Diagonal, Iowa Weather Forecast Discussion

686
FXUS63 KDMX 051705
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
1205 PM CDT Fri Sep 5 2025
...Updated for the 18z Aviation Discussion...
.KEY MESSAGES...
- Diminishing winds through today with an isolated shower
  possible in northern Iowa.
- Cool and dry into the weekend.
&&
.UPDATE...
Issued at 300 AM CDT Fri Sep 5 2025
By 3am the cold front is nearly through Iowa, draped across northern
Missouri and into southern Illinois. With dry air in place
precipitation fizzled out last evening and has been unable to
redevelop along the boundary overnight. As temperatures warm this
afternoon some redevelopment is expected, though this will be south
of the area. Meanwhile, an embedded lobe of vorticity will
rotate around the parent low and skim across northern Iowa this
afternoon. Model soundings continue to indicate decent dry air
to overcome, however a ribbon of better moisture near the
700-800mb layer, paired with daytime heating, may be enough to
squeeze out a few sprinkles or isolated showers in northern
Iowa. Low levels remain dry and a cumulus field or virga remains
the more likely scenario for most locations.
Aside from low end precipitation chances across the north
today, the rest of the area will remain dry and see diminishing
winds through the day as the pressure gradient relaxes. Cool and
dry conditions remain in place through the weekend, providing
an early taste of fall as temperatures are nearly 10 degrees
below average for this time of year (average highs are around
80 for much of the area this time of year). Lows will be in the
40s with parts of northern Iowa likely to see a few locations
dip to the upper 30s at times overnight this weekend.
Temperatures warm back to the upper 70s and low 80s by early
next week with a return of shower and thunderstorm chances.
&&
.DISCUSSION...
Issued at 201 PM CDT Thu Sep 4 2025
Surface high pressure is quickly departing to the southeast today as
a fast-moving wave drops southward through the upper midwest and
toward Iowa this afternoon and evening. The deepening surface low
will increase pressure gradients overhead as it approaches,
resulting in breezy southwest winds with gusts around 20 to 25 mph
this afternoon. Wildfire smoke has also filled in ahead of the
approaching wave, with some visibility reductions around 8 to 10 SM
at the surface. This is expected to continue filling in through the
evening, but will be quickly pushed aside by the cold air mass
advecting southward with the surface low.
Pressure gradients will continue to strengthen as the wave
approaches the state this evening, with the strongest winds expected
in proximity to the surface low and along the cold front. Wind gusts
of 25 to 35 mph expected in northern Iowa in proximity to the low.
In addition to the gusty winds, strong lift and moisture pooling
along the front will produce scattered showers and thunderstorms
over northern Iowa as the system passes through. These will be most
likely near the surface low over northern and northeastern Iowa,
while area farther south and west will struggle with moisture
availability and weaker forcing. That said, although the
likelihood of showers/storms is lower, could still see a few
isolated showers or storms over central Iowa this evening as
well.
The potential for severe weather with these storms appears fairly
low for much of the area this evening, as instability will be quite
low (less than 500 J/kg) and shallow. Most of this instability will
be pooled right along the boundary, and will be dwindling after
sunset. That said, while the storms themselves may not be overly
rigorous, the wind fields within the environment surrounding them
will be quite strong. This of course suggests higher amounts of
shear for storm organization (0 to 6 km shear values will be over 70
kts), but moreso the potential for the strong winds aloft to be
mixed down to the surface through downward momentum transfer with
storms. Models indicate a roughly 45 to 55 kt jet aloft nosed into
the cold front, which could be transported to the surface by what
would otherwise be fairly shallow and relatively benign storms. For
this reason, an isolated wind threat will be in place with storms
this evening, especially around or slightly after sunrise. The storm
prediction center has issued a targeted marginal risk for severe
wind (level 1 of 5) over northwestern Iowa, where winds are most
likely to be mixed down to the surface. After sunset, the likelihood
for these strong winds with storms will diminish some, but the
possibility for an isolated gust to be mixed down will exist with
any storms tonight.
Once the front passes through, shower and storm chances will subside
but gradient winds will remain breezy through the night as cold air
advection fills in behind. The wave will depart to the east through
the night and the cold front begins to sputter over southern Iowa
toward Friday morning. Short range guidance is kicking out some
showers along the boundary as it slows and leaks south, introducing
rain chances to southern Iowa by Friday morning. However, lack of
forcing and very dry low levels should keep most of this
precipitation at bay, with only sprinkles or light rain
expected. The front then falls south of the state by mid-morning
on Friday.
Breezy conditions remain through Friday, but will diminish into the
afternoon and evening as pressure gradients weaken and high pressure
begins to fill in. Model soundings show a shallow layer of
instability developing as the boundary layer mixes out tomorrow
afternoon, suggesting afternoon cumulus is likely and even a few
sprinkles are possible, despite the extremely dry low levels. These
should dissipate after sunset.
Cool and dry conditions prevail through the weekend, with overnight
lows falling into the upper 30s to low 40s and daytime highs only in
the upper 60s to near 70. It`s possible elevated surfaces could see
some minor frost buildup in the mornings this weekend, but ground
temperatures and overnight lows should generally remain warm enough
to limit frost concerns. Rain chances return in the first half of
next week, so make sure to get out and enjoy the dry and pleasant
conditions this weekend!
&&
.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1205 AM CDT Fri Sep 5 2025
Few aviation weather impacts are forecast during the TAF
period. SCT- BKN clouds will affect the area this afternoon,
with ceilings most prevalent at the northern terminals. A brief
MVFR ceiling will be possible in the next hour or two, mainly at
MCW, but heights will continue to rise and VFR conditions are
otherwise expected. Gusty northwest winds will diminish with
sunset, becoming lighter overnight into Saturday.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
UPDATE...Hagenhoff
DISCUSSION...Dodson
AVIATION...Lee

NWS DMX Office Area Forecast Discussion