496
FXUS63 KDMX 222046
AFDDMX

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

 ...Updated with Hydrology Discussion...

.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 346 PM CDT Sat Jun 22 2024

Heavy rainfall in northeastern Iowa last night has led to flooding
on multiple area rivers, primarily along the Des Moines River and
Cedar River basins. As of right now, hydrographs are either on track
or slightly less than what was expected yesterday.

Those that have fallen below the forecasted rises are mainly along
the Cedar/Winnebago Rivers which missed out on some of the heavier
rainfall that was previously expected through that basin. This is
most notable at MCWI4, which will see a significant downward trend
from the previous forecasted crest of major flood stage to a now
forecasted moderate flood stage. That being said, precipitation
fallen in the basins and water routing down from Minnesota along the
Cedar River, will still result in moderate to major flooding in
areas along the Cedar River, notably at CCYI4, JANI4, and CEDI4,
among other sites also seeing minor flooding on the Cedar River and
it`s tributaries.

Further west, river forecasts are mostly on track, with major
flooding already ongoing at ESVI4 on the West Fork Des Moines River.
Rises at ESVI4 have slowed some, but not expecting much improvement
at this time with more water upstream still to be routed down.
Forecasted crests of moderate and major flooding downstream at EMTI4
and HBTI4, respectively. HBTI4 is also forecasted to near and exceed
the flood of record of 15.4ft. Flooding is also ongoing along the
East Fork Des Moines River, with moderate flooding at AGNI4 and
minor flooding forecasted at DAKI4. Consequently, flooding is likely
along the main stem of the Des Moines River north of Des Moines as
water routes downstream. This will lead to significant rises at
Saylorville Lake, with an estimated lake elevation of just over 870
ft.

Additional showers and thunderstorms are expected over portions of
northeast Iowa this evening, which may slightly delay improvement on
rivers. Those along the Cedar River basins will be most affected by
any rainfall tonight, while those further west should mostly get a
break. That being said, more rainfall is forecast at the beginning
of next week so will continue to monitor the impacts these may have
on area rivers.

Make sure to stay up to date on the latest forecasts and river
information, and use extreme caution in flooded areas.

&&

.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...Dodson