087
FXUS63 KDMX 231741
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1141 AM CST Mon Dec 23 2024

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

.KEY MESSAGES...

- Above normal temperatures will persist for the next week,
   with highs most days in the mid 30s to upper 40s.

 - From today through Christmas the forecast is dry, but clouds
   moving in tonight will persist during much of that period
   making for a couple of gray days through the holiday. Areas
   of fog may also develop at times, especially tonight into
   Tuesday morning.

 - A generally more active weather pattern sets in later this
   week, with multiple chances for rain between around Thursday
   night and Saturday.

&&

.DISCUSSION...
Issued at 211 AM CST Mon Dec 23 2024

A mid-level shortwave is currently swinging over the eastern
Dakotas and will cross Minnesota and Wisconsin today. In
response, a surface trough currently extending from
northeastern Kansas across western Iowa and into southern
Minnesota will close off and push slowly through our service
area during the day. Temperatures remain relatively mild ahead
of this trough, currently in the mid to upper 30s across most of
our area, but will be moderated behind the front. Still, cold
air advection is notably weak and no strong winds or
precipitation are anticipated in associated with the passage of
the trough.

The indirect effects of the system moving through today will
come home to roost tonight through Tuesday, in the form of a
modestly cooler airmass and areas of stratus clouds that are
already covering much of northern Minnesota and the eastern
Dakotas. Once the trough moves through Iowa today, the upstream
surface ridge will build down into Iowa tonight and, in fact,
remain fairly entrenched through Christmas. Initially this
progression will draw the stratus to our north down into Iowa,
where it will then likely persist for several days given the
light flow within the ridge as well as the weak insolation at
this time of year. High-resolution model guidance illustrates
the inexorable advance of the stratus into Iowa tonight into
Tuesday, however, the devil is in the details as there are
several implications associated with this scenario:

1.) As the surface ridge spills in tonight, areas that are clear
ahead of the stratus will see ideal conditions for radiational
cooling and, given the low dewpoint depressions within the
post-trough airmass, some fog development is likely. However,
the location of any such fog is uncertain as it will be heavily
dependent on where the stratus is not. In terms of density of
fog, some of the models such as the HRRR and RAP indicate
potential for dense fog, however, probabilistic output from the
HREF Grand Ensemble shows very low probabilities (~20%) of
visibility below a half mile. With so many questions as to the
areal extent and magnitude of fog, have introduced it into the
forecast primarily in our western and southern counties later
tonight (where stratus will take the longest to reach) but have
not considered Dense Fog Advisory issuance at this time.

2.) Aside from fog potential, the temperature forecast tonight
is very tricky. As mentioned above, in clear areas outside/ahead
of the stratus tonight radiational cooling should send
temperatures plummeting, and have nudged forecast MinT downward
in our western and southwestern areas where stratus should take
the longest to reach, and which also overlap with the favored
cooling belt of the Nishnabotna valley. Meanwhile, in areas
already covered by stratus at sunset temperatures will be very
slow to fall overnight, and have trended the MinT forecast up in
our northeastern counties as a result. However, it is expected
that a pronounced temperature gradient could develop after
sunset tonight between the cloudy and clear areas.

The good news is that outside of the possibility of dense fog,
these various forecast uncertainties are exclusive of any
hazardous weather threat. As referred to above, the persistence
of the surface ridge across Iowa through Tuesday and Christmas
day should allow the stratus to remain entrenched, resulting in
a couple days of gray weather with little diurnal temperature
variation and have lowered forecast MaxT on Tuesday by several
degrees, and by less on Christmas.

The large-scale weather pattern is still expected to become more
active in the latter part of this week as a wave train at 500 mb
eventually carves out a large trough. The first wave will emerge
over the southern High Plains on Thursday, swinging across the
Midwest and near or over Iowa around Thursday night or Friday.
However, long-range models continue to have difficulty resolving
the sharpness and track of this wave, with the GFS remaining
somewhat stronger and further north, moving a closed 500 mb over
Iowa, while the EC remains more open with the wave focused more
over Missouri. Subsequent shortwaves rounding the developing
large-scale trough will then move over the central U.S. in rapid
succession. Each wave will provide forcing and support for
precipitation, but the timing and location of each round becomes
more uncertain moving into the future. What can be said with a
high level of confidence is that precipitation is expected
between Thursday night and Saturday, with the most likely (~60%)
period coming during the day Friday, and that the precipitation
will be mostly or all in the form of rain as temperatures are
warmer than normal and dewpoints remain above freezing during
this time. While there may be some minor travel impacts from wet
roads at times, as of now no winter precipitation or hazardous
weather is expected in this scenario.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1133 AM CST Mon Dec 23 2024

A large area of low stratus clouds will continue to move swd
into the area this afternoon, and persist for the foreseeable
future. High confidence on seeing MVFR to IFR ceilings based on
upstream trends and the latest guidance. There is lower
confidence on the potential for VSBY restrictions and fog, with
the guidance showing large spread in values. At this point, for
consistency, kept generally MVFR BR for now, but there is
potential for IFR or even LIFR tonight.

&&

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

&&

$$

DISCUSSION...Lee
AVIATION...Fowle