923
FXUS63 KGRR 190239
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1039 PM EDT Sat May 18 2024

.KEY MESSAGES...

- Warm Weekend Continues

- Unsettled Weather at Times Monday through Wednesday

- Drier and cooler Thursday and Friday

&&

.UPDATE...
Issued at 1038 PM EDT Sat May 18 2024

Cold front is moving east through Wisconsin at 1030pm and can
clearly be seen in regional radar loops. Convection on the front
has trended down (fading diurnally), but it has not come to an
end. There is enough instability on the front that storms
continue. MUCAPE values at present are on the order of 1000-2000
j/kg in Wisconsin. The front makes steady progress eastward
tonight and moves through much of the forecast area between
midnight and 800am. The 3km NAM shows MUCAPE values remaining
around 1,000 j/kg through the night in Southwest Lower Michigan.
Consensus of short term models suggest we need an
isolated/scattered chance (20-30 pct) for showers and storms
through the night. Most places will likely not see any
precipitation but the front is strong enough and instability great
enough to warrant the chance. The HREF that is now coming in
suggests an uptick in showers and storms near and after 500am. It
will be a warm night with lows in the lower 60s in many areas. The
normal low at GRR tonight is 50.

&&

.DISCUSSION...
Issued at 258 PM EDT Sat May 18 2024

- Warm Weekend Continues

"Cold" front over Wisconsin slips our way tonight but it is
really nothing more than a wind shift line. High temps will
actually be similar or even a few degrees warmer on Sunday
despite its passage.

A band of showers and thunderstorms along the front in WI weakens
considerably as it drifts southeast and arrives in our area later
tonight, and may even dissipate completely before getting here.
Will maintain 20-30 pops toward/after midnight for areas
north/west of GRR but latest CAMs look to be decreasing the threat
of convection since the shortwave stays well northwest of us over
the Lk Superior region and instability wanes quickly.

A few showers and storms may redevelop by Noon Sunday south and
east of GRR where the frontal boundary stalls out and sfc
instability rebuilds. RAP has SBCapes rising to near 2000 J/KG
where sfc dew pts near 60 will be present along with some weak
sfc convergence. Pops only 20-40 pct again (highest near JXN),
and any storms will be the pop-up/pulse-variety in weakly sheared
environment. Any diurnal showers/storms will dissipate with
sunset although returning warm front late Sunday night could touch
off a few storms again by daybreak Monday.

- Unsettled Weather at Times Monday through Wednesday

The ridge that will be over the area on Sunday will slip east of
the area by Monday. This occurs as a long wave trough will dig and
amplify across the western half of the country. Our area will end
up under deep flow from the SSW which usually results in a warm
and humid/wet period. This period looks about as expected with
periods of showers/storms, although it will not be raining all of
the time by any means.

The first chance of rain will come as early as Monday as the
leading short wave ejecting from the long wave trough skirts by to
our NW. Ahead of this wave, we will see a surge of warm and moist
air ahead of it with a low level jet. Showers and storms look
like a decent bet into Monday night before that wave moves away
from the area.

We are expecting another more significant wave to lift NE around
the long wave trough in the Tue/Wed time frame. A front will
remain over the southern half of the area behind the Mon system,
and will then move back north as a warm front on Tue ahead of the
stronger wave. This will bring additional chances of
showers/storms. This system is the center of the SPC DY4 outlook
that clips the area. It appears that the main threat may stay SW
of the area with the entire system wrapping up a bit to the west.

We will remain out ahead of the cold front on Wednesday, leaving
the area in the warm sector for at least a part of the day.
Getting out that far, details are a bit gray. Timing of features
with this system with respect to peak heating as late as Wed may
ultimately determine the severe weather threat. The potential does
exist with some strong wind fields aloft and plenty of warm and
humid conditions present.

- Drier and cooler Thursday and Friday

It does look like the front should sweep out the heat and
humidity by late Wednesday, leaving a mild and drier air mass in
it`s wake. The instability and showery weather with the cold pool
aloft should stay north, while the front moves SE of the area.
This leaves some ridging in place, and it looks likely to stay dry
through at least Friday. There remains some small chances of rain
in the NBM, but we are thinking it is more likely to stay dry.

Rain chances will increase some by Saturday, but nothing
significant is on the horizon yet for the beginning of the
upcoming holiday weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 747 PM EDT Sat May 18 2024

VFR prevails across the area with clear skies. Expect continued
VFR through the forecast period with no major forecast concerns to
note. Wind will be primarily southerly through much of this
evening and early overnight, then a dry front will pass through
resulting in a wind shift to the northwest between 06 and 12z
Sunday. A line of 5 to 7k ft clouds will accompany the front then
clouds will scatter out after sunrise. Diurnal cumulus is
expected to develop tomorrow afternoon.

&&

.MARINE...
Issued at 258 PM EDT Sat May 18 2024

Winds and waves remaining below advisory levels through the
remainder of the weekend, although a frontal boundary slipping
through tonight will shift the winds from southerly this afternoon
and tonight to northwesterly on Sunday. There may be a period
Sunday afternoon where the NNW winds increase to near 15 kts or
so for a time. The fog threat appears to have ended over the lake.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Air Quality Alert until midnight EDT tonight for MIZ050-056-057-
     064-071.
MARINE...None.

&&

$$

UPDATE...Duke
DISCUSSION...Meade/NJJ
AVIATION...Thielke
MARINE...Meade