277
FXUS63 KDMX 242340
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
540 PM CST Sun Nov 24 2024

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

.KEY MESSAGES...

- Increasing clouds with drizzle/fog north tonight.

- Temperatures fall into Monday with highs in the 30s to 40s and
  increasing winds out of the northwest 15-25 mph, gusts to
  30-35 mph making for a brisk and chilly day.

- Temperatures in the 30s-40s through mid week with even colder
  temperatures (80+% confidence) arriving to end the week (highs
  in the teens to 20s Sat-Sun).

- Precipitation chances have broadened on Wednesday due to
  location uncertainty. Highest confidence remains in southern
  Iowa. Rain and snow both in play. Continue to monitor the
  forecast.

&&

.DISCUSSION...
Issued at 258 PM CST Sun Nov 24 2024

Today has generally progressed as expected with temperatures warming
nicely across the area as the thermal gradient moves into and
through the area ahead of the approaching shortwave and associated
cold front. As of early afternoon, temperatures have climbed into
the 40s north, where cloud cover is more widespread, to low 60s in
several locations south, where cloud cover is more variable. This
will be the last pleasant day temperature-wise for the foreseeable
future.

Besides the cloud cover continuing to increase through the rest of
the day, precipitation chances return to parts of the area tonight.
As noted in the previous discussion, confidence is much higher than
this time yesterday with low level saturation occurring in the
lowest 4-6kft coinciding with the lift/forcing moving across the
area tonight. The better moisture, lift, and isentropic ascent
are all to the north and east where QPF values are much higher.
However, there is much more agreement amongst deterministic and
ensemble models that portions of the area, especially north or
east, will see at least a trace of precipitation and around a
30-40% chance for measurable precipitation, 0.01" or more,
along the Iowa/Minnesota border in northern Iowa towards
northeast to east central Iowa (locations like Waterloo). This
precipitation would be in the form of drizzle as this low-level
moisture sets up below a mid-level dry layer with the most
likely location in northern to northeastern Iowa, as noted
previously, though it is important to note although less likely
(~10-15% chance), it is not impossible for this drizzle to occur
further south into greater parts of central Iowa, if low level
saturation can occur before the lift departs. Will have to watch
trends closely tonight for the loss of saturation/lower level
lift with temperatures cooling (more on that in a moment), but
generally much of the drizzle chances are between 6pm and 6am
(starting in the north/northwest, ending in the east/northeast)
and should end in areas prior to temperatures dipping below
freezing. However, impacts should generally be mitigated if it
became close with road temperatures this afternoon in the 40s
to 60s F. In saying this, there is a trend in some of the latest
hi-res model runs, notably the RAP, but increasingly so the
HRRR, for saturation to occur through the ice introduction layer
before lift/saturation fully depart, with even a push of extra
moisture through far northern Iowa around the 09-13Z time frame
(3a to 7a CST). If all of these things were to line up, this
would result in flurries to light snow early Monday morning,
right before the morning commute towards the IA/MN border.
Although a low probability scenario, there is around a 10%
chance for trace to light measurable snow (0.1") along the
IA/MN border, again, if everything could come together. Road
temperatures should overarchingly mitigate once again should any
snow occur, but this would also be as/after temperatures have
fallen below freezing allowing for some potential slick spots on
elevated surfaces like bridges (where road temperatures will
drop more quickly) as well as when winds are increasing leading
to some snow (again, if it occurs) blowing around. Confidence is
higher in the drizzle scenario, as noted previously, so
forecast overarchingly reflects that thinking, but a few snow
mentions were included along the IA/MN border in this
afternoon`s forecast with run to run consistency remaining in
the hi-res models from this morning to this afternoon. Other
feature of the night would be the possibility for fog
development with low cloud bases and the aforementioned low-
level moisture. This could be an instead of to even an addition
to the drizzle depending on location, with localized visibility
reductions possible. Generally any fog development looks limited
and transient but probabilities are 10-20% in portions of
northern Iowa for locally dense fog with visibilities less than
0.5 miles.

As the aforementioned cold front moves through the area
northwest to southeast tonight into Monday morning, much cooler
air will filter in behind dropping high temperatures Monday
around 10 to 20 degrees from today. It will feel even colder as
winds shift to be out of the northwest and increase through the
night remaining breezy through the day Monday sustained around
15-25 mph and gusts up to around 30- 35 mph. The colder
temperatures are here to stay through the next week with highs
in the 30s to 40s through midweek before another colder punch of
air arrives for Thanksgiving and beyond dropping high
temperatures into the 20s to 30s with low temperatures in the
single digits to teens! Find those winter coats if you haven`t
already as big changes are on the way Monday and even more so to
end the week!

As for the mid-week precipitation chances, low run to run continuity
is keeping confidence lower in overarching precipitation chances and
where. The Euro remains wetter and more robust while the GFS remains
weaker and drier. Ensemble trends have backed off considerably in
the Euro but overarchingly timing is trending towards during the day
Wednesday and less so continuing into Thanksgiving. Highest chances
remain across portions of southern into southeast Iowa (30-40%) with
precipitation generally rain or a rain/snow mix, with lower chances
(around 25%) further north for precipitation which would come mainly
in the form of snow. It`s still a bit too early to have much
confidence in snow totals or exact precipitation amounts, especially
given the inconsistency in the models.

After this mid week wave moves through the otherwise zonal upper
flow, troughing will develop to the east putting us back in
northwest flow with another system arriving around the Saturday time
frame bringing another chance (around 20%) for snow given the
expected temperatures. Continue to monitor the forecast for updates
as confidence increases for both the mid-week and late week
systems.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 540 PM CST Sun Nov 24 2024

MVFR stratus will expand across all sites through the evening
and overnight with isolated pockets of IFR ceilings possible
mainly in northern Iowa. Light drizzle is also expected in
northern Iowa this evening, ending early Monday morning. There
is less confidence in drizzle reaching into central Iowa so for
now have kept from KDSM but will monitor closely for
adjustments. Patchy fog will also be possible with the drizzle
overnight in northern Iowa, however confidence is low in this
development so have kept mentions out of TAFs for now. By Monday
morning northwest winds will increase with gusts around 25 kts
anticipated much of the day.

&&

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

&&

$$

DISCUSSION...KCM
AVIATION...Hagenhoff