929
FXUS63 KGRR 191852
AFDGRR

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

.KEY MESSAGES...

- Chance for Storms Tomorrow

- Several Chances of Showers and Storms into Next Week

&&

.DISCUSSION...
Issued at 252 PM EDT Thu Sep 19 2024

- Chance for Storms Tomorrow

Clouds begin to increase late tonight as an upper low tracking from
the Northern Plains into Manitoba sends a trailing front towards
lower MI, arriving in our area Friday afternoon and evening.
Spotty showers will begin to move through the area around late
morning to mid- day, then storms will be possible during the
afternoon. Diurnal instability up to 1000 to 1400 J/kg (highest
over southern lower MI) and bulk shear to 35 to 40 knots will
support a few strong storms, with the highest chance in far
southwest lower Michigan. Damaging wind gusts will be the primary
concern with any strong storms.

- Several Chances of Showers and Storms into Next Week

The weekend starts off dry as a period of brief ridging and high
pressure advects dry air across the area. Precipitation chances then
increase later Sunday and especially Sunday Night into Monday. This
is driven by a closed low over the southwest US ejecting into the
main flow pattern and bringing ample moisture with it. Can`t even
rule out a bit. Ensemble CAPE forecasts suggest the chance of
isolated thunderstorms also exists. Grand Ensemble precipitation
probabilities show the chance for a needed soaking rain is definitely
there. Chances of over 0.5" of rain Sunday Night into Monday night
is 40-60 percent and over 1 inch is around 10 percent.

This weekend into into next week marks the start of a pattern shift
as the ridge that has brought dry weather for over a week breaks.
This change makes troughing the predominant upper-level setup.
Several additional chances of rain are possible over this time
period, however as is common with these setups, the timing of the
shortwaves and associated rain showers is still not nailed down.
Thermal troughing also causes 850mb temperatures falling to around
10C, translating to a near normal low to mid 70s for highs next week.

&&

.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 252 PM EDT Thu Sep 19 2024

A weak cold front will bring a chance for scattered showers early
Friday morning. Otherwise, expect offshore winds starting Friday
with winds shifting southerly then westerly Friday afternoon
following the passage of the front. Wave heights stay generally low
through this weekend, with the highest waves to around 2 feet Sunday
evening.

&&

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

&&

$$

DISCUSSION...Thielke/Thomas
AVIATION...NJJ
MARINE...Thielke