Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241741
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1141 AM CST Tue Jan 24 2017

.SHORT TERM...(Today through Wednesday afternoon)
Issued at 446 AM CST Tue Jan 24 2017

Developing winter storm remains the focus through the next 24-36
hours. As of 09Z, the surface low was located in far western Kansas,
with a dry surface ridge just northeast of the Minnesota River,
toward the I-94 corridor. Easterly flow between the two features
reinforcing a dry layer aloft, which will likely delay onset of
precipitation across the northeast half of the forecast area until
midday to mid-afternoon. Numerious high-resolution models showing
very good agreement on evolution of this system throughout the day,
lending confidence in potential for a heavier snowband developing,
likely south of I-90, with a much sharper northern gradient to the
snow accumulations than coarser models have been able to depict up
to this point. All of this has led to some adjustment to existing
Winter Storm headlines, which will be discussed further below.

System beginning to have the makings of a frontal banding event, and
seeing the first effects of this across northern Nebraska early this
morning, with narrow band of precipitation lifting north on regional
radar. This aligns well with the 700-750mb frontal surface, and
anticipate this will be focus through at least this evening as the
strengthening mid-upper low tracks east from southeast Wyoming
across Nebraska. While strength of the mid level frontogenesis
varies, largely due to model resolution, placement appears to be
coming into line this morning. Leading frontal surface appears to
weaken as it lifts north-northeast into the drier airmass. This
should be able to produce some light snow into south central South
Dakota this morning, but anticipate little development east of the
James River initially, due to the aforementioned dry air. Models
then have a secondary area of frontogenesis rapidly strengthen
eastward along/just south of the Missouri River and the US Highway
20 corridor through Sioux City by 18Z. Expect this will become the
primary focus for development midday through the afternoon as it
lifts northeast into north central Iowa by 00Z. Snowfall rates in
the heaviest band could easily reach 1"/hour or more, and right now
it looks like axis of this band will stretch from the SD/NE border
area around Wagner/Tyndall/Yankton, east-northeast toward the Iowa
Great Lakes. However, models are still showing minor fluctuations,
so further adjustments to that are likely through the day.

Models showing a second area of frontogenesis north of this into
southern Minnesota by 00Z as well, but wonder if this is a
reflection of evaporative cooling effects on temperatures as the
lift works northward into the drier air mass, and thus any
enhancement to lift will work more on saturation than productive
snowfall. As a result of the dry air and southward shift of the
frontogenetical forcing, have tightened up the north-south snowfall
gradient, and lowered amounts in through the Highway 14 corridor to
under 6 inches through the event. Some of the high-res models
suggest Brookings-Marshall may only see an inch or two total, but
with some lingering uncertainty, will hold with 2-4 inch amounts for
these areas. Interstate 90 corridor amounts didn`t change much from
our previous forecast, generally in the 8 to 11 inch range from
Chamberlain-Sioux Falls-Worthington. Far eastern portions of I-90
toward Jackson may see more prolonged snowfall as the system pivots
northward later Tuesday night/Wednesday, so Jackson may nudge closer
to a foot. Areas south of I-90 through Sioux City/Storm Lake have a
significant adjustment as areas to the north, just in the opposite
direction. QPF forecasts from high-res models point to potential for
localized pockets of well over an inch liquid equivalent, and again
the favored area for this right now looks to be from that Missouri
River bend area toward Spencer/Iowa Great Lakes. Currently have
amounts in the 12-14" range through this region, but would not be
surprised to see some local amounts closer to 18".

On top of the snow, strengthening system will bring increasing
winds, especially later this afternoon through tonight, and into
Wednesday afternoon across our eastern areas. Do not anticipate
winds to reach blizzard levels, and much of the snow should be on
the wetter/heavier side, which should limit full-fledged blowing
snow after it`s on the ground. However, with the amount of snow
anticipated, will carry mention of blowing snow through the day
Wednesday, and even into Wednesday evening in the far east.

As far as headlines, primary change was to drop our northern tier
from Huron-Brookings-Marshall down to a Winter Weather Advisory due
to the lesser snow accumulations. Elsewhere we realigned some of the
warning segments to delay the start time, especially for northeast
portions of the area where snow is not expected to begin in earnest
until this afternoon. Also extended the end time for most segments
by 6 hours due to slower progression, and potential for some
lingering blowing snow through the day Wednesday.

.LONG TERM...(Wednesday evening through Monday)
Issued at 446 AM CST Tue Jan 24 2017

As storm system exits the area Wednesday evening, the forecast area
will be set up for dry conditions through the end of the week with
near normal high temperatures and normal to above normal lows as we
sit in northwesterly flow between the low to the east and the high
to the west. A series of weak short waves will then rotate around
Great Lakes low and drop through the region later this weekend into
early next week, bringing a slight chance of light snow every 24 to
36 hours from Saturday evening through Tuesday. No way to really pin
down any exact timing or location of snow with these clippers this
far out, so will stick pretty much with model blends for including
slight chances. Temperatures should still average above normal for
the later periods with highs in the 20s to mid 30s, and lows
generally in the teens.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1140 AM CST Tue Jan 24 2017

Snow system will continue to impact the area through the TAF
period. Could see some mesoscale banding, especially at the KSUX
and KFSD taf sites that reduce visibilty to 1/4 or less through
06Z/Wednesday morning. Thereafter, looking at more steady snowfall
rates, but increasing northerly winds will create periods of
blowing snow. Snow will be relatively wet and heavy, so may not
see a lot of lofting once it reaches the surface.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning until noon CST Wednesday for SDZ050-052>071.

     Winter Weather Advisory until noon CST Wednesday for SDZ038>040.

MN...Winter Storm Warning until noon CST Wednesday for MNZ080-089-097-
     098.

     Winter Weather Advisory until noon CST Wednesday for MNZ071-072.

     Winter Storm Warning until 6 PM CST Wednesday for MNZ081-090.

IA...Winter Storm Warning until noon CST Wednesday for IAZ001-012-020-
     031.

     Winter Storm Warning until 6 PM CST Wednesday for IAZ002-003-013-
     014-021-022-032.

NE...Winter Storm Warning until noon CST Wednesday for NEZ013-014.

&&

$$

SHORT TERM...JH
LONG TERM...MG
AVIATION...BT


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