586
FXUS63 KDMX 212347
AFDDMX

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

 ...Updated for the 00z 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 /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 644 PM CDT Sat Sep 21 2024

VFR conditions to largely prevail with the exception of some
localized vsby restrictions from rain tonight. A line of
thunderstorms will approach KDSM and KALO over the next 2-4
hours and may pose some gusty variable winds for an hour. A
front will pass over and shift winds to northwesterly. Rain will
gradually fill in from the south after sunset and bring steady
rain and a few non-severe storms overnight. Stratus may form
behind, but enough dry air present to keep VFR. Gusty winds
expected in the morning hours, lessening as the front moves
farther away after 18z.

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Jimenez