Grimes, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

719
FXUS63 KDMX 201945
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
245 PM CDT Sun Jul 20 2025

.KEY MESSAGES...

- A few storm possible tonight, mainly over the far south and
  northwest. A strong storm may occur over the northwest.

- Hot and humid conditions expected Tuesday and Wednesday with
  heat index values in excess of 100 degrees in the afternoons.

- Additional storm chances arrive at the end of the week.

&&

.DISCUSSION...
Issued at 243 PM CDT Sun Jul 20 2025

Convective trends through Monday are the near term forecast
challenge after the past several nights of MCSs moving across the
state with some severe weather and heavy rainfall. This afternoon,
much of the state is under scattered to broken stratocumulus clouds
with a warm layer above keeping convection in check for the most
part. There are a few isolated areas of bubbling but this has been
mostly short lived. Also monitoring thunderstorms that are ongoing
over eastern South Dakota with one primary supercell currently
southwest of Huron. This storm should be slightly elevated and is
beginning to make a turn southeast into the instability axis which
would fit the conceptual model for this. This is the area to watch
should stronger convection reach the area, perhaps as early as early
this evening. Otherwise, not expecting much for areal coverage in
storms overnight with the other focus region just south of the Iowa
border into Missouri.

As mentioned in the aviation discussion, stratus should move back
into the state overnight and into early Monday and it could be
further supported with an increasing elevated mixed layer aloft,
trapping moisture below it. Some fog could develop as well though at
this time, much of the moisture will support the stratus. High
temperatures Monday may depend on how quickly this cloud cover can
mix out on Monday. The Southern Plains upper ridge will extend lift
north with ridging and thermal ridging moving into Iowa later Monday
night and into Tuesday which will bring hot and humid conditions to
central Iowa. Expected heat index values for Tuesday afternoon are
in the 100 to 110 degree range. The current dewpoint forecast for
the afternoon is in the mid to upper 70s. That may be a bit high
considering the mixing potential though lower mixed dewpoints could
result in high actual temperatures. Heat headlines are possible for
Tuesday. Heat indicies back in the 95 to 105 range again on
Wednesday.

The upper ridging will move east Wednesday night into Thursday
leading to southwest flow into Iowa with another short wave
approaching the area and a boundary dropping into the state. This
will bring the next chance for thunderstorms to the region. Those
chances will be on and off into the weekend. There is the potential
for even stronger upper ridging becoming re-established early next
week which would bring another round of very hot conditions though
being on the northeast quadrant of this feature could also lead to
MCSs dropping into the state.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1228 PM CDT Sun Jul 20 2025

A large swath of stratocumulus is over Iowa early this afternoon
with bases rising to MVFR while some scattering has resulted in
VFR at times. Most sites should become VFR this afternoon as
bases rise further. The low level steering flow comes from the
southeast tonight and that is a source region of MVFR to IFR
cigs that are currently over parts of Illinois and Ohio. Those
cigs will move into central Iowa overnight into early Monday.
Thunderstorm chances have decreased during the period therefore
no mention in this forecast.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Donavon
AVIATION...Donavon