Jefferson, Iowa Weather Forecast Discussion

866
FXUS63 KDMX 111956
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
256 PM CDT Thu Sep 11 2025
.KEY MESSAGES...
- Warming trend through early next week with highs in the upper
  80s to mid 90s this weekend. Hottest day Saturday.
- Precipitation chances return late Sunday into Monday and
  intermittently next week.
&&
.DISCUSSION...
Issued at 249 PM CDT Thu Sep 11 2025
Ample sunshine is in place early this afternoon with only a few high
clouds around, though area webcams continue to show some hazy skies
in places with upper level wildfire smoke still around today. This
smoke will finally start to move out west to east later tonight
through the daytime Friday, but until then we`ll still have another
night and morning of vibrant sunsets and sunrises. Fog is not as
much of a concern heading into Friday morning with slightly higher
winds and less low level moisture to work with. However, the heat
will continue to build. We`ve already seen temperatures reach the
upper 70s to upper 80s early this afternoon with even warmer
temperatures incoming for the weekend as the thermal ridge
continues to push eastward through the weekend. The hottest day
looks to be Saturday when mid-level temps (H850) peak around
20-25C, and temperatures at the surface reach toward to into the
low to mid 90s. The heat with temperatures in the 80s to low
90s will be more prolonged though, from tomorrow through about
midweek next week. With dew points `only` in the 60s through the
weekend, heat indices will remain within a few degrees of high
temperatures, keeping it not nearly as hot and humid as other
heat events earlier this summer, but it won`t be a completely
dry heat either. The pressure gradient will increase into
tomorrow and remain through the weekend keeping daily afternoon
breezes out of the south that may also help take the edge off of
the heat. As has been done and discussed over the last several
days, did lower high temperatures from NBM Friday through
Tuesday with NBM still at the high end of the temperature
distribution thanks to the warm bias of the GFS/GEFS and related
overmixing. As one example, the GFS surface temperature is
nearly 10 degrees warmer than other guidance on Saturday
afternoon in portions of southern and western Iowa. Besides the
extreme overmixing (a known bias in heat events in the GFS
suite), we`ll also have cloud cover to contend with at times
further spoiling the extent of the heat. It will certainly be
warm, just not likely to be as warm as the GFS suite is
advertising.
As the upper ridge continues to move eastward today through the
weekend, nearby shortwave energy will round the ridge at times with
increasing mid to upper level moisture. This is what will help drive
the aforementioned cloud cover. With increasing warm air advection
and moisture, CAMs continue to try and kickoff showers at times on
Friday but forecast soundings are not supportive of precipitation at
the surface with low level dry air and high cloud bases. This is
consistent with previous forecasts with trend towards clouds vs.
precipitation given the set up. Should some additional moisture
lower in the atmosphere be realized, maybe a rogue sprinkle could
occur, but this is unlikely with cloud bases of 10-12kft or more and
dry air below. Thus, the heat is paired with a dry forecast
through much of the weekend.
Our next better chance (~20-40%) for precipitation arrives in the
region Sunday night into Monday morning as the upper trough from the
western U.S. arrives and the upper ridge is finally pushed off to
the east. Will have to continue to evaluate moisture profiles
with low-level dry air still in place potentially limiting the
spatial and/or temporal extent of these shower chances.
Additional precipitation chances exist at times next week as
shortwaves move through the our otherwise southwest flow prior
to a frontal passage late week bringing more precipitation
chances but also some relief from the heat.
&&
.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1204 PM CDT Thu Sep 11 2025
VFR conditions are forecast through the period. Southeast wind
less than 12 kts is expected. Some scattered cumulus AOA 5kft is
possible this afternoon. Additional redevelopment of mid-level
scattered clouds may occur overnight into Friday morning.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...05
AVIATION...Donavon

NWS DMX Office Area Forecast Discussion