Duncombe, Iowa Weather Forecast Discussion

142
FXUS63 KDMX 201707
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
1207 PM CDT Sat Sep 20 2025
...Updated for the 18z Aviation Discussion...
.KEY MESSAGES...
- Scattered showers south this morning, then north this
  afternoon. Thunder potential is highest north (25-30%) and
  only around 20% south. No severe weather expected at this
  time.
- Fog potential tonight, mainly in southwest Iowa.
- Lingering showers and thunderstorms will gradually clear out
  toward the east on Saturday, with a brief spell of quiet
  weather around Saturday night. Rain chances then return from
  around Sunday afternoon through much of next week. Severe
  weather chances remain low on Saturday, and for next week will
  be determined in the coming days.
&&
.DISCUSSION...
Issued at 158 PM CDT Fri Sep 19 2025
A large upper low continues to spin over the region, primarily
centered over eastern South Dakota down into northwest Iowa. A
shortwave trough is rounding the low over central and eastern
Nebraska, generating renewed cloud and precipitation development
across that area this afternoon. Further east, a surface
boundary (effective dryline) is sweeping through central into
eastern Iowa at this time, generating a thin line of showers
stretching from north to south roughly from around Mason City to
Centerville. As we progress through this afternoon these showers
will continue to push eastward producing spotty light rainfall,
and probably isolated thunderstorms as instability gradually
increases. While surface-based CAPE of 1000-1500 J/kg is
forecast, deep-layer shear is very weak (10-15 KT over the 0-6
KM layer) and mid-level lapse rates are only around 6.0-6.5
C/KM, greatly reducing any severe weather threat. Weak low-level
convergence along the boundary may provide a slight enhancement
to justify isolated small hail/gusty winds however. Also, 0-3 KM
CAPE of 100-125 J/kg and enhanced stretching potential near the
boundary may support some brief weak funnel clouds, although
widespread cloud cover and modest low-level lapse rates would
seem to mitigate that potential. Overall any impacts from
showers/thunderstorms today will be minimal, and POPs are capped
around 40% in our northeastern and eastern counties.
Tonight, as the aforementioned shortwave aloft moves through the
region, the showers and storms over Nebraska will translate east
southeastward into parts of Missouri and Iowa. Most of the
models have been having difficulty resolving both the timing and
latitudinal position of the resulting swath of rainfall, with
most of them initializing the precipitation too far west/slower
than indicated by regional radar trends. In addition, a low-
level jet is expected to develop this evening over Kansas and
veer into Missouri overnight, which may generate an additional
cluster of convection in that region that could cut off some of
the push for storms further north in Iowa. However, there is
near-unanimous agreement in a slug of Theta-E advection and
region of diffluence extending over southern Iowa, and still
believe rain will move through parts of our forecast area
overnight accordingly. Have increased POPs to 60% in some of our
southern counties, but resisted the temptation to go higher
because confidence in the areal coverage and northward extent of
the rain is lower. While severe weather is not anticipated with
the overnight convection, locally heavy rainfall may be possible
given the expected orientation of the band. Precipitable water
values will only be around 1 inch however, and overall the rain
should be progressive, mitigating the heavy rain potential.
One other item of note for tonight is that winds will become
light and variable and near-surface moisture will remain high,
with dewpoints in the upper 50s to lower 60s, so patchy fog may
redevelop. Some high-resolution models are indicating this, but
generally outside our forecast area. Any dense fog development
would likely be contingent on areas of clearing that should be
few and far between, and cannot be predicted with any confidence
at this range, so will not mention fog in the official forecast
but it will be something to keep an eye on as the night
progresses.
On Saturday the upper low will finally begin to pull out to the
northeast, taking the clouds and rain with it. Precipitation
will move out slowly but steadily during the day, and there may
be a bit of a resurgence in the afternoon/evening if diurnal
heating is sufficient to destabilize enough for a few showers or
storms to form. Once again severe weather will be unlikely, as
deep-layer shear will be a bit stronger in the 20-30 KT range,
but instability will be more limited. Saturday night will be
relatively quiet, then on Sunday one last lobe of vorticity will
dig down over Minnesota on the backside of the departing low,
and may approach near enough to northern Iowa to generate some
showers or thunderstorms late Sunday into Sunday night.
Thereafter, the forecast for next week remains relatively (and
unfortunately) active as another large-scale upper level trough
develops right on the heels of the departing one, eventually
forming a closed 500 MB low somewhere near Missouri by the
middle of next week. This synoptic progression results in POPs
being carried through next week, every day in at least portions
of Iowa, with any details of severe weather/heavy rain hazards
to be sorted out in the coming days. Meanwhile, temperatures
will remain nearly steady with daily highs in the 70s to perhaps
lower 80s at times.
&&
.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1206 AM CDT Sat Sep 20 2025
Scattered SHRA/TSRA will affect portions of the area today. The
highest rain chance (60-80%) is at OTM where VCSH will be
included this afternoon, and at MCW and ALO (30-40%) where VCTS
will be carried for a few hours. Any impacts should be minimal
and brief, however. Otherwise, mainly VFR ceilings will prevail
today into early tonight, but then fog and low stratus will
develop overnight into Sunday morning. It is likely that patches
of clearing skies will lead to the development of dense (

NWS DMX Office Area Forecast Discussion