$$

UPDATE...MJS
DISCUSSION...Duke/Hoving
AVIATION...NJJ
MARINE...Hoving