741
FXUS63 KDMX 230021
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
621 PM CST Fri Nov 22 2024

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Fog possible again tonight, could lead to some isolated slick
  spots into early Saturday morning.

- Above average highs this weekend in the 40s to 50s.

- High confidence (90%) in highs in the 30s next week.
  Increasing confidence (>75%) in below normal temperatures
  after Thanksgiving.

&&

.DISCUSSION...
Issued at 302 PM CST Fri Nov 22 2024

Similar story to this time yesterday with the cloud cover extending
well away from the low now over the east coast but influencing much
of the eastern half of the U.S. However, the outcome of the cloud
cover was the opposite of yesterday with stratus slow to erode over
the eastern 2/3rds of the state this morning, and still covering the
eastern half of the state as of about 2 PM. As a result,
temperatures have been slower to warm over eastern into parts of
central Iowa but are still in the 30s to mid 40s west, overall
similar to yesterday, though a few degrees cooler in central
portions. In a somewhat rinse and repeat of yesterday, the main
weather impacts headed into this evening are the lingering and
incoming cloud cover as well as fog potential. For those keeping
track at home, everything except the wind!

Lingering low level moisture, winds which become light and variable
overnight and the surface high moving across the state into
tomorrow morning all create an overall more favorable set up
for fog development overnight/early Saturday. However, although
confidence in fog development overall may be >50% in some
portion of the area, confidence in exact impacts/lowest
visibilities and location in space and time is much lower. The
river valleys towards western Iowa are certainly one favorable
location, with fog development underneath the stratus in the
north to east another possibility, as occurred this morning, but
this fog was mainly patchy and never became dense.
Probabilities for visibilities less than 1 mile are around
30-40% in isolated locations of northern into east central Iowa
from the HREF, with NBM members lower, 10-20%, in roughly the
same location. Meanwhile, other hi-res guidance is much more
robust with fog development further west. The location and
thickness of the cloud cover with the lingering stratus and
additional incoming cloud cover will be the ultimate determinant
of fog development so like yesterday, will let next shift
continue to monitor trends. Temperatures tonight will be warmer
underneath the cloud cover and cooler in the clear skies, but
still generally near or in the 20s. It should be noted that any
fog development could create some slick spots given these
temperatures.

Mid-level flow becomes out of the west to southwest through the day
Saturday behind the departing high pressure warming temperatures to
the 40s to low/mid 50s. Dry conditions continue Saturday with partly
sunny skies, though cloud cover is quick to move in again ahead of
an incoming wave for Sunday.

The forecast for Sunday into Monday has become a bit more murky in
the last 24 hours. The aforementioned wave does have more
saturation with it at times, though models are highly variable
on timing. The NAM is about 12-24 hours earlier than other
models bringing rain with the system later Sunday into early
Monday. Meanwhile, the GFS is much slower to saturate and
waiting on the northern stream which moves in on Monday bringing
some light snow or a rain snow mix with it early Monday
morning. The Euro favors the two pronged southern stream and
northern stream waves of the GFS, saturates both, but at
different times and with different results, the one that would
impact northern Iowa looks to loose its ice introduction and
could end up more drizzle-like depending on its phasing with the
forcing. All this to say, there are multiple different outcomes
that could happen to end the weekend and start the new week.
The combination of the cloud cover and potential precipitation
could spoil the extent of our warmth Sunday with 25-75th
temperature ranges actually decreasing in the last several runs
indicating we may not get quite as warm, which is further seen
in the decreasing H850 temperatures from 24 hours ago around
peak heating Sunday.

Behind the overarching Sunday system, temperatures cool
significantly into Monday as highs fall back into/near the 30s to
start the week and then falling further with highs in the 20s to end
the week. Confidence in the temperature forecast is fairly high with
a 75-85% likeliness of below normal temperatures as seen in the
latest 6 to 10 Day outlook from the Climate Prediction Center,
covering the period of Thursday November 28 to Monday December 2.
For reference, "normal" highs are in the mid 30s to low 40s for late
November into early December. All this to say, use the warmer
temperatures this weekend to finish any fall outdoor projects!

The precipitation forecast for the upcoming week remains highly
variable as multiple waves move through the overarching flow.
Details remain limited at this time range with high model
variability between deterministic and ensemble solutions. Will
continue to monitor trends and provide additional updates in the
coming days.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 623 PM CST Fri Nov 22 2024

Fog potential, timing, and coverage is the primary
concern for terminals overnight. At issuance time, sharp west
edge of 700-1100ft AGL stratus extends from just east of MCW
south to near TNU to just west of OTM. VFR and high clouds were
evident west of this line. Latest model output suggests moves
gradually east through the period, resulting in high-confidence
that stratus persists east. The fog potential & uncertainty
arises just to the west of this sharp stratus back-edge where
lessening winds beneath the surface high pressure will combine
with dew point depressions that were already in the 3-8 degree
range at issuance time. So confidence is high in patchy fog
formation, however areal extent and timing remain uncertain. As
of now, HREF depict highest probs from MCW to DSM to OTM, so
have added mention into those terminals for this issuance. ALO
should hold into the stratus deck the longest which should
mitigate widespread fog potential there. FOD should be far
enough west & more influenced by drier air. Will continue to
monitor trends closely. Expect conditions to VFR everywhere by
late tomorrow morning and thru the rest of the period.

&&

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

&&

$$

DISCUSSION...KCM
AVIATION...Hahn