918
FXUS63 KDMX 261743
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1243 PM CDT Wed Jun 26 2024

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

.KEY MESSAGES...

- Drying out today with cooler and lower humidity air moving in

- Storms return late Thursday into Friday with locally heavy
  rainfall. Severe storm risk may develop later Friday.

- Hydrologic discussion below main discussion details more on ongoing
  river flooding and future locally heavy rainfall potential

- Drying out this weekend with more seasonal temperature and
  humidity

- Intermittent storm chances return late Sunday night into early
  next week

&&

.DISCUSSION...
Issued at 320 AM CDT Wed Jun 26 2024

An active evening with severe storms across a good portion of
central and southern Iowa, but now activity has largely shifted
into Missouri with the storms in Nebraska diving along the
downdraft instability gradient into northeastern Kansas and
northwest Missouri. Lingering showers and elevated
thunderstorms do not pose any more severe risk as the cold
front slowly pushes through the state through this morning.
There is a secondary cold front up in Minnesota that will drop
through the state today that will help to flush out the higher
dewpoints/humidity air through the day with cooler conditions as
well. This secondary cold front could be the focus for a few
showers over the north this morning as well. Otherwise, a
period of dry weather is forecast as a high pressure zone
transits the area through Thursday.

The rain and storm chances do return late Thursday and more so
Thursday night into Friday morning as strong theta-e advection
develops into the state as the 850mb low level jet ramps up.
Instability is insignificant Thursday night so not seeing much for
severe risk in this time frame, but widespread showers and storms
are likely in this period. By Friday afternoon, new storm
development is possible ahead of a cold front as MLCAPEs top 2000
J/kg and 35 knots of deep layer shear point to some severe risk.
Forecast soundings show a stout cap, though this is weaker in
southern Iowa. Colorado State University`s machine learning random
forest outlook does paint low severe probabilities over the state
and SPC`s day 3 has a slight (2 out of 5) risk over southern into
portions of central Iowa. On the heavy rainfall side, precipitable
water (PW) values will be on the increase Thursday night pushing
above 1.5 inches with warm cloud depths (WCD) of 4000m favorable for
efficient rainfall processes across much of the forecast area. Mean
flow is 25 knots and should lead to the storms being progressive
with rainfall amounts looking between 1 and 2 inches. PW and WCD
values increase topping 2 inches and 4500m respectively on the day
Friday. An isolated flash flood risk may evolve depending on a)
rainfall Thursday night and Friday morning and b) orientation of
surface front with the mean flow. For now, agree with the slight
risk (2 out of 4) of excessive rainfall over southern Iowa from
WPC. More details in the hydrologic section below.

As this cold front pushes through Friday night into Saturday
morning, we`re looking at a nice weekend setting up weatherwise. It
will feature cooler and lower humidity air with no rain forecast as
a big zone of high pressure passes through the upper Midwest. The
high`s departure late in the weekend will mean the return of storm
chances as soon as later Sunday night as theta-e advection pushes
back into the state. A more amplified trough largely staying north
of our state may provide more widespread chances of storms Monday
night into Tuesday morning along with the possible return of 90
degree temperatures in portions of the state.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1243 PM CDT Wed Jun 26 2024

VFR conditions are expected throughout the period across the
terminals. Mainly clear skies are expected for the rest of the
day, before high clouds increase from the west and north before
gradually spreading eastward through the end of the period.
Slightly breezy winds from the north will diminish into this
evening, before shifting SE Thursday as winds increase, gusting
to 15-20 knots. A small signal is in place for patchy fog over
the eastern terminals, though confidence on occurrence is low
at this time, and will continue to be monitored.

&&

.HYDROLOGY...
Issued at 320 AM CDT Wed Jun 26 2024

Major to record flooding is ongoing or continues to be forecast on
the West Fork of the Des Moines River early this morning. The upper
portions of the West Fork and the East Fork are either leveling off
or in recession from their crest. Farther downstream on the West
Fork, rises are still forecast. On the Cedar River, Janesville
has crested and Cedar Falls and Waterloo are nearing their
crests later today. The Shell Rock and Winnebago are also in
recession.

Rainfall yesterday/Tuesday fell largely south of Highway 20 and away
from the basins mentioned above. There were pockets of rainfall of 2
to 3 inches along and south of I-80. With about 36 to 48 hours
before new rain falls over these areas, there should be some
recovery of infiltration capacity. Not surprising, NASA SPoRT
relative soil moisture (RSM) has changed little since this time
yesterday showing more limited capacity/saturation over northern
Iowa with some limited capacity of 50 to 60% in the 0-40cm layer.
Rivers and streams did respond with the rain that fell yesterday
with half foot to foot rises generally at worst (e.g. NRWI4,
DESI4, DMOI4, DEMI4, PRMI4, CHTI4, BDFI4) so capacity remains in
many central and southern Iowa streams after this rainfall.

The next chance for rainfall will be later Thursday through Friday
night. The flash flood risk remains uncertain in this period with
WPC excessive rainfall outlooks painting broad slight risks (2 out
of 4) Thursday night into Friday. Depending on where the rain falls
with the expected more widespread storms Thursday night into Friday
morning and if the storms later Friday fall over the same area, then
there could be an isolated flash flood risk if things line up just
right, or wrong depending on your perspective.

On the river flooding, QPF only goes out 24 hours so this late week
rainfall is not reflected in forecasts and won`t be until Thursday
morning`s updates. For areas that are not currently experiencing or
forecast to go above flood stage, namely central and southern Iowa,
the experimental Hydrologic Ensemble Forecasting Service (HEFS)
based on GEFS QPF continues to paint within bank rises at the 30%
chance exceedance level. The 10th exceedance level would bring a few
locations on the Iowa, Raccoon, and Des Moines into flood stage, but
that would need the higher end rainfall. For northern Iowa and
rivers with above flood stage conditions or forecasts (Upper Des
Moines, Cedar, Shell Rock, and Winnebago), the rainfall
Thursday night into Friday may prolonged the high flows or slow
the recession.

Make sure to stay up to date on the latest forecasts and river
information, avoid flooded areas and rivers, and never drive around
barricades or through flooded areas.

&&

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

&&

$$

DISCUSSION...Ansorge
AVIATION...Bury
HYDROLOGY...Zogg