Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 091029

Area Forecast Discussion
National Weather Service Sioux Falls SD
429 AM CST Tue Jan 9 2018

.SHORT TERM...(Today and Tonight)
Issued at 428 AM CST Tue Jan 9 2018

Another mild day today, and despite some high cloud cover, should be
able to warm as much or perhaps slightly more than yesterday given
increased southerly flow to aid mixing of the lower levels. Favored
warmer end of guidance, which is generally around 40 northeast to
near 50 in Gregory County. Could see some very patchy fog again this
morning, but with the cloud cover and light wind flow, do not expect
to see the low visibility such as occurred in northwest Iowa Monday

Southerly flow continues tonight ahead of approaching cold front,
drawing additional low level moisture into the area on top of any
snowmelt moisture produced during the day. Consensus, largely NAM
dominated, has model visibility dropping quite low later tonight
into Wednesday morning. Confidence in fog development over just a
stratus deck is not high, but opted to include a mention of patchy
fog for now, given potential for road impacts from frosting with
temperatures near freezing.

.LONG TERM...(Wednesday through Monday)
Issued at 428 AM CST Tue Jan 9 2018

Focus for the mid-long range portion of the forecast will be the
return of winter, and an impactful storm system still on track to
affect the region late Wednesday into Thursday.

Will be monitoring two main players bringing the threat of moderate
to potentially heavy snow. First will be a southern stream trough
near the southern California coast today, which then tracks across
the southern Rockies and lifts northeast through the Central Plains
Wednesday and Wednesday night. This wave will focus strong lift
along the lower level front beginning Wednesday evening, with 850mb
frontogenesis, topped by strong lift within the dendritic growth
zone aloft, along/east of the Highway 60 corridor from 11/00Z
through 12Z when approaching northern stream wave kicks the boundary
southeast. Main question at this time is how quickly lower levels
will cool sufficiently for snow vs. rain or possibly mixed precip.
Model consensus at this time keeps low levels above freezing well
into the evening, so initial 0.10"+ of QPF in this area would fall
as liquid and not contribute to the snow total. If cooling develops
earlier, this could boost snow totals from the current 3-6" we have
across that area.

Second player will be a northern stream wave crossing the northern
Rockies on Wednesday, then pushing east across the Northern Plains
Wednesday night. This appears to be a little slower than at this
time yesterday, which could leave a gap between the northwest Iowa
rain/snow band and a mid-level based snow band on the leading edge
of the wave across our western CWA Wednesday evening. In fact, 00Z
runs of the GFS/NAM/ECMWF showed decent agreement in maintaining
relatively dry air within the ice-bearing layer aloft between the
two snow bands Wednesday evening. This could result in only very
light snow, or possibly freezing drizzle, through the center of the
CWA in an axis from KYKN-KFSD-KMML. Despite the consensus among this
model set, the placement, or even existence of this lack of ice
introduction has been less consistent from run to run. Thus after
coordination with surrounding offices, opted to limit mention of any
freezing precip potential to just a narrow transition zone as rain
changes to snow. However, it`s worth noting that the potential for
more widespread freezing drizzle may need to be added in later
forecasts. Even if freezing drizzle is minimal, there is a decent
chance that most areas will see moist pavement conditions before the
cold air moves in, either due to drizzle or just sweating pavement
earlier in the day. So we could still be dealing with icing on
untreated surfaces as temperatures will quickly fall below freezing
Wednesday night.

As far as snowfall amounts across our western areas, strong 700mb
frontogenesis will help drive the band which moves into our western
counties Wednesday evening. This mid-level front should translate
eastward into southwest Minnesota/northwest Iowa by daybreak
Thursday, and progression of this band should limit potential snow
amounts west of Highway 60. For now, looking at broad 2-4" amounts,
but again these could vary depending on how quickly the band moves
east and the availability of moisture aloft to produce snow in the
first place.

In addition to the snow, we see a strong pressure gradient develop
behind the cold front Wednesday night into Thursday, which of course
will further complicate travel across the region. Will continue to
carry a mention of areas to widespread blowing snow until winds
begin to diminish late in the day Thursday.

The added impact of the strong winds is also the driving force
behind issuance of a Winter Storm Watch for the entire CWA this
morning. Despite the lower snow totals across our west, felt the
combination of drier snow and strong winds could create hazardous
travel conditions which may warrant a high-end advisory or low end
warning for snow and blowing snow. Strongest winds, possibly topping
40 mph at times, are expected across southwest Minnesota, southward
through the Iowa Great Lakes and Storm Lake area later Wednesday
night through much of the day Thursday. After coordination with
offices in the Twin Cities/Des Moines, opted to include a mention of
possible blizzard conditions in the watch for these areas.

Beyond Thursday, models still indicating another northern stream
wave moving through the region on Friday, bringing another fast
moving area of light snow. The cold air will stick around for a
while again, with sub-zero highs for parts of the area on Saturday,
and well below normal temperatures through the end of this forecast


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1043 PM CST Mon Jan 8 2018

Although patchy fog is still possible, high level clouds continue
to stream across the region. This along with light winds should
keep widespread or dense fog out of the area. Chances for fog
in TAFs have been lowered accordingly. Light winds overnight will
switch to southerly Tuesday afternoon. No precipitation is
expected over the next 24 hours.


SD...Winter Storm Watch from Wednesday evening through Thursday
     morning for SDZ038>040-050-052>071.

MN...Winter Storm Watch from late Wednesday night through Thursday
     afternoon for MNZ071-072-080-081-089-090-097-098.

IA...Winter Storm Watch from Wednesday evening through Thursday
     morning for IAZ001-012-020-031.

     Winter Storm Watch from late Wednesday night through Thursday
     afternoon for IAZ002-003-013-014-021-022-032.

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



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