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581
FXUS63 KDMX 212007
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
307 PM CDT Fri Jun 21 2024

.KEY MESSAGES...

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- Showers and thunderstorms in northern Iowa tonight. Heavy
  rainfall and flash flooding likely, with the chance for a few
  strong to severe storms capable of producing small hail,
  damaging winds, and a few brief tornadoes.

- River flooding expected for multiple area rivers in response
  to heavy rainfall tonight. See hydro discussion below.

- Additional severe chances expected tomorrow afternoon, with
  wind and tornadoes being the primary threat, although some
  small hail is possible.
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&&

.DISCUSSION...
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Issued at 303 PM CDT Fri Jun 21 2024

Synoptic Overview

The frontal boundary that brought heavy rainfall and flash flooding
to the far northwest portions of Iowa last night remains in place
near the Iowa/Minnesota border and will again be the focal point for
the weather over the next 12-18 hours. This boundary is one part of
a larger synoptic pattern which has a surface low to our west slowly
tracking north and east along with some weak upper level troughing.
As this surface low approaches the state, an increase in forcing and
moisture will provide fuel for showers and thunderstorms across the
northern portions of the state, resulting in a high likelihood of
locally heavy rainfall and a chance for a few strong to severe
storms.


Heavy Rainfall Tonight

The much advertised heavy rain threat continues to trend south into
Iowa with the most recent guidance. Convection overnight last night
has brought the surface front slightly further south than expected,
which has pushed the most likely axis for heavy rainfall further
south as well. As has been mentioned in previous discussions,
convection will initially develop along the border but will start to
leak south with the thunderstorm outflow and new development favors
the better, less worked over air to the south. However, at the same
time the low level jet will begin to oscillate east and increase in
strength, which will eventually hold new development along wherever
the surface boundary sets up. This, in conjunction with slow storm
motions to the east, will result in storms training over the same
general areas through much of the night, up until the shortwave
brings an MCS along the boundary and washes out any new development
tomorrow morning. This will ultimately result in a prolonged period
of rainfall roughly along and south of the Iowa/Minnesota border,
which becomes problematic given how moist the environment is. Strong
moisture transport will push PWAT values over 2 in northern Iowa
(almost 200% of the normal value for this time of the year) making
for very efficient rainfall. As a result, expecting rainfall amounts
of 2 to 4 to be common in northern Iowa with a high likelihood for
a band of locally higher amounts pushing 7 to 8 or more. With rain
amounts of this magnitude, flash flooding will be likely, especially
in urban areas where drainage is poor, and significant flooding is
forecasted for multiple area rivers.

While the conceptual model certainly checks out for heavy rainfall
this afternoon, the exact location for the heaviest rainfall is
still not trivial. As alluded to earlier, the heaviest rainfall will
be where the front stalls, which will be strongly influenced by
thunderstorm outflow and how convection plays out this afternoon and
evening. It may also be augmented by the synoptic evolution of the
surface low and LLJ, which could lift things further north. That all
being said, convection has already begun along the Iowa/Minnesota
border, which will start the process of pushing the boundary south
and will favor the southern solution for heaviest rainfall. Models
have started to key onto this trend and have started to place the
heaviest rainfall over our northern two tiers of counties.
Therefore, to account for this continued southerly shift, have also
expanded the flood watch down to the Highway 20 corridor to allow
for some buffer in case the models continue with their northern bias
and mesoscale processes end up pushing precipitation further south.

Severe Chances Tonight

Not to be overshadowed by the heavy rainfall and flash flood
chances, there is also a chance for a few strong to severe storms
this evening. Instability on the warm side of the boundary will be
in the 2000 to 3000 J/kg range with shear values in the 20 to 30 kt
range, which would support a few organized storms and a
multicellular storm mode. Similarly, there will be vorticity
generated along the surface boundary with 0-3 km CAPE values in the
100 to 200 J/kg range and LCLs basically on the ground, which would
promote some brief spin up tornadoes as the vorticity is stretched
in the vertical. DCAPE values arent overly high near the
convection, but wind could become a factor as storms grow upscale
into an MCS overnight and begin to ingest the surface boundary,
leading to locally enhanced winds. That all being said, there are
also multiple limiting factors, including warm profiles, marginal
shear, and poor mid level lapse rates. Likewise, heavy rainfall will
work to stabilize the boundary layer, further limiting any surface
based hazards later in the night. Therefore, with weak shear and
warm cloud processes, large hail is unlikely, but the environment
will still be supportive of some isolated strong wind gusts and even
a brief tornado. The slight risk for severe weather from SPC remains
in generally the same area as this time yesterday, with just a
slight expansion further south to account for the southerly trend in
guidance.


Severe Chances Saturday

The fun doesnt quite end on Saturday morning, as the surface low
will pass overhead and bring its cold front through the area.
Fortunately, at this point the system will be more progressive,
which should help to mitigate flash flooding concerns despite the
heavy rainfall caused by moisture pooling ahead of the front.
Although rainfall will be less of a concern, the severe environment
does look a bit better. Instability values will be similar to
todays with 2000 to 3000 J/kg of SBCAPE in the afternoon, as well
as a bit more deep layer shear of 30 to 40 kts, which will favor a
few organized storms. Warm cloud processes will likely still negate
hail chances, but high DCAPE values in the afternoon will make wind
a threat with any stronger storms. There will also be marginal
amounts of low level shear and increasing low level lapse rates as
the surface warms in the afternoon, suggesting tornadoes could be
possible, although the better threat will be nearer to the low
northeast of our forecast area. The day 2 outlook from SPC has
upgraded us to a slight risk for much of eastern Iowa.
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.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1248 PM CDT Fri Jun 21 2024

Primarily VFR conditions over the area today, although scattered
cumulus have developed a roughly 3k to 4k ft deck overhead with
some scattered showers and isolated thunderstorms as well. This
may lead to brief MVFR flight conditions should the cumulus
become more widespread, as well as the possibility for a few
strikes of lightning this afternoon. Since storms are
scattered, no mention of thunder in the TAFs at this time, but
will amend should the need arise.

Later this evening, showers and thunderstorms will increase in
coverage over the north, with training storms and heavy rainfall
likely. The most likely site to be impacted is KMCW, but both
KALO and KFOD may see periods of heavy rainfall. This may lead
to a prolonged period of thunder, but, in the TAFs, have tried
to limit thunder mention to only the most likely times. That
being said, anytime there is precipitation mentioned, there will
be a chance for at least isolated thunder. In addition to the
thunder, low ceilings and reduced visibility from heavy
rainfall will be likely with any storm passing over a terminal.

&&

.HYDROLOGY...
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Issued at 307 PM CDT Thu Jun 21 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.
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&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Saturday afternoon for IAZ004>007-015>017.
Flood Watch from 7 PM CDT this evening through Saturday
afternoon for IAZ023>028-033>039.

&&

$$

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