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485
FXUS63 KDMX 181949
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
249 PM CDT Wed Sep 18 2024

.KEY MESSAGES...

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- An increase in precip chances later tonight into Thursday
  morning, but warm and humid with the potential of strong-
  severe storms late Thursday afternoon and evening.

- Marginal (1/5) to Slight (2/5) Risk of severe weather late
  Thursday afternoon and evening

- Mainly dry Friday, but chances for showers and storms return
  this weekend, becoming likely (60+%) in many locations Sunday
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&&

.DISCUSSION...
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Issued at 248 PM CDT Wed Sep 18 2024

Our pattern will become increasingly more active Thursday, and
again over the weekend into early next week as influences from
the current western CONUS mean trough continue to progress
eastward. The weather is relatively inactive right now however,
with the first lobe of mid-high level kinematic forcing
associated with the current MT/ND upper low producing mid clouds
and light precip, which is mainly aloft due to high cloud
bases. The next lobe of deeper forcing ahead of the SD PV
anomaly will increasing forcing through the Mid MO Valley later
tonight however and attempt to activate the fairly deep
moisture fetch from the central Plains through the Siouxland
area. Effective shear will remain fairly weak this far east into
Thursday morning however, so while scattered nocturnally
influenced convection is expected overnight into early Thursday,
it should remain on the weaker side.

More appreciable convection is anticipated late Thursday
afternoon and evening however. The aforementioned forcing will
continue to slowly advance into Iowa with peak heating
development anticipated along a weak frontal boundary. Mixed
layer CAPEs are expected to reach 2000-3000 J/kg with negligible
CINH, and with a modest but strengthening jet segment ejecting
into the Plains ahead of what will be our weekend system,
effective shear will increase to 30-40kts resulting in an
increased potential for strong-severe storms. With that
effective shear oriented near normal to the boundary, that
should support the potential for discrete/supercell development,
at least initially, but drier low levels and higher LCLs would
seem to diminish tornado potential somewhat with wind and/or
hail the more likely severe weather threats. Convection should
exit overnight leading into a break from precip Friday with
subsidence as the initial upper low lifts into Canada, and a
bit more southern CONUS ridging influence.

Precip chances will increase once again overnight into Saturday
morning however with a nocturnally influenced theta-e advection
surge into IA associated with fairly deep forcing also
increasing ahead of a trough along the US/Canadian border.
Chances will linger later in the day and night, although there
may be a break later in the day ahead of the Four Corners system
which will sharply increase rain chances Saturday night into
Sunday as the upper low passes through the region. While the
location of appreciable rainfall is uncertain north to south
depending on model solution, both EC and GEFS 24hr median QPF
suggest 1"+ amounts somewhere in the state, with locally higher
amounts possible in convective elements. Neither GFS
deterministic shear and instability parameter space, or
experimental CSU ML/AI algorithms suggest much in the way of
severe weather this weekend. There isn`t a whole lot to latch
onto in terms of forcing with short wave strength and timing
ill-defined in various guidance sources, but the central CONUS
mean trough will linger into the middle of next week keeping at
at least low precip chances in the forecast, and temperatures
fairly close to seasonal values.
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&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 114 PM CDT Wed Sep 18 2024

VFR conditions were in place across central IA at 18z with areas
of mid level cloudiness and some light precip mainly aloft
west. Confidence is high that VFR conditions should persist
into at least the evening. Weak convection may increase a bit
overnight into Thursday morning, with some low potential for
brief MVFR or less, but confidence in that occurrence is
insufficient to mention at this lead time. More appreciable
convection may occur beyond the valid period however, after
19/18z.

&&

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

&&

$$

DISCUSSION...Small
AVIATION...Small