591
FXUS63 KDMX 020442
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1142 PM CDT Wed May 1 2024

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

.KEY MESSAGES...

- Rain and storms moving in through this afternoon and
  overnight. Severe chances low and focused along the IA-MO
  border. Locally heavy rain possible in the south.
- Rain continues Thursday with a focus in the southeast half of
  the state. Localized heavy rain impacts possible.
- Dry Friday with additional precipitation expected Saturday and
  then again Monday.

&&

.DISCUSSION...
Issued at 253 PM CDT Wed May 1 2024

High pressure kept dry air in place through today with remnant
midlevel cloud cover left aloft. The surface high will continue to
depart this afternoon and make way for an approaching low from the
southwest, bringing with it a strong push of theta-e advection from
the wide open Gulf of Mexico. The main instability axis will be
hugging along the IA-MO state border. CAMs have been differing
on convection evolution this evening, but generally have some
rain and storms moving in from the southwest through this
afternoon. An MCS may form at the jet nose and move across
southern Nebraska this evening and follow along the instability
axis. A rogue severe wind gust or hail would be possible in the
far south with this outcome, hence the marginal risk for severe
weather on Day 1.

As the convection moves into the area tonight, synoptic forcing from
the approaching system takes over and spreads additional rainfall to
much of the remainder of the state with much of this being in the
southeast half. The LLJ will inch into the state and veer across it
through Thursday, remaining parallel to the cold front and focusing
the heaviest rainfall to the southeast half. Some Gulf transport
will be obstructed from a trough in the south. Storm motion is also
fast enough to limit heavy rain impacts from one specific storm.
Some instability will work its way into the state Thursday with
MUCAPE values nearing 1000 in the southeast. Profiles illustrate
long, skinny CAPE profiles, meaning this instability will primarily
contribute to more efficient rainfall production. The amount of
instability is conditional to how the potential MCS modifies the
near storm environment tonight. One would theorize the instability
axis would be shoved further south from this outcome. A rogue severe
storm cannot be ruled out, but with the better shear displaced
behind the surface low, long-lived organization seems unlikely.
Cannot rule out any mode of severe weather, but the risk is low. Any
tornadoes that would form would be in the east where cloud bases
will be low and the low level wind profile is elongated from the
LLJ.

Can expect rainfall amounts in the west totaling a half inch to
an inch. The east and especially southeast will see the most rain in
the area with 1-2 inches possible. Locally higher amounts will be
possible in this region if storms can mature from better
instability. Given the recent rounds of rainfall across southern
Iowa, localized ponding and hydrologic impact is possible,
especially since this time of year is in a favorable window for
runoff. HEFS and NCRFC Ensemble output bring some river gages to
action stage with higher river action possible if the higher amounts
fall over the same basin (see hydro discussion for more).

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1142 PM CDT Wed May 1 2024

Scattered showers and thunderstorms have spread across central
Iowa this evening with more widespread activity anticipated
overnight and Thursday morning. Ceilings will drop to IFR as
precipitation becomes more widespread, along with occasional
visibility reductions within heavier rain. Thursday morning
will see occasional gusts of 20-25 kts across all sites. As
precipitation moves east of the area later Thursday ceilings
return to VFR and winds shift to out of the northwest and
diminish.

&&

.HYDROLOGY...
Issued at 253 PM CDT Wed May 1 2024

Recent heavy rainfall may lead to some flash flooding concerns over
the next day or two along with longer term river flooding concerns.

In terms of short-term response (flash flooding potential and urban
flooding potential), the main concern would be later tonight through
Thursday night. Antecedent conditions show the soil moisture is
highest across the southern 2/3rds of the CWA with the highest
values across the southwest quarter. Runoff would be increased where
soil moisture is highest, however higher rates may lead to local
issues regardless.

As far as longer-term concern (rivers), hydrologic ensemble and QPF
ensemble guidance is showing significant within-bank rises are
likely on many rivers with several locations in the Des Moines,
Raccoon, Skunk, Cedar and Iowa river basins reaching action stage. A
few locations may reach flood stage especially if the rainfall comes
in on the high side mainly in the Des Moines River basin above Des
Moines. The peaks would occur later this week into this weekend.
Conditions will have to continue to be monitored.

&&

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

&&

$$

DISCUSSION...Jimenez
AVIATION...Hagenhoff
HYDROLOGY...Zogg