Grimes, Iowa Weather Forecast Discussion

603
FXUS63 KDMX 211707
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
1207 PM CDT Sun Sep 21 2025
...Updated for the 18z Aviation Discussion...
.KEY MESSAGES...
- Marginal Risk for severe weather in northern Iowa today.
  Large hail the main threat. Storms will arrive close to
  sunset.
- Marginal Risk for severe weather for much of Iowa on Monday.
  Both damaging winds and large hail are in play.
&&
.DISCUSSION...
Issued at 125 PM CDT Sat Sep 20 2025
An upper level trough over southern Minnesota and northeastern Iowa
this afternoon will gradually shift northeastward away from the area
tonight. A shortwave impulse rounding the base of the trough is
currently generating showers in our southern and southeastern
counties, which will linger into the afternoon before slowly ending
as the forcing moves off. Meanwhile, modest diurnal heating will
result in weak instability (200-400 J/kg of CAPE) later this
afternoon and evening, and weak lift associated with the departing
upper trough should be sufficient to generate a few additional
showers and thunderstorms roughly across the northeastern half of
the forecast area. Will include low rain chances (20-30%) in that
area into the evening before ending them tonight as the trough moves
away and the atmosphere stabilizes. A lack of instability and weak
shear fields preclude a severe weather threat today.
Overnight tonight we will finally have a brief respite from rain,
while a weak surface high pressure ridge builds across Iowa. This
will bring nearly calm winds, with a saturated near-surface layer
and dewpoints in the upper 50s to lower 60s. That is a recipe for
fog formation, but the limiting factor will be prevailing cloud
cover. Even beneath the clouds some fog formation is likely to
occur, at least across portions of the area, with visibility falling
to perhaps 1-3 miles. However, if there are areas where skies are
able to mostly clear, then dense fog formation may be possible with
visibility falling to well below a mile. Most high-resolutions
depict areas of such dense fog forming in Iowa, however, the
location of any such features is unpredictable at this range. For
now will include patchy fog, but not dense fog, in the official
forecast and will monitor near-term satellite and observational
trends tonight.
On Sunday another lobe of upper level vorticity will swing around on
the backside of the departing trough, moving across Minnesota and
exerting some influence in northern Iowa late in the day. This
should be sufficient to generate some thunderstorms, mainly around
Sunday evening, and deep-layer shear profiles are somewhat stronger
with around 25-30 KT of 0-6 KM Bulk Shear predicted by most models.
However, the degree of clearing and insolation remains unclear and
there is a high degree of variability in the magnitude of resulting
destabilization. Most solutions depict CAPEs nearing 1000 J/KG in
our northern and northwestern counties, however the bulk of the
dynamic support associated with the vorticity lobe should remain
just north of our area. Even so, have included ~50% POPs in our
north and northwest late Sunday into Sunday night, and SPC has
updated to include a Marginal Risk of severe weather in that
area which is prudent. Hail and gusty winds would be the primary
threats with any severe storms that due occur.
On Monday a broad surface low pressure trough will develop down
the High Plains, with a northern lobe over Nebraska and a
southern lobe near the Oklahoma panhandle. Modest zonal steering
flow over Iowa coupled with light south southwest low-level flow
will promote warn air advection, pushing temperatures into the
lower 80s in parts of our service area. A weak surface boundary
will also morph into an effective warm front during the day,
stretching eastward from the Nebraska trough/low somewhere near
or over northern Iowa in the afternoon and evening. Strong
destabilization will occur during the day due to low-level
warming and persistent cooler temperatures aloft, resulting in
CAPE values of approximately 2000-3000 J/kg by late afternoon,
likely pooling up against the boundary in the north. While
deep-layer shear late Monday will be weaker than on Sunday,
more in the 15-20 KT range in the 0-6 KM layer, the degree of
instability in proximity to a surface boundary certainly
supports some level of threat in terms of severe weather. Any
such potential will be sensitive to chances in the instability
forecast due to temperature and cloud cover fluctuations, so
will revisit tonight and Sunday to determine any trends in the
threat level.
From Monday night into Tuesday the northern (Nebraska/Iowa)
portion of the High Plains trough will weaken as the southern
(Oklahoma) portion deepens and becomes the dominant surface
cyclone. This will occur in response to a 500 MB low sinking
over Colorado during this time, which will subsequently broaden
over Missouri and Iowa on Wednesday and Thursday before moving
into the southeastern U.S. at the end of next week. Within this
broad scenario we will see a continuation of rain chances in
Iowa from Tuesday onward, however, they will gradually become
more concentrated over southern portions of the state and
depending on how far south the low progresses, it is possible
much of Iowa could become dry during this time with Missouri
seeing the bulk of any prolonged rainfall. It is also likely
that any severe weather threat will remain low in Iowa in the
latter half of next week, on the northern periphery of the
cyclone. These details will be better determined in the coming
days.
&&
.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1207 PM CDT Sun Sep 21 2025
A few low clouds remain from the fog and stratus that formed
last night, mainly at OTM where MVFR ceilings are still being
reported. These will gradually rise and break up this afternoon,
with VFR conditions prevailing at most times/locations. There
are low (20-30%) rain chances this evening and early tonight at
some terminals, but confidence/probability is too low to include
in the TAFs at this time. Expect more fog and stratus formation
on Monday morning and have indicated this in the 18Z TAFs,
though widespread ceiling/visibility restrictions are not
expected to be as bad as this morning, with MVFR more likely on
Monday.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
DISCUSSION...Lee
AVIATION...Lee

NWS DMX Office Area Forecast Discussion