Mason City, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

957
FXUS63 KDMX 281740
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1240 PM CDT Sat Jun 28 2025

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Line of showers and storms moves across northern Iowa this morning
  dissipating with an otherwise warm and humid day forecast.

- Storm chances late tonight over northern Iowa, but the higher
  and more widespread storm chances are Sunday afternoon into
  Sunday night. Damaging wind gusts and locally heavy rainfall
  are the primary hazards.

- Warm conditions around 90 degrees this weekend, then 80s next week.
  Today`s heat indicies well into the 90s to near 100 degrees.

- Additional storm chances mid to late next week.

&&

.UPDATE...
Issued at 217 AM CDT Sat Jun 28 2025

Convection over Siouxland around midnight is now entering north
central Iowa similar to how last night`s 0z HRRR and RRFS
advertised. An outflow boundary has pushed out ahead of these
storms per KFSD radar with wind gusts topping out around 40 mph
or so. These storms will continue on a weakening trend as they
outrun the instability and kinematic support, but will likely
push through much of our north central service area early this
morning. The other area of activity we`ll be monitoring for
later this morning is over western Minnesota. Convective
allowing models (CAMs) show this activity largely tracking
eastward across northern and central Minnesota. However, a few
global models and CAM solutions show this turning and diving
southeastward along the instability gradient into Minnesota and
northeast Iowa toward mid to late morning. If this turn does
occur (15% chance), the storms should be in the diminishing
phase as it reaches northeast Iowa so not anticipating anything
severe, but lightning would certainly be a possibility. Today
otherwise will be warmer than yesterday as a warm front pushes
northeast of the state. Temperatures will peak well into the 80s
to the low 90s. Dewpoints will also be higher well into the 60s
and low 70s and the combination of this moisture and
temperatures will push heat index values well into the 90s with
a few places near 100 degrees. In addition to the heat and
humidity, winds from the south will turn breezy by later this
morning into this afternoon averaging 10 to 20 mph with gusts
topping out between 15 to around 25 mph this afternoon.

A shortwave trough over the Alberta province will push eastward over
the southern Canadian prairies today into tonight. As it does so, a
surface low pressure and cold front will begin to move east and
southeastward, respectively. This front will be the focus for storm
development late this afternoon into this evening over Minnesota.
Most guidance - both global and high resolution - keeps Iowa dry
through midnight. However, storm chances do creep upward after
midnight through the morning hours over northern Iowa. As the cold
front settles into the central part of the state by afternoon
Sunday, there will be strong instability in place along with steep
mid-level lapse rates. While effective and deep layer shear is
higher closer if not behind the boundary, it is generally under 15
knots and will limit storm organization. Storms therefore will
develop quickly, which may result in some small hail. However,
storms will struggle if not fail to sustain themselves before
collapsing. With downdraft CAPE values over 1000 J/kg due to mid-
level dry air, strong, gusty downdraft winds will be possible and
are reflecting in the 0z HRRR, RRFS, and FV3 runs. Given past
experience, the 0-3km CAPE values over 150 J/kg per GFS forecast
soundings do point to potential of a few funnel clouds with LCLs
just above 1km, but tornadoes are not expected given the overall
environment. Of equal concern is the locally heavy rainfall threat.
The heavy rainfall parameter space shows precipitable water values
of 1.5 to around 2 inches, particularly near the boundary where
moisture will pool, and deep warm cloud depths over 3500m. To
quantify the low level moisture, the ECMWF 925mb specific humidity
is pushing the 99.5th percentile along the boundary, though the
NAEFS is not nearly as high. Finally, mean flow will be nearly
parallel to the boundary with storm motions around 15 to at most 20
knots. Thus, storms repeating over the same area are possible along
with efficient rain rates. Now, 0-10cm soil moisture percentiles -
good for assessing rapid response flooding - from NASA SPoRT have
recovered some from late week, but still are above the 80th to 90th
percentile over northern into parts of central and less so southern
Iowa. While the next day or so will add capacity through evaporation
and plant use, capacity will be limited. Therefore, we will need to
monitor the potential for flash flooding, especially over urban
areas that have less capacity due to their more common impervious
surfaces. Most global models and CAMs show rainfall of 1 to 2 inches
with isolated 3 inch amounts. However, a few global models such as
the NAM and regional Canadian paint higher bullseye and will have to
monitor this potential as we get more within the CAM and HREF
forecast windows.

&&

.DISCUSSION...
Issued at 300 PM CDT Fri Jun 27 2025

Quiet weather persists this afternoon as weak high pressure remains
overhead, along with northwesterly flow keeping temperatures a bit
cooler. These cooler temperatures are also due to lingering low
scattered clouds over the northeastern half of the state this
afternoon, with current temperatures in the low to mid 70s northeast
and in the upper 70s to low 80s further southwest. Late tonight into
early Saturday, with a low level jet increasing over Nebraska into
South Dakota and a shortwave passing across Upper Midwest, this
looks to bring showers and storms at least back into the region.
HRRR and RAP guidance runs today continue to depict gradually
decaying showers and storms tracking out of the Dakotas through
southern Minnesota, just scraping into far northeastern Iowa around
sunrise and through the rest of the morning. However, the NAM keeps
this activity in southern Minnesota into western Wisconsin,
completely missing Iowa. Model soundings indicate that any storms
would be elevated in nature, with shear values around 20-25 knots,
though MLCAPE values are at least more favorable around 1500 J/kg.
Mid-level lapse rates look to top out around 7 J/kg, though with
fairly high freezing levels, would expect at most a very low
potential for small hail. However, damaging winds could be a threat
as DCAPE values around 1000J/kg Saturday morning. Overall, the
potential for severe weather looks low but a few storms could bring
sub-severe winds at most. Considering the track differences as
discussed above, impacts look to remain only confined far northeast
Iowa for potential sub-severe (at most) storms into mid morning as a
worst case scenario, but otherwise all areas remain dry for at least
much of the day.

A more defined mid-level shortwave passing through the Upper Midwest
by Saturday evening will be the next focus, with an increasing low
level jet into the area resulting in the development of a broader
area of showers and thunderstorms along a front over South Dakota
and Minnesota into northern Nebraska and continuing into Sunday
morning. Models are generally trending on keeping the bulk of
showers and storms into southern Minnesota, though Euro and NAM
guidance places some of this convection into northeast Iowa by early
Sunday, potentially skirting parts of far northern Iowa in the early
morning. A notable warm layer in the low levels would lead to
another potential for elevated convection, with favorable
instability, though limited shear into northern Iowa to really allow
for much organization to occur. A Marginal Risk for a few isolated
severe storms remains over northwest/northern Iowa to account for
the potential, with even a Slight Risk further northwest into SD/MN
where the parameter space is looking much better for severe weather
to be more likely. Beyond this morning activity, more scattered
showers and storms into Central Iowa are possible into Sunday
afternoon to evening as the system slides through the area, though
models are still struggling with the coverage of this activity. The
GFS and Euro depict scattered showers and thunderstorms across the
state, whereas the NAM has the bulk of activity in portions of
southern into eastern Iowa with the frontal passage. Sounding
guidance shows plenty of instability overhead with values generally
around 3000-4000 J/kg, with shear values around 30-35+ knots that
would allow for storms to develop and potentially bring a wind
threat given DCAPE values around 1000 J/kg given dry low levels and
potential some small hail. Warm cloud depths around 15kft with PWATS
over 1.5 inches will also introduce the potential for heavy rain as
well, as GEFS shows generally widespread 1 inch values, though
cannot rule out locally higher values. In terms of hydrologic
impacts, please refer to the hydro discussion for more details.
Details more specifically on severe weather potential and overall
expectations should be better known through the weekend as models
better capture this activity. Outside of on and off showers and
storm chances this weekend, returning southwest flow will increase
high temperatures back into the upper 80s to low 90s through the
weekend, with heat indices reaching near 100 degrees in portions of
western Iowa Saturday. Though not quite hitting Heat Advisory
criteria, heat impacts are still possible especially for more
sensitive populations, so proper heat safety should still be
practiced, especially if outdoor plans are in place.

Beyond Sunday, lingering showers and storms may continue into
Monday, followed by dry weather Tuesday and Wednesday before more
showers potentially return Thursday, with temperatures generally
topping out in the mid to upper 80s through the next work week. More
details on the forecast next week will be shared in the coming
days.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1237 PM CDT Sat Jun 28 2025

VFR conditions expected to prevail at area terminals through the
period as south winds of 5-10 knots persist across the state.
Thunderstorm chances will return to some terminal sjust beyoond
this TAF cycle and will likely need to be included in the next
update or two.

&&

.HYDROLOGY...
Issued at 300 PM CDT Thu Jun 26 2025

The rainfall from yesterday afternoon into last night came in a
little lower than QPF, so overall the river forecasts were adjusted
downward accordingly. In fact the new river forecasts were low enough
that we were able to cancel some river flood watches because those
locations are no longer expected to reach or exceed flood stage.

Soil moisture remains high though across much of the CWA owing to
heavy rainfall over the past several days. NASA SPoRT soil moisture
percentiles for the top 40 cm of the soil (usually considered for
river forecasting purposes) are much above normal to near record
highs everywhere north of US 30, while there is a mix of near normal
to near record high values south of US 30. These above normal values
will tend to relax over the next several days in the absence of
heavy rainfall due to increased water demands from the ag crops in
place across the state, thus lessening additional flooding concerns.

Additional rainfall is forecast Sunday into Sunday night, however.
Right now the rainfall looks to be seasonally moderate. Latest tools
suggest this additional rainfall will have a minimal impact on
existing river trends especially at those locations where levels are
or will be running high. The rainfall may add minor amounts to the
crests but at this time no significant impacts are expected from
that rainfall.

&&

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

&&

$$

UPDATE...Ansorge
DISCUSSION...Bury
AVIATION...Hahn
HYDROLOGY...Zogg