Smith Cemetery, Iowa Weather Forecast Discussion

098
FXUS63 KDMX 171731
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
1231 PM CDT Wed Sep 17 2025
...Updated for the 18z Aviation Discussion...
.KEY MESSAGES...
- Not as warm with scattered showers and storms. A few stronger storms
  may produce gusty winds this afternoon or early this evening.
  Locally heavy rainfall is possible, but widespread flooding
  is not expected.
- Continued shower and storm chances Thursday and Friday
  lingering into a part of this weekend. It will not rain
  during this entire period, but rain is likely each day in at
  least some portion of the state.
- Temperatures more seasonal late this week into this weekend in
  the 70s to low 80s.
&&
.UPDATE...
Issued at 409 AM CDT Wed Sep 17 2025
Showers and thunderstorms continue at this hour over portions of
west central and southwest Iowa fueled by a weak, 20 knot low level
jet overriding an old outflow boundary. The rain rates have
lessened, but there was a period of time where radar estimated rates
over 2 inches in an hour fell in Guthrie County. Convective allowing
models (CAMs) earlier in the night were abysmal in their handling of
the situation, though may have finally turned the corner and caught
onto trends. As the low level jet further weakens and veers off,
this should result in coverage becoming less with time. At the same
time, synoptic scale forcing will start to influence the weather
pattern over Iowa. This will be due to a trough over the northern
Rockies/northern Plains, which has a pair of closed lows with one
over Wyoming, which will move eastward, and another over
Saskatchewan, which will drop in behind the first closed low. This
trough will bring modest pulses of low level theta-e advection and
QG convergence that spreads over the region through the end of the
week. In addition, kinematics will slowly increase as well. What
this means to sensible weather will be an increase and a thickening
of the cloud cover with limited sunshine and chances for showers and
thunderstorms each day through the end of the week perhaps lingering
into a part of this weekend
For today, there may be spotty showers and storms over western Iowa
after sunrise this morning, but a wave of theta-e advection will
bring an area of showers and storms across the state this afternoon
and evening. Forecast soundings show that the effective shear will
be 15 knots or less and would point toward no severe weather
risk and pulse storm type. However, if the last two days are a
teacher, there could once again be a low severe weather risk
today with clues in the forecast soundings and CAMs, which paint
the potential for gusty winds that may reach severe criteria.
The 0z NAM and 3z RAP soundings show a warmer, drier boundary
layer, but a more moisture laden sounding in the mid and upper
levels. The 0z HRRR is the opposite with a cooler, more
saturated boundary layer, but very dry mid-levels. While they
are different, the NAMNest, FV3, ARW cores, and HRRR all show
wind gusts anywhere between 30 to near 50 knots. While I
wouldn`t rule out small hail either, the CAPE profile looks more
restricted than in the previous days. For Thursday and Friday,
there is no apparent severe weather risk in the parameter space,
but will note that Thursday does have sufficient instability
and favorable low level lapse rates with deep layer shear still
weak (aka similar to what we have seen earlier this week).
As for hydrology and flooding potential, locally heavy rainfall will
be possible with seasonally high warm cloud depths and precipitable
water values coupled with 850-300mb flow that does not look to
exceed 20 knots through Friday. While there were localized areas
that saw substantial rainfall yesterday and tonight (e.g. parts of
Webster, Calhoun, and Guthrie Counties), dry antecedent conditions
still prevail in many parts of central Iowa. Thus, isolated water
issues are possible, mainly in urbanized areas, but widespread
flooding is not expected.
Conditions will turn cooler late this week into this weekend with
highs in the middle and upper 70s to low 80s. There is a bit more
uncertainty into this weekend with regards to rain and storm chances
as the global models are beginning to show a different evolution of
the trough and closed lows. For example, the 0z GFS and ECMWF take
the first closed low over the state Friday into Friday night and
then lift the trailing closed low into Canada. This would still
result in rain/storm chances this weekend, but perhaps not as long
or as widespread. Conversely, the 12z ECMWF and last night`s 0z
guidance was more similar in that they brought both closed lows
through the region with chances for showers and storms higher
and more persistent into this weekend.
&&
.DISCUSSION...
Issued at 327 PM CDT Tue Sep 16 2025
Radar returns this afternoon shows widely scattered pulsing showers
and storms this afternoon. The environment is characterized by
temperatures in the mid 80s to low 90s and dewpoints in the 60s,
which has resulted in higher instability values around 2000-3000+
J/kg, though given the high pressure overhead has led to fairly weak
flow and forcing overhead, resulting in storms struggling to
maintain themselves. This activity is expected to continue through
the afternoon to evening tied to peak heating, but should gradually
wane into tonight. Soundings continue to indicate low level dry air
and DCAPE values around 1000J/kg, which would indicate the potential
for gusty winds with collapsing storms. Reports with current
convection however have generally remained below 35 mph so far, but
cannot rule out a few sub-severe to brief severe criteria winds.
Given lapse rates around 7-7.5 C/km, a hail threat also remains but
would be short-lived given the pulsing nature of this activity. In
terms of any funnel potential, low level instability around 50-
100J/kg covers much of Iowa this afternoon, though surface vorticity
is very minimal if any, which per guidance looks to change little
through the rest of the afternoon and into the evening. Cannot rule
out a weak funnel or two, but the overall environment is trending
less favorable.
Looking at the larger scale synoptic setup today depicts a number of
waves in or near the CONUS, the first as a midlevel low pressure
system circulating over the southeastern CONUS, the second as
another mid-level deepening low pressure tracking across the
Intermountain West, and a shortwave near in Southern Canada
approaching the Upper Midwest. Into Wednesday morning, the system
out of the Intermountain West is expected to track into the Dakotas,
with increasing moisture and forcing into Iowa that will result in
more scattered shower and storm activity into Iowa at times through
Wednesday. Temperatures will be on the warm side in the mid to upper
80s, with instability values around 1000-1500 J/kg and shear just a
touch higher around 15-20 knots, which would suggest another
diurnally driven pulse storm environment once again. The severe
threat at this time looks low, but again cannot rule out the
potential for a stronger storm that could produce some gusty winds
and smaller hail. By Thursday, the mid-level trough and Canadian
shortwave phase together and become a larger system that looks to
generally remain over the western part of the Plains before breaking
apart into two waves again. This will result in several additional
periods of on and off shower and storm activity through the
remainder of the work week. Given that the larger scale system
overall remains over the same area per general model trends, the
moisture axis in association also moves very little, with better
moisture into western Iowa, where the higher rainfall amounts look
to occur before gradually moving eastward across the state by
Friday. Rainfall amounts through Friday of around 0.5-1 inch
generally are expected, though locally higher amounts are
certainly possible in areas that see repeated rainfall and/or a
stronger storm.
As the larger mid-level trough breaks down over the eastern CONUS
into the weekend, the system over the Plains finally moves through
the region and lifts into the Upper Midwest. Additional on and off
scattered showers and storms will remain through the weekend with
this, though details become even less defined in coverage and timing
given this is several days out and will depend on how the system
evolves earlier in the week.
&&
.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1231 PM CDT Wed Sep 17 2025
Scattered thunderstorms across Iowa today are the primary
forecast challenge. These will persist into the evening and
perhaps overnight. The widespread scattered nature of the storms
has made timing a challenge so the most likely near term window
has been highlighted in TAFs. Additional thunderstorms are
likely, but confidence in placement and timing is too low to
include at this time and will be added as need in future
issuances.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...Ansorge
DISCUSSION...Bury
AVIATION...Hagenhoff

NWS DMX Office Area Forecast Discussion