010
FXUS63 KDMX 270423
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1123 PM CDT Wed Jun 26 2024

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

- Quiet weather continues rest of today into Thursday afternoon

- Shower and storm chances return west into Thursday evening, with
severe weather possible by Friday

- Dry and generally quiet conditions return Saturday and especially
Sunday

- Active weather potential into early next week, with gradual
warming

&&

.DISCUSSION...
Issued at 258 PM CDT Wed Jun 26 2024

Fairly mild conditions today across Iowa, with surface analysis
showing a large area of high pressure centered over the Dakotas and
extending across the Upper Midwest. Satellite imagery shows clear
skies across Iowa, though scattered cumulus can be seen across much
of Minnesota and Missouri, which is expected to diminish into the
evening. Despite a weak boundary now over southern Iowa,
temperatures this afternoon have increased into the upper 70s to
80s, though dewpoints are more varied with most areas in the upper
50s to low 60s, while southern Iowa sits in the mid to upper 60s.
Dry conditions are expected to persist through the rest of the day
and into Thursday, with overnight low temperatures falling into the
mid 50s to low 60s. Given expected quiet conditions with very light
winds and residual moisture, especially east, a few areas of patchy
fog cannot be ruled out into early Friday, though uncertainty is on
the higher side given gradually increasing clouds, but monitoring
will continue.

Much of Thursday is looking to be dry as high pressure gradually
tracks east/southeast out of the region, with slightly cooler
temperatures as values in the upper 70s to low 80s are expected. Low
level flow per guidance is expected to increase out of the
southwest, especially through the evening and into Friday as a 40+
knot LLJ sets up over the region. This is ahead of a developing low
pressure system centered over Montana, which looks to drop a cold
front into the state later Thursday afternoon to evening, gradually
track eastward into Friday. An increased pressure gradient will lead
to breezy winds late Friday morning to afternoon, with gusts upwards
of 25-30 mph expected. Further analysis in the environment depicts
low-end CAPE and mid-level lapse rate values, along with bulk shear
values around 30-40 knots Thursday evening into Friday.
Deterministic models struggle with the timing of arrival, which is
expected to track west to east, as RAP, NAM and GFS have little if
any activity over western Iowa until late Thursday, while the CAMS
have showers and storms arriving in the afternoon to evening. Have
taken the middle ground solution at this time, given some potential
for dry air to be a limiting factor in the earlier arrival.
Regardless, not expecting severe weather with this initial activity
into Friday. As the front gradually tracks east through Friday
afternoon, showers and non-severe storms are expected. However, a
boundary is expected to set up over southern Iowa Friday evening
into Saturday, with additional showers and storms expected. Despite
the remaining timing differences, the overall setup for strong to
severe weather is looking higher, with higher CAPE values generally
in the 1000-1500 J/kg range, bulk shear values around 40 knots and
mid-level lapse rates around 7 C/km, supporting a higher potential
for strong to severe storms. This is highlighted with the Day 3 SPC
outlook which outlines a Slight Risk across southern and
eastern Iowa into Missouri. The main concerns at this time
include damaging winds and some possible larger hail, along with
a few tornadoes. On the hydrological side, PWATs around 1.75``
with warm cloud depths just over 15kft are suggested per model
guidance, which would allow for an increased potential for heavy
rain with this activity. Ensemble guidance has the highest
rainfall in southern Iowa into Missouri, with total rainfall
amounts 1-2 inches expected and locally higher amounts possible,
with areas further north seeing lesser values.

Though the differences in timing remains into Saturday regarding how
quickly any remaining showers and storms depart, dry conditions are
gradually expected to return and remain through the rest of the
weekend as high pressure settles overhead. Highs in the upper 70s to
80s are expected Saturday, before cooling Sunday in the 70. Into
early next week into the beginning of July, an active weather
pattern is signaled to return, with the Colorado State Machine
Learning site pinging portions if not all of Iowa with at least a
lower end potential for severe weather. Further analysis shows
moderate CAPE and shear over the region, though uncertainty on this
potential remains on the higher end given timing ahead of this
activity, as changes are surely expected. Warmer temperatures also
look to return Tuesday/Wednesday with values in the upper 80s into
the 90s. Stay tuned for updates as more information becomes
available.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1118 PM CDT Wed Jun 26 2024

VFR conditions with light and variable winds continue through
the night. Patchy fog chances look low, but still can`t rule
out some brief ground fog, especially in areas over northern
Iowa (KMCW, KFOD, KALO) where more surface moisture is present.

Cloud cover will increase through Thursday, with the chance for
a few sprinkles or an isolated shower in the morning, followed
by increasing chances for scattered showers Thursday afternoon
into the evening. Some lightning is possible later Thursday
into Thursday night, but have kept thunder mention out of the
TAFs for now.

&&

.HYDROLOGY...
Issued at 259 PM CDT Wed Jun 26 2024

The river forecasts have transitioned mainly to maintenance mode for
our CWA. Having said that, major to record flood is still ongoing or
forecast. Major to record flooding continues to be forecast on the
West Fork Des Moines River. 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. Significant flooding is also forecast on the Des Moines
River at Fort Dodge. On the Cedar River, Janesville has crested and
Cedar Falls and Waterloo are nearing their crests. The Shell Rock
and Winnebago are also in recession.

Rainfall on Tuesday fell largely south of US 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 up to a
couple foot rises generally at worst (e.g. NRWI4, DESI4, DMOI4,
DEMI4, PRMI4 and CHTI4) so capacity remains in many central and
southern Iowa streams after this rainfall. The one exception was
BDFI4 where the river rose significantly but still crested below
action stage.

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...Bury
AVIATION...Dodson
HYDROLOGY...Zogg