141
FXUS63 KDMX 150545
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1245 AM CDT Thu Aug 15 2024

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- Strong to some severe convection this evening into the
  overnight, biggest threat southwest forecast area.

- Locally heavy rainfall also possible overnight.

- Additional storm chances Thursday afternoon into evening.

- Decent weekend weather with relatively dry conditions.

&&

.DISCUSSION...
Issued at 227 PM CDT Wed Aug 14 2024

MCV currently moving into southern Minnesota from northwest Iowa
this afternoon with extensive area of showers across northern into
eastern Iowa.  Meanwhile, some clearing has been underway across the
southwest third or so of the forecast area in the wake of the
morning activity.  Instability has been slowly building in this area
with more robust destabilization farther southwest into Nebraska and
Kansas where insolation is a bit more robust. CAMs models continue
to focus development early this evening in eastern Nebraska near the
nose of surface instability axis as low level flow converges in that
area.  The 0-3km portion of the hodographs remain rather robust in
southeast Nebraska into southwest Iowa with a large curving sweep
and SRH values of 300-400.  This is also where heating appears to be
sufficient for convection to become surfaced based and the best
tornado threat.  However, soundings heading east into the forecast
area have less robust near surface thermal profiles with near
surface parcel acceleration becoming more subdued and anticipate the
tornado threat to drop off quickly heading into the DMX forecast
area. This doesn`t mean there isnt a threat, just that the it will
become much more unlikely.  Some hail threat will exist given the
rather large positive areas in our far west.  Otherwise, the
convection appears to consolidate and move eastward into the
forecast area mid to late evening.  Some wind threat does exist but
the somewhat unfavorable low level profiles may inhibit the overall
wind potential.  Once again, the best threat remains in the far west
where storms are likely to be near surface based with the activity
becoming more elevated with eastward travel. Finally, ample moisture
exists for heavy rainfall during this event with some models
indicating localized 3+ inch amounts.  A hydro discussion below will
address this potential.

The storms are likely to pass east overnight with some lull in the
wake of the activity into Thursday.  Some insolation during the day
leads to increasing instability once again as upper low passes north
of the state.  While the deeper convection and severe potential are
likely to exist primarily to the east of the forecast area, some
scattered showers and storms are expected during peak heating as
another shortwave heads east southeast towards the state. Some of
these may have some gusty winds along with marginally severe hail
given the decent instability which arises by mid to late afternoon.
This activity lingers into the early evening but should die off
quickly with the approach of sunset.

Thereafter, a higher amplitude eastern trof develops with Iowa on
the backside.  This suppresses hotter airmasses to the southwest of
the state with temperatures remaining at or below normal into the
weekend with generally limited precipitation chances.  Warm
advection does increase by late in the forecast period with
temperatures climbing into the middle of next week with some threat
of storms by this time as well with increasing theta-e
advection.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1235 AM CDT Thu Aug 15 2024

Showers and storms are expected to continue pushing across much
of central Iowa through the overnight hours with much of the
storm activity lifting into northern Iowa and mainly showers
with occasional lightning/thunder across southern Iowa. This
shower/storm activity is accompanied by some drops in
visibility and low ceilings, which will allow for MVFR/IFR, or
even LIFR at times, conditions to persist across much of the
state, at least into the morning hours, before gradually
improving as conditions dry out. However, there is the potential
for additional showers and storms into Thursday, especially in
the afternoon to evening southeast, though uncertainty is on the
higher end so have left out at this time. Breezy winds are
expected to remain as the system passes through, gradually
shifting southwest/west.

&&

.HYDROLOGY...
Issued at 227 PM CDT Wed Aug 14 2024

Seasonally moderate to heavy rainfall is expected through Thursday.
At this time the primary concern will be ponding of water and
potential flooding in urban areas. Outside of urban areas, there may
be some isolated issues but we do not expect any widespread issues.
SAC-SMA soil moisture values are over 50 percent across the far
southwestern CWA (mainly Taylor and portions of Ringgold counties).
Anytime those soil moisture values exceed 50 percent the risk of
flash flooding increases significantly. The elevated values there
are mainly from the rain last night into early this morning. Thus
all other things being equal, we will have to monitor those counties
more closely than normal with any future rainfall for the flash
flood threat.

Longer term, the river flood threat appears to be minimal at least
on a large scale.  The QPF ensemble hydrographs and HEFS output
suggest several sites may reach or exceed action stage if precip
comes on the heavier side vs. expected. Plus, some sites may even
see minor flooding--but again if precip comes in on the heavy side.
National Water Model (NWM) 10-day annual exceedance probability
(AEP) guidance suggests the greatest rises would be on streams to
the southeast through south and southwest of Des Moines where 50
percent chance AEP values (roughly bankfull) and spot 20 percent
chance AEP values (flood stage or higher) are indicated. The most
likely scenario across our region would be minor to moderate within-
bank rises with perhaps some more significant rises in isolated
areas or on smaller streams.

&&

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

&&

$$

DISCUSSION...Cogil
AVIATION...Bury/KCM
HYDROLOGY...Zogg