951
FXUS63 KDMX 201116
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
616 AM CDT Fri Sep 20 2024

...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Continued warm today with a mostly sunny sky and dry conditions
  through this evening.

- Rain and storm chances return after midnight tonight and are
  off and on through Saturday with it still warm. There is a low
  chance of a few severe storms Saturday afternoon or evening.

- More widespread rainfall and non-severe storms later Saturday night
  through Sunday, mainly over southern and central Iowa. Rain
  totals of 1 to 2 inches remain possible in these areas.

- Highs Sunday in the 60s - 20 degrees lower than Saturday! Cool
  to seasonal temperatures next week.

&&

.DISCUSSION...
Issued at 244 AM CDT Fri Sep 20 2024

Upper level GOES-East water vapor imagery shows one shortwave trough
spinning over the Manitoba province with the surface cold front
associated with this feature causing our thunderstorms Thursday
evening. A western US longwave trough remains with a shortwave near
its base over southern California, which is more subtle on satellite
imagery, with another wave cresting the Rockies in the Alberta
province. These shortwaves will be our main contributors to our
upcoming active pattern this weekend into early next week. Behind
the cold front this morning, winds are light and given the recent
rainfall, patchy fog has formed most notably over northeastern Iowa.
This fog will dissipate shortly after sunrise. The aforementioned
latter shortwaves will begin to move towards the central US while
Iowa resides underneath the top of the southern Plains ridge. This
will mean dry and mostly sunny conditions as subsidence in the wake
of the cold front passes overhead along with another warm day.

Southwesterly flow will increase this evening with a strong surge of
theta-e advection, which will bring rain and storm chances into the
southern to perhaps central part of the state after midnight tonight
into Saturday morning. There may be a break before the shortwave
that is over Alberta this morning dips into North Dakota and
northern Minnesota with a cold front trailing into Iowa being the
focus for renewed development. Strong to perhaps a few severe storms
may develop with favorable instability and mid-level lapse rates
with more marginal amount of deep layer shear. Hazards would likely
be gusty winds or hail. Locally heavy rainfall is also possible with
a ribbon of precipitable water values near or over 1.75 inches near
the front.

The front will move out of the state Saturday evening, but the
present California shortwave will be arriving with its forcing. This
should bring a period of showers and non-severe storms later
Saturday night into Sunday. There is growing clarity from
deterministic, probabilistic, and experimental AI data that this
rainfall will be focused over southern and central Iowa with
southern Iowa have the highest chances at around 80% dwindling to 20
to 30% over northern Iowa. With the dreary, cloudy, and rainy
weather on Sunday, it will be much cooler with highs 20 degrees
lower than Saturday. Rain chances will decrease Sunday night through
Monday as the shortwave trough passes Iowa and subsidence increases,
though this could be sooner given the dry high pressure pressing
into the area. As for rain totals, deterministic models show 1 to 2
inches over southern Iowa with possible higher amounts associated
with convective elements. For the 48 hour period ending Monday
night, the chance of one inch or more of rainfall is around 70 to
90% over southern Iowa and 50 to 80% over central Iowa from the
various ensemble means while the probability of two inches of
rainfall is 20 to 30% over southern Iowa from the GEFS and GEPS/CMCE
and 40 to 60% from the ECMWF ensemble.

Another shortwave trough will drop into the region on Tuesday,
though moisture recovery may be limited and thus rain chances have
lowered below 15% in this forecast cycle for Tuesday into Wednesday.
After the cool Sunday, temperatures will rebound with highs in the
middle 60s on Monday increasing to the low and middle 70s by mid to
late next week.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 616 AM CDT Fri Sep 20 2024

Primary aviation concern is the shallow, patchy fog that will be
dissipating within the next 2 or so hours. Early morning
satellite imagery shows this fog over central Iowa from near EBS
down towards IKV/DSM and PEA/OXV and LWD. Our TAF sites, namely
OTM, DSM, and perhaps ALO, may bounce into restrictions briefly.
Otherwise, no restrictions are expected through the period, but
clouds will increase late tonight and this is reflected in
DSM/OTM. Scattered showers and storms are possible after 6z, but
confidence in location and more so timing precludes any mention
beyond this.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Ansorge