Richards, Iowa Weather Forecast Discussion

038
FXUS63 KDMX 211141
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
641 AM CDT Sun Sep 21 2025
...Updated for the 12z Aviation Discussion...
.KEY MESSAGES...
- Dense Fog Advisory until 9am for parts of southern and central
  Iowa.
- Marginal Risk for severe weather in northern Iowa. 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.
&&
.UPDATE...
Issued at 358 AM CDT Sun Sep 21 2025
A Dense Fog Advisory is in effect until 9am this morning as fog has
spread across the cleared areas, mainly in southwest Iowa. Fog may
still be found on the fringes of the exiting cloud deck on the
northern extent of the advisory.
Water vapor imagery included a few features to note for the next 40
hours: (1) An upper closed low located in North Dakota (2) An
extension of the trough in the form of a PV anomaly in western
Nebraska and (3) an extension of the 850mb trough reflection in the
OK/TX Panhandle region. (2) and (3) will steer the low-level flow
and theta-e advection (and resultant instability axis while (1) is
the main source of synoptic support for storms in the north.
To start with the more straightforward kinematic setup: The low
center will pull an upper jet around it, increasing deep-layer shear
in the ballpark of 25-35kt in northern Iowa. At the same time, the
front extending to (2) will act as a surface catalyst. Most of the
shear will be focused above 1km with straight hodographs pointed
orthogonal to the front. There has been a northern trend in the
placement of the front and a lag with when CI occurs on the tail end
of it. Therefore, the time of arrival in northern Iowa is after 7pm.
Hail is the main concern with elevated storms, but gusty winds can`t
be ruled out early in the event. There is a Marginal Risk for hail
across northern Iowa.
The thermodynamic setup is the more challenging part of the
forecast, and it`s thanks to (3) having first dibs at the LLJ and
moisture. As mentioned before, some moisture is leaking this far
north, evidenced by the clouds collecting on the front in western
Nebraska. We`ll largely lose the LLJ for the day, and by the time it
tries to push into southern Iowa tonight, it will quickly veer into
Illinois, spawning an MCS in northern Missouri in the process. This
is why PoPs were cut back in southern Iowa today, some of it being
maintained to account for precipitation loading from the isentropic
upglide.
The front will sag into the state for Monday with (2) pulling the
LLJ back west as well. Marginally favorable deep-layer shear
(~25kts) points orthogonal to the front again Monday afternoon.
Initiation in the daytime puts damaging winds back into the mix
of severe weather threats as DCAPE exceeds 1000 J/kg and theta
e differences are around -20K, a sign of strong cold pools.
There is now a Marginal Risk for wind and hail on Monday.
&&
.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 /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 636 AM CDT Sun Sep 21 2025
Challenge is the dense fog this morning. Removed vsby
restrictions from FOD as obs have not dropped, but MVFR vsbys
remain possible this morning. Have kept TEMPO at MCW due to
nearby FXY with LIFR conditions. Similar conditions possible at
MCW as clouds clear out.
Further south, fog will lift to an IFR/MVFR cloud deck that will
become SCT to FEW through the morning. Showers and storms with
possible hail at KFOD and KMCW after 00z. Added in fog mentions
for KDSM and KOTM as signal remains for more fog tonight.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning for IAZ046>048-
057>060-070>075-081>086-092>097.
&&
$$
UPDATE...Jimenez
DISCUSSION...Lee
AVIATION...Jimenez

NWS DMX Office Area Forecast Discussion