Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200924

Area Forecast Discussion
National Weather Service Des Moines IA
424 AM CDT Mon May 20 2024


- Generally non-severe MCV associated convection lingers into
  the morning east, then chances temporarily diminish later in
  the day

- More storms expected tonight into early Tuesday morning. Severe
  storms possible, mainly a wind treat.

- Peak severe potential Tue afternoon with another round of
  storms. Tornadoes, damaging wind, and hail all possible.
  Enhanced Risk of severe weather (3/5).

- Break from precip follow midweek, but additional chances
  return late in the week


Issued at 424 AM CDT Mon May 20 2024

Little change in the overall weather pattern is anticipated
through the period with relatively lower amplitude and
progressive flow bringing systems of varied strength through the
Midwest every few days keeping us active. At onset this
morning, the parent short wave and combined generative MCV
continues to swirl along the IA/MO border with the last of the
healthy forward flank convection exiting the area to the east.
Non-severe showers and a few storms are then expected to linger
for several more hours before a temporary break with little to
no precip in the MCV`s subsident wake into at least early

Some regeneration may occur during the late afternoon and
especially early evening hours northwest however as the current
NE/SD boundary sags into the state. This may start what could be
an active night with 2-3K J/kg MLCAPEs building into the state,
and effective shear increasing as well as the western long wave
trough nears and mid level flow increases. This convection may
persist and expand into the night, fueled by increasing low
level moisture transport through the MO Valley. This may also be
reinforced by an advancing MCS generated from what was High
Plains peak heating convection as it maintains intensity from
the low level jet associated moisture transport and convergence
crossing NE into IA. Hail and especially damaging wind appear to
the primary threats with 00Z HREF high wind probs quite high into
IA, even several hours past daybreak. Highly anomalous moisture
parameter space, coupled with the strong moisture transport,
suggests locally heavy rainfall is possible with isolated
several inch maxes.

The CAM consensus suggests a brief break toward midday, but
then the airmass recovers quickly into the afternoon as what is
now a short wave off the Baja coast reaches the MO Valley and
intensifies. Strong synoptic scale forcing should aid continued
low level theta-e advection into the warm sector and also what
is expected to be a line of storms forming along the associated
cold front, with more widespread weaker precip to the north and
west along the deformation zone. The precip should form into
discrete supercells initially shortly after onset with the
latest RAP suggesting uncapped 3000+ J/kg MLCAPEs with 50-60kts
of deep shear normal to the boundary. Thermodynamic questions do
remain however with recovery potentially delayed by the morning
convection, possibly resulting in the genesis region being
pushed farther east than depicted by hi res guidance. While
confidence in supercells is fairly high, confidence in tornadoes
is certainly there, but less. Examination of several CAM
soundings suggests a spectrum of possibilities. While all have
strong 500m shear, some seem more effective than others ranging
from alarming SRH and streamwise vorticity values and high
percentages of ingest, which could certainly be enhanced
depending on remnant outflow boundaries, to less favorable
crosswise SR ingest in broader SSW surface wind. It is curious
that various CAM 0-3km updraft helicity tracks during the
afternoon are not as ominous as one might think with a few
tracks but not numerous ones, unlike what was depicted on 26
Apr. Nevertheless, large hail and damaging wind expected, along
with the potential for tornadoes considering the overall severe
weather parameter space.

Any convection should exit by early evening leading to more
tranquil weather and seasonal temps for the next few days.
Albeit not as deep as our Tuesday system, our active pattern
will cycle another northern Plains trough passage Friday
bringing additional chances for showers and storms.
Deterministic GFS solutions aren`t overly enthusiastic on
severe weather potential, with weaker instability and deep
shear. However 00Z 19 May CSU GEFS based machine learning
solutions do reflect at least some severe potential, especially
south. Other weaker waves will keep at least some precip chances
into the forecast through the weekend as well.


Issued at 1144 PM CDT Sun May 19 2024

A line of showers and storms continues east of I-35 early this
morning and is expected to move out of the area over the next
few hours. However, a large cluster of additional showers and
storms are expected to lift north across the state after 06z,
which will allow for active weather to continue through much of
the morning. Periods of lower ceilings and reduced visibilities
from heavier rainfall with these storms may allow for MVFR
conditions at times. A drying period looks to occur by late
morning through the afternoon, before additional showers and
storms look to move into the area towards the evening. Specific
mention of timing and location of storms Monday evening remain
unclear and will largely depend on the previous storm evolution,
which will be monitored closely. Winds will gradually shift
south/southwesterly with slightly breezy winds through the
daylight hours.