523
FXUS63 KDMX 211800
AFDDMX

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

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Flood Watches now starting this morning for northern counties
  as heavy rain moves in from the west. Heaviest rain still
  anticipated for tonight across northern Iowa, especially
  nearest to the state line.

- Rainfall amounts along the front will be 3 to 5 inches with
  locally higher amounts at or exceeding 7 inches possible.
  Flash flooding and river flooding impacts possible.

- In addition to heavy rain, severe weather will be possible
  this evening for the north as well as Saturday afternoon for
  much of the area. Both days: Damaging winds and a tornado or
  two possible. Hail possible, but confidence is lower.

&&

.DISCUSSION...
Issued at 402 AM CDT Fri Jun 21 2024

The nocturnal LLJ has veered over into NW Iowa overnight with an
MUCAPE axis near 1000 J/kg supporting an MCS entering into the
state. The MCS has slowed its progression, following the Corfidi
vectors and sinking to the southeast and feeding more into the
instability. The cold pool from the MCS is forming training storms
as the mean wind carries the new updrafts northeast. CAMs have not
handled tonight`s storms well and continue to initialize poorly, so
here`s the theory on evolution over the next few hours: seeing that
the MCS is halting its southward extent, current thinking is
that overall cluster will take an eastern track and turn
northeast. This outcome would bring heavy rain into northern
counties this morning before the H850 boundary is pushed into
Minnesota. Have elected to start the Flood Watch this morning
instead of tonight to capture the heavy rain potential. Can
expect 1-2 inches with morning storms with some locally higher
amounts, but as the storms have decreased in intensity over the
last couple of hours, not expecting the 5+ inches found on
personal weather stations near Sioux Falls. Through the morning
and afternoon, a cold pool from the overnight convection will
sink into the state today as the nocturnal jet weakens. Have
kept rain chances in the north for all of today as soundings
hold onto moisture in a weakly capped environment. At the very
least, northern Iowa can expect to be in cloud cover today with
highs held into the low 80s and perhaps even some 70s. Further
south, cumulus clouds will blanket the skies by the afternoon
with a non-zero chance of rain. Highs will be in the 80s.

A shortwave will approach from the west later this evening with deep
layer shear values exceeding 30kts. 0-3km shear will be further
enhanced by the return of the LLJ, elongating the lower portion of
the hodograph. MUCAPE axis with values near 2000 J/kg will build in
through the afternoon in the warm sector. Wherever the surface
boundary finds itself this afternoon, all modes of severe weather
are on the table for this evening. Confidence in hail is lower due
to the warm clouds depths and saturated profiles. Wind threat
remains as potent cold pools from rain-cooled air surge out. For
tornadoes, LCLs are predictably very low with all of the present
moisture and the LLJ-influenced hodographs yield favorable SRH
values. The best chance for tornadoes will be immediately along the
surface front, the location of which is estimated by the 5% outline.

As storms evolve more into a heavy rain threat overnight tonight,
the LLJ angles across Iowa, with its front located somewhere near
the IA-MN border. PWATs exceed 2 inches, warm cloud depths are over
4000m and the entirety of the profile appears saturated. Speaking to
how models have handled the overall pattern for most of this week,
they have consistently been too far north in QPF placement. So in
turn, not confident in a solution such as the 06z HRRR that tries to
trend northward. WPC Excessive Rainfall Outlooks have accounted for
this well with an southern expansion beyond the main QPF output
axis. Areas in the southern part of the moderate risk and in the
neighboring slight risk region (or counties just south of the flood
watch) should monitor for changes in the forecast as details become
more refined today. 3-5 inches of rain is forecasted to fall along
the H850 warm front with locally higher amounts in play. The HREF
max QPF spread speaks to the possible upper bounds of those local
extremes with contours between 7 and 10 inches of QPF on the map
near the state line. The atmosphere has proven itself capable of
this outcome already tonight with a personal weather station south
of Sioux Falls recording 9 inches of rain this morning.

The wave will remain organized and push east Saturday with its cold
front draped across the state. The jet remains enhanced through the
day, increasing low level SRH. Shear values exceed 30 kts and MUCAPE
values will near 1500 J/kg by the afternoon. The moist profiles will
once again benefit damaging wind chances and hinder hail chances. A
tornado or two will be possible from the aforementioned low level
shear. Confidence on storm initiation timing is low and will be
better capture by high resolution guidance as less short term
convection augments the near storm environment. An upper level
ridge blocks the Gulf Sunday and Monday with rain chances
returning by the middle of next week.

&&

.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...
Issued at 402 AM CDT Thu Jun 21 2024

More of the highest QPF axis is anticipated to fall in northern
Iowa counties with the highest amounts near the state line. This
in turn will lead to faster initial responses from sites as
more rain falls into local basins with additional water flowing
in from upstream basin areas in Minnesota. The river of most
concern remains the Des Moines River north of Saylorville Lake,
including both its west and east fork. High confidence in at
least Minor Flood Stage for ESVI4, HBTI4, EMTI4, and AGNI4. Higher
stages uncertain due to QPF placement discrepancies. The East
Fork will be more in play if the more southward trend in
rainfall is realized. The Cedar and Winnebago rivers are also of
concern as their reaches both extend into southern Minnesota.
River forecasts currently available at the time of issuance
include 48 hours of rainfall and morning forecasts are also
expected to include QPF beyond the usual 24 hours window due to
the extended duration of the rainfall event.

&&

.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...Jimenez
AVIATION...Dodson
HYDROLOGY...Jimenez