307
FXUS63 KDMX 171911
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
211 PM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Scattered thunderstorms expected at intervals later today
  through Wednesday, then a chance of storms north on Thursday
  night. Isolated severe weather and locally heavy rainfall will
  be possible during both time frames.

- High confidence in hot and humid conditions from Friday into
  Monday. The most intense heat is likely over the weekend; Heat
  Index values may approach 105 degrees on Saturday afternoon.

- Multiple rounds of thunderstorms likely in the region next
  Monday through Wednesday, but low confidence in whether this
  will occur in our forecast area or farther north and west.

&&

.DISCUSSION...
Issued at 211 PM CDT Tue Jun 17 2025

A weak, nebulous surface ridge covers most of the region this
afternoon. A mostly washed-out stationary front is draped across
northern Iowa but becoming difficult to discern, while a weak
warm front lies across northern Missouri and it attempting to
nudge northward toward southern Iowa. Meanwhile, a mid-level
shortwave trough is approaching from the High Plains, crossing
Nebraska and neighboring states. As the atmosphere continues to
destabilize with heating this afternoon and evening, MLCAPE of
around 1000 J/kg will become commonplace across central Iowa and
we should see initiation of scattered thunderstorms. These will
mostly remain elevated, but if the aforementioned warm front
over Missouri gets into southern/southeastern Iowa there may be
some surface-based storms in that area. Regardless, while
instability is sufficient for some strong updrafts initially,
deep-layer shear is very weak with 0-6 KM Bulk Shear only in the
5-15 KT range so no sustained organization is expected. Pulsey
updrafts and disorganized storm clusters should be the mode,
with an attendant low threat of hail and gusty winds primarily
being localized. SPC has maintained a Marginal Risk of severe
weather across all of Iowa accordingly, with the previous Slight
Risk now trimmed back into just the far south/southeast in
deference to the possible warm front influence. Aside from the
traditional severe weather threats, there will also be the
potential for locally heavy rainfall given slow storm motions
and efficient rainfall rates, but any resulting impact threat is
low and would likely rely on a heavy rain core setting up over
an urban area. Also, with strong low-level lapse
rates/stretching and light flow there is a possibility of weak
funnel clouds, mainly in our western/northwestern counties where
skies have been clearer today and there may be more of a surface
vorticity enhancement due to the approaching shortwave trough.
As this trough crosses our area overnight, convection/rain
should become somewhat more widespread. However, confidence in
timing and location of showers and storms is very low given the
nebulous nature of the forcing mechanisms at play. Have
maintained an upward POP trend during the night, but it is hard
to find anything to hang your hat on regarding favored
areas/times. Regardless, the severe weather threat should be
even lower overnight although the threat of locally heavy rain
and isolated flooding impacts will increase in turn as coverage
increases.

Once the trough and associated convection clear out of our area late
Wednesday a surface ridge will briefly cross the area, bringing dry
and quiet conditions from Wednesday night into Thursday. A large 500
MB ridge will also build over the southwestern U.S. during this time
and push up into the Midwest, bringing notably warmer temperatures
by Thursday afternoon. By Thursday evening a surface warm front will
lie across central Minnesota, on the fringe of the 500 MB high, with
thunderstorms initiating and being sustained/intensified by a
developing nocturnal low-level jet. While most model solutions
depict the resulting complex of thunderstorms moving
east/southeastward along the warm front and remaining north of our
area, some solutions have more of a southward component to its
motion and just scrape our far northern counties, or develop
additional storms well south of the warm front in northern Iowa, or
both. The latter scenario seems less likely given the lack of an
apparent focus mechanism and warmer nose aloft in our area, however,
with plenty of elevated instability in place, not to mention
stronger deep-layer shear with 0-6 KM BS values of roughly 30-40 KT,
will maintain POPs across about the northern half of the forecast
area with some potential for severe weather as outlined by an SPC
Marginal Risk on Day 3.

On Friday the southwestern U.S. 500 MB ridge will slide eastward and
then this weekend will rapidly build over the eastern and
southeastern U.S., exerting its dominance over Iowa as well. This
will preclude precipitation but lead to a hot, humid, and breezy
forecast for Friday through Sunday with the hottest conditions
likely on Saturday. The NBM is pegging highs in the mid-90s across
much of our service area on Saturday afternoon, with little spread
indicating high confidence. This is reasonable given the synoptic
pattern and good agreement among the long-range input model runs
with the GFS/EC/GEM all depicting heights of 591-594 decameters
across most of our area. With dewpoints likely in the lower 70s or
so, Heat Index values should exceed 100 degrees at times and may
approach 105 degrees. Given the preponderance of outdoor activities
on summer weekends and the relative lack of substantial heat thus
far in the season, impacts may be exacerbated and it is likely some
flavor of heat headlines will be needed in the coming days.

Looking into the way-out machine, in the first half of next week the
500 MB ridge will stubbornly cling to the eastern U.S. with a belt
of southwesterly flow on its western flank somewhere over the High
Plains/Midwest. This will result in a proverbial "ring of fire" set-
up in which multiple rounds of thunderstorms develop on the western
fringe of the ridge for several days, leading to some potential for
strong storms and heavy rain. However, the location of this belt
will depend on the strength and orientation of the ridge, so it is
difficult to tell at this time range where it will manifest. The EC
is keeping most of the storms/rain northwest of our area, at least
next Monday and Tuesday, while the GFS is bleeding more of it
into our area resulting in substantially higher QPF. We will be
keeping an eye on this over the next week, because if the belt
of convection remains more or less stationary for several days
then total rainfall amounts could be heavy somewhere in the
region.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1221 PM CDT Tue Jun 17 2025

Scattered showers and thunderstorms are forecast at intervals
through the 18Z TAF period. Confidence in timing and coverage is
unusually low, but in general the most likely time frames will
be late this afternoon and evening, and overnight into Wednesday
morning. Have included PROB30 groups at all terminals in the
former period, and VCSH in the latter period, but expect
refinement via subsequent TAF issuances and amendments today and
tonight. Aside from SHRA/TSRA, VFR conditions will prevail
through this evening, but then lowering cloud bases overnight
should result in prevailing MVFR ceilings by Wednesday morning,
with pockets of IFR possible but more unlikely. Within
SHRA/TSRA, visibility will be highly variable and ceilings may
be lower at times as well.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

DISCUSSION...Lee
AVIATION...Lee