235
FXUS63 KDMX 232027
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
227 PM CST Mon Dec 23 2024

.KEY MESSAGES...

- Expect dry conditions through Christmas Eve, although it will
 be mostly cloudy with areas of fog. Drizzle is possible on
 Christmas Day, but temps will be above freezing.

- High confidence of above normal temperatures for at least the
 next 7 days with highs most days in the mid 30s to upper 40s,
 although a few 50s are possible by late this week.

- A more active period with several chances for rain, yes rain,
 from Thursday through the upcoming weekend.

&&

.DISCUSSION...
Issued at 218 PM CST Mon Dec 23 2024

This afternoon and through tonight...

The upper shortwave that passed through the area this morning is now
over the Great Lakes region. In its wake, broad upper ridging is
developing over the central CONUS, with strong troughing taking
shape off the west coast.   At the sfc, high pressure over the
Dakotas and MN will build swd into Iowa tonight.

Skies have been variably cloudy this afternoon, with mainly high
clouds over southern Iowa, a bonus area of mostly clear skies
thru central Iowa, with the next batch of stratus pushing swd
into nrn Iowa. The location of this stratus will be important
overnight, and will likely dictate if, or where, any fog
develops. A majority of the guidance has the stratus taking
residence over much of the area, which as mentioned previously
will hinder radiational cooling, and likely dense fog
development. However, areas that remain clear/mostly clear could
see fog quickly develop. The CAMs and various statistical
guidance are all over the map with regards to when and where,
so a low confidence forecast. Thus, felt the best course of
action right now was to remain consistent and simply mention
patchy fog areawide overnight and wait this out a bit. With
expected clouds, lows should only drop into the 20s.

Christmas Eve and Christmas Day...

Broad upper ridging and the corresponding "dirty" sfc high will
remain over the upper Midwest tomorrow. With light sfc winds and the
persistent low level temperature inversion, its possible/likely
the stratus deck will be locked in over all, or at least a
portion of the forecast area. Tis the season for this sort of
pattern. As a result, the temperature forecast will be
predicated on the extent of cloud cover /or fog/, or lack
thereof. Will continue on the cooler side of guidance with
readings in the 30s. The good news is conditions will be dry,
so no travel issues for anyone heading out to Grandmas house, or
anywhere else. On Christmas Eve night and Christmas Day, the
sfc high will slide off to the east, and allow for a return
southerly flow to develop. Model cross sections and soundings
show deeper low level moisture advecting nwd from the southern
Plains and arriving sometime on Christmas Day. Depth of the low
level saturation is likely to increase above 1 or 2km, which is
typically sufficient for areas of drizzle to develop. The good
news is that temperatures will also rise to above freezing, thus
any drizzle should remain the liquid variety. Another mostly
gray day appears on track for Christmas, with perhaps a few
breaks in the clouds in the far west. High temperature on
Christmas will remain above normal in the 30s to lower 40s.

Extended Forecast - Thursday into the Weekend

A broad upper trough will eject out of the west on Thursday, with a
series of shortwaves poised to kick northward across the Midwest.
Deeper moisture will continue to advect nwd out of the Gulf of
Mexico ahead of this system, accompanied by anomalously warm air in
the low levels, with H9/H8 temps from +3C to +5C. There will be
several periods of rainfall late week into the weekend, most notably
Thursday night into Friday, with another round likely on Saturday.
Rainfall amounts are expected to be moderate, with the various
ensembles showing high probabilities /70-90%/ of 0.10" to 0.25"
but generally less than a 20% chance of 0.50". As noted,
temperatures will remain well above normal, with highs well into
the 40s, and even a few 50s possible on Friday into Sunday. On
Thursday into Friday, almost all of the CONUS will have 850mb
temps at or above 0C - pretty unbelievable for late December!

&&

.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...Fowle
AVIATION...Fowle