789
FXUS63 KDMX 011124
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
624 AM CDT Tue Sep 1 2026

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Hot and humid conditions will continue through the week,
  especially over southern Iowa where heat index values will
  peak around 100 degrees each day. A Heat Advisory remains in
  effect for portions of southern Iowa through Wednesday
  afternoon.

- Thunderstorms will be possible at times for the next several
  days. Any storms today will be very isolated and rain chances
  are 20% or less. Tonight thunderstorms will be more likely in
  northern Iowa, close to the Minnesota border, where severe
  weather will be possible after dark.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 207 AM CDT Tue Sep 1 2026

A large heat dome is still smothering the southern/southeastern
U.S., while a low pressure system is moving over parts of
Saskatchewan and western Ontario early this morning. Between
these systems, Iowa lies beneath modest zonal flow, while out to
the west a 500 MB shortwave trough is moving across the Colorado
and Wyoming Rocky Mountain region. At the surface, a weak low
pressure trough is draped across western Kansas and Nebraska,
while high pressure is centered over the Dakotas. A nebulous,
washed out stationary boundary lies across Iowa roughly from
around Omaha up to Dubuque, though it is very difficult to
locate precisely given the total absence of surface flow or
convergence. Radar imagery depicts several tiny showers and a
couple of brief thunderstorms developing near the boundary, but
they are small, short-lived, and of little impact. Also of note
is that patchy fog has developed in our northeastern counties
overnight, where winds are calm and moisture is pooling along
and just north of the aforementioned boundary. This is resulting
in bouncing visibilities with a couple of sites dropping below
a mile at times. However, any dense fog observations have been
sporadic and brief, and webcams indicate the fog is generally
shallow and not restricting visibility much at eye level. Will
hold off on an advisory for now, but monitor observations this
morning in case one should become necessary.

As the low over Canada moves eastward today, in its wake the
heat dome to our southeast will reassert itself. This will bring
mid-level ridging across our area during the day and also allow
the surface low over the Plains to move northward, reaching
central Nebraska by this evening. The nebulous boundary overhead
will also move northward in response, reaching northern Iowa by
early tonight. Isolated showers and thunderstorms will likely
continue to bubble near the boundary this morning and move
northward with it, but then largely fade out during the day as
the ridging aloft sets in. This evening, however, the 500 MB
shortwave trough currently moving across the Rockies will emerge
over the High Plains and interact with the surface low,
generating more vigorous thunderstorms over parts of Nebraska
and South Dakota. These storms will then track roughly eastward
along the boundary and pushed by the advancing shortwave, moving
along or near the Iowa/Minnesota border after dark tonight. A
nocturnal 850 MB jet will intensify after sunset with its exit
region pivoting over far northern Iowa, and also bringing very
warm 850 MB temps around 23-25C up toward northern Iowa. This
should help the thunderstorms remain in the north, and perhaps
even send the bulk of them slightly east northeastward into
southern Minnesota. Any southward expansion of thunderstorms
should also be hampered by the weak mid-level lapse rates
stemming from the elevated warm layer advecting in. That being
said, wherever the northern edge of these warm 850 MB temps and
wherever the surface boundary reaches, that region will be
conducive to severe weather especially if the thunderstorm
clusters can develop an organized cold pool. MUCAPEs of
1500-2500 J/KG and 0-6 KM Bulk Shear around 50-60 KT support
strong updrafts and a threat of damaging straight-line winds,
and SPC has updated the Severe Weather Outlook to include a
Slight Risk for southern Minnesota and northern Iowa, roughly
north of Highway 20 in our service area.

The 500 MB shortwave and thunderstorm system moving by on
Tuesday night will pass east/northeast of our area by early
Wednesday morning, but will likely leave the old frontal
boundary dangling nearby through Wednesday, where it may once
again generate a few showers and thunderstorms though these will
once again be suppressed, similar to today, by another brief
period of mid-level ridging and a general lack of large scale
forcing for ascent. Aside from thunderstorm concerns over the
next couple of days, oppressive heat will also continue given
our proximity to the large southeastern heat dome as well as
mostly sunny skies and southerly winds south of the nebulous
frontal boundary. The ongoing Heat Advisory thus continues
unchanged and remains in effect through the day Wednesday. It
may eventually need to be extended further toward the end of the
week, however, with increasing time there is increasing
uncertainty regarding the potential for additional thunderstorms
inhibiting warming at times, especially around Thursday.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Issued at 242 PM CDT Mon Aug 31 2026

Into the end of the work week, the ridge over the southeastern CONUS
flattens out somewhat, but still generally keeps its influence over
Iowa, which will allow for the continuation of rather uncomfortable
conditions with the heat and humidity. Forecast highs in mid 90s and
heat indices around 100-105 degrees over much of southern Iowa are
expected, which could lead to an extension in current heat
headlines into Friday if forecast trends continue to lean that
way, so stay tuned for updates. Additional shortwaves traveling
through the western periphery of this ridge will continue to
bring continued periods of showers and thunderstorms,
particularly during the overnight and morning Thursday and
Friday. Location of the associated boundary(s) and the influence
of the cap overhead are still generally uncertain in this
range, and even more so through the weekend considering how
earlier activity in the week plays out, so will need to monitor
trends over the coming days. For now, have left at least low PoP
mentions over portions of the forecast area through the period
given this overall potential.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 624 AM CDT Tue Sep 1 2026

Patchy fog will linger through 13Z or a little after, primarily
near MCW and ALO where IFR or lower conditions may occur for a
brief period. There will be rapid improvement after sunrise,
with VFR conditions then prevailing for the rest of the day. A
few tiny thunderstorms may occur during the day, again, near the
northern terminals, but probability/duration of any impacts is
far too low for TAF mention. There is a somewhat higher chance
of storms (20-40%) in northern Iowa tonight, but only at MCW is
it high enough to justify a PROB30 group at this range. Finally,
a strengthening low-level jet late tonight into Wednesday
morning justifies inclusion of LLWS groups at all terminals
after around midnight.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...

Dense Fog Advisory until 8 AM CDT this morning for IAZ027-028-
038-039-049-050.

Heat Advisory until 7 PM CDT Wednesday for IAZ059>062-072>075-
082>086-093>097.

&&

$$

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