Grimes, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

600
FXUS63 KDMX 061131
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
631 AM CDT Sun Jul 6 2025

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Mainly quiet today with highs in the 80s.

- Thunderstorm chances return on Monday over western Iowa and
  across the entire area by Monday night.

- A few other chances for storms through the week. A few periods
  may contain strong storms and/or heavy rain potential.

&&

.UPDATE...
Issued at 339 AM CDT Sun Jul 6 2025

The precipitation has mostly ended across the forecast area
early this morning outside of diminishing light showers over
north central Iowa. A patch work development of fog and stratus
has occurred into central Iowa and there have been brief
instances with visibilities at 1/4sm. All of this will dissipate
early this morning.

For today, weak high pressure ridging will move into Iowa and
should result in a dry day. Disclaimer the word should here as
atmospheric profiles do become convectively unstable this
afternoon, especially west of Interstate 35. However, there is a
lack of any forcing mechanisms today to force vertical ascent
outside of surface heating. The most likely outcome will be
bubbly cumulus though there is the potential for an isolated
thunderstorm to develop but there is a low predictability in
pinpointing this location if it occurred.

Beyond today, the main forecast challenge is thunderstorm
chances throughout the week. There is a PV anomaly/short wave
currently over southern California that will move across the
Northern Colorado Rockies tonight with an MCS potentially
developing over northeast Colorado and western Nebraska and
moving eastward overnight. It is possible this MCS and PV
anomaly will reach west central/southwest Iowa on Monday morning
and could lead to renewed development of storms during the day
Monday. This will be followed up by another short wave and
surface boundary later Monday and Monday night with additional
storm potential. Storm chances linger into Tuesday. A reprieve
from storms may arrive Wednesday into Thursday. Another strong
system arrives at the end of the week and bring more storm
chances Thursday night and Friday. There will be chances for
severe weather and some areas of heavy rainfall with these
systems. PWAT values will be near 2" at times. The severe
weather threat likely will be highlighted by damaging wind
potential. Temperatures this week will be seasonal and mainly in
the 80s for highs.

&&

.DISCUSSION...
Issued at 257 PM CDT Sat Jul 5 2025

Our morning convection produced some healthy rainfall over the area
today, with a few bands of 2 to 3 inch rainfall over the northern
and western portions of the forecast area. This convection was
fueled by a very moist low level jet nested within an upper level
wave (and coincident surface low pressure) which is currently
drifting north and east of the area early this afternoon. This
morning activity has all but departed our forecast area, with
showers and storms developing sporadically in it`s wake. However, as
this first round departs, our second round of storms is already
starting to develop along the cold front draped behind the surface
low. While most of the storms today have only produced brief gusty
winds and heavy rainfall, this second round of storms will have the
potential to produce a few stronger storms now that skies have
cleared out some and the atmosphere is destabilizing. Fortunately,
shear still remains fairly low, and storms will struggle to
organize. The main severe risk will be small hail, as well as
damaging winds as the initial cores develop and collapse. However,
as convection occurs, cold pools/thunderstorm outflow will race out
ahead of storms. New storms will then develop on the progressing
outflow, but any severe threat should be limited to new storms
developing in the untapped air ahead of the outflow. Also a product
of the low shear environment will be the potential for funnel clouds
and even a brief landspout with initial updrafts along the boundary
this afternoon, especially in northwest Iowa. That said, expect this
threat to diminish as more storms develop and cold pools negate the
processes involved in stretching vorticity into a funnel. Finally,
heavy rainfall is expected with additional convection this evening.
Storms today should remain progressive with the movement of the
boundary and outflow, but will still continue to monitor for
hydrological concerns, especially as storms move through areas that
already saw heavy rain this morning.

Storms move out of the area through the overnight hours, although
the boundary will stall out just to the southeast of the forecast
area. This may lead to additional scattered showers in far southeast
Iowa through tomorrow, but these should generally be weak and
transient. As mentioned in the previous update, weak surface high
pressure fills in behind the wave on Sunday. Northerly flow will
keep temperatures seasonal in the low to mid 80s through the day and
drier conditions behind the boundary will drop dewpoints into the
60s, helping to alleviate the humid feel we`ve had recently. Some
model output does still imply the development of diurnally driven
showers and isolated storms, mainly south and east on Sunday
afternoon as the atmosphere destabilizes. However, this would be
contingent on there being a source of lift to displace parcels,
which there is little of in our area. If anything is going to do it,
it would be the slight wind shift over the eastern half of the
state. Regardless, instability will generally be around 1000 J/kg or
less and wind shear will be non-existent, meaning any convection
would be pulsey and disorganized. This type of convection could
still produce gusty winds and small hail, but generally won`t pose a
significant severe risk. Aside from this low potential for isolated
precipitation, Sunday will remain dry for most, giving us brief
reprieve before another active week starting on Monday.

The upper level pattern overhead becomes more zonal through the
first half of the week as ridging sets up over the southwest CONUS.
This will see multiple waves of convection developing around the
northern periphery of the ridge and over the Plains region, which
will progress eastward toward the state, either in the form of an
MCS or remnant MCV/shortwave which will refire over the state. The
first of these waves begins Sunday night into Monday morning. Short-
range guidance implies this will grow upscale over Nebraska but
eventually dissipate as it loses support from the low level jet on
Monday morning. However, remnants from this system then drift into
Iowa, becoming the source for convection on Monday afternoon. That
said, it`s get too attached to this solution, as it`s dependent on
multiple upstream factors. Since this zonal pattern is so weakly
forced, what happens in Iowa on Monday will depend on the mesoscale
factors that play out with thunderstorms firing off the front range
all the way over in eastern CO/WY Sunday afternoon then it`s
progression and evolution through Nebraska Sunday night and then
into Iowa Monday morning. If you`re familiar with the butterfly
effect, that applies here. Needless to say, it`s a low confidence
scenario, so will be keeping an eye on model and observational
trends through the coming days. Fortunately, this will once
again be an environment with plenty of CAPE but minimal shear,
helping to negate organization and sustainment of storms.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 628 AM CDT Sun Jul 6 2025

A few areas of LIFR cigs and vsbys around central Iowa to start
the period. Those conditions are expected to improve through
14z. This is some potential that a broken MVFR cumulus deck
could develop mid morning as this dissipating near surface
moisture lifts. Widespread VFR conditions are anticipated this
afternoon as any lingering MVFR bases lift further. Light
northerly winds at less than 12kts are expected through the
period.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Dodson
AVIATION...Donavon