219
FXUS63 KDMX 241953
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
253 PM CDT Fri May 24 2024

.KEY MESSAGES...

- Some additional showers and storms this afternoon over eastern Iowa

- Returning chances for showers and storms late Saturday into
Sunday; some strong to severe storms possible

- Additional active weather into early next week

&&

.DISCUSSION...
Issued at 253 PM CDT Fri May 24 2024

After a busy morning with a QLCS passing across Iowa, which has
weakened significantly as it tracks into Lake Michigan early this
afternoon, conditions quieted down by mid to late morning outside of
a few lingering storms across the eastern parts of the state. As of
this afternoon, radar returns over the past hour or so have shown
redevelopment of additional showers and storms along the frontal
boundary, which is draped from southwest to northeast over eastern
Iowa. A favorable environment with MUCAPEs around 1000-1500J/kg and
favorable shear and lapse rates could allow for a few storms to
potentially become strong at times, though the better severe
potential lies further east where more favorable conditions reside.
Small hail and gusty winds look to be the main hazards, along with a
few heavy downpours at times. Breezy west/southwest winds this
afternoon will shift more northwesterly and are expected to
gradually decrease, along with skies clearing out from west to east
this evening. With plentiful radiational cooling and the
aforementioned NW flow, overnight lows are expected to fall into the
40s across the state.

High pressure quickly passing across the region will allow for dry
and quiet weather to remain into Saturday, before exiting the area
ahead of a shortwave and deepening low pressure system tracking
across Nebraska/Kansas into Missouri through the day. Increasing
theta-e advection along with southwesterly flow and resultant warm
air advection per guidance is expected to stream into the state
throughout the day, with increasing chances for showers and storms
by Saturday evening and into Sunday. Looking closer into the
environment during this timeframe, steep lapse rates, effective
shear values around 30-40+ kts are highlighted over southern
Iowa, though CAPE values are on the lower end below 1000J/kg.
While more favorable environmental conditions are looking likely
into Missouri, the highest potential for strong the severe
storms are expected over portions of southern Iowa where a
Slight risk is highlighted per SPC guidance, while a Marginal
risk covers the central into portions of southern Iowa. In terms
of rainfall, model soundings have shown PWAT values up to
1.25-1.50 inches along with deep warm cloud depths, which would
allow for a higher end potential for heavy widespread rainfall
across the state. Showers and storms are expected to largely
continue through Sunday morning, while model members are a bit
less agreeable on when and where the rain ends, as a few members
depict a lingering boundary remaining over southern Iowa that
may allow for continued chances for showers and storms.
Widespread rainfall amounts with the late Saturday into Sunday
system look to bring widespread 1-2 inch amounts, with a few
locally higher amounts possible as well.

Into early next week, models indicate continued potential for active
weather, though details regarding location and impact remain more
uncertain. In general, guidance shows a positively tilted trough and
associated developing low pressure system pivoting across the
upper midwest, with a few embedded waves dropping into the state
which look to bring continued chances for showers and storms.
In specific terms, the NAM generally shows developing activity
over largely the northeast half of Iowa Monday afternoon to
evening and into Tuesday, while the GFS and Euro depict more
widespread activity across Iowa through the same time periods.
Overall, the severe potential looks low end at this time, along
with generally lower end rainfall amounts, though will be
watching closely for any changes. Upper level ridging looks to
follow by midweek, with some quieter weather looking to result
from this. Temperatures through the week generally are expected
to be rather mild, with highs in the 70s and lows in the 50s.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1233 PM CDT Fri May 24 2024

Redevelopment of showers and storms, some possibly strong to
severe, is expected into this afternoon, mainly impacting KOTM
while lighter shower activity and a few weaker storms may occur
over KDSM and KALO briefly. Mid to low level clouds remaining
this afternoon will lead to continuing MVFR conditions, though
will clear into the evening and remaining TAF period, with skies
clearing and conditions improving. Breezy winds shifting
northwest will gradually lighten up into the evening hours, then
shifting more southerly into Saturday and becoming slightly
breezy.

&&

.HYDROLOGY...
Issued at 253 PM CDT Fri May 24 2024

An active hydro pattern continues and is expected to continue over
the next several days at least for rivers and larger streams. With
forecast rainfall last night into early this morning and during the
day today coming in a little higher than what was expected
yesterday, a flood watch was issued for much of the CWA for last
night through this afternoon. That flood watch will expire this
afternoon as the showers and storms gradually migrate east.

Locally heavy rainfall this morning led to decent runoff and some
localized flooding issues mainly across the southeastern CWA.
Elevated SAC-SMA soil saturation values have increased across the
southeastern CWA. Values are in the 40 to 60% range across the
eastern CWA roughly bounded by IA 92, US 218, US 20 and I-35. In the
western CWA they are also elevated in the Audubon-Jefferson County
area. When soil saturation values exceed 50% the flash flood concern
increases markedly. Thankfully with a relative lack of heavy rain in
the forecast for the next couple days the flash flood threat will
subside.

In spite of the decrease in flash flood concerns, river flooding
continues on some streams in the CWA and will continue over the next
several days. River flooding continues or is forecast across
portions of the Cedar, Iowa, Des Moines and Raccoon River basins as
well as their tributaries. Further analysis of observed rainfall
trends have led to decreases in the forecasts this afternoon for
parts of the Des Moines River basin above Saylorville Lake. Below
Saylorville Lake and Lake Red Rock, changes to outflow plans by the
U.S. Army Corps of Engineers at both locations has led to
adjustments at some locations as well.

As is usual this time of year, the river forecasts take into account
24 hrs of QPF. Additional rainfall is in the forecast beyond
Saturday. Looking beyond 24 hrs using our QPF ensemble hydrographs,
the  Hydrologic Ensemble Forecast System (HEFS) and National Water
Model (NWM), the most likely scenario in most places is that the
rain over the next several days will slow the fall of area rivers
and possibly result in secondary crests. Those secondary crests
should be lower than the levels we saw earlier this week. It is not
outside the realm of possibility, however, that some locations may
see higher crests than we saw earlier this week--but again that
would be heavily dependent on future rainfall. If the rainfall comes
in higher than forecast then we would see long, broad crests and/or
higher secondary crests.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 4 PM CDT this afternoon for IAZ027-028-038-
039-049-050-061-062-074-075.

&&

$$

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