177
FXUS63 KGRR 210751
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
351 AM EDT Tue May 21 2024

.KEY MESSAGES...

- Severe Storms Possible Late Tonight

- Dry for Thursday; Rain Chances Return Friday

- Rain Chances Linger Through the Holiday Weekend

&&

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

- Severe Storms Possible Late Tonight

A rapidly amplifying shortwave ejects out of the Central Plains into
the Great Lakes this afternoon through late tonight. In response, a
surface low deepens as it moves into northern Wisconsin and the
UP while a trailing cold front becomes the focus for thunderstorm
development. A cluster of storms currently working through
Nebraska and western Iowa will be the focal point to monitor
throughout the day today. Starting off semi-discrete, these storms
will organize and transition into an organized line of storms by
the time they reach central Iowa and western Wisconsin.

Meanwhile across lower Michigan ahead of the storms, return flow
moisture bumps dewpoints into the mid to upper 60s with afternoon
highs peaking in the low to mid 80s. Diurnal cumulus may give way
to a few showers or a stray thunderstorm ahead of the main line
of storms late tonight. Hi-res cams are in good agreement on a
midnight time of arrival of the main line of storms, and are also
favoring a quick weakening trend into lower Michigan given the
late hour and strong nocturnal limiting factors. The progression
of SPC Enhanced in far western Michigan to Slight/Marginal Risk
in central to eastern Michigan nicely depicts the weakening trend
with eastward progression. Damaging wind will be the main hazard
to monitor as the storm mode transitions into a well organized
QLCS to our west and travels east towards lower Michigan.

The eastward advancing cold front will be the focal point for any
additional convection Wednesday, but as the boundary becomes
parallel to the mean flow convection will be very limited and
mainly shunted off to our far southeast. Therefore the risk of
storms has shifted southeastwards.

- Dry for Thursday; Rain Chances Return Friday

Sfc frontal boundary will settle south of MI for Wednesday night
through Thursday night with a sfc high and drier air mass sliding
through, yielding dry conditions during this time.

Current guidance indicates that the boundary lifts back north as a
warm front on Friday afternoon and night as the next shortwave
tracks across the nrn Plains into MN/WI. Pops returning as this
occurs and this is faster than in previous model runs.

- Rain Chances Linger Through the Holiday Weekend

Depending on the timing of the Friday wave, its possible that
Saturday ends up being a mostly dry (and cooler) day due to a period
of subsidence behind it. Confidence is too low at this time though
to advertise a dry start to the weekend and 20-40 pct pops will be
maintained. It is possible that those rain chances end up being
restricted top just the morning-time.

A zonal flow aloft is progged throughout the holiday weekend with
frontal zone/baroclinicity still nearby. This pattern will result in
continued chances of showers and possible storms although the timing
and coverage of convective events remains low confidence. Upshot is
for a continuation of daily 20-40 pops but the weekend will
certainly not be a washout and temperatures will average near to
slightly above normal.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 152 AM EDT Tue May 21 2024

Stratus and fog will produce IFR/LIFR conditions through 14Z or
so before mixing out, mainly north of a AZO to LAN line. Otherwise
VFR conditions prevailing from mid morning into the evening
before a possible squall line races across Lk MI toward Midnight.
Southerly winds will increase substantially after 02-03Z tonight
in front of that approaching line, with LLWS becoming likely.

The timing of the line of strong to severe storms tonight is
toward the very end of the TAF period, but if the line holds
together there could be gusts as high as 50 kts associated with
it shortly after Midnight.

&&

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

Winds stay mainly offshore today, then will shift southwesterly this
evening while building. Strong to severe thunderstorms will move through
the region late tonight resulting in locally strong winds and choppy waves
in the vicinity of storms. Apart from storms, conditons become
hazardous to small craft late this evening as winds gust towards
20 to 30 knots through tomorrow morning. Meanwhile, waves will
build late tonight through Wednesday morning, gradually subsiding
from south to north Wednesday afternoon.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory from 10 PM this evening to 8 PM EDT
     Wednesday for LMZ844>849.

&&

$$

DISCUSSION...Meade/Thielke
AVIATION...Meade
MARINE...Thielke