Mason City, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

059
FXUS63 KDMX 200807
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
307 AM CDT Thu Aug 20 2026

.KEY MESSAGES...

- Areas of fog this morning could become dense.

- Scattered thunderstorms tonight, mainly north and again Friday
  night, mainly over the south. A strong storm is possible with
  both periods.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 257 AM CDT Thu Aug 20 2026

The upper level flow will continue to be driven by an upper level
low over northeast Canada and by an intensifying sub-tropical high
over the Four Corners region. The evolution of these two systems
will lead to increasing northwest flow over Iowa through Saturday
before the upper high ridging begins to move eastward. A couple of
short wave impulses will be passing through this flow later today
and tonight and again Friday night and each will bring a chance for
showers and thunderstorms to the state. In the immediate term,
surface high pressure centered over northern Iowa. Sky conditions
across Iowa range from mostly clear over the majority of the state
to scattered mid-level clouds over central to south central Iowa.
Dense fog is the immediate challenge this morning. Areas of dense
fog have developed over portions of northwest Missouri into eastern
Nebraska in addition to a few localized locations over far southeast
Iowa. These areas align fairly well where the more widespread
precipitation has occurred over the past 24-48 hrs and it is these
area with wet soils that are the first to have fog impacts. The
Nighttime Microphysics satellite imagery is showing some river valley
fog occurring in the forecast area near Taylor and Ringgold counties
in the south and in the west central near the Nishnabotna valley.
The southern valley fog has actually diminished some per the
satellite trends and that may be in part to drier dew points from
the high pressure system still advecting into the state. That is the
challenge this morning is how widespread dense fog will become under
mostly favorable radiational cooling conditions along with the
lowering dew points in addition to several days of dry weather
north. Currently no Dense Fog headlines for the area but will add if
trends change.

The short wave impulse to impact the area later today is showing up
on water vapor imagery over southern Manitoba. The main kinematic
energy associated with that system will focus over Minnesota and
Wisconsin. The main forcing processes will thermally and moisture
driven as return flow develops into northwest Iowa as the high
pressure departs and a weak surface boundary approaches from the
north. The storms will be moving into an increasing strong elevated
mixed layer (EML) farther south through Iowa. Storms may persist but
will become increasing elevated above the EML which will decrease
available instability. A strong storm could occur over the far
north/northwest but for the reasons listed above, that potential
will decrease further into Iowa.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 222 PM CDT Wed Aug 19 2026

The ridge-trough pattern across the CONUS will continue to amplify
as we head into the weekend, with Iowa continuing to be positioned
on the interface of the two, under enhanced NW flow. A similar
pattern appears to be in place into early next week as well. There
is still significant uncertainty timing of the frontal passage
Friday, with the NAM even suggesting it holds off until Friday
evening. NAM/GFS/ECMWF all suggest some storm chances Friday night,
though more focused to our west under better instability. These
storms would have a moderate amount of instability and bulk shear
overlap and could perhaps pose a severe threat, hence the marginal
across our area and slight risk to our west. GFS/ECMWF suggest the
next shortwave (or waves) passing through the enhanced flow sometime
Saturday night through Sunday, though timing still varies. With the
passage of the cold front on Friday, and replacement with high
pressure through the weekend, gulf moisture flow seems unlikely.
However, evapotranspiration is still significant and may provide
enough moisture for some marginal instability and low end storm
chances. All things considered, the weekend seems drier overall,
with a few marginal storm chances with uncertain timing.
Temperatures remain relatively unchanged from this past week, with
highs in the upper 70s and low 80s. More significant instability
appears to return early next week, ahead of another cold frontal
passage according to deterministic models. There is still
uncertainty in timing of that front, and any further details given
by deterministic models at this stage are certain to change.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1044 PM CDT Wed Aug 19 2026

VFR conditions prevail over the area late this evening, but
will see the potential for visibility reductions as patchy fog
develops through the morning. The greatest fog potential will
be over southern and western Iowa, with KFOD and KDSM on the
fringes of these chances. Therefore, have added 6SM BR for both
these sites and will amend as conditions evolve. KOTM will have
a better chance for fog, especially with recent rainfall and
clouds clearing out of the area, so have went with a few hours
of 5SM visibilities. Cnoditions improve at all sites shortly
after sunrise, with light winds and mostly clear skies on
Thursday. There is a low potential for isolated
showers/thunderstorms near KMCW and KALO tomorrow evening, but
no mention in TAFs at this time.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SHORT TERM...Donavon
LONG TERM...De Bruin
AVIATION...Dodson