Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

534
FXUS63 KDMX 290632
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
132 AM CDT Mon Jun 29 2026

.KEY MESSAGES...

- A prolonged period of hot and humid conditions will continue
  through the coming week and potentially the Fourth of July
  weekend. An Extreme Heat Warning is in effect for the entire
  area.

- Thunderstorms will be possible tonight and again Tuesday
  night, mainly over the northwestern half or so of Iowa. Some
  storms could be strong to severe, with damaging winds and hail
  the primary threats. However, confidence in the occurrence and
  placement of any storms is relatively low at this time.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 131 AM CDT Mon Jun 29 2026

A much anticipated and well-advertised 500 MB blocking pattern
has become established across the Continental U.S., with a large
trough in the west and a "heat dome" of high pressure in the
south and southeast. Iowa remains beneath southwesterly steering
flow between these two systems, and while we are more under the
influence of the high and thus seeing much warmer and more humid
conditions, we are far enough on its periphery to be potentially
subjected to convective effects from shortwaves ejecting out of
the western trough. For today this will not be an issue and with
a relatively tight surface pressure gradient promoting gusty
southerly breezes, a hot and muggy day is in store across our
service area. With temperatures peaking in the low to mid-90s
and sweltering mid-70s dewpoints, Heat Index values will peak
around 104-108 degrees area-wide. Not only will this be quite
unpleasant for anyone venturing outdoors, it will also be
hazardous for those not taking appropriate precautions,
especially in vulnerable populations. With the heat expected to
persist into Tuesday and at least to some extent beyond, an
Extreme Heat Warning remains in effect for the entire area.

By this evening a positively tilted shortwave will be rounding
the southeastern quadrant of the western U.S. 500 MB trough
over the Dakotas. The southeastern periphery of this shortwave
will move from northeastern Nebraska across southeastern South
Dakota, far northwestern Iowa, and into Minnesota overnight and
will likely force convective initiation with clusters of
thunderstorms affecting parts of the region. However, there is
general agreement that the nocturnal 850 MB southwesterly jet
will be located across eastern Kansas and roughly central Iowa
overnight, well displaced from the forcing associated with the
500 MB impulse. Central Iowa, closer to the influence of the
southeastern U.S. heat dome, will maintain a good EML overnight
as evidenced by forecast soundings, which should inhibit any
convection. However, there may be a metaphorical Venn diagram
overlap in northwestern Iowa and adjacent states where the
forcing on the periphery of the 500 MB shortwave, strong
instability, and the left exit region of the LLJ create a window
of opportunity for thunderstorms which could be severe. With
strong winds just off the deck (40-50 KT at 3000 FT at
Estherville) and ample CAPE (2000-4000 J/KG depending on which
model one looks at), any storms that are able to develop may be
capable of damaging winds and hail. That being said, there are
so many uncertainties in the location of the LLJ,
location/magnitude of the mid-level lift, and strength of the
cap, that confidence in the convective forecast for tonight is
quite low. CAM solutions illustrate this as they vary widely
both between models and from run-to-run between no thunderstorms
at all, thunderstorms well northwest of our forecast area, and
only occasionally depict thunderstorms in our northwestern
counties. That being said, there is enough environmental support
for some strong storm potential that SPC is justified in
maintaining a Slight Risk severe outlook for our far northwest.

On Tuesday, in wake of the Monday night 500 MB impulse, weak
ridging will slide across the High Plains to our northwest and
decrease the pressure gradient in our area. This will result in
somewhat lighter south southwest breezes during the day. While
this will decrease warm air advection, the airmass in place will
already be sufficiently warm and humid that combined with good
insolation, temperatures/dewpoints/heat index values will be
similar to those of Monday. The proverbial fly in the ointment
here will be whether there are any lingering effects from
overnight convection, i.e., a remnant cold pool or clouds that
may inhibit warming (most likely in our northwestern counties).
With heat index values still expected to peak around 105 degrees
in our south and east, will maintain the Extreme Heat Warning
through Tuesday evening for now and if convective effects do
reach the northwest tonight and are expected to lower the heat
impacts on Tuesday, then some portion of that area could
potentially see the Heat Warning cancelled tonight. However, if
any storms tonight remain well northwest of our area then it is
likely the warning will continue right on through Tuesday and
possibly beyond (see Long Term discussion below).

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 312 PM CDT Sun Jun 28 2026

There remains the signal for another period of showers and
storms as another wave within the larger scale trough lifts
near Iowa Tuesday evening into Wednesday, along with an
increasing low level jet overhead. Showers and storms are
suggested to develop across portions of northern and western
Iowa per Euro, while the GFS and NAM are on the drier side as
any activity is further north into Minnesota closer to the warm
front. These details are more on the uncertain side at this
time, so will need to monitor this over the next few days. A
Marginal Risk covers this area per SPC, with hail and gusty
winds the main hazards where storms do develop and grow upscale.
Otherwise, the main story will continue to be the long stretch
of extreme heat as the upper ridge moves very little through
the mid to late week period, including the 4th of July holiday.
General highs are expected through the 90s each day, with heat
indices above 100 degrees for much of the area, while overnight
lows generally ``bottom out`` in the mid to upper 70s,
providing little relief from the heat. Additional heat headlines
are certainly possible in the coming days, but will need to
monitor potential limiting factor such as overnight storm
activity prior to each day, which models indicate are certainly
possible over parts of Iowa. Specific details regarding coverage
and intensity of these periods of active weather will be better
known in the coming days.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1048 PM CDT Sun Jun 28 2026

Removed gusts in the short term due to lack of observations. Stratus
has formed over the northeast part of the state, affecting KMCW
and KALO the most. Currently expecting these to remain VFR.
LLWS common overnight. VFR conditions with gusty winds during
the day with gusts over 30kts common after 18z, easing winds
after 00z. Have reflected chances for stratus with a FEW deck
in the north in this same timeframe.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...

Extreme Heat Warning until 9 PM CDT Tuesday for IAZ004-005-015-
023>025-033>037-044>050-057>062-070>075-081>086-092>097.

Extreme Heat Warning from noon today to 9 PM CDT Tuesday for
IAZ006-007-016-017-026>028-038-039.

&&

$$

SHORT TERM...Lee
LONG TERM...Bury
AVIATION...Jimenez