Jolley, Iowa Weather Forecast Discussion

620
FXUS63 KDMX 181747
AFDDMX
Area Forecast Discussion
National Weather Service Des Moines IA
1247 PM CDT Thu Sep 18 2025
...Updated for the 18z Aviation Discussion...
.KEY MESSAGES...
- An arc or area of showers and storms lifts into the state
  after noon today with a few stronger storms capable of
  producing gusty winds or small hail through early this
  evening. Locally heavy rainfall of 1 to 2 inches is possible,
  but widespread flooding is not expected.
- There will be dry intervals, but additional scattered shower and
  storm chances persist Friday into this weekend.
- Near seasonal to perhaps slightly above normal temperatures
  through through this weekend with highs in the upper 70s or
  low 80s.
&&
.UPDATE...
Issued at 231 AM CDT Thu Sep 18 2025
Our upstream closed lows are readily apparent as cyclonic spins
near the shared border of the Saskatchewan province and Montana
and over Nebraska in the upper level water vapor channel of
GOES-East imagery this morning. While there is a good fetch of
moisture over Iowa with precipitable water (PW) values 125 to
150% of normal, the echoes on radar are few this morning. This
will change as we move into the daytime hours, particularly this
afternoon and evening, as the closed lows within the parent
trough over the northern Plains send shortwave energy over the
state. This will be paired with low level QG convergence and
modest surges of theta-e advection through tonight. While there
may be a few spotty showers or rumbles of thunder this morning,
the higher chances arrive after noon into this evening as an arc
or an area of showers and storms rotates and lifts
northeastward through the state. Forecast soundings by early
afternoon show a skinny, but deep CAPE profile with values over
1000 J/kg with around or a little over 100 J/kg of this in the
0-3km layer. No surprise, but our effective inflow layer (EIL)
shear is around or less than 10 knots so another day of pulse
storm mode is expected. Low level lapse rates of 7 C/km are
common across models with mid-level lapse rates weaker. The RAP
and more so the HRRR show a more well mixed boundary layer and
varying degree of an inverted V sounding while the NAM is more
saturated. It figured then that the evening runs of the HRRR
have been focusing isolated, near severe wind gusts over
southern into central Iowa this afternoon with this just
supported by the WRF-ARW with no gusts noteworthy in the
NAMNest or FV3. Given how the last few days have played out,
isolated severe wind gusts along with mainly sub-severe (less
than 1" diameter) hail is possible as storms go up and down this
afternoon and early evening. Locally heavy rainfall with spotty
1 to 2 inches will be possible per high resolution
deterministic and ensemble models within an environment
characterized by seasonally deep warm cloud depths and the
aforementioned above normal PW values. The flow does look
slightly faster at 20 to 25 knots compared to last night`s model
data so this may lower the residence times of storms over a
particular location. Any flooding issues would likely be
restricted to a populated urban area.
Chances for showers and storms will diminish overnight as they move
away from the area. The light surface low and recent rainfall could
result in areas of fog once again. For Friday, the two closed lows
will have phase into one as the parent trough axis moves over the
state through the day into the night. This will result in
another day of primarily afternoon scattered showers and storms.
The EIL shear does not look appreciably any different from the
rest of this past week so another round of pulse mode storms is
likely. With more cloud cover forecast, this may limit low
level lapse rates and boundary layer mixing such that the
potential for gusty winds and hail may end up lower. As we move
into this weekend, the parent trough will lift away, but with
another shortwave trough dropping into the region by Sunday,
additional scattered showers and storms will be possible at
times along with periods of no precipitation.
&&
.DISCUSSION...
Issued at 208 PM CDT Wed Sep 17 2025
Widespread scattered thunderstorms have persisted across the area
with the surface low to the west slowly easing east. An axis of
instability in the form of 1000-1500 J/kg of MLCAPE has developed
from central to southern Iowa where clearing earlier in the day
allowed for quick heating. Along this axis the thunderstorms are
more robust with periodic stronger cores. Even so, shear is quite
weak at 15-20 kts or less, limiting more robust and persistent storm
development. As such the severe threat remains low today, however a
few stronger cores may produce hail or gusty winds as those cores
collapse. While a few stronger cores have had pixels of 60-65dbz,
this has generally been shallow and short-lived today. These may
have produced brief hail, however no reports have been received so
far. A few of these stronger cores have produced gusty winds with
isolated gusts of 30-40 mph observed so far.
Aloft, the upper level cutoff low will remain across the area for
the next several days (through Friday and into the weekend)
with continued chances for scattered showers and thunderstorms.
As we saw yesterday and have seen so far today, activity is more
widespread than CAMs would have us believe and expect that this
trend will continue until the low finally lifts out of the area
early next week and the pattern changes. The environment
remains the same through the end of the week and weekend, so
pulsey storms producing hail or gusty winds will remain possible
while widespread storms are less likely. With little flow,
storm motions are fairly slow. These slow moving storms are
efficient rain producers in an environment with deep warm cloud
layers of 3500+ m and PWATs over 1.5". After several fairly dry
weeks, flood concerns are low at this time. While convection
remains scattered, pockets of heavy rain with some of the
stronger cores may produce areas of ponding. Any potential for
flash flooding would come from stronger cells setting up in town
where drainage is slower.
Another upper level cutoff low drops across the area early next
week, pushing the current one out of the area. Differences
remain with regards to timing and placement of this low,
lending towards uncertainty in precipitation chances into early
next week.
&&
.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1247 PM CDT Thu Sep 18 2025
Scattered showers with a few areas of thunderstorms are moving
across southern into central Iowa this afternoon. This will
continue to expand north this afternoon with an additional
round of showers and storms lifting across the area later this
afternoon through the evening. It will not be a washout
throughout the entire period, however there will be shower and
storm chances at times through the period due to the scattered
nature of the anticipated convection. As such, have attempted to
assign SHRA/TSRA mentions in the more probable windows for
precipitation, however additions and adjustments are likely. By
Friday morning much of the precipitation for this round should
be tapering off and patchy fog will likely develop once again as
it did this morning. This will be accompanied by low stratus
with MVFR to possible IFR impacts at most sites.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...Ansorge
DISCUSSION...Hagenhoff
AVIATION...Hagenhoff

NWS DMX Office Area Forecast Discussion