271
FXUS63 KGRR 191700
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
100 PM EDT Thu Sep 19 2024

.KEY MESSAGES...

- Mostly sunny and hot today, chance of storms Friday

- Cooler next week with chances for showers or storms, most likely
  Sunday night into Monday

&&

.DISCUSSION...
Issued at 332 AM EDT Thu Sep 19 2024

- Mostly sunny and hot today, chance of storms Friday

One more day of abundant sunshine and hot temperatures as the
blocking pattern begins to break down. Mixing will bring down
drier air aloft again today which will keep the humidity in check.
Highs in the mid 80s are expected again.

Increasing clouds tonight will limit how much temperatures fall.
I`d expect lowest temperatures...upper 50s...to occur over the
eastern cwa.

A short wave over North Dakota will lend some support for the
surface low over the upper MS Valley today as it tries to push a
weak cold front eastward. Given the downward diurnal instability
trend late tonight, anything that moves across the lake will
likely be showers around sunrise.

Instability will rise during the day Friday and SBCAPE values
increase to around 800-1200 j/kg...highest over the southern cwa.
As the frontal boundary moves eastward, it will encounter the
higher instability and a broken line of showers/storms may
develop. Bulk shear values rise to 35-40 kts across the far
southern cwa after 18z giving rise to the possibility of some
organization and potentially a few strong/severe storms. CAMs
forecast soundings Friday afternoon show the classic inverted V
signature suggesting that downdraft winds will be the greatest
threat.

- Cooler next week with chances for showers or storms, most likely
Sunday night into Monday

The spread among the ensembles remains rather large with varied
plausible outcomes in the day-to-day weather next week, but overall
it is a pattern that supports daytime highs more often in the 70s
and occasional showers or thunderstorms. The complex interaction
between a northern jet stream wave pattern which is fast-flowing,
amplified, and progressive against a southern stream which is
showing a tendency for blocking may be a culprit behind the lesser
predictability toward the middle of next week.

There is still medium confidence for rain, potentially over 1 inch
in spots and the best chance of rain in quite a while, late Sunday
into Monday. This is as an upper-level low over California and the
Desert Southwest moves toward the Midwest while becoming more
incorporated into the northern stream. Northward transport of
moisture in the lower atmosphere ahead of this low and a frontal
zone over the region would support showers, and an assortment of
ensemble members have enough instability to support a chance of
thunderstorms. The 00Z run of the ECMWF ensemble provides a 40-60
percent chance of 0.50+ inch rain totals in 24 hours between Sunday
evening and Monday evening.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 100 PM EDT Thu Sep 19 2024

Much of this forecast period will see a continuation of the
conditions over the past few days with mostly sunny/mostly clear
skies and lighter winds. We will see some rain chances later in
the period, with some potential MVFR conditions with the rain
chances.

Once again, mainly high clouds in place at the beginning of the
forecast period. Cumulus clouds are starting to form, but will
remain VFR. The cumulus clouds will be more concentrated for the
eastern terminals. Those cumulus clouds will once again dissipate
after sunset. Fog is not likely tonight with a light wind around
4-5 kts remaining at the sfc, and better winds 1-2k ft agl.

We will see some mid level clouds start to move in just before
sunrise Friday at KMKG and spreading east. Can not rule out a
sprinkle with this, but the very dry lower levels should keep the
sfc dry. The better chance of showers and isolated thunder will
arrive at the western sites after 16z or so. The eastern sites
will not see this potential until after the end of this period.
Some MVFR will be likely with the showers and thunder, with some
isolated IFR possible also. Will stick with the MVFR wording for
now, and fine tune the forecast in subsequent issuances.

&&

.MARINE...
Issued at 332 AM EDT Thu Sep 19 2024

A weak pressure gradient will result in onshore winds during the
afternoon for the next few days. A weakening cold front may push a
broken line of showers and/or storms across the lake late tonight
and Friday morning.

&&

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

&&

$$

DISCUSSION...04/CAS
AVIATION...NJJ
MARINE...04