Colo, Iowa Weather Forecast Discussion

878
FXUS63 KDMX 151120
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
620 AM CDT Mon Sep 15 2025
...Updated for the 12z Aviation Discussion...
.KEY MESSAGES...
- Remaining very warm through Wednesday with temperatures averaging
  around 10 degrees above normal. Cooler by week`s end.
- A few showers possible early this morning, otherwise isolated storms
  this afternoon or early evening (20% or less) and Tuesday
  afternoon and evening (30% or less).
- Not a washout, but a prolonged period of clouds with
  intermittent shower and storm chances from Tuesday night
  through Friday that may linger into a part of the weekend.
&&
.UPDATE...
Issued at 305 AM CDT Mon Sep 15 2025
This morning`s upper level water vapor imagery shows that the ridge
has been shoved over the western Great Lakes with a trough that is
lifting northward through the Dakotas. This trough has kept broad
swaths of mid and high clouds over the state with a very broken line
of sprinkles or weak showers developing over central Iowa at
this hour. As the trough continues to lift away today, the
forcing will continue to be rather nebulous and weaken and
shift over the eastern half of the state. Further, forecast
soundings this morning show, that unlike yesterday`s soundings,
the convective temperature will be a few degrees out of reach.
Both of these features has resulted in an overall decrease in
chances and/or coverage of showers and storms this afternoon and
evening. In spite of the cloud cover, temperatures will still
be around 10 or so degrees above normal peaking in the upper 80s
to low 90s.
As the one trough lifts away, there is another shortwave trough that
has moved over the Northern Central Basin this morning. This will
slowly saunter over the northern Rockies on Tuesday with additional
weak shortwave energy ahead of it passing near and over Iowa. This
along with forecast soundings that show temperatures may be
closer to the convective temperature, we may have a slightly
higher chance of developing isolated storms in the afternoon to
early evening hours. While severe weather is unlikely given the
weak shear, will need to assess potential of funnel clouds given
the high amount of low level instability and low LCLs. Highs
will be comparable to Monday in the upper 80s to low 90s.
From Tuesday night through Friday, the forecast is trending
slower with shower and storm chances as the trough slowly
advances towards Iowa with it further slowing as it closes off
in the mid-levels. With this sitting west of the state, waves
of low level theta-e advection will pass overhead along with an
area of low level QG convergence. Thus, there will be prolonged
chances for showers and storms from midweek into at least a
portion of this weekend as these chances slowly migrate across
the state. It won`t be a washout, but there will be plenty of
opportunities to bring some needed moisture to the region. In
addition, there remains no apparent signal for severe weather in
this period with the deep layer shear overall weak. The cloud
cover will result in cooler conditions as we head toward and
into next weekend.
&&
.DISCUSSION...
Issued at 252 PM CDT Sun Sep 14 2025
Out of the sequence of troughs seen over the Western CONUS and
Pacific today, the first feature continues its negative tilt into
the Upper Midwest today. Moderate height falls on the order of -5 to
-6 dam will stay just to the west of the state later today. An upper
jet will round the base of the feature through today, boosting deep-
layer shear values over 40kts in western Iowa. Convective trends
have slowed with time, meaning that less storms will inch into
western Iowa tonight and, when they do, less of the shear will be
utilized thanks to the nocturnal inversion. With the synopic-driven
convection getting held to the west, there`s another region of lift
to watch tonight: the airmass boundary over east-central Iowa.
Convergence is driven by the clashing of the easterlies from the
Ontario high and the warm conveyor belt of the aforementioned low to
the west. This boundary can be seen this afternoon in the southwest
to easterly surface winds creating a cumulus field.
While shear this far east will be low, MUCAPE values over 1000 J/kg
reside over eastern Iowa. Overnight CAM runs, mainly the HRRR,
produced 40kt winds. This occurs in the pocket of the highest DCAPE
values (around 400 J/kg, but decreasing through the night). Cloud
layer shear is valued at less than 20kts, so storms that do become
strong will be short-lived. This gradient hangs around Monday and
Monday night and may once again provide enough support for showers
and storms, although this time with less moisture at play. Most all
HREF members have convection across north central Iowa Monday
afternoon, but keep in mind that earlier runs that covered
today/tonight period had convection going by now, so it`s possible
that coverage is overdone again in a similar setup. Have at least
kept non-zero PoPs in to cover potential.
Broad troughing swirls over the Western CONUS and the upper ridge
will angle into the Great Lakes region by midweek. Given the
proximity to the forcing, rain and storm chances are not far away
with some potential beginning once convective temperature is
reached. The mass of synoptic forcing gets close enough to have
higher confidence rain and storms for the second half of the
week. It will be wrapped up enough at that point to only have
height falls around -2 dam, so less dynamic.
&&
.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 620 AM CDT Mon Sep 15 2025
While VFR conditions are shown to prevail through the period,
will be monitoring spotty shower and storm chances. There is
currently a broken line of sprinkles and showers from near MCW
to IKV to CNC that are lifting northward. Confidence in any one
shower moving over a terminal (MCW/ALO) let alone any impact is
too low to include. Isolated storms may develop this afternoon,
but again chances are too low to include so will monitor for
possible AMD.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...Ansorge
DISCUSSION...Jimenez
AVIATION...Ansorge

NWS DMX Office Area Forecast Discussion