Ottumwa, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

768
FXUS63 KDMX 201732
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1232 PM CDT Sun Jul 20 2025

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

.KEY MESSAGES...

- Flood Watch expanded in southern Iowa, will need to monitor
  for continued rain chances through the day.

- Marginal Risk for severe weather mainly Highway 20 and south.
  Damaging winds will be the main threat.

- Becoming very warm this next week with heat index values
  topping 100 degrees at times.

&&

.UPDATE...
Issued at 236 AM CDT Sun Jul 20 2025

Forecast remains largely on track overnight. The main boundary was
easily visible with the line of training thunderstorms forming in
southwest Iowa, with a slight lift northeastward from the LLJ.
Speaking of the LLJ, isentropic ascent prompted more thunderstorms
to fire north of that boundary, this also outlining the northern
extent of the MUCAPE axis. Heavy rain remains the primary concern in
the short term, driven by the near daily climo max for PWATs in
OAX`s 00z sounding this past evening. The Flood Watch has been
slightly expanded in the south to account for the backbuilding seen
behind the departing MCS. Have already seen 2" per hour rainfall
rates with storms training/angling into the front.

The MUCAPE axis will make a push north today as upper-level ridging
attempts to intrude on the state, its gradient setting up somewhere
in northern Iowa this evening. H85 flow is poised to continue to ram
into the stationary front in southeast Iowa, this requiring
continued monitoring for heavy rain issues through the day. If
storms continue, they`ll pose a risk for damaging winds and some
smaller hail due to ~2k J/kg of MLCAPE building up on the line. The
Marginal Risk covers this possibility as well as the chance for
storms to fire on the MUCAPE gradient mentioned earlier in the
evening. Monitoring the potential for an MCS to traverse down this
same gradient tonight which would bring an additional round of rain
and storms to northern Iowa.

&&

.DISCUSSION...
Issued at 258 PM CDT Sat Jul 19 2025

Outflow from this morning`s thunderstorms did reach northern
Missouri this morning but it has started to washout this
afternoon with a return to southeast wind. This is re-
establishing the northern boundary that is extending northeast
from western Kansas surface low to near the Omaha area and to
near Des Moines. The main warm front is still to the south over
northern Kansas and near the Kansas City metro area. An elevated
mixed layer (EML) that developed over central Iowa in the wake
of the early morning MCS is still holding this afternoon. Some
cumulus/stratocumulus has developed over west central Iowa.
Slightly more agitated cumulus is over far southern Iowa,
however, that development is above the EML and is elevated.

The challenge for the remainder of today and tonight is convective
trends. The first uncertainty is if afternoon surface based
convection can develop as the EML erodes and static stability
becomes near zero. Instability does increase during this time and
the latest SPC mesoanalysis is starting to indicated uncapped ML
instability over west central Iowa. Increasing surface convergence
and repeated waves of theta-e advection over the boundary should be
enough to initiate convective development. Conditions are favorable
for a few organized storms to form should development occur with the
potential for damaging winds, large hail and possible a tornado.

Additional thunderstorms are expected to develop overnight as a 40+
kt low level jet (LLJ) points into the region and at this time,
monitoring the region between the surface boundary and the location
of the 850 mb boundary for numerous overnight thunderstorms. The 850
mb boundary will be in the Highway 20 to Highway 30 corridor region
with the surface boundary along or south of Interstate 80. Deep
layer shear is the greatest it has been for awhile and at 35 kts or
greater. Storm motions should follow the orientation of the
instability axis and eventually following the corfidi vector lines.
The 0-3 km bulk shear around 30 to 35 kts should help keep cold
pools from outrunning storms right away and this shear will be near
line normal at times so may see more mesovortices along the line
overnight.

Heavy rainfall is the other potential hazard overnight with ample
moisture lifting into the state. PWAT values of 2 to 2.25 inches do
develop along the ribbon of instability mentioned above. Warm cloud
depths of 13-14 kft with support efficient warm rain processes
overnight. Intense rainfall rates are anticipated with storms and 1
to 2 inches may occur over a very short period of time. Total
amounts may exceed 5 inches in a few areas. Under normal soil
conditions for July, these rainfall amounts would not be an issue,
however, with recent heavy rainfall, these additional amounts may
produce flooding. See the hydrology section below for more details.

The boundary will settle further south on Sunday with the influence
of the Northern Plains high pressure. Thunderstorms will still
possible, in particular Sunday night as the LLJ increases again.
Upper Level ridging will begin to build into the region early next
week and will lead to a return to very warm and humid conditions
over Iowa with highs back into the 80s and 90s. The next more
significant chance for storms is late Thursday into Friday as upper
southwest flow develops and a boundary settles south.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1228 PM CDT Sun Jul 20 2025

A large swath of stratocumulus is over Iowa early this afternoon
with bases rising to MVFR while some scattering has resulted in
VFR at times. Most sites should become VFR this afternoon as
bases rise further. The low level steering flow comes from the
southeast tonight and that is a source region of MVFR to IFR
cigs that are currently over parts of Illinois and Ohio. Those
cigs will move into central Iowa overnight into early Monday.
Thunderstorm chances have decreased during the period therefore
no mention in this forecast.

&&

.HYDROLOGY...
Issued at 258 PM CDT Sat Jul 19 2025

Per the weather-related discussion above, the potential for flash
flooding and subsequent river flooding is ramping up. Flash Flooding
is generally a shorter term concern while river flooding is
generally a medium to longer range concern.

As for antecedent conditions related to flash flood potential,
despite recent heavy rainfall, the ground has recovered nicely
overall. With crops rapidly growing in the rural areas, their
moisture demands are at or near peak for the growing season. NWM
soil moisture saturation values are generally the highest from US 30
northward. It shows several areas of 80+% saturation with some
scattered areas of 90+% saturation. Overall the NWM soil moisture
saturation values appear to be too high which would affect NWM
forecasts in those areas, again mainly from US 30 northward. NWM
forecasts in those areas would tend to be too high. To the south of
US 30, those values appear to be more realistic, albeit a little on
the high side as well. CREST and SAC-SMA soil moisture values have
already begun recovering from the rain earlier this morning. They
are showing only isolated to scattered areas of 50-60% saturation,
which would be on the low end of when flash flooding concerns would
begin to ramp up significantly.

That being said, with the amount of potential rainfall later today
into tonight, those relatively non-concerning soil moisture
parameters could be overcome quickly especially in intensity-driven
heavy rainfall events. Based on latest meteorological parameters,
combined with antecedent soil conditions, from a conceptual
perspective the area most likely to see flash flood issues (if they
were to occur) would be roughly from US 20 to US 34 later today into
tonight. NWM 18-hr rapid onset flooding (ROF) probabilities tend to
concur, with the highest probabilities in this zone, although they
do appear to be overdone especially from US 30 northward likely for
the reasons mentioned earlier. Flash flood potential also exists
outside of that zone, however the greatest potential is within that
zone. Given high moisture content of the atmosphere and resulting
locally high QPF values, the potential exists for higher end flash
flood concerns in isolated areas, especially if the heavier rain
were to fall over urban areas. Some of those larger urban areas
include but are not limited to Des Moines metro and Ames.
Waterloo/Cedar Falls would be included as well but looks to be on
the periphery.

In the medium to longer term, river flooding is a potential issue as
well. River levels across much of the CWA are running above to much
above normal for this time of year, although overall runoff has
begun trending downward over the past few days. Morning runs of the
QPF ensemble hydrographs and HEFS output suggest the most likely
(i.e., expected or median) scenario would for for many locations to
see significant rises, with scattered locations seeing rises to
between action stage and flood stage at the most. Those forecasts
are likely on the low side, however, since the input QPF for them
seems to be on the low side. Thus, the more realistic solutions are
likely closer to the 95% max QPF traces for the QPF ensemble
hydrographs, and closer to the ~10% chance QPF traces for HEFS--at
least in the areas favored to receive the highest QPF later today
into tonight. Thus, based on those thresholds, scattered action
stage to minor flooding would be expected, with some isolated
locations possibly seeing minor to moderate flooding.

&&

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

&&

$$

UPDATE...Jimenez
DISCUSSION...Donavon
AVIATION...Donavon
HYDROLOGY...Zogg