Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

994
FXUS63 KDMX 232329
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
629 PM CDT Mon Jun 23 2025

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Heat Advisory remains in effect from around Des Moines
  southward and eastward until late this afternoon/evening. Warm
  temperatures in the 80s-90s for the rest of this week.

- Multiple rounds of thunderstorms are forecast this afternoon
  through at least Thursday. Scattered severe storms remain
  possible this afternoon into tonight across much of central
  Iowa with strong winds and heavy rainfall the primary threats.
  Additional strong to severe storm threats exist on subsequent
  days but activity is somewhat dependent on the previous day.

- In addition to the severe weather potential, heavy rain will
  be possible at times this week and flooding concerns increase
  with subsequent events, especially in urban areas or where
  multiple rounds of heavy rain fall successively.

&&

.DISCUSSION...
Issued at 248 PM CDT Mon Jun 23 2025

Another active week has begun with shower/storm development having
already occurred over northern into north central Iowa early this
afternoon near to just south of the surface boundary that is a main
driver of our storms today, and will continue to be a driver over
the next several days as it stalls out and wobbles north to south
and back with time through much of the upcoming week. Although
temperatures to the north of this boundary in northwest Iowa are
only in the 70s, temperatures have warmed again into the 80s or 90s
with near widespread dewpoints in the 70s. Although in the short
term this means another very warm day is ongoing in much of southern
to eastern Iowa with heat indices near to over 100, it also means
the environment is becoming increasingly unstable with anywhere from
1000-2500 J/kg of MLCAPE, and up to another 1000 J/kg more than that
at the surface per 18Z SPC Mesoanalysis. Although shear remains
lackluster south of the boundary in the warm sector with profiles
largely unidirectional, still expecting storms to continue to
develop as H700 temperatures decrease and in turn inhibition
continues to weaken over the next several hours and into this
evening. The strongest storms/biggest severe threat will be with the
initial storms that develop this afternoon into evening. Can`t
fully rule out some hail or a brief tornado with these initial
storms or any storm mergers near/along the boundary prior to
having to compete with other updrafts, but these are generally
less likely threats with any cold pools in the warm sector
likely struggling to stay in balance, as noted in the previous
update. Thus, the main threat remains strong wind gusts as
storms pulse up and then collapse down, with gusty winds also
possible along storm outflows, or if storms can actually
organize into some kind of line. If this later threat occurs,
the wind threat could continue later into the evening, as
starting to be noted by some of the CAMs in far southern Iowa,
and will continue to be monitored closely.

With the unidirectional flow previously mentioned, the boundary
remains largely parallel to the mean flow which will also allow
for storms to train over the same area increasing the hydro
threat with time. Although not overly robust, the LLJ redevelops
overnight allowing the rain to continue through much of the
night leading to rain totals of 1-3" over portions of central
into southern Iowa with localized higher totals certainly
possible. Rain totals to the north remain on the lower side, but
could near up to 2" by 7 AM Tuesday per the HREF localized
probability matched mean. With the large bulk of this QPF
through these first 24 hours of our active week in southern Iowa
that has more hydro capacity, did not pursue a flood watch for
flash flooding at this time, but for any of these larger QPF
values that fall in an urban area, flash flooding is certainly
possible. Ponding in any poor drainage or low lying areas is
expected and conditions will be monitored closely. See the hydro
discussion for additional information on the hydro threat.

Depending on any MCV activity and associated outflow that may/may
not develop tonight, the surface boundary stays stalled out in the
area, as noted above, allowing for additional storm development
tomorrow afternoon and evening once the morning activity moves out.
At this time, activity looks largely isolated but with additional
diurnal instability developing, more strong to severe storms are not
out of the question near the boundary or any other lingering
features. Exactly where this occurs is somewhat dependent on how
things play out tonight into tomorrow morning, but with shear still
not overly robust, main threat would once again be gusty winds with
activity not overly organized but should that occur, then maybe some
hail.

By overnight into Wednesday morning, additional rain or storms move
in along a shortwave with additional afternoon/evening storm
development possible as well. Each event will continue heighten the
hydro threat from a flooding stand point (again, see hydro
discussion) with the severe threat in time and place dependent each
day on the day before and the location of the lingering boundary
with this active weather continuing through the work week though not
necessarily at every time step as noted by the extended NBM
forecast. Will continue to refine PoPs with time through the week.
Although warm in the 80s to 90s, the daily precipitation/storm
chances will keep heat indicies lower this week.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 626 PM CDT Mon Jun 23 2025

A line of storms have developed across central IA this evening
and will continue moving SE. The primary threat with this
convection is damaging winds. The line will taper off this
evening but the state will remain inundated with rain through
the overnight hours as a stationary boundary stalls. Some
additional rumbles of thunder tonight can`t be fully ruled out.
Ceilings will drop to MVFR/IFR with the overnight rain and are
expected to linger into Tuesday. High res guidance shows a 30%
chance of MVFR ceilings staying through the afternoon hours.

&&

.HYDROLOGY...
Potentially wet conditions this week will lead to above average
hydrologic concerns involving both flash flooding and river
flooding. There has been a persistent signal of heavy rainfall
however there is uncertainty in terms of amounts and location. See
the above sections for the meteorological aspects. The hydrologic
response will depend heavily on the meteorological activity. In
terms of short term hydrologic response, flash flooding will be a
possibility through at least the middle to late part of this week.
Although soil moisture values are near to above normal, the heavy
rainfall this week has the potential to still generate flash
flooding due to either its intensity or the sheer volume of
rainfall.

In terms of longer term hydrologic response, present thinking is the
most likely scenario will be significant rises on many streams--with
potential flooding--especially across the northern half of the CWA.
Right now the river basins of most concern include the
Cedar/Winnebago/Shell Rock, Iowa, Skunk, Des Moines and Raccoon
rivers. We are also concerned that QPF going into our river model
guidance may be underdone, thus the true flooding threat may be
higher than indicated by our current river forecast guidance such as
HEFS and the NWM. The primary QPF inputs into HEFS and NWM are GEFS
and GFS respectively. In higher-end rainfall events the QPF tends to
be underdone which is our concern this week. Thus our partners and
users are strongly encouraged to follow subsequent watches, warnings
and advisories from our office and not necessarily take the online
river forecast guidance at face value.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for IAZ050-062-075-
085-086-095>097.

&&

$$

DISCUSSION...05
AVIATION...Britt
HYDROLOGY...Zogg