507
FXUS63 KDMX 221750
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1150 AM CST Fri Nov 22 2024

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

.KEY MESSAGES...

- 50% confidence for fog in the southwest half of the area
  tonight. Otherwise, partly to mostly cloudy elsewhere.

- Above average highs this weekend.

- High confidence (90%) in highs in the 30s next week.
  Increasing confidence (>80%) in below freezing highs
  following Thanksgiving.

&&

.DISCUSSION...
Issued at 408 AM CST Fri Nov 22 2024

Stratus expanded over the eastern 2/3rds of the state overnight
which will keep morning lows closer to 30 degrees this morning.
further west where clouds have less influence, lows in the morning
will drop into the 20s. Expect the opposite warm/cold divide this
afternoon as stratus will hang on in the north and east. Have
lowered highs in these areas with mid to upper 30s expected. The
sunnier southwest will warm into the 40s. Have raised lows for
tonight since mid and upper level clouds are expected to arrive from
the north around the midnight hour. It`s also probable (~40%
confidence) that stratus will still have some influence in the far
northeast. If clouds arrive more sparse, winds will be light enough
to allow for fog to develop. This scenario appears most likely in
the southwest where minor surface ridging will take place. The 06z
HREF from last night has trended in this direction, with less fog
coverage in the partly cloudy north, but with more widespread fog
generally around the Des Moines River Valley and westward. Have
decided to forego adding fog mentions in the official forecast for
this issuance, but anticipate better certainty heading into
today`s updates.

A passing upper level system in Canada keeps the midlevel cloudiness
in the north Saturday, thus stunting some of the warming over the
weekend. Nonetheless, highs in the south will return to the 50s
Saturday afternoon. A cloud shield surrounding a surface cyclone in
the Central Plains will overspread much of the state Sunday, but the
WAA regime beneath the clouds keeps the warmer temperatures coming.
Perhaps a 60 degree high temperature could be recorded nearest the
IA-MO border (about a 40% chance of this happening). The upper level
pattern for Monday remains uncertain. The GEFS and EC ensemble camps
differ on how the wave behaves over the Midwest. The GEFS prefers a
slower, more intense wave and the EC settles on a more open wave.
However this wave takes shape, it will energize the aforementioned
surface cyclone and direct the main axis of moisture more towards
the eastern half of the Corn Belt, leaving Iowa more moisture
starved. Therefore, the ~10% chance for rain for the northeast seems
appropriate. Depending on the synoptic forcing present, it
cannot be completely ruled out to see precipitation on Monday,
but it remains to be known whether or not accumulation (mostly
speaking in liquid equivalent form) will be realized this day.
Trends will be monitored. The 00z deterministic GFS offered a
unique solution that had appreciable QPF north of here, but a
quick glance at the rest of the GEFS suite quickly deemed this
solution an outlier.

Otherwise, CAA is a certain outcome of the passing system with highs
falling back into the 30s Monday. It`s plausible (90% confidence) at
this point that northern Iowa won`t see temperatures above freezing
for the foreseeable future. Upper level zonal flow sets in for the
midweek and offers little in the way of temperature variation.
Deterministic runs still illustrate a nearby trio of waves around
Thanksgiving day. Looking in ensemble land, the southern stream is
becoming the favorable candidate for moisture, decreasing confidence
in precipitation Thanksgiving day (current forecast is likely too
generous). There`s plenty of time for that outcome to change, so
stay tuned for changes to the forecast. A stout upper level closed
system in Canada expands its reach southward after Thanksgiving and
brings with it a shot of even colder air for the region, with most
guidance keeping high temperatures for much of the state below
freezing for the following weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1138 AM CST Fri Nov 22 2024

Stratus lingers across all TAF sites early this afternoon with
aviation conditions ranging from LIFR to even VFR in western
Iowa where clearing has occurred. Although KFOD followed by
KDSM will become increasing scattered in the next hour or two as
the clearing slowly reaches each location, sites further east
will continue to have LIFR to MVFR ceiling conditions through
much of at least the rest of the daytime hours. However,
improvements may not last long with additional cloud cover
moving in tonight and the current cloud deck likely become
thicker/lower again per forecast soundings in especially eastern
Iowa. Although some fog/visibility reductions also remain a
possibility overnight into early Saturday morning, confidence
in impacts to any one TAF site remains too low to include
mentions at this time with uncertainty in development underneath
the various cloud decks. Will continue to monitor trends and
provide updates in future issuances. Winds out of the northwest
today will become light and variable overnight before settling
out of the south to southeast on Saturday, but generally
remaining near to under 10 knots through the period.

&&

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

&&

$$

DISCUSSION...Jimenez
AVIATION...KCM