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816
FXUS63 KDMX 171737
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1237 PM CDT Tue Sep 17 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Staying warm, higher than typical humidity for mid-September
  through week`s end before temperatures lower this weekend

- Rain and storm chances over western Iowa today and Wednesday,
  then gradually spreading eastward later this week into this
  weekend. A few severe storms are possible on Thursday.
  Forecast uncertainty grows with regards to timing of rain and
  storm chances this weekend.

&&

.DISCUSSION...
Issued at 256 AM CDT Tue Sep 17 2024

Upper level water vapor imagery from GOES-East shows a large area of
low pressure within the deep western US trough with several areas of
convection on the downstream side of this trough along the North
Dakota/Minnesota/Canadian border early this morning. Farther to the
east, a more compact low pressure is over the Carolinas, which
is undercutting the departing northeastern US high pressure.
Iowa is between these two regions and with the Carolinas low
meandering over that region through much of this week, it will
block and slow down the western US trough progression.
Conditions for much of this week will remain warm with highs in
the 80s and higher than typical mid-September humidity.

A piece of shortwave trough energy has broken off and advanced
ahead of the western US trough. It is arriving this morning
with clouds noted on the Nighttime Microphysics RGB imagery with
showers and thunderstorms beginning to develop over Siouxland.
Forecast soundings show quite a bit of dry air at the moment,
but do show some moistening as this wave moves over the state,
particularly in the mid-levels and a bit less so in the low
levels. There may be enough remaining moisture depth to squeeze
out a few showers or storms over our western Iowa forecast area
towards and around the Highway 71 corridor. Deterministic
global and convective allowing models (CAMs) along with
probabilistic data all support this idea with the highest
probabilities of measurable rainfall up towards 60% over
northwest Iowa in the latest GEFS and CMCE means, which rapidly
drops off into our northwestern service area.

The large area of low pressure presently at the base of the western
US trough will lift out northeastwardly into Montana tonight
into Wednesday with another area of low pressure replacing this
one in the base of the trough. The former will bring weak low
level QG convergence into western Iowa with the better forcing
staying farther west over South Dakota and Nebraska. This may
result in a few showers or storms over western Iowa on
Wednesday. Probabilities of measurable rainfall are highest
between 50 to 80% - depending on ensemble choice - along and
west of the Missouri River, which tapers off by Highway 169. A
strong storm could form over western Iowa in a plume of
instability, though the deep layer shear and mid-level lapse
rates are rather marginal.

As we move into the latter portion of this week, the Carolina low
will finally be moving eastward towards the Eastern Seaboard, and
this will allow for rainfall and storm chances to become more
widespread over our area during this timeframe into this weekend.
In a bit more detail, the low over Montana will lift into
southern Canada. The stronger QG convergence mentioned in the
previous paragraph spreads over the state on Thursday into
Thursday night. This is also a period where the instability,
shear, and lapse rates are painting a more favorable environment
for possible severe storms. SPC`s day 3 outlook as well as the
Colorado State University`s machine learning random forest
outlook paint probabilities across much of central Iowa with
these peaking over northern Iowa into Minnesota.

There should be a relative minimum in rain chances around Friday as
we await the western US trough`s slow arrival. However, difference
abound as they often do at this time range. While cluster analysis
does not provide the answer, it does provide potential outcomes, at
least on Saturday. The first scenario is that if the ridge over
the region is a bit stronger than the grand ensemble, then
there will be more of a delayed onset of rainfall on Saturday.
However, the second scenario is if the western US trough is
deeper, then this would result in rainfall sooner on Saturday.
Cluster analysis is less clear on Sunday with each cluster having
much of the day littered with precipitation chances. With most
clusters having around a 30% outcome chance and no clear
favorite, this speaks to the uncertainty of the forecast at this
time range.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
-- Changed Discussion --
Issued at 1237 PM CDT Tue Sep 17 2024

VFR conditions are forecast through the TAF period with no
aviation impacts anticipated. A few light showers may be
possible around FOD this afternoon, but probability is too low
to carry in the TAFs.
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Lee