623
FXUS63 KDMX 281918
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
218 PM CDT Fri Jun 28 2024

.KEY MESSAGES...

- Showers and storms likely through the remainder of the day
  into the early overnight hours, including the potential for
  strong to severe storms and locally heavy rainfall.

- Terrific weekend with seasonably comfortable temps and
  humidity, but weather pattern becomes active again early next
  week with increasing humidity and chances for storms,
  including the potential for strong to severe storms and heavy
  rainfall.

- River flooding continues within the Des Moines and Cedar River
  basins in northern Iowa.

&&

.DISCUSSION...
Issued at 212 PM CDT Fri Jun 28 2024

19z GOES water vapor imagery shows a compact upper shortwave
pushing off the northern Rockies into the high Plains. At the
surface an area of low pressure was noted over far northeastern
Nebraska, with an attendant cold front draped to the
south/southwest and a warm front positioned southeast through
Iowa. Morning shower/storm activity within a broad elevated
warm air advection regime is finally exiting east, although
widespread low cloud cover remains in it`s wake as highly moist
low levels are trapped beneath a very warm mid-level environment
with 700mb temps pushing 14C. Elevated convection has managed
to develop within a zone of mid-level convergence over northwest
Iowa. This activity will generally ride the northern edge of
the warmer mid-level air across northern Iowa this afternoon.
Some model guidance suggests a few robust core may materialize
and support a threat of large hail.

The most favorable window for severe weather likely holds off until
the stout mid-level cap erodes toward early to mid-evening, allowing
for CI along the cold front as it moves into western Iowa. Hires
models initially show discrete storm modes as updrafts work within
effective shear values of 30-40 kts. All severe hazards could
be on the table. Model soundings show steep mid-level lapse
rates and ECAPE near 3000 J/kg which would be supportive of
large hail. Low level winds gradually veer with height, and may
be enough to squeeze out a tornado or two especially as the LLJ
increases and enhances the low level shear profiles. Storms
will eventually congeal into one or more convective complexes as
cold pools merge and support a threat for damaging wind gusts
before the severe weather risk begins to wane due to decreasing
instability with time and eastward extent. The environment is
primed for heavy rainfall rates given warm cloud depths pushing
4000m+ and PW 1.75-2"+. HREF LPMM product advertises pockets of
2-3"+ rainfall amounts. The good news is the convection should
be progressive, limiting the extent and magnitude of the heavier
rainfall amounts. The threat should also mostly remain south of
the heavily saturated portions of northern Iowa.

Cold front and any associated storm chances clear the CWA tonight.
Model guidance suggests some patchy fog potential into the early
morning hours within a zone of light winds behind the front. Beyond
that it is shaping up to be a fantastic weekend as a cooler and dry
Canadian airmass settles into the state with highs in the 70s and
80s and comfortable humidity levels. Pattern activity picks
right back up as a broad upper level trough with numerous
embedded lower amplitude waves is re-established over the
northern Rockies into the northern Plains. A broad warm air
advection wing should spur elevated shower/storm activity Sunday
night into early Monday. 12z model guidance is a bit slower
with pattern progression beyond that point as a surface low
deepens over the Dakotas and a deep but narrow plume of moisture
is pulled northward up the Plains. This may hold additional
convection to our west until Monday night into Tuesday as a cold
frontal boundary slowly pushes through the state, including the
potential for strong to severe storms and heavy rainfall.
Models are in fair agreement with stalling out the front just
south of the state and serves as the primary focus for
additional storm activity through mid-week. Should the boundary
hang up a bit further north, then we would possibly face a
threat of additional episodes of heavy rainfall and impacts to
our ongoing river flood concerns. See the hydro section for more
details.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1225 PM CDT Fri Jun 28 2024

Messy aviation forecast over the next 12 to 18 hours as the
area deals with variable cig restrictions, rounds of shra/tsra,
then some patchy fog tonight into early Saturday morning. As of
1730z, cigs over much of the area are MVFR with pockets of IFR.
Radar imagery shows a complex of showers and storms moving into
southeast Iowa. This will continue to push east, with additional
development already noted over northwest Iowa. Expecting
further development by later this afternoon and evening,
impacting terminals from west to east into the evening hours.
Timing of tsra and extent of categorical impacts to individual
terminals remains uncertain at this time, although sporadic
strong wind gusts and heavy rain reducing visby below 2sm is
possible at times.

Conditions should improve as storms push east by late evening into
the early overnight hours. Model guidance suggests patchy fog may
develop east of I-35, and have included mention of MVFR visby at
KALO and KOTM. VFR conditions prevail area-wide by mid-morning
Saturday and through the rest of the TAF period.

&&

.HYDROLOGY...
Issued at 200 PM CDT Fri Jun 28 2024

The observed rainfall from Thursday night into Friday morning ended
up being higher than expected in some areas which led to some
adjustments to the river forecasts. The most drastic examples are
the Shell Rock River where renewed flooding is now expected at Shell
Rock with the river going back above flood stage this evening. In
addition, renewed flooding is expected on the Winnebago River with
the Mason City area going back above flood stage on Saturday. In
other locations the adjustments primarily involved either keeping
the stages steady for longer or slowing the fall.

Concern in the short term then turns to later today into tonight. We
may see some locally high QPF amounts however we are not expecting
widespread heavy QPF amounts. Our river forecast updates this
morning take into account the QPF through Saturday morning.

In the longer term, concern is about 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 half of the state. 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
flash flood potential next week given the favorable heavy rainfall
parameters Monday night into Tuesday.

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...Martin
AVIATION...Martin
HYDROLOGY...Zogg