Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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869
FXUS63 KFSD 081727
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1127 AM CST Thu Dec 8 2016

.SHORT TERM...(Today and Tonight)
Issued at 327 AM CST Thu Dec 8 2016

Clouds continue to stream down in northwest flow across the
northeast half of the forecast area including southwest Minnesota
into portions of northwest Iowa and southeast South Dakota. Very
efficient atmosphere with dendritic layer from the surface to 700 mb.
Moisture is most plentiful across the northern edge of the forecast
area, and clouds in the 2000-3500 ft layer are readily producing
light snow. Have ramped up pops across northeast half of the
forecast area through the morning, but lift in the low levels really
dies away by mid-day as upper low over the Great Lakes lifts
northeast. As this occurs, the atmosphere also dries out and expect
clouds to gradually decrease as high pressure builds into the
region.

This will set the stage for a cold night tonight with clearly skies
and light winds. Have lowered lows tonight closer to guidance.
Despite the lack of snow cover, lows in the past have still been
able to fall to near -10 below with pristine radiative conditions.
Given great radiative conditions will exist west of I-29 tonight,
have lowered lows towards the cooler side of guidance.

.LONG TERM...(Friday through Wednesday)
Issued at 327 AM CST Thu Dec 8 2016

The focus in the medium and extended portions of the forecast
remains on the snow chances this weekend, and potential for brutally
cold temperatures and wind chills towards the middle of next week.
The difficulties in this forecast remain on the progressive nature
of the flow pattern through the weekend, and the resultant impacts
on track and location of precipitation.

Friday-Friday Night: Guidance continues to show a narrow zone of
isentropic lift moving through the area, especially during the
afternoon hours. Despite the absence of QPF in several of the
models, inspection of soundings continues to show that light snow or
flurries should be possible as this initial corridor of lift moves
through. Would not be surprised to see a dusting or couple tenths of
snow.

Saturday-Saturday night: Models have converged towards a new soln,
bringing an initial PV anomaly out of the Pacific NW on Saturday,
with a second progressive wave nipping at it`s heels for Sunday into
Monday. Current model preference falls with the GFS/EC/GEM
alignment, with the NAM currently a northern outlier with the wave
and 850 baroclinic zone early on Saturday. The resultant soln brings
a corridor of impressive warm advection snow to the area quickly
Saturday morning. Cross sections through the QPF axis show potential
for increased banding structures focused at or around 800 mb.  This
lift slowly begins to wane by the late afternoon hours as the PV
anomaly moves into Minnesota.  Regarding QPF, feel the low
resolution ECMWF may not be catching the potential intensity of the
lift with this wave, and with PWAT values up around the 75-90
percentile, have falling more in line with WPC/GFS QPF values. SLR
will likely be on the higher side given the low DGZ, and could
average from 14:18-1 depending on location. The end result could be
a narrow corridor of 3-6" of snow across the area by Saturday
evening. Again, the exact track of this wave is still a bit up in
the air, and based on ensemble guidance and probability plumes from
the SREF and GEFS, a 50 mile shift north or south or even
slightly higher localized snow amounts are still quite possible!

Sunday: As mentioned earlier, a secondary wave is expected to follow
closely in behind Saturday`s system. GFS/EC/GEM all deeper with this
second wave, but what`s left to be determined is how much of a stage
setter the first wave will be in regards to the thermal field it
leaves behind. The GFS shoves this wave through central and southern
Nebraska keeping the highest risk of modest snow accumulations south
of the CWA. The EC/GEM are all 100 miles further north, and would
produce a much higher impact to those along and south of I-90
(potentially inches of snow accumulation) into Sunday night. The
GEFS members are split, but there is more of a preference towards
the further north solns of the EC/GEM, which is where the forecast
has trended to attm.

Monday-Thursday: Our attention then turns towards another arctic
blast of cold air that will arrive Monday night into Tuesday. When
this front moves through, believe there will be an increased risk
for snow along the increasing thermal gradient aloft as the cold air
surges southward late Monday and Monday night.  In coordination
with surrounding offices, have boosted PoPs to advertise this risk.

From Tuesday onward, we`re looking at the coldest airmass we`ve seen
this winter. It`s very likely to have several days of single digit
high temperatures, with overnight lows well below zero in some
locations given the impacts of fresh snow. Have again trended the
blended guidance lower, but even now, overnight lows could be even
colder.  I wouldn`t rule out double digit below zero lows in some
sheltered locations next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1127 AM CST Thu Dec 8 2016

Lingering patches of MVFR stratus producing a few flurries
near/east of the Buffalo Ridge into tonight. Moisture layer
gradually diminishes through the late afternoon and evening, and
thus the potential for flurries should diminish, but patchy MVFR
ceilings will remain possible through US Highway 71 corridor from
KMWM-KSLB into tonight. Low-mid level moisture will return from
the southwest after 12Z Friday, but sub-VFR conditions will remain
southwest of TAF locations through the end of this period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...BT
LONG TERM...Dux
AVIATION...JH



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