681
FXUS63 KDMX 290801
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
301 AM CDT Sat Jun 29 2024

.KEY MESSAGES...

- Lingering showers/patchy fog early today; then mild afternoon
- Much less humid and cooler tonight/Sunday
- Seasonal temperatures Monday through Friday with more active
  boundary nearly stationary near Iowa/Missouri Wed to Friday
- River Flooding continues with monitoring of evolution and
  location of upcoming rainfall next week which may lead to
  additional issues for both rivers and localized flash
  flooding.

&&

.DISCUSSION...
Issued at 301 AM CDT Sat Jun 29 2024

.Short Term /Today through Sunday/...

Confidence Short Term:  Medium to High

Low pressure troughs over Iowa and Minnesota at 04z tonight with
numerous weak boundaries/troughs trailing both systems. A weak
trough over Nebraska continues to be the focus of a small area of
thunderstorms over north central Nebraska. This is expected to move
east into Iowa and track into central Iowa between US20 and I80
this morning; weakening quickly with time. Aloft, an H850 low
over northern MN will move east with a stronger trailing front
entering northwest Iowa by mid to late afternoon. This will
finally bring some cooler and drier air into the forecast area
tonight into Sunday. Though we continue with a ribbon of +15C
dewpoints at H850 over our area this evening, this will be
replaced by much drier air tomorrow. Surface dewpoints will drop
from the lower 70s this evening to a much more comfortable
upper 40s to lower 50s by tonight and only recover to the mid
50s by Sunday. Along with that high pressure will settle into
the area with dry conditions and occasional fair weather
cumulus. Highs today will reach the lower to mid 70s north while
central to southern areas will see highs in the lower to mid
80s. Tonight mins will cool to the upper 40s to lower 50s north
with mid to upper 50s south. Sunday will be a pleasant day with
only some high level clouds approaching the region later in the
day. Highs will remain pleasant in the lower 70s north to the
mid to upper 70s central to south.

.Long Term /Sunday Night through Friday/...

Confidence: Medium

With the high sliding east Sunday night, a return of more southerly
flow with mark the onset of a warmer and more humid week ahead.  Lee
side troughing will begin Sunday night and continue Monday as an
area of low pressure migrates into southern Canada and a weak wave
develops over western Nebraska. Both will help increase H850 flow on
Monday. Though Sunday night appears mainly dry, an impulse will move
into western Iowa by daybreak and bring showers and storms into
western sections Monday morning. Current medium range models differ
somewhat on the magnitude of the cluster of storms tracking north
into Monday morning. The EC is more robust with a stronger H850 LLJ
while the GFS is weaker. With even the GFS advertising PWATs of 1.5
to 2.0 inches across the western half of the state by 12 to 18z
Monday, it seems somewhat reasonable that a stronger response would
be seen by the models. Will wait on qpf amounts, but will increase
PoP and thunder chances west during the morning/afternoon west and
north. Forcing increases over northern Iowa Monday afternoon and
evening which results in another area of showers and storms across
areas that dont need any additional rainfall. There remains a fairly
strong signal for more moderate to locally heavy rainfall Monday
night into Tuesday morning. PWATs increase along the front that
lifted north Monday morning but are now aligning along the boundary
as it sags south across central Iowa overnight. With PWATs
increasing to 2 to 2.25 inches and warm cloud depths to near 15kft,
warm rain processes appear to be maximized somewhere in central Iowa
between I80 and US20 or so, given the forecast at this time. Though
we still have a few days to monitor any changes, any signal for
locally heavy rain in this corridor would not be welcomed. As the
overnight rain diminishes early Tuesday morning, another round is
expected as the main upper level trough finally pushes through the
region Tuesday night into early Wednesday morning. This would
certainly bring another night of moderate to locally heavy rainfall
across the region with this round slightly farther southeast of the
previous event. By Wednesday a brief break is anticipated with a
weak area of high pressure making an appearance over the region.
This may be short lived, however, as another trough is anticipated
to establish over the Western Plains by 00z Thursday, forcing the
front back north. At this time, there is some uncertainty as to how
far north the front will lift. There is however, support from the
GEFS and the EPS for continued wet conditions Thursday 4th into
Thursday night. This will be closely monitored moving forward. We
continue with cooler highs Monday in the 70s, Tuesday has the
potential to see highs central and south in the 80s to around 90.
However, with the repeated chances for showers/storms Tuesday,
Wednesday and Thursday, it may be difficult to reach the lower to
mid 80s. The current forecast guidance is somewhat optimistic in
that regard and may need adjustments moving forward. Warm and rather
humid will be the rule for much of later Monday through Thursday
with the expected pattern.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1156 PM CDT Fri Jun 28 2024

Main aviation concern through the overnight is from patchy fog
that has started to develop over northern Iowa. Guidance is not
handling the current fog well so confidence in timing and exact
impacts to TAF sites through the overnight remains low as most
guidance suggests fog to be transient, but also is much slower
to develop the fog than has occurred. Updates likely, but any
fog should dissipate by around sunrise with otherwise VFR
conditions looking to prevail afterwards. Winds will be light
and variable overnight before settling out of the northwest and
increasing through the afternoon hours Saturday.

&&

.HYDROLOGY...
Issued at 301 AM CDT Sat Jun 28 2024

Despite the rainfall over some of the region into early Friday,
little additional rain fell in the Winnebago and Shell Rock River
watersheds late this afternoon or evening. So far, the rivers have
not risen as quickly as forecast. We may see the Winnebago River
remain just below FS at Mason City while the Shell Rock River at
Shell Rock may yet just get to or slightly above flood stage this
afternoon.

In the longer term, a much bigger concern is QPF next week and its
potential impact on rivers. Again, with our current river
forecasts taking into account 24 hrs of QPF they do not include
the QPF for next week. The last few meteorological model runs
have been trending higher in QPF especially across the southeast
two thirds of the state with mean ensemble values of 2 to 2.5
inches over much of the region.

Our latest QPF ensemble hydrographs and Hydrologic Ensemble Forecast
System (HEFS) runs are latching on to this QPF. Unlike the rainfall
a week ago, the QPF next week will affect more of the state so
additional river basins may come in to play. The Cedar/Iowa River
basins look to be the most affected however our guidance is showing
possible rises on tributaries to those rivers as well as portions of
the Des Moines and Skunk River basins. At this point the most likely
scenario will be slowing the rate of fall of our area rivers,
however some ensemble members are suggesting the possibility of
additional or new crests with additional minor flooding. Some of the
lower probabilities are suggesting moderate to major flooding at
some locations. We cannot totally rule out these possibilities.
Ultimately the location, amount and timing of the rain next week
will be the biggest factor.

In addition to the river flooding, we will also need to monitor for
localized flash flood potential next week given the favorable heavy
rainfall parameters Monday night and Tuesday night through Thursday.
Though this is a rather broad overview for now, refinement of
details will likely be a 6 to 12 hour process at most and will be
much better defined prior to each potential event.

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...REV
AVIATION...KCM
HYDROLOGY...Zogg/REV