185
FXUS63 KDMX 272349
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
649 PM CDT Sat Apr 27 2024

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

.KEY MESSAGES...

* Threat for strong to severe storms today, primarily south of
  I-80 this afternoon and evening. Primary threats for
  large/damaging hail and damaging straight line winds. A
  tornado or two cannot be ruled out.

* Lingering hydrologic threat with potential for multiple rounds of
  storms over same areas of southern Iowa possibly yielding 3"+ of
  rainfall. Flood Watch has been issued.

* Another round of strong to severe storms possible Sunday.
  Primarily over southern/southeast Iowa, with hail and winds
  leading threats.

* Multiple opportunities for showers/storms next week as a
  handful of shortwaves progress through the large scale flow.

&&

.DISCUSSION...
Issued at 145 PM CDT Sat Apr 27 2024

A welcome quiet start to the day after the events of yesterday
afternoon and evening. Multiple survey teams from the office are out
in the field assessing damage and tornado paths.

For today, another round of strong to severe storms remains expected
over roughly the southern half of the area, namely in/around a
surface warm front draped across the state. Compared to yesterday,
trends have pushed the surface front, and by virtue the "juiciest"
air, southward a bit. Aside from that, much of the advertised
environment remains with moderate lapse rates and supportive
deep layer shear for organization. SBCAPE values remain
forecast into the 2000- 3000+ J/kg range with dew points into
the mid 60s near/south of the front. Low level wind fields
certainly are relaxed comparatively to yesterday, though
hodographs still have suggestions of a tornado or two possible
with good low level curvature and streamwiseness to go along
with supportive low level CAPE, even as streamwise vorticity
values are on the lower end/more marginal. That said, large hail
and damaging straight line winds are primary risks this
afternoon and evening as activity. Morning model and CAM runs
have come into much better agreement as to the anticipated
progression of convection this evening, favoring upscale growth
out of NE Kansas and depicting a bowing complex of storms riding
along the frontal boundary and instability gradients. This
would thusly suggest that damaging winds may ultimately be the
primary threat, with hail most likely early in the event with
any initial discrete/semi- discrete cells. As additional lift
overspreads the area, there remains the potential for additional
storms, but that scenario appears more likely into areas of
northern Missouri in/along outflow boundaries/southward shunted
boundary. Regardless, wherever that scenario sets up will have
an additional hydrologic/flash flooding threat. With that in
mind, have hoisted a Flood Watch across the southern two tiers
of the state.

Into Sunday, some uncertainty remains with activity this
afternoon/evening playing a role, but there does remain another
potential window for strong to severe storms across southern Iowa as
an upper wave and associated surface low track northeastward across
the area. Cannot rule out any mode of severe weather, with tornado
potential near the low as it tracks across NW Iowa, as well as hail
and winds with any strong or organized convection within warmer/more
unstable airmasses.

Initially quiet start to the week next week will be disrupted by
multiple shortwave troughs moving through the larger flow with
periodic opportunities for showers and storms beginning as early as
Tuesday afternoon and evening.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 644 PM CDT Sat Apr 27 2024

Showers and storms early in the period will diminish in areal
coverage for a period this evening then a few more rounds will
move through the are into Sunday afternoon. Some uncertainties
on specific timing thus best estimates of more widespread
periods are represented in this forecast. Cigs are expected to
lower and become widespread MVFR/IFR with some improvement late
Sunday. A mix in wind directions eventually becoming southeast
Sunday.

&&

.HYDROLOGY...
Issued at 145 PM CDT Sat Apr 27 2024

Last night`s rainfall, while welcome, was locally heavy in some
areas.  The highest amounts per MRMS QPE extended from our southeast
to northeast CWA where widespread amounts of at least one inch
occurred.  Some areas especially southwest and northeast of Des
Moines received 2-3 inches.  River and stream response was as
expected with us being a drought -- mostly minor responses with some
more elevated responses in urban areas like Des Moines.  Smaller
streams showed more responses.  Some larger streams such as the Iowa
River are still rising.

Last night`s rainfall also helped to increase our soil moisture
especially in the heavier rainfall areas.  Some locations within the
heavier rainfall band especially southwest of Des Moines are showing
upper-level soil saturation values approaching or slightly exceeding
50% per FLASH CREST data.  The FLASH SAC-SMA data is slightly more
muted which is expected as that model tends to take longer to
saturate the soil.  Regardless, when upper-level soil saturation
approaches or exceeds 50% then the flash flood concern significantly
increases.  Thus we will have to monitor for runoff and flash flood
issues overnight especially across the south and into the eastern
CWA where the heavier rainfall occurred and upper-level soil
moisture is higher.  Thankfully the areas with 50% saturation in
CREST are relatively small.  Any issues we may see tonight would be
primarily confined to smaller streams as well as urban areas.

Looking further ahead, the combination of last night`s rainfall
along with forecast rainfall tonight as well as later this coming
week may lead to more substantial rises on area rivers.
Probabilistic river forecast models are suggesting rises to or above
action stage at multiple locations at the 50% exceedance probability
level mainly in the Des Moines and Raccoon River basins.  At the 30%
level, several locations in the Des Moines, Raccoon, Cedar, Iowa and
Skunk basins are showing rises to action stage or higher.  While
rises to or above flood stage are possible, the exceedance
probabilities are relatively low at this time but will have to
continue to be monitored.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Sunday morning for IAZ070-081>086-092>097.

&&

$$

DISCUSSION...Curtis
AVIATION...Donavon
HYDROLOGY...Zogg