Tracy, Iowa Weather Forecast Discussion

832
FXUS63 KDMX 121123
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
623 AM CDT Fri Sep 12 2025
...Updated for the 12z Aviation Discussion...
.KEY MESSAGES...
- Hot conditions into middle of next week with highs 10 to 15
  degrees above normal. Conditions hottest on Saturday.
- 20 to 30% chance of a few showers or storms generally north of Highway
  20 this afternoon and evening. Lightning possible, but no
  severe weather forecast.
- Additional storm chances Sunday night and then at times mid to
  late next week as conditions turn more seasonable by next
  Friday.
&&
.UPDATE...
Issued at 247 AM CDT Fri Sep 12 2025
The upper air pattern over the CONUS features a trough over the
Eastern States, ridge over the central US, and a longwave trough
over the western CONUS with both troughs having a positive tilt
at the moment. In the lower levels, a 850mb moisture axis is
advancing northeastward across Iowa with the 700mb 12C
temperature contour lagging toward the western side of this
moisture axis near the Iowa/Nebraska border. This increase in
moisture along with a weak, lead shortwave trough topping the
ridge today will bring cloud cover, mostly high and mid-level,
over the state. Forecast soundings show this moisture in the
profile and incoming guidance this evening/tonight is also
showing low QPF of a few hundredths over northern Iowa. While
this will be fighting the dry sub-cloud layer air with bases
8000 feet or higher, have introduced low end PoPs less than 30%
over northern Iowa this afternoon and evening. While lightning
may occur, no severe weather is forecast. Otherwise, the heat is
on today into about mid next week as the thermal ridge pushes
over the region with unusually, but not unprecedented, warm
conditions for this time of September. As has been done in
previous forecasts, initial National Blend of Models guidance is
too high and have blended this down toward the 50th percentile.
This results in high temperatures about 10 to 15 degrees above
normal in the upper 80s to low 90s in most places on most days.
The hottest conditions will be on Saturday with highs around 90
degrees over northern Iowa to low and middle 90s elsewhere. Peak
evapotranspiration season is behind us and with dewpoints in
the low and middle 60s, heat indices will be just a few degrees
higher than the air temperature with heat index values below 100
degrees. HeatRisk is picking up on and highlighting the late
season heat with minor to moderate impacts over Iowa today
through mid next week with Saturday have patchy major impacts.
Beyond the heat this weekend, conditions will be dry on Saturday
through the daylight hours of Sunday. As the West Coast trough
swings and lifts northeastward over the Dakotas and Nebraska
becoming slightly negatively tilted later Sunday, this will bring a
surge of moisture into the state with weak low level thermal lift
and modest mid-level QG convergence. The stronger forcing will
be northwest of the state, but the weak forcing is enough for at
least low end PoPs Sunday night for now. The shear is weak so
not expecting anything severe at this time.
Looking into next week, the heat will relent toward midweek as a new
trough slowly advances from the Pacific Northwest. This brings
scattered storm chances into the area mid to late week with
conditions turning seasonable by late in the 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 /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 623 AM CDT Fri Sep 12 2025
High level clouds are overhead now and expect mid-level clouds
with bases around or above FL090 to fill in through the day with
VFR conditions prevailing. Winds from the south-southeast will
turn breezy with sustained winds of 10 to 15 knots with gusts
of 20 to 25 knots. There remains a less than 30% chance of a
storm passing near or over the northern terminals mainly after
18z. With low confidence in an impact at any given terminal,
have made no mention. Toward the end of the period, low-level
wind shear of around 30 knots may develop over western Iowa.
Given that this is marginal and near the end of this period,
will defer any inclusion to later issuances.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...Ansorge
DISCUSSION...05
AVIATION...Ansorge

NWS DMX Office Area Forecast Discussion