-- Discussion containing changed information from previous version are highlighted. --
660
FXUS63 KDMX 211131
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
631 AM CDT Sat Sep 21 2024

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Warm and humid today with storms early and again
  afternoon/evening
- Marginal Severe Risk today
- Lingering showers/storms southeast half tomorrow/Monday
- Pleasant and Mainly Dry Remainder of Week

&&

.DISCUSSION...
Issued at 257 AM CDT Sat Sep 21 2024

.Short Term /Today through Sunday/...

Confidence: Medium to High

Though most of the details through tomorrow seem well defined, highs
today may pose a few challenges due to cloud cover. Subjective 03z
surface map shows elongated trough/areas of low pressure extending
from South Dakota north to Northern North Dakota with cool front
connecting the two lows and a stronger cold front trailing the
northern low. Two areas of warm air advection/warm fronts extend
from the two systems this morning. A warm front over northern MN has
developed an area of showers and storms from west central MN to
northern MN while another weak warm front developing over Iowa has
developed an area of mid level warm air advection showers over
northern Iowa at 0630z, which may expand somewhat this morning
through 12-14z in the far northeast. Have added some showers/iso
thunder due to expanding area of mid level clouds on nighttime
microphysics image. The southern low over South Dakota is pulling a
ribbon of 60 to 70F dew points into Kansas at this time and already
initiating some scattered showers/iso storms over southeast Nebraska
and northwest MO. This area is anticipated to expand and track
northeast into southern/southeast Iowa by 12-13z and continue to
track northeast into the late morning hours. This should be mainly
south of I80 this morning. As we move into the afternoon hours, the
approaching cool front will eventually have a decent airmass to tap
into for storm development. Sometime between 18z and 20z, initiation
will take place along and behind the boundary. HIRES models are in
generally good agreement with scattered storms initially, then
filling in toward the I35 corridor between 21 and 22z with a line
from near Atlantic to Charles City. Though sufficient CAPE over 2000
J/kg and ECAPE over 1500 J/kg is forecast this afternoon, 0 to 6 km
BWD is running about 30kt during the afternoon hours ahead of the
boundary. Organization will likely not be significant, though some
linear organization is expected with time. The main threats will be
some wind or hail as the LCLs are forecast to be near 4500 feet or
higher. Though there is no real concern of any hydro issues today,
along the approaching boundary this afternoon we briefly reach
efficient rainfall criteria with warm cloud depths of 13kft and
PWATs of 1.5 to 2.0 inches. This will continue as the line of
showers and storms drifts southeast into the evening. With mean
storm motion of 20 to 25kt, rainfall totals may exceed 1 to 1.5
inches at some locations. As the boundary sags south into the
evening hours, another weak ripple may enhance some of the showers
and storms over the south with rainfall continuing into day Sunday;
especially over the southeast. Highs today will be warm and will
range from the lower 80s north to the mid to upper 80s south.
Tonight will turn much cooler, as breezy northwest winds usher in
more fall like air into the region. Temps will range from the lower
50s northwest to the lower 60s in the southeast. Highs tomorrow will
be especially cool over the southeast half where showers will
continue; ranging from the lower to mid 60s there to the mid to
upper 60s in the far northwest. This will be some 20 to 25 degrees
cooler than today.

.Long Term /Sunday Night through Friday/...

Confidence: Medium to High

There is generally good agreement with the exodus of the system to
the southeast by Monday, though some timing differences persist
between the deterministic and ensemble solutions. The ensembles are
holding onto a precipitation chance a bit longer on Monday, but
otherwise any lingering precipitation will be light. A lot will
depend on whether an apparent southern stream system can strengthen
in Missouri Sunday night. If it does, this would extend the light
showers south into Monday afternoon. Regardless of some uncertainty
early in the extended, the bulk of the week will feature a broad
area of high pressure expanding across the Great Lakes south
into the mid Mississippi River Valley through late week. Our
extended blend has brought back some slight rain chances beginning
Thursday night and extending into Friday. There seems to be little
support in either the 00z deterministic or ensemble guidance for
any precipitation by late week. We will likely revisit those rain
chances as we near the end of the extended forecast over the next
few days. Otherwise, temperatures will gradually warm from the
60s Monday to the lower 70s Tuesday and then remain in the mid
70s through Thursday with dry and pleasant conditions.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
-- Changed Discussion --
Issued at 631 AM CDT Sat Sep 21 2024

Several challenges today. Area of shower/possible iso thunder
expected to track through southeast Iowa prior to 18z;
affecting mainly OTM and possibly DSM. South southwest winds
increase to above 12kt gusts 15-18 most sites as cool front
approaches from northwest. Aft 19z, a line of showers/storms
will gradually fill in and track southeast across the region
with most sites seeing showers and possible thunder; continuing
through end of period southeast half. Northwest winds also
increase north sites aft 07z. /rev
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...REV
AVIATION...REV