Coon Rapids, Iowa Weather Forecast Discussion

351
FXUS63 KDMX 212347
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
647 PM CDT Sun Sep 21 2025
...Updated for the 00z Aviation Discussion...
.KEY MESSAGES...
- Chance of thunderstorms north late this afternoon into
  tonight, but better chances across southern Minnesota.
  Marginal Risk of severe weather, with large hail the primary
  threat.
- Some fog redevelopment early Monday morning, but dense fog
  less likely than this morning.
- Another round of thunderstorms around Monday evening into
  Monday night, initiating in western-to-northern Iowa then
  moving southward into southwestern-to-central Iowa. There is
  again a risk of severe weather, with damaging winds and large
  hail the primary threats.
&&
.DISCUSSION...
Issued at 200 PM CDT Sun Sep 21 2025
A weak surface ridge is sliding across Iowa today, providing a
brief spell of quiet weather. Conditions were conducive for the
formation of fairly widespread stratus and dense fog early this
morning, but these have largely dissipated as insolation has
increased and skies are now partly to mostly sunny across the
service area. This will not hold for long, however, as
thunderstorms return to the forecast as early as this evening. A
500 MB low remains centered over the northern half of
Minnesota with one more shortwave trough rounding its
southwestern periphery later today and tonight. Meanwhile, at
the surface a broad low pressure trough has developed along the
lee of the Rockies with a stationary boundary gradually
developing from near the Nebraska panhandle northeastward into
western Minnesota, effectively providing the northwestern
terminus of the aforementioned weak surface high over Iowa.
Around this evening, upper-level forcing for ascent from the
approaching shortwave will overlie the surface boundary just
after peak heating, allowing for destabilization to be utilized
for convective initiation. This will occur northwest of our
forecast area, likely over southwestern Minnesota or far eastern
South Dakota, with any storms sinking east southeast with the
shortwave and boundary tonight. Modest instability (CAPEs near
1000 J/kg), 0-6 KM Bulk Shear of 25-35 KT, and fairly steep
mid-level lapse rates will support a Marginal Risk of large hail
around the SD/MN/IA tri-state region, possibly bleeding into our
northwestern counties as outlined by SPC. However, with a steady
northward trend in convective area forecasts today, will cap
POPs in our area at around 50% and confined to our northern
sections. There is also an area of light rain showers over
northeastern Kansas that will translate east northeastward this
afternoon into tonight and have maintained 20-30% POPs in our
southern counties to account for this, but instability is much
lower there and thunderstorms will be sparse or non-existent
with no impacts anticipated.
Some fog and stratus redevelopment is possible tonight as Iowa
remains within a moist airmass characterized by dewpoints in the
upper 50s to lower 60s. Winds will once again be light, but
slightly more organized with the weak surface high departing. It
remains to be seen how much any storms/convective outflow will
be able to penetrate into our area as well. Will advertise a
couple hours of patchy fog in the official forecast, but overall
conditions are not as favorable as this morning and the
probability of dense fog is lower overall.
During the day on Monday, modest low-level warm air advection
will translate into somewhat higher temperatures, peaking around
80 degrees. Destabilization will be more significant, with CAPE
of 2000-2500 J/kg predicted by Monday afternoon and evening
when thunderstorms are likely to initiate along the surface
boundary, which by then should reside somewhere from western
into north central Iowa. While deep-layer shear will be a bit
weaker on Monday evening, perhaps only around 20 KT in the 0-6
KM layer, the strong instability will still support a risk of
large hail and damaging winds. The lack of storm organization
or a balanced cold pool could result in storms congealing into
less severe clusters thereafter, as they head southward into
central and southwestern Iowa. However, there is certainly
support for another severe weather risk as outlined by SPC in
their Day 2 Outlook.
By Tuesday another 500 MB low will be sinking over Colorado, and
subsequently slide eastward across Kansas and Missouri on
Wednesday and Thursday. It will then interact with the previous
500 MB low, the one over Minnesota today, which will by then be
weaker and stationary/retrograding over the Great Lakes region.
Long-range models differ in their solutions for this evolution,
but overall we can expect continuing rain chances
Tuesday/Wednesday, mainly over southern Iowa as the second upper
low slides by to our south. Thereafter things may trend drier,
but it depends on the interaction between the upper lows
occurring nearby. Confidence in such details decreases with
time in this scenario, but in any event, given the nature and
location of the systems the overall threat of severe weather
appears low from Tuesday onward.
&&
.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 634 PM CDT Sun Sep 21 2025
Not a ton of change in thinking from 18z TAFs, with main
aviation concerns being a low probability (30% or less) for
a shower or isolated storms tonight, mainly near KFOD and KMCW.
Given low confidence, still keeping mention out of TAFs at this
time. The remainder of the area should remain dry overnight,
turning the concern to more fog and stratus potential into
Monday morning. Fog and low stratus looks most likely at KDSM,
KALO, and KOTM, although KMCW and KFOD may see restrictions as
well. Visibilities down to a mile or less are possible, but will
be less widespread than Sunday morning. Stratus could develop
below 1000kft, leading to a period of IFR conditions. Fog
should begin to dissipate shortly after sunrise, but the MVFR to
IFR ceilings may linger through the morning hours. Regardless,
improvement to VFR is expected by mid-day.
Later Monday afternoon and into the evening there will be more
chances for showers and storms, mainly north and west. That
said, this mostly occurs at the end of this TAF period and
after, so have held off on any mention in the 00z TAFs.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Lee
AVIATION...Dodson

NWS DMX Office Area Forecast Discussion