132
FXUS63 KGRR 051826
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
226 PM EDT Wed Aug 5 2026
.KEY MESSAGES...
- Periods of Rain and Thunderstorms Into Next Week
&&
.DISCUSSION...
Issued at 226 PM EDT Wed Aug 5 2026
Broadly, forecast expectations have not changed in any significant
way. The most plausible outcome in the short term is for another
round of showers and non-severe thunderstorms to move into the area
late afternoon/early tonight and exit early Thursday morning.
This is far from a certain forecast, however.
The challenge continues to be timing rounds of showers and
thunderstorms across the area, mostly south of a diffuse quasi-
stationary front that extended northeast to southwest through the
northwest forecast area this afternoon. Scattered rain showers
are already ongoing across the forecast area. More widespread
ongoing convection is noted across IA/MO/IL ahead of upper PV
forcing depicted by both satellite water vapor imagery and short
range model guidance.
Convection allowing guidance has shown significant run to run
variability, which suggests large forecast sensitivity to antecedent
convective trends. It is interesting to note that even the ECMWF
deterministic model hints at this; it erodes the PV max (and
therefore organized upper forcing) as it moves east towards Lower
MI. This is likely due to modeled deep moist convection and
attendant mid-level latent heat release that has the effect of
decreasing both static stability and cyclonic vorticity (the
ingredients of PV) at the upper levels. The substantial influence of
diabatic forcing we see here amidst weak synoptic forcing is what
really hurts the predictability of this forecast.
As noted previously, conditional instability is not favorable for
strong storms and will be further limited by ongoing precipitation.
12Z HREF guidance does indicate a continued chance of thunderstorms
overnight trailing off closer to sunrise. Another item of concern is
locally heavy precipitation towards I-94. This is advertised by HREF
LPMM. Having said that, the 12Z REFS is completely different,
showing greatest (but far less impressive) precipitation amounts
towards Clare County. This all speaks to the low forecast certainty
we can expect even in the first 24 hours, but there is the possibility
of localized significant rain, leading to at least some nuisance
flooding overnight.
Thursday afternoon now looks a little drier than before with the
focus for most precipitation, including thunderstorms, shifted east
towards southeast Lower Michigan. But again, this will depend a lot
on convective evolution beforehand.
After Thursday, broad upper troughing will slowly migrate from the
western Great Lakes to the central Great Lakes with perhaps some
flattening by the beginning of next week. As noted before, the
timing of convection will remain uncertain, yielding a now familiar
repeating theme of 20-40 PoPs in the extended forecast. It appears
unlikely that we will be seeing much in the way of strong/severe
convection or significant areal flooding.
&&
.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 212 PM EDT Wed Aug 5 2026
Showers have returned with a chance for thunderstorms this
afternoon and early evening. Another lull is possible during the
late afternoon with another round moving in between 00-06Z
Thursday and lingering into Thursday. Localized heavy rain is
possible with heavy rainfall rates and accumulations of 1 to 3
inches. Reduced visibilities will be possible with any shower or
storm.
MVFR ceilings will slowly move in this evening and overnight.
Showers and storms may bring brief periods of MVFR conditions. Fog
may also develop as showers exit the area or taper off overnight.
There could be another round of showers and storms mid to late
Thursday morning which could affect most, if not all, TAF sites.
&&
.MARINE...
Issued at 226 PM EDT Wed Aug 5 2026
Scattered showers and a few thunderstorms are expected through
tonight into at least early Thursday. Showers and thunderstorms will
continue to be possible this weekend into next week. No severe
thunderstorms are expected. Winds and waves should generally remain
below Small Craft Advisory and Beach Hazard Statement levels.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...TJT
AVIATION...Ceru
MARINE...TJT
NWS GRR Office Area Forecast Discussion