794 
FXUS63 KFSD 020449 AAA
AFDFSD

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Sioux Falls SD
1049 PM CST Fri Jan 1 2021

.UPDATE...
Issued at 1042 PM CST Fri Jan 1 2021

Expanded Dense Fog Advisory further south and west, now
encompassing much of the region. While fog is not as widespread in
parts of southeast SD and areas south of I-90, webcams show
pockets of dense fog in lower coverage areas and model guidance 
heavily favors further expansion of dense fog overnight.

UPDATE Issued at 607 PM CST Fri Jan 1 2021

The latest satellite imagery shows an area of very low stratus and
dense fog expanding southeast into areas north of I-90, from
northeast SD and western MN. As winds go light and a strong low 
level inversion sets up, most model soundings are in agreement 
that this expansion will continue overnight with fog lingering 
until at least mid morning Saturday. Webcams within the area of
fog indicate very low visibility already occurring along parts of
the Hwy 14 corridor. Issued a Dense Fog Advisory for areas that 
will be affected by more widespread dense fog early this evening 
and will continue to monitor for further expansion.

&&

.SHORT TERM...(This Evening through Saturday)
Issued at 235 PM CST Fri Jan 1 2021

The biggest forecast challenge in the short term will be the 
potential for fog and stratus.

Per satellite, a wide area of stratus presently exists through 
eastern ND and northeastern SD. This stratus has been drifting 
closer and closer to our area through the afternoon on a 
north/northwesterly low level flow, and models suggest these low 
clouds overspreading portions of our CWA during the late afternoon 
and evening hours. They will then linger through the overnight 
hours, and with light winds, increasing near surface moisture, and a 
strong inversion, fog development is likely. Based on observations 
to the north, some of this fog will most likely be dense, especially 
along the edge of the stratus deck.  With low level warm air 
advection and fairly widespread cloud cover tonight, lows will be a 
touch milder than last night - in the low to mid teens.

The question for Saturday is how quickly stratus will move out of 
the area - if at all. While model RH time sections indicate it 
moving out, soundings point toward an inversion holding across the 
area until at least Sunday. If clouds hold through the day, 
temperatures may be a little cooler than currently forecasted - in 
the upper 20s to mid 30s.

.LONG TERM...(Saturday Night through Friday)
Issued at 235 PM CST Fri Jan 1 2021

Not much change for Sunday with highs in the upper 20s to mid 30s. 
An upper level shortwave/surface frontal boundary slides through the 
region on Sunday night into Monday, though will have little impact 
on the weather - as readings remain in the mid to upper 30s for 
Monday. Tuesday will be a repeat as a southerly flow develops across 
the region with surface ridging to our east and a trough to the west.

Models are still struggling with the midweek upper level pattern, 
though are trending more toward a southern stream dominant system 
for Wednesday into Thursday. This would keep more of the moisture 
through the Central/Southern Plains, though cannot completely rule 
out some light snow in our area on those days - time will tell. With 
no real cold air in sight, temperatures will remain at or above 
normal through the mid/end of next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1042 PM CST Fri Jan 1 2021

VFR conditions will erode to low IFR overnight as an area of 
dense fog and stratus continues to spread south into the region.
A difficult forecast as HON remains on the west edge of the
fog/stratus deck, but expect IFR conditions to prevail. Stratus 
and fog will reach FSD shortly, but models lean more toward MVFR
than IFR at SUX overnight.

Expect IFR conditions and fog to prevail for much of the region until
at least mid morning. Remained optimistic for Saturday's outlook,
but there is potential for IFR conditions to linger throughout 
Saturday, Saturday night, and early Sunday.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 10 AM CST Saturday for SDZ038>040-
     053>056-059>062-065>071.

MN...Dense Fog Advisory until 10 AM CST Saturday for MNZ071-072-080-
     081-089-090-097-098.

IA...Dense Fog Advisory until 10 AM CST Saturday for IAZ001>003-
     012>014-020>022-031-032.

NE...Dense Fog Advisory until 10 AM CST Saturday for NEZ013-014.

&&

$$

UPDATE...BP
SHORT TERM...JM
LONG TERM...JM
AVIATION...BP