882
FXUS63 KDMX 220447
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1147 PM CDT Sat Sep 21 2024

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

.KEY MESSAGES...

- A few strong storms with large hail and damaging wind possible
  into mid evening.

- Much cooler Sunday. Chances for showers and non-severe storms
  over the southeast half.

&&

.DISCUSSION...
Issued at 239 PM CDT Sat Sep 21 2024

The well advertised cold front has reached northwest Iowa this
afternoon and it will move slowly southeast through the
remainder of the afternoon. Temperatures and dewpoints ahead of
the boundary have exceeded forecast values. Dew points in the
low 70s have reached into southwest Iowa while temperatures are
in the 90s in some areas of central Iowa. Waterloo was at 92
degrees at 2 pm. Southern Iowa was still cooler where more
clouds and some weak elevated showers and thunderstorms have
persisted. Coincidentally, the higher dew points have been in
that part of the state and that has also been an area where the
mid-level moisture has been highest. In fact the 700-500 mb
relative humidity values has a minimum in vicinity of the
boundary which has been a limiting factor in elevated convective
initiation currently. The elevated mixed layer has been capping
surface based convection this afternoon but that warm layer
will continue to erode the remainder of the afternoon for a
narrow corridor of mixed layer instability for surface based
convection to develop some time between 21-23z.

There remains a chance for a few strong to severe storms this
afternoon and early evening. Mixed-layer CAPE values near 2000 J/kg
with ECAPE values near 1500 J/kg. Shear values are relatively low in
the below 500 mb with speed shear increasing above that level, which
overall will just result in anvil spreading and shadows leading to
the potential cooling surface temperatures which actually could lead
to some stabilizing. Additional instability fuel will come from
modest kinematic forcing and the additional convection overnight
will likely become even less organized and mushy. The severe
weather chances will be confined to large hail and damaging wind
gusts, especially with the initial storms.The potential for
efficient rainfall remains though with the PWAT values of 1.5 to
2 inches over central and southern Iowa and good warm cloud
depths. Pockets with rainfall amounts of 2 to 3 inches may occur
through 06z.

The shallow surface boundary will be undercutting the moisture
overnight as well so temperatures will cool and stratus should begin
to form. This will lead to a much cooler Sunday with highs only in
the 60s expected. Precipitation chances will continue over the
southeast half of the area to begin the day then gradually settling
southeast during the day.

The more fall like weather will prevail through the week though
highs in the mid to upper 70s will return by next Wed. Precipitation
chances will be minimal with a few low chances over the southeast
Monday and again late week.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1142 PM CDT Sat Sep 21 2024

Rain with a few thunderstorms expected to continue across the
south overnight and into the morning hours. Over the next 6
hours, some scattered thunderstorms will be possible over KDSM
with locally MVFR cigs possible. Gusty winds expected after 12z,
especially in the north where 25kt gusts are possible. Winds
will ease and skies will gradually clear after 18z.

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Jimenez