820
FXUS63 KGRR 190740
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
340 AM EDT Thu Jun 19 2025

.KEY MESSAGES...

- Showers moving out today with the upper trough

- Convective chance small but not zero tonight/Friday night

- Building weekend heat

&&

.DISCUSSION...
Issued at 340 AM EDT Thu Jun 19 2025

- Showers moving out today with the upper trough

Radar shows the back edge of the rain shield associated with the
surface low moving across Lake MI attm. The GFS and ECMWF are
consistent in moving the low into Ontario and Quebec later today.
The showers will end once the upper trough axis (currently over
the MS Valley) moves through this afternoon.

Latest IR loop shows clearing skies over western Wisconsin and we
should share in that by this afternoon. Thus, showers this morning
followed by skies becoming mostly sunny this afternoon. Today will
be the last day with highs in the 70s for a while.

- Convective chance small but not zero tonight/Friday night

A building ridge will develop over the Plains later today and that
will drift east to the MS Valley tomorrow. Short waves riding the
ridge are expected to combine with an increasing LLJ during the
late evening and fire convection over the upper MS Valley tonight
and Friday night. The NAMNest moves this convection southeast
along the thickness contours and over Wisconsin late tonight and
into the northern cwa Friday morning. The HRRR does something
similar but fires convection a bit quicker and moves it over the
northern cwa by midnight while simultaneously firing an MCS over
MN and moving it south across northern IL by Friday morning. That
seems a bit too far south given the CAPE gradient and convective
firing location farther north over MN. For now, we`ll retain the
30-40 PoPs across the northern cwa.

A stronger wave develops in the strong warm advection regime over
MN Friday night and uses the nocturnal LLJ as fuel to push it
east. I think the GFS is too far north and like the HREF solution
better that moves in a bit farther south near the CAPE gradient.
The NAMNest develops a bowing MCS and moves it ESE across northern
Wisconsin to northwest Lower by Saturday morning and clips the
northern cwa. This evolution will need to be watched. For now we
are going with 40-50 PoPs over the northern cwa late Friday night.

- Building weekend heat

The building upper ridge will drift toward Michigan this weekend.
H8 temps are progd to rise from 12c to near 24c by Saturday and
Sunday. This will send temperatures well into the 90s. When
combined with dewpoints in the lower 70s, apparent temperatures
climb into the lower 100s: a heat advisory appears inevitable
Friday through Sunday, but it looks we may just miss extreme heat
warning criteria, but it will be close.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 130 AM EDT Thu Jun 19 2025

A large rain shield covers much of Lower MI early this morning as
low pressure moves across the peninsula. We`ll see showers and
general IFR conditions through much of the rest of the night. Look
for ceilings and visibilities to improve this morning as the low
moves away from the region. VFR conditions are expected this
afternoon.

&&

.MARINE...
Issued at 340 AM EDT Thu Jun 19 2025

A cold front is moving down the peninsula and winds will be
shifting to the north shortly. There may be a brief period prior
to 12z when waves over the far southern nearshore zone climb into
the 2-4 foot range but should be brief; there`s not much of a push
of cooler air behind this front.

Wind and waves will increase again Friday afternoon and a SCA will
probably be needed at that point.

&&

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

&&

$$

DISCUSSION...04
AVIATION...04
MARINE...04