714
FXUS63 KDMX 221922
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
222 PM CDT Sat Jun 22 2024

.KEY MESSAGES...

- Showers and thunderstorms are expected this evening with the
  potential for strong winds, heavy rainfall, and a few
  tornadoes. Isolated severe hail may also be possible.

- Mostly dry on Sunday with highs in the 80s

- Hotter with additional shower and thunderstorm chances Monday
  and Tuesday.

&&

.DISCUSSION...
Issued at 219 PM CDT Sat Jun 22 2024

>> Severe Weather Threat Today

The system that brought heavy rainfall across northern and
northwestern Iowa last night has begun to make its way east over
the region this afternoon and will depart the area this evening.
However, as it passes through, showers and thunderstorms will
develop initially across northern Iowa ahead of the surface low
early this afternoon, then also fill in along a trailing cool front
oriented roughly from southwest to northeast later this afternoon.
This will bring a threat for severe weather through the afternoon
and into the early evening hours, with all hazards possible
mainly across the eastern and northeastern portions of the
area. SPC mesoanalysis data indicates favorable instability
values of 2000 to 3000 J/kg, and shear values of 40 to
potentially 50 kts of effective shear. This will support
organized convection and potentially a few supercells, given the
20 to 30 kts of low level shear. Hodographs also show a few
hours of the traditional sickle shaped profile with near 100
m2/s2 of SRH, although the streamwise ingest does still seem to
be lacking some. Combine this low level shear with LCLs around
500 to 1000 m, steep low level lapse rates, and 0-3km CAPE
values of 150+, J/kg could certainly see a tornado this evening.
Strong flow aloft and marginal DCAPE values of 500 to 1000 J/kg
will also make damaging wind gusts a consideration with any
organized storms. This is especially true if discrete convection
congeals and grows upscale, although this will be a greater
concern further east of our forecast area. Regardless, the
chance for strong winds and a tornado are the primary concerns,
but also cant rule out some severe hail with any supercells
that do develop. The large hail threat does look somewhat
limited by warm and moist profiles, but an organized, rotating
updraft would still be able to support large hailstones, which
are less susceptible to melting than smaller hailstones found in
less organized storms.

With moisture pooling in the warm sector ahead of the approaching
system, PWATs will again be in the 2+ range, which will promote
efficient rainfall rates with any storm that develops. There also
looks to be a brief period this afternoon where storms may even
train along the boundary in northeastern Iowa, which could cause a
few areas to pick up  2-3+ inches of rainfall, especially in areas
further east along the boundary. Fortunately,  the overall synoptic
system will be quite progressive and storm motions will pick up to
around 30 to 40 kts, so extremely high rainfall amounts are not
anticipated. Regardless, given the higher rainfall rates in
proximity to areas that already received heavy rainfall last night,
have decided to extend the flood watch through 7 PM this evening.
Another extension for some of the eastern counties may also be
needed later this evening, but these areas received less rainfall
last night so will want to evaluate how convection plays out before
continuing the flood watch into the night.


>> Hot Temperatures With More Storm Chances Next Week

After the low pressure departs today, central Iowa gets a brief
break from showers and storms through tomorrow, aside from an
isolated shower or two in the afternoon. Unfortunately, this break
is relatively short lived with more shower and thunderstorm chances
expected both Monday and Tuesday as weak waves pass through the
northwest to zonal flow overhead. These will be accompanied by high
instability values as temperatures increase into the 90s both days.
Shear values at this point dont look great, at least on Monday,
which currently has a marginal risk for severe weather in the
current SPC day 3 outlook. Tuesday has slightly higher forecasted
shear values and also has some greater severe weather probabilities
from the CSU machine learning algorithm, but at this point not
going to dig too much into the minutia. In addition to the
precipitation chances, highs climbing into the low 90s over
southern Iowa and increasing dew points may lead to some
uncomfortable apparent temperatures both Monday and Tuesday, so
will be closely monitoring the heat risk in future forecast
packages.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1222 PM CDT Sat Jun 22 2024

Scattered to broken cumulus at roughly 2k to 3k ft have
developed over the western half of the state this afternoon.
These precede a system bringing shower and thunderstorm chances
through the afternoon and evening, potentially impacting all TAF
sites. However, storms will be scattered and therefore
confidence in impacts vary for each site. Therefore, have
included mention of thunder during the most likely periods for
each location and will refine as needed.

Overnight, an MVFR to potentially IFR cloud deck looks to
develop over much of the area, again impacting all sites. These
ceilings then lift in the morning hours, and are followed by a
quieter and drier day on Sunday.

&&

.HYDROLOGY...
Issued at 317 AM CDT Sat Jun 22 2024

We are looking at a high-end river flood event especially across our
far northwestern CWA. The river basins of most concern include the
West Fork Des Moines River, East Fork Des Moines River, mainstem Des
Moines River above Saylorville Lake and the Cedar/Winnebago Rivers.
This higher end river flooding is due to the heavy QPF axis shifting
south and now affecting many of our river basins.

The West Fork Des Moines River basin is of most concern. At some
locations the expected river flooding will approach the flood of
record and will exceed the levels seen in 1993 (which is the
benchmark major flood for many locations in Iowa). The river is
rising rapidly and is above flood stage already at some locations.

As for the East Fork Des Moines River, significant flooding is
expected there too. The river flooding will be in the top three
crests of all time at some locations.

River forecasts presently include QPF through Sunday morning due to
the widespread, heavy nature of this rainfall event. The river
forecasts are based on our best forecast rainfall amounts and
locations. That being said, location is everything in terms of
rainfall and river response. If the QPF axis shifts even farther
south than expected then the river responses may be even more
significant and include more rivers/locations. If the QPF ends up
being higher then the same thing may happen.

Saylorville Lake on the mainstem Des Moines River above Des Moines
is expected to rise significantly in response to this runoff.
Presently the lake elevation is just under 840 ft. The latest U.S.
Army Corps of Engineers forecast has it rising to between 865 and
870 ft in early July, or a rise of between 25 and 30 ft. As a point
of comparison the lake reached an elevation of 860.1 ft in late May
in response to the heavy rainfall from April into May. Although the
forecast rise with this event will be significant it will still be
well below the spillway elevation of 884 ft. Of course like the
river forecasts the Saylorville Lake forecast depends heavily on the
forecast rainfall. If the rainfall ends up being heavier or further
south then expected then the response may be more significant than
presently expected.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 7 PM CDT this evening for IAZ004>007-015>017-
023>028-035>039.

&&

$$

DISCUSSION...Dodson
AVIATION...Dodson
HYDROLOGY...Zogg