288
FXUS63 KDMX 221149
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
649 AM CDT Sat Jun 22 2024

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Heavy rain to continue for northern Iowa this morning. Storm
  total rainfall generally between 3 and 5 inches expected with
  locally higher amounts possible.

- Severe weather possible for primarily southern and eastern
  portions of the state this afternoon. Damaging winds will be
  the primary threat with a tornado or two possible. If storms
  can initiate early, heavy rain will again be a factor for
  northern counties.

- Storm chances return late Monday as well as Tuesday.

&&

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

Synopsis:

Early morning shortwave was located over the Siouxland area with an
associated warm front across northern Iowa which connected back
southwest to a surface low in Southwest Nebraska. The H850 jet with
speeds over 40kts continued to advect in dew points around +15
degrees C from the southwest. Its magnitude and direction is similar
to the mean wind and parallel to parts of the aforementioned
boundary in western Iowa, leading to training storms on the more SW
to NE oriented portions of the front. An MCS tracked slowly
southeast in northern Iowa with advection fueled updrafts sustaining
it.

Saturday Morning:

With the warm front stalled over northern Iowa, expect heavy rain to
continue in at least the pre-dawn hours of Saturday. Previous
rainfall rates of 2-3 inches per hour have relaxed into the 1-2 inch
per hour range. Severe weather chances have waned as storms have
decreased in intensity overnight, but gusty winds are possible along
the leading edge of the MCS. Storms further down along the line show
signs of weakening with warming clouds tops on IR in Nebraska. This
area of rain will move northeast into northern Iowa over the coming
hours, focusing with the forcing from the shortwave and developing
surface cyclone over southern Minnesota. Have kept rain mentions in
the north through the morning as the cold pool from the MCS will
behave similar to yesterday in that it will sink further south into
Iowa and fester convection except now with more forcing in play.
Would expect rain to be lighter compared to overnight, but adding to
an already impressive rainfall total. H850 jet sustains itself
through the morning and will lead to gusty winds in the clear skies
of the warm sector in the far south.

Saturday Afternoon:

MLCAPE values in the warm sector exceed 2000 J/kg especially where
skies are allowed to clear this afternoon. Deep layer shear values
have trended towards 40 kts, especially nearest the triple point at
the state line. Low level SRH values remain high due to the jet and
hodographs are straight at higher levels, making rotating updrafts
favorable and also sustainable. CAMs illustrate updraft helicity
swaths nearest the triple point in the north early this afternoon,
but some factors of uncertainty are in play with this outcome. The
main one namely being what the near storm environment will be this
afternoon. If the cold pool from overnight`s storms does not retreat
towards the state line, the aforementioned outcome is less favorable
and would at least offset the initiation time of the convection,
delaying it until the northeast part of the boundary almost exits
the CWA. Further southwest along the front, convection will be
possible, but will be farther away from the better forcing. As for
threats: hail appears to be the least favorable threat as columns
remain very saturated. This however will favorable strong cold pools
for damaging winds. The tornado threat looks more favorable closer
to the triple point. Storms will be more progressive in nature due
to the cold front, but there remains lingering concerns for heavy
rain in the already saturated northern counties. If storms initiate
earlier on in the afternoon, another 1-2 inches of rain could fall,
leading to more flooding issues. Confidence is lower in this outcome
at this time so the Flood Watch has been preserved at its original
1pm expiration, but trends will be monitored through the morning.

Long Term:

The upper level ridge retreats into the Desert Southwest region
after today and will keep Iowa dry Sunday into much of Monday. A
passing shortwave to the northeast might at least yield clouds
Sunday afternoon. Southerly flow builds Monday with warmer
temperatures returning. Somewhere nearby in the state will be a
surface boundary with enough instability and forcing to cause
concern for more severe weather, hence the Marginal Risk.
Details on this day will be better defined as mesoscale features
are better captured. Storms will again be a possibility on
Tuesday. Highs for much of the area are likely overdone both of
these days due to the factors mentioned.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 643 AM CDT Sat Jun 22 2024

Scattered showers and a few storms possible in the next 6 hours
for northeast terminals, especially KALO as a boundary lifts
towards the terminal from the south. Some MVFR cigs are possible
with these showers. Thunderstorms to form near the same area
after 18z with gusty winds in excess of 30kts possible from
storms. Less confident in storms at KDSM and KOTM at this
time. MVFR to IFR stratus to build in tonight in the northeast
near the end of the TAF period.

&&

.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 1 PM CDT this afternoon for IAZ004>007-
015>017-023>028-033>039.

&&

$$

DISCUSSION...Jimenez
AVIATION...Jimenez
HYDROLOGY...Zogg