Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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341
FXUS63 KFSD 222053
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
253 PM CST Sun Jan 22 2017

.SHORT TERM...(This evening through Monday)
Issued at 248 PM CST Sun Jan 22 2017

Difficult sky forecast tonight with deterministic models not
handling current situation all that well. CAM models a little bit
better, with stratus working back into the region from the north. As
surface ridge builds into the area, expect winds to go light.  With
low lying stratus across the region, could see ceilings build toward
the ground during the overnight hours. Initially thought the favored
area for fog would be along and east of I-29 as fairly think
stratus builds south and east. Already seeing visibilities in this
region falling as low as a mile or so, and likely won`t see any
improvements in visibility as sun sets.  Other area to watch will be
across south central south Dakota, where winds go light and variable
tonight.  Have raised lows tonight with the expectation of low
clouds across the area through much of the night.

With surface ridge across the area through at least the morning
hours on Monday, will be a struggle to shake the stratus across the
region.  Again, have gone higher than deterministic models would
suggest for cloud cover.  Will see increasing southeasterly flow as
next wave approaches from the west. With stratus lingering, should
be a struggle to warm appreciable throughout the day. With
relatively dry air below 700 mb, expect precipitation to hold off
until Monday evening across central South Dakota.


.LONG TERM...(Monday night through Sunday)
Issued at 248 PM CST Sun Jan 22 2017

The only story in the medium and extended portions of the forecast
is with the increasing potential for a winter storm Tuesday into
Wednesday.

Model Trends:  Generally slight deviations today in guidance.
GEM/NAM remain the furthest south with the 850 low track, likely
under developing upper system as it enters the Plains.  GFS remains
the northern most track, but has trended southward slightly
following it`s ensemble members which have shown some potential for
a continued southern shift.  The ECMWF has remained very consistent
in the past days, owing more confidence to it`s stronger more
dynamic solution with jet interactions. Forecast in general was
trended highly towards WPC QPF/Ratios to which Superblend numbers
seems too broad and unfocused due to the spread in model
solutions. It must be noted that there is still considerable
uncertainty that will hopefully be resolved further by late
Monday.

General Forecast: Initial lift should be rather light Monday evening
as warm advection begins in advance of the upper low. With the
initial PV anomoly rotating northeast and likelihood of increased
left exit jet dynamics aloft, snow should blossom Tuesday morning.
There are also minor signs of elevated instability and increased
frontogenesis pointing towards increased banding potential as we
move into the afternoon hours. As the 850 low tracks from Omaha to
Des Moines into the evening, the individual wobbles of the
developing upper low will continue to develop lingering snow well
into the night, especially near the I-90 corridor.

Drier air will begin to advect into this system Wednesday morning,
but snow showers or light snow may continue into the mid-day hours.
Another concern with this system will be winds.  Models continue to
point to increasing SPG as the sfc low initially develops over NW
Missouri. However, the channel of winds around the backside of this
system has tended downward slightly since Saturday.  Nevertheless,
sustained 20 to 30 mph winds look to cause significant visibility
issues as snow falls into Wednesday morning.

Early QPF/Snow Forecast: Probabilities through ensembles have really
pushed likelihood of 0.50" of QPF higher since yesterday, with a
southern shift as well. Snow ratios during this event will be
very tricky. Temperatures warming into the lower 30s may limit
snow accumulations in the southern CWA before heavier snow arrives
in the morning. Given slightly more southern solutions, have
bumped up SLR a bit overall, but not nearly as high as WPC
numbers. The end result in snow totals is a widespread area of 4
to 8" of snow, still a third lower than WPC values which is
something to monitor.

Product Highlights:  Given increasing forecast numbers, and
potential impacts of snow and winds, will go ahead an issue a large
winter storm watch. There is still considerable uncertainty in the
track of the system, but confidence is high enough in the impacts to
issue a watch into Wednesday morning. The actual highest impact area
can be refined as the forecast is refined in the next 48 hours.

Rest of the Week:  New snow, plus northerly flow will hold
temperatures near to below normal into next weekend. No major
systems anticipated into next weekend.


&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1128 AM CST Sun Jan 22 2017

Another difficult aviation day today especially along and east of
the James River Valley. Secondary push of stratus surging
southward across the area. have seen some 3-5 sm visibilities in
fog with latest push of the stratus, but expect a little gradual
improvement through the afternoon. Under weak flow, could see fog
try to redevelop tonight, mainly east of I-29. Best threat for
this will be across northwest Iowa, which may impact KSUX TAF.
Will need to keep a close eye on this through the overnight.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from Tuesday morning through Wednesday
     morning for SDZ040-055-056-061-062-066-067-070-071.

     Winter Storm Watch from late Monday night through late Tuesday
     night for SDZ038-039-050-052>054-057>060-063>065-068-069.

MN...Winter Storm Watch from Tuesday morning through Wednesday
     morning for MNZ071-072-080-081-089-090-097-098.

IA...Winter Storm Watch from Tuesday morning through Wednesday
     morning for IAZ001>003-012>014-020>022-031-032.

NE...Winter Storm Watch from Tuesday morning through Wednesday
     morning for NEZ013-014.

&&

$$

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



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