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139
FXUS63 KDMX 211942
AFDDMX

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

.KEY MESSAGES...

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- 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.
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&&

.DISCUSSION...
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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.
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&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1244 PM CDT Sat Sep 21 2024

A boundary moving into northwest Iowa currently will move slowly
southeast during the period. Scattered thunderstorms are
expected to develop along the boundary late this afternoon and
into the evening. Additional showers and thunderstorms are
possible over mainly the southeast half (KDSM/KALO/KOTM) tonight
into Sunday morning. Cigs will be mainly VFR so any restrictions
will be with vsbys in rain. Some stratus is likely to form later
tonight and Sunday in areas mentioned above with precipitation.
The wind will become northwest with fropa.

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Donavon