641
FXUS63 KGRR 190549
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
149 AM EDT Wed Jun 19 2024

.KEY MESSAGES...

- Small Risk for Stronger Storms into Late This Afternoon

- Backdoor Cold Front may drop down from the north Thursday

- Heat/Humidity Persists into Saturday

- Cooler Weather Behind a Cold Front Sunday

&&

.UPDATE...
Issued at 1047 PM EDT Tue Jun 18 2024

While there was some instability aloft as indicated by some
altocumulus, overall the divergent pattern kept any convection
from forming. Still keeping an eye as long line of showers and
storms moves into Wisconsin. Latest CAMS continues the trend of
showing this line weakening as the night continues. Have left
slight chance POPS in the far northwest in case the remnants clip
the region overnight.

&&

.DISCUSSION...
Issued at 240 PM EDT Tue Jun 18 2024

- Small Risk for Stronger Storms into Late This Afternoon

Low level convergence has been relatively weak so for and the
instability has been building steadily. Surface based instability
ranged from around 1000 J/kg south to 2000 J/kg north with ML
values about half those values. A mid level vort max was tracking
through western parts of the CWA and that feature will push
northeast over the next few hours. Based on the instability values
and lift, a few storms could get going, primarily over eastern
parts of the CWA in the next couple of hours. They could have the
potential to produce locally stronger wind gusts, but even that
risk looks low given the lack of DCAPE. On the backside of this
mid level wave, NVA will generate some subsidence which would
limit the storm risk for western parts of the region.


- Backdoor Cold Front may drop down from the north Thursday

This feature could keep temperatures down a few degrees Thursday,
lessening the risk for heat related impacts. Ensemble output from
the Canadian and GFS suggest the Grand Rapids area will likely
fall short of 90 degrees for high temperatures Thursday but the
12z ECMWF still has central parts of the CWA reaching into the low
90`s. We will not change the headline for Thursday just yet, but
confidence on reaching criteria has trended lower.

- Heat/Humidity Persists into Saturday

No changes to headlines planned. As noted previously, Saturday
looks quite hot, perhaps even more so than the preceding days.
This will be at the end of a prolonged and deleterious stretch of
heat and humidity. Moreover, being the weekend, there will be even
more exposure to the risk. All of this supports the notion of
extending the heat advisory into Saturday - but as it currently is
only Tuesday, we still more time to assess trends.

- Cooler Weather Behind a Cold Front Sunday

As noted previously, guidance is starting to converge on a solution
that brings showers and thunderstorms through the forecast area
Saturday night. As is often the case, the best upper PV forcing will
be shifted a bit poleward of the best instability, so it`s unclear
how strong convection will get. But, as I noted previously, the odds
favor at least some strong/severe convection during this time given
the pronounced airmass change.

Predictability really falls off the shelf after Monday, although
generally speaking, we can expect another round of above normal
temperatures and precipitation chances right around climatology
starting Tuesday.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 149 AM EDT Wed Jun 19 2024

VFR continues with cloud bases at or above 12,000 ft. Widely
scattered tstms developing this afternoon and continuing into
early evening. Local/brief IFR conditions possible should a storm
with heavy rainfall pass over a terminal, with the main risk of
storms being south and east of GRR between roughly 20Z and 02Z.
Southwest winds 10 to 20 kts (higher at MKG), diminishing later
in the day and especially this evening.

&&

.MARINE...
Issued at 1059 PM EDT Tue Jun 18 2024

While there have been periods of SCA criteria the winds have
slackened However LDM and Big Sable Point remains gusty up to
25kts so will keep SCA going overnight. Latest models keep
marginally hazardous conditions through tomorrow morning.

Previous Discussion:
The southwest winds at 925/850 mb are progged to increase this
evening as the pressure gradient at those levels tightens. This
will likely result in increasing southerly surface winds
especially north of Grand Haven. Based on this we will maintain
the headlines for small craft and swimmers in those locations. We
will need monitor locations further south as conditions are likely
to be become marginally hazardous.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through this afternoon for MIZ037-043-
     050.
     Heat Advisory until 8 PM EDT Friday for MIZ043>046-050>052-
     056>059-064>067-071>074.
     Air Quality Alert until midnight EDT tonight for MIZ050-056-057-
     064-071.
MARINE...Small Craft Advisory until 4 PM EDT this afternoon for
     LMZ847>849.

&&

$$

UPDATE...Ceru
DISCUSSION...MJS/NJJ
AVIATION...Meade
MARINE...MJS/Ceru