FXUS63 KDMX 201741
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1241 PM CDT Mon May 20 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- 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

&&

.DISCUSSION...
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 
afternoon.

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.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1241 PM CDT Mon May 20 2024

VFR conditions will persist through the afternoon and into the
early evening. By late evening scattered thunderstorms will
develop across northern Iowa, expanding through the overnight
and resulting in MVFR ceilings. Further visibility and ceiling
reductions possible within heavier thunderstorms which will be 
evaluated in better detail in later updates. Note that
additional robust thunderstorms are also anticipate on Tuesday
afternoon after a brief break in the morning convection.

&&

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

&&

$$

DISCUSSION...Small
AVIATION...Hagenhoff