Severe Thunderstorm Warning 40, NWS Grand Rapids, MI

Area Forecast Discussion
National Weather Service Grand Rapids MI
300 PM EDT Thu Jun 29 2023

LATEST UPDATE...
Short Term/Long Term/Marine

.SHORT TERM...(This evening through Friday night)
Issued at 300 PM EDT Thu Jun 29 2023

Shower and thunderstorm chances, along with the persistent smoke,
remain the primary forecast challenges.

As expected, temperatures warming through the 80s along with dew
points inching up through the 60s /to low 70s in some areas/ has
resulted in MLCAPES in excess of 1.5k J/kg. In spite of modest
instability, an overall weakly forced environment continues to
result in uncertainty with respect to storm location/coverage.

Very weak front over Wisconsin has made little progress today,
and won't pass through the region until Friday. Aloft, quasi-
neutral flow is in place, with one embedded ripple currently
crossing far northern Michigan as another /MCV/ exiting IA and
takeing aim on the lower Great Lakes. Expect scattered storms to
develop next few hours over parts of Lake Michigan and western
Lower Michigan and then push eastnortheastward. Given instability
and respectable 500 mb flow aloft, marginally severe wind and hail
will be possible with these storms through the evening.

By Friday, aforementioned weak front will move across Michigan.
It's really more of a modest wind shift than a cold front. In
fact, highs on Friday will reach the upper 80s. Still a small
risk for scattered covection Friday in presence of continued
warmth/moisture and weak boundary, but coverage should remain
small and be limited to far eastern and southern parts of forecast
area. Lows Friday Night will fall into the 60s, and a small risk
for showers/storms will persist mainly along and south of I-94.

.LONG TERM...(Saturday through Thursday)
Issued at 300 PM EDT Thu Jun 29 2023

A progressive, quasi-zonal flow regime will continue this period
with the best chances for rain coming on Sunday over the southern
forecast area and by late next week across all of the forecast
area.

A baroclinic zone across the Ohio Valley will persist into the
weekend and focus more convection in the form of clusters of storms
and possibly more of the progressive derechos that are currently
ongoing there. Most of this will remain south of Lower Michigan but
the rain and storms could expand north on Sunday as an upper low or
shortwave trough with weak sfc reflection tracks across or just
south of the forecast area.

Weak surface and upper ridging builds in for early next week with
only slight chance POPs, but moisture pooling in front of an
advancing cold front/seasonably strong shortwave trough on Thursday
will set the stage for what could be widespread rains and convection
lasting from Thursday into Friday. Precipitable water values progged
to be above 1.8 inches by that time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 141 PM EDT Thu Jun 29 2023

Still anticipate scattered thunderstorms to develop this
afternoon across West Michigan. Though we continue to
destabilize, the lack of low level focus/boundaries is making
timing and location of convection a bit uncertain. In general,
expect scattered storm development after 20-21Z. Gusty winds and
hail may accompany any of these storms.

Storms should abate by 04Z, and the weak synoptic pressure
gradient will cause the current SSW 10-15 kt winds to be become
light and variable. Smoke continues in most areas, with borderline
IFR/MVFR visibiltiies being reported. Both fog and smoke will be
possible overnight, before visibilities finally improve after
13-14Z Friday.

&&

.MARINE...
Issued at 300 PM EDT Thu Jun 29 2023

Visibility has slowly improved today and the smoke concentrations
are expected to diminish on Friday as winds become southwest. Waves
will continue to be relatively light into the weekend as winds
remain below 20 knots. There could be some locally higher wind gusts
in thunderstorms from this evening into the weekend, mostly across
areas south of Grand Haven.

There could be a push of north winds on Sunday as high pressure
builds in. The high is rather weak so winds and waves are not
expected to pose a hazard to small craft, although there could some
choppy water around Big and Little Sable points.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

SHORT TERM...Smith
LONG TERM...Ostuno
AVIATION...Smith
MARINE...Ostuno

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