218
FXUS63 KDMX 240142
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
842 PM CDT Thu May 23 2024

.KEY MESSAGES...

- A potentially large MCS is expected to move through Iowa tonight.

- Severe weather is possible, with the primary risks being damaging
winds and large hail, but tornadoes are possible as well, especially
in the southern end of the QLCS/squall line.

- The Storm Prediction Center has a Level 3 Enhanced Risk western
counties, and a level 2 slight risk through central Iowa, lowering
to a marginal level 1 risk for eastern counties.

- Heavy rainfall could occur tonight, and with that, flash
  flooding could be a concern in isolated areas.

&&

.DISCUSSION...
Issued at 218 PM CDT Thu May 23 2024

/Tonight through Saturday/

A beautiful day is found across Iowa today, with seasonal
temperatures in the mid to upper 70s, low dewpoints in the upper 40s
to lower 50s, and winds of 10 to 20 mph.  This pleasant air is found
to the north of a developing warm front, as there is widespread 60 -
70 degree dewpoints found throughout locations to the south in
Kansas through Missouri and the lower Ohio Valley.

In a typical MCS set up, the advection of this warm unstable air
will force strong to severe storms near and to the north of the
developing warm front tonight. The upper wave triggering storms in
along this boundary is found well to the west, but should help
initiate storms in eastern Nebraska by mid to late evening. A deeply
sheared atmosphere will be found over eastern NE through Iowa
tonight, allowing for organized storms. These will most likely
form into a QLCS over eastern Nebraska and race eastward,
generally north of Interstate 80, as there is a notable
EML/capping inversion found to the south. That southern edge of
the convection will be important overnight, as this both could
keep the event from occurring in southern Iowa, and also allow
the southern end of the line of storms to retain a favorable
profile for supercells, including tornadoes well into the night,
possibly through daybreak. Thus, the chance for storms is
somewhat lower south of Highway 30 late tonight, but the overall
chance for severe weather occurring with them is higher than to
the north where the depth of a stable boundary layer approaches
4000 ft. In a QLCS mode, +30kts of 0-1km shear will allow for
possible spin up tornadoes along the leading edge, generally in
areas that have dewpoint values over 55 degrees, keeping the
boundary layer somewhat neutrally buoyant, vs confidently
stable (highway 20).

QPF tonight will be generally under 1 inch in most of the CWA, due
to the fast moving storms, with low training potential.  There
certainly could be a warm advection wing of storms once a mature
QLCS develops, but those leading cells should lift northeast rather
quickly without significant QPF.  This progressive scenario suggests
that no flood watch be issued, despite areas that are sensitive to
heavy rainfall, especially west and east CWA counties. We`ll watch
those areas carefully tonight for any localized flooding threats.

Friday, a cold front will sweep through during the morning hours,
with any new storm development possibly only affecting eastern
counties towards early afternoon, followed by a dry night Friday
night and Saturday.   A "windows open" night is expected Friday
night, with lows in the mid to upper 40s. Saturday, highs in the 70s
are expected before another strong wave approaches the region
Saturday night and Sunday.

Following section issued at 335 AM...
.Long Term /Saturday Night through Wednesday/...

Confidence: Medium

As we approach Saturday night, another area of low pressure will
track into the Central Plains. The bulk of the energy is still
forecast by the medium range models to be just southeast of Iowa,
though the Euro does bring some slightly more unstable air into the
southeast/south by 12z Sunday. Though severe storms are not
expected, there may be a few stronger storms in far southeast and
southern Iowa. Rainfall may be the bigger issue, with another half
inch to inch across the south half of the state. Certainly not
needed at this time. Lows Saturday night will be in the  50s to
around 60 with highs Sunday in the 60s to lower 70s. As we round out
the weekend, there is lower confidence heading into Monday as
operational model runs diverge. The current EPS is more supportive
of the operation GFS with the light rain lingering Sunday night into
Monday over the region. This will likely keep the region cooler and
somewhat cloudy/wet with highs in the 60s to lower 70s again. From
Tuesday through Wednesday night, a break from the wet weather is
expected. Highs should gradually warm back to the mid to upper 70s.
By Thursday some scattered showers and storms are possible with
cooler highs in the lower to mid 70s. We may need to make some
temperature adjustments at that time.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 624 PM CDT Thu May 23 2024

VFR conditions with breezy south to southeasterly sfc winds through
late-evening. A dramatic change to occur after midnight/early
Friday morning as a line of strong-severe storms race eastward
across the state of Iowa. This linear convection may include
some bowing structures with damaging/high winds as it propagates
eastward across the TAF network of Central Iowa through 13-15z.
Most sites will see a 1-2 hour window of these strong storms
with higher gusts, mostly in the 30-40 kt range, but with
potential gusts as high as 60 kts. Along with the high wind
potential with these storms, there exists a potential for large
hail, and even a tornado or two. Also a chance 10-30% coverage
of higher based/ elevated showers and storms developing ahead of
the main line this evening/overnight, but these will be hard to
pin-point for TAF occurrence or VCNTY wording. CIGS generally
VFR, but with likely dip to MVFR or even brief IFR levels with
the storm line passage. Also, much guidance continues to
indicate higher coverage MVFR stratocu in the wake of the main
line Friday morning. VSBY also drops during the passing storms.
Clearing is expected from west to east during the afternoon
hours on Friday. Winds to remain elevated behind the main line
of storms as they become more westerly during the day on Friday.

&&

.HYDROLOGY...
Issued at 218 PM CDT Thu May 23 2024

Both short-term (flash flooding) and longer-term (river flooding)
hydro concerns will continue over the next few days due to the
combination of current hydrologic conditions and QPF.

In terms of flash flooding concerns, we may see some isolated issues
tonight into Friday morning with the thunderstorm activity. With the
storms expected to be rather progressive the rainfall is not
expected to be too heavy. Thus based on present information a flood
watch should not be needed. That being said, SAC-SMA soil saturation
values remain in the 40 to 60% range across the eastern CWA roughly
bounded by I-80, US 218, US 20 and I-35. In the western CWA they are
also elevated in the Audubon-Guthrie-Jefferson County area. When
soil saturation values exceed 50% the flash flood concern increases
markedly. Thus, those areas will be the most likely areas to watch
tonight into Friday morning. Fortunately in those areas the smaller
streams have begun receding or have receded already.

River flooding is more of a concern especially regarding how the
forecast rainfall will affect already high river levels. River
flooding continues or is forecast across portions of the Cedar,
Iowa, Skunk, Des Moines and Raccoon River basins as well as their
tributaries. Moderate flooding is occurring along portions of the
Iowa River as well as Black Hawk Creek in northeast. The latest
river forecasts updated this morning take into account 24 hrs of QPF
ending Friday morning. Additional QPF is in the forecast beyond
Friday morning. Looking beyond 24 hrs using our QPF ensemble
hydrographs and Hydrologic Ensemble Forecast System (HEFS), the most
likely scenario in most places is that the rain over the next
several days will slow the fall of area rivers and possibly result
in secondary crests. Those secondary crests would be lower than the
levels we are presently seeing except for portions of the Iowa River
and the South Skunk River across the eastern CWA where a long, broad
crest is expected. That is due mainly to the timing of crests
working their way down both rivers combined with runoff from future
rainfall. Along both rivers the broad crests may extend well into
the Memorial Day weekend before the river levels begin falling.

In the Des Moines area as well as the Eddyville/Ottumwa areas, river
levels on the Des Moines River are affected heavily by the
Saylorville and Lake Red Rock reservoirs operated by the U.S. Army
Corps of Engineers (USACE). Coordination with the USACE indicates
they plan to adjust (minimize) outflows from the reservoirs to try
to minimize flooding issues downstream. Having said that, minimizing
outflows will result in building pool elevations more quickly on the
lakes. As a result, we will see decent rises on both Saylorville and
Lake Red Rock again. The rises on Saylorville will likely be similar
to what we saw in the last event but they will be higher on Lake Red
Rock. In the last event Lake Red Rock crested around 747 ft but with
this event it is expected to go to around 754-755 ft. Those
forecasts may change going forward due to the actual rainfall
amounts.

If the rainfall comes in higher than forecast then we would see
long, broad crests at more locations in our CWA and higher secondary
crests. We will continue monitoring.

&&

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

&&

$$

DISCUSSION...REV
AVIATION...Percha
HYDROLOGY...Zogg