690
FXUS63 KGRR 211756
AFDGRR

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

.KEY MESSAGES...

- Severe Storms Possible Late Tonight

- Dry for Wednesday and 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 Wednesday and 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 /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 156 PM EDT Tue May 21 2024

VFR conditions should predominate except for a brief period of
MVFR/IFR conditions with the passage of thunderstorms tonight.

We continue to expect a north-south oriented line of strong to
severe storms coming onshore at MKG around midnight tonight and
then moving east. The storms should weaken quite a bit as they
move onshore followed by gradual additional weakening over Lower
MI as the line moves east. Wind gusts in excess of 40 kts seem
like a good bet at MKG and over 50 kt is not out of the question;
farther inland, gusts may end up being considerably weaker. In
any case, any given terminal can expect less than an hour of
thunderstorms. The storms will be preceded and also followed by a
period of LLWS that will peel off to the east and exit the area
before 12Z.

&&

.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...TJT
MARINE...Thielke