578
FXUS63 KGRR 181917
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
317 PM EDT Tue Jun 18 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


&&

.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 /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 146 PM EDT Tue Jun 18 2024

Main changes were to refine timing for thunder threat this
afternoon and to include LLWS for MKG and GRR terminals. A north
to south axis of showers and thunderstorms may develop in the
18-19Z timeframe, originating from an approximate AZO to GRR line
and moving east towards the BTL/JXN/LAN terminals later this
afternoon. Detail certainty is lacking so went with VCTS at the
mentioned terminals for now.

A distinct upper level disturbance swinging through the area will
be responsible for this different low level wind profile overnight
that justifies some LLWS. Additionally, there is less concern for
precipitation overnight compared to the past couple of nights due
to unfavorable upper level support.

&&

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

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 Wednesday 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 from midnight tonight to midnight EDT
     Wednesday night for MIZ050-056-057-064-071.
MARINE...Small Craft Advisory until 4 PM EDT Wednesday for LMZ847>849.

&&

$$

DISCUSSION...MJS/NJJ
AVIATION...TJT
MARINE...MJS