-- Discussion containing changed information from previous version are highlighted. --
289
FXUS63 KDMX 162330
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
630 PM CDT Mon Sep 16 2024

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Above normal temperatures and mostly dry conditions continue
  for the next few days.

- Multiple chances for showers and storms over the latter half
  of the week, and particularly late week into this weekend. The
  timing of precip chances and threat of severe weather is
  uncertain at this time.

&&

.DISCUSSION...
Issued at 215 PM CDT Mon Sep 16 2024

Synoptic pattern remains dominated by rex blocking with a ridge
over a broad upper low out east and a deep trough to the west.
With little movement to the wave train aloft, the weather over
Iowa is relatively unchanged with above normal temps and
humidity for mid-September and mostly dry conditions.
Widespread shallow cumulus has developed once again with diurnal
boundary layer heating. Some of this convection could be deep
enough to produce a brief shower or two before the boundary
layer cools again this evening. A weak shortwave moving out of
the central Rockies spurs additional showers/storms west of the
area later today into tonight. This activity should be in a
state of decay as it progresses eastward toward the western edge
of the CWA by Tuesday morning, as primary forcing mechanisms
and low level moisture transport remain focused to our west.
Kept mention of ~20% chance PoPs for some of our far western
counties, however ensemble data would suggest even lower
probabilities for any measurable precip.

A large upper low pivots northeast out of the desert southwest
toward mid-week, eventually taking on a negative tilt before
occluding over eastern Montana. Models continue to paint light QPF
over the state Wednesday, although key forcing mechanisms and
moisture convergence are still displaced well to our west. Thus
trending toward a drier forecast for Wednesday except for
possibly our far western counties. A more probable window for
precip arrives Thursday into Thursday night as the upper low
begins to crest and suppress the ridge as it moves into south
central Canada and attendant sfc boundary pushes into the state.
Above normal temperatures likely continue through at least the
middle of the week, although cloud/precip coverage could hinder
peak heating potential.

Substantial model differences hamper confidence wrt to the
timing and magnitude of precip chances toward late week and
into the weekend. While models unanimously agree that another
upper low will emerge from the western US late in the week,
there has been little run-to-run and cross-model consistency
with the evolution of the upper low as it traverses the central
conus. 12z GFS now advertises a more progressive open wave while
the EC is prefers a slower, more wrapped up circulation.
Substantial ensemble spreads also provide little clarity to the
forecast at that time range. The latest NBM is loaded up with
higher chance PoPs Friday night into Saturday, but this may be
overdone as some guidance suggests our most likely window for
rain holds off until Sunday into Monday.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
-- Changed Discussion --
Issued at 627 PM CDT Mon Sep 16 2024

VFR conditions prevail through the period. Winds generally out
of the south southwest diminish this evening, then pick up out
of the south again through mid day tomorrow. A few gusts near or
exceeding 20 kts may be possible, primarily further north and
west. Some guidance continues to hint some fog development
tonight, but with very little agreement, have kept out of TAFs
for this issuance.
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...Martin
AVIATION...Dodson