636
FXUS63 KGRR 131813
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
213 PM EDT Sun Sep 13 2026

.KEY MESSAGES...

- Greater chances of rain, mainly this afternoon

- Showers, thunderstorms, possible heavy rain starts Tuesday

- Unsettled into this weekend

&&

.DISCUSSION...
Issued at 213 PM EDT Sun Sep 13 2026

- Greater chances of rain, mainly this afternoon

Based on radar and satellite trends, combined with model
guidance, have raised PoPs to near categorical this afternoon.
Thunderstorms are not expected but there should be a fairly
extensive coverage of measurable precipitation. Rain should be
mostly, if not totally, out of the forecast area by this evening.

As noted before, winds should decrease rapidly this evening as the
surface high moves into Lower MI. We are still looking at lows in
the 40s Monday morning with upper 30s not out of the question in the
favorably cooler sandy soil regions to the north.


- Showers, thunderstorms, possible heavy rain starts Tuesday

A cyclonically breaking upper shortwave trough originating from the
northern Rockies with a surface low tracking into Canada by Tuesday.
This will amplify the downstream upper ridge and push it eastward
across the Great Lakes. The associated eastward progression of the
surface high will introduce increasing southerly flow in its wake.
The cold front an associated low level theta-e ridging will enter
the forecast area early Tuesday morning. With MUCAPE approaching
1000 J/kg, there will be a chance for thunderstorms.

As noted previously, the cold front is expected to stall somewhere
in the vicinity of the southern CWA. There will be a sharp theta-e
and conditional instability gradient with this feature. ECMWF ensemble
(ECE) mean guidance indicates precipitable water values approaching
2 inches, which is highly anomalous with respect to both climatology
for this time of year and with respect to the propensity of the ECE
to forecast values this high for this time of year. WPC is indicating
a marginal risk for excessive rainfall, which is quite reasonable.
However, accurate placement of the actual threat area will be tricky
and highly location-dependent given the sharpness of the boundary.


- Unsettled into this weekend

Ensemble guidance really speaks to the poor predictability leading
into this weekend. Often it`s possible to carve out a particular day
that`s likely to be dry. Not so this time. While there is a signal
for a dry day happening at some point, which day that is has varied
from run to run. Hence, we are featuring persistent 20-40 percent
PoPs through the period.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 130 PM EDT Sun Sep 13 2026

VFR conditions will prevail through the TAF period at all TAF
sites. Mid clouds /bases 10-12kft/ will quickly increase next few
hours, as a fast moving impulse moves W to E across the lower
Great Lakes. This will also yield a small window of light
showers, per upstream obs and short range model guidance, likely
lasting no more than 1-2 hr at any given TAF site between roughly
19-00Z. Given dry air mass and high cloud bases, don`t expect
vsbys in rain showers to drop below 6SM. Skies will then clear by
early this evening, and continue for the duration of the night through
18Z Monday.

NW breezes gusting to 15-20 kt this afternoon will quickly
diminish to around 5 kt this evening as high pressure builds
overhead, then become E/SE 5-10 kt early Monday as high slowly
moves east of the area.

&&

.MARINE...
Issued at 213 PM EDT Sun Sep 13 2026

No changes to the marine headlines at this time. As noted
previously, gale conditions are expected to commence Monday
night. However, to prevent confusion, we will wait for current
headlines to expire tonight before addressing this next threat
window. After the onset of gale conditions Monday night, hazardous
conditions should persist through Tuesday and into Tuesday night.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 11 PM EDT this evening for MIZ037-
     043-050-056-064-071.
MARINE...Small Craft Advisory until 11 PM EDT this evening for LMZ844>849.

&&

$$

DISCUSSION...TJT
AVIATION...Smith
MARINE...TJT