621
FXUS63 KGRR 011829
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
229 PM EDT Sat Aug 1 2026
.KEY MESSAGES...
- Rain showers, low chance of thunder today and tonight
- Dry Monday and Tuesday, then chances for rain
&&
.DISCUSSION...
Issued at 229 PM EDT Sat Aug 1 2026
- Rain showers, low chance of thunder today and tonight
Satellite water vapor channels and Airmass RGB product show a deep
upper PV intrusion curling around the eastern flank of a vertically
stacked low over east-central IL. This intrusion extends into far
southwest Lower MI as well. There is some question regarding how
much conditional instability we will realize this afternoon and
evening. The general consensus is for parts of our forecast area,
particularly far SW Lower and south central Lower to make a run
towards 1000 J/kg MUCAPE. Insolation should be minimized by
extensive mid/upper cloudiness associated with broad upper
deformation (and concomitant divergence) over most of southern
Lower MI.
As noted previously, we are in an environment characterized by
seasonably large precipitable water values (greater than 90th
percentile). When combined with light flow in the cloud-bearing
layer and a warm cloud depth well in excess of 10000ft AGL, we
are looking at a setup with very high precipitation efficiency.
Thus, any showers or thunderstorms will be able to produce large
rainfall rates with a risk of at least some mostly nuisance
flooding.
Earlier CAM guidance advertised a narrow, generally west to east
band of precipitation developing this afternoon/evening across the
central CWA. The forcing for this has been a bit uncertain but could
possibly be related to a deep wind col over this area between the
closed upper low immediately southwest and an open trough immediately
northeast. In any case, newer CAM model solutions are collectively
less consistent about developing this feature. What seems most
plausible is that potentially heavy rain could brush the southeast
forecast area, including Jackson County, later tonight into Sunday
morning in conjunction with low level fgen oriented northeast of
the progressing lower tropospheric low and this precipitation
could linger into early Sunday afternoon. This scenario agrees
with the WPC excessive rainfall forecast showing a greater flood
risk just south of the forecast area.
- Dry Monday and Tuesday, then chances for rain
Forecast certainty is high (greater than 90 percent) for dry conditions
Monday and Tuesday. By Tuesday night, a broad upper trough is
expected to be positioned over the northern Plains with a surface
front poised to enter the forecast area from the northwest. There
is some question regarding the timing of this front reaching
southern Lower MI; several of the deterministic solution indicate
an upper PV max swinging through Wednesday evening, which in turn
would help with the southeastward progression of the front. The
coupled forcing for upward vertical motion supplied by the front
and PV max will justify greater PoPs.
As noted before, we will be in a persistent broad upper troughing
pattern, meaning continued chances of showers and thunderstorms with
poor predictability regarding timing and coverage. It is worth
noting that the latest available medium range machine learning and
CFSv2 ensemble guidance point to increasing severe convection
chances in the Friday/Saturday timeframe and an active pattern for
strong/severe convection that could persist into next week.
&&
.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 149 PM EDT Sat Aug 1 2026
There is a mix of VFR/MVFR/IFR out there with cloud ceilings and
showers associated with a slow-moving low pressure system that is
centered over Illinois. In general, can plan on prevailing MVFR
today and IFR tonight into Sunday morning. The main concern for
the rest of this afternoon is the potential for a band or two of
heavier rain showers, possibly with an embedded thunderstorm, with
IFR visibility. Presently there is one such isolated thunderstorm
in the vicinity of OZW (Howell). Models have been favoring the
vicinity of LAN for the greatest potential for a heavy rain shower
or TS over the next several hours, but other aerodromes could
also get in on heavier showers temporarily.
&&
.MARINE...
Issued at 229 PM EDT Sat Aug 1 2026
As noted previously, offshore flow associated with gusty northeast
winds will grow waves mostly beyond the 5 nautical mile offshore
distance that marks the western edge of our nearshore zones.
However, recent runs of the WW3 show that our southernmost zone
(south of South Haven) may approach 4+ feet at its most
southwestward extent, so decided to issue a Small Craft Advisory
lasting until 4 AM. Have not issued a beach hazards statement given
the suboptimal weather right now for visiting the beach. Winds will
likely subside a bit towards sunrise Sunday and we may need to
revisit the need for beach hazards given we will be in an advancing
surface high setup that can produce locally strong shoreline
parallel winds.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory until 4 AM EDT Sunday for LMZ844.
&&
$$
DISCUSSION...TJT
AVIATION...CAS
MARINE...TJT
NWS GRR Office Area Forecast Discussion
