Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

364
FXUS63 KDMX 061733
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1233 PM CDT Tue Oct 6 2026

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

.KEY MESSAGES...

- Highs warm today to near 80 degrees and winds become breezy
  this morning, especially in western Iowa.

- Localized patchy fog is possible Wednesday morning, but the
  day will again be dry and mild.

- Scattered showers and thunderstorms possible Thursday night

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 159 AM CDT Tue Oct 6 2026

Clear skies stretch across Iowa tonight with the continued
domination of ridging across the southern US. The difference today
is the trough approaching from southern Canada, and associated
surface trough stretching from North Dakota to Colorado. This trough
will advance across Iowa this afternoon and bring across a wind
shift boundary. The 850mb trough can be seen across South Dakota and
Nebraska tonight, and here in Iowa a 30kt LLJ is forecast overhead
by morning as the 850mb moves east. This is the reason for lower fog
concerns tonight, as the the added mixing should prevent a river fog
scenario like what was seen last night. With clear skies expected
this morning, PBL growth will be unrestricted and there will be a
window of a few hours this morning where gusty LLJ winds mix down to
the surface. Gusts of 20 and perhaps up to 25 knots are forecast
this morning, mainly in northwest Iowa, but lingering 15-20kt
gustiness may continue across the state through the afternoon. Dry
moisture profiles will likely keep precip chances near-zero with
this frontal passage.

Highs this afternoon have a few added factors from yesterday. On the
one hand, NBM highs have come in too low, and dewpoints too high,
for the last several days, and forecast 850mb temps are several
degrees higher than yesterday with decent mixing likely. On the
other hand, moisture advection will be present and dewpoints may
approach 60 locally, and there is also a frontal passage. As a
result, have opted to raise highs to ~78-80 and lower dewpoints.
Although with the caveat of knowing that the continued NBM bias may
be less applicable today as the previous few days given the synoptic
pattern change.

On Wednesday, the front continues to advance south and exits the
state by evening. Surface winds are expected to be calm Wednesday
morning and skies mostly clear, which could introduce the potential
for morning river fog. Highs in the mid to upper 70s are expected
once again, with the highest temps in southern Iowa. Cirrus may
limit high temps in central to northern Iowa.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 234 PM CDT Mon Oct 5 2026

Little change in the overall pattern through Friday as the upper
level flow remains a modified westerlies leading to mild and mostly
dry conditions. The caveat being Thursday night and Friday morning
as a few showers and storms could develop by another round of
moisture return with surface low pressure moving to the east and
return flow commences. Upper low pressure will descend into the
Pacific Northwest this weekend leading to a gradual amplification of
southwest flow into Iowa. This will lead to warmer temperatures this
weekend, with more widespread 80s possible. A bit beyond the 7 day
forecast and perhaps the next widespread weather potential for Iowa
arrives mid next week as the upper low shifts east into the Plains
and may bring thunderstorms back to Iowa.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1231 PM CDT Tue Oct 6 2026

VFR conditions are expected through much of the period. Breezy
southwest winds will diminish mid to late afternoon. A boundary
will move across the state late tonight into Thu morning and
will bring a switch to light northwest wind. Local fog is
possible along the leading edge of the boundary though
confidence remains low. Any fog will dissipate with the boundary
passage.

&&

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

&&

$$

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