-- Discussion containing changed information from previous version are highlighted. --
651
FXUS63 KGRR 131802
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
202 PM EDT Thu Jun 13 2024

.KEY MESSAGES...

- Strong storms remain possible for this afternoon and early evening

- Becoming hot and humid

&&

.UPDATE...
Issued at 1200 PM EDT Thu Jun 13 2024

Lowered POPs for early this afternoon based on current
model/satellite trends. Rest of forecast unchanged. We still
expect the main threat of storms to be later afternoon into early
evening. Latest model guidance shows storms across central and
southern zones between approximately 4 pm to 8 pm.

&&

.DISCUSSION...
Issued at 349 AM EDT Thu Jun 13 2024

- Strong storms remain possible for this afternoon and early evening

Our main focus and challenge initially will be on convective trends,
and what kind of severe potential will be present later today.

One factor with potential severe weather later today is the ongoing
convection this morning. We have a narrow broken line of showers and
a rumble of thunder embedded coming through the area right now.
Then, there is the leftover convection across Northern WI. The
convection over the area right now is associated with a weak short
wave and the southern edge of the low level jet nosing into the
area. The WI storms are better associated with the low level jet
dynamics with the core of the jet supporting the convection.

To start with, we have some outflow boundaries very evident on the
radar loops that are propagating to the south and east. The low
level jet will be weakening a bit as we approach sunrise as is
typical. Severe weather is not expected with any storms this
morning. What we do anticipate is the remnants of the storms and the
outflow boundaries will continue to progress toward the area. These
will likely have a decent say in where convection develops.

A majority of the models and their ensemble members have been fairly
consistent in showing that most of, if not all of the convection
will be south and east of the Grand Rapids area. In addition to
where the outflow boundaries end up, other factors will likely be
detrimental for widespread and severe convection for the vast
majority of the area. These are as mentioned last night, divergent
flow downwind of Lake Michigan, a cap in place, and the low level
jet moving away.

That seems like the more likely scenario. However, the HRRR and RAP
have been consistently showing the outflow boundary and effective
cold front igniting convection a bit further NW and more widespread.
This may have to do with instability being a bit higher with higher
sfc dew points. If the convection does ignite along the cold front
to our NW, it will have 2,000 J/kg of CAPE, and deep layer shear
values in the 30s. A unidirectional wind flow in the lowest few
thousand feet would support a wind threat, justifying the
conditional Slight Risk. We will continue to monitor the multiple
parameters mentioned above.

The front will sweep through the area no later than overnight. A
much drier and a bit cooler air mass will filter in over the area.
Much more stable conditions should keep the area dry on Friday, with
maybe some cumulus clouds mixing out.

- Becoming hot and humid

High pressure will be centered over the state Friday night and will
result in our coolest temperatures of the forecast period. We`re
looking at lows in the lower to mid 50s by early Saturday morning.
after that, the high moves east and south flow develops and warmer
and more moist air will advect north across Lower MI. A large
expansive upper high is progd to develop over the Ohio Valley/Great
Lakes next week and that usually means a heat wave this time of
year. H8 temps are progd to rise from 10c Saturday morning to 19c by
Tuesday. Sixty degree plus surface dewpoints won`t arrive until
Sunday which is when surface temperatures will close in on 90. Highs
Monday will be in the lower to mid 90s and heat indices will climb
to 95-100. Similar conditions are expected Tuesday through Thursday
too; we may be looking at an extended period of Heat Advisory
Criteria.

Given the juicy airmass in place, any minor short waves that ripple
across the region in the upper flow may be able to generate a shower
or storms, but nothing organized is expected. Thus, low
chance/isolated PoPs will carry the day.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
-- Changed Discussion --
Issued at 155 PM EDT Thu Jun 13 2024

Showers and storms still expected this afternoon and evening. Have
increased the onset timing considering the current line moving
over Lake Michigan. Expect them to reach the lakeshore between 19
to 20Z with MKG begin affected first. The line should move through
quickly so expect any showers and storms to be done between 00Z to
03Z.
 There is a period of MVFR cigs and vsbys overnight into Friday
morning. Especially between 09Z to 13Z.
-- End Changed Discussion --

&&

.MARINE...
Issued at 349 AM EDT Thu Jun 13 2024

We continue to monitor the situation this morning for a possible
Small Craft Advisory/Beach Hazards event. Latest consensus of model
guidance indicates that the predominate conditions will fall just
short of both wind and wave criteria. There may be a few isolated
gusts touching the 22 knot criteria, and waves approaching 4 ft.
This has been the expectation all along, and will not deviate from
the plan at this time. The peak of the wind should come mid-morning
just ahead of the front coming through, before dropping off the
remainder of the day.

We are not looking at any additional periods of concern at this time
until at least Saturday night/Sunday, if not later as the gradient
starts to pick up.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

UPDATE...Ostuno
DISCUSSION...NJJ/04
AVIATION...Ceru
MARINE...NJJ