749
FXUS63 KFSD 232335
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
535 PM CST Mon Dec 23 2024

.KEY MESSAGES...

- Above normal temperatures are expected through next weekend.

- Dense fog is anticipated this evening. Visibility to less than
  1 mile at times is possible. Patchy dense fog continues
  through Tuesday afternoon.

- Precipitation chances look to return directly after Christmas
  on Thursday night and Friday. High confidence (80-90% chance)
  that temperatures remain above freezing, keeping the
  precipitation type as rain.

- Travel impacts this week look to be minimal across the  Northern
  Plains.

&&

.DISCUSSION...
Issued at 243 PM CST Mon Dec 23 2024

Looking at GOES water vapor imagery, we can see the wave responsible
for today`s cloudy conditions situated over Minnesota. Lows for
tonight will be in the teens to 20s. Winds will become light and
variable this evening. Near surface moisture, the result of melting
snow and some moisture advection from the Great Lakes region, will
increase surface dewpoints to at or near saturation this evening.
Patchy dense fog will be the likely result. Model soundings support
this by indicating saturation from the surface up through 1000 to
1500 ft AGL beginning this evening and continuing for most of the
day Tuesday. Just above the mixed layer, a stout and persistent
inversion will keep a layer of warm air over top of this low
stratus/fog, holding it in place for most of the day Tuesday.
With this in mind, have issued a Dense Fog Advisory for portions
of east central South Dakota and southwestern Minnesota, as
well as for counties along the Missouri River in central and
southeastern South Dakota. With low temperatures below freezing,
be on the lookout for areas of freezing fog that could create
slick areas on roads. Clouds and fog will slowly burn off
Tuesday afternoon as an upper ridge and surface high pressure
begins to approach from the west. Highs for Tuesday will be
slightly limited due to the lingering clouds. But thanks to
continued WAA on light, southerly winds, we can still look
forward to highs above the average for December 24, in the 30s-
40s for southeastern South Dakota and northwestern Iowa.
Southwestern Minnesota will be cooler as the clouds will take
longer to burn off in the afternoon. Highs there will be in the
low 30s. Overnight lows for the region will be in the 20s. Areas
along and east of I-29 may experience fog redevelopment
overnight.

Wednesday, the aforementioned ridge and surface high will build
in. Winds will be south to southeasterly and increase slightly
for the afternoon, with gusts around 15-20 mph possible. Along
and west of the James River valley could see gusts up to 25 mph.
Looking at ESATs tables, both geopotential heights and
temperatures in multiple levels will be on the upper end of
climatology. With good mixing into the 925 mb level, we have the
potential for warm surface temperatures in the upper 30s to
40s. However, there is some model disagreement in cloud cover
and high temperatures. The GFS and CAN have us with partly to
mostly sunny conditions by afternoon, while the EC and NAM keep
thicker clouds over the region for most of the day. Ensemble
guidance is split on whether or not temperatures will break
freezing. Though most agree that the cut off for where
temperatures will be warmer are south of a line from Huron-Sioux
Falls-Spencer. If the more pessimistic models win out, north of
that line will struggle to warm above freezing, and south of
that line will remain in the 30s. The gradient between above
and below freezing temperatures also looks to be steep, with the
change happening over a short geographical distance. All this
to say confidence in afternoon highs is low. Therefore, have
opted to keep the NBM highs as they are, with highs in the mid
30s-40s. Lows will fall into the upper 20s.

Late Wednesday night into early Thursday morning the ridge begins to
break down as an upper trough closes off into an upper low over the
central and southern Rockies. This low is expected to eject out onto
the southern Plains Thursday afternoon/evening and accelerate
northeast. This brings our next best chance for widespread
precipitation beginning Thursday night into Friday morning. Enough
differences exist in model guidance on timing and track to keep
confidence low in coverage of precipitation. However, temperatures
continue to be in the 90th percentile for climatology in the mid to
upper levels, with surface temperatures remaining well above
freezing. Leading to the conclusion that the precipitation type will
most likely be rain that will continue through most of the day
Friday. Again, depending on which guidance you consider, there is
potential for rain to continue into Saturday as well. If that is the
case, during the overnight hours there is a possibility that
p-type will become a wintry mix. Ensemble guidance tends to keep
the majority of the precipitation along and east of I-29, with
an 80- 100% probability of measurable precipitation (0.01
inches). As you move west of I-29 probabilities drop off
quickly. Totals are expected to be on the light side, with
ensembles indicating between 0.1 and 0.2 inches of liquid. Highs
for Thursday will be in the upper 30s to 40s with lows in the
30s. Thursday night there is potential to tie or break some warm
low record temperatures. Highs for Friday will be in the 40s
and lows will be in the upper 20s.

Things dry out for the weekend with continued warm temperatures in
the 40s for Saturday and Sunday. The early part of next week brings
more potential for precipitation.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 528 PM CST Mon Dec 23 2024

Mostly MVFR conditions this evening will transition to mostly
IFR and lower tonight as areas of low level stratus and fog
become more widespread. While this could lead to vsbys of 1/2
miles or less at times, won`t see things really begin to tank
across the area till after 03z as the surface high moves
overhead. Otherwise, light and variable winds will continue
across the area through Tuesday morning before becoming light
southerly winds to end the TAF period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 9 AM CST Tuesday for SDZ039-040-055-
     056.
     Dense Fog Advisory from midnight tonight to 9 AM CST Tuesday
     for SDZ050-052-057>059-063>065-068>070.
MN...Dense Fog Advisory until 9 AM CST Tuesday for MNZ071-072-080-
     081-089-090-097-098.
IA...None.
NE...Dense Fog Advisory from midnight tonight to 9 AM CST Tuesday
     for NEZ013.

&&

$$

DISCUSSION...AJP
AVIATION...Gumbs