Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 132223
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
423 PM CST Sat Jan 13 2018

.SHORT TERM...(This evening through Sunday)
Issued at 421 PM CST Sat Jan 13 2018

Quick moving upper wave across eastern MT will move into the central
Plains tonight, giving a glancing blow of lift with briefly enhanced
mid-level warm advection. Dry air to the northeast will limit the
eastward progression of the organized band. Amounts toward south
central SD and into northeast Nebraska could reach around a half
inch. As clouds will increase from this feature, there is also some
lower cloudiness working northward through central and eastern
Nebraska which represents the moisture transport below the
inversion.  One thing to watch for late tonight and tomorrow morning
is the potential for very light precipitation from the low-level
moist layer given fairly persistent warm advection through lower
levels, as the mid levels dry out. Profiles are on the fence between
being cold enough for flurries or freezing drizzle, approximately -8
to -11C. Not convinced that the depth of moisture near surface will
truly be up to 5000 feet, but presence and expansion of clouds this
afternoon suggests the feature is indeed real. Signature of lift
forcing is weak, so kept the mention out.

Southerly winds and the increase in clouds will bring warming
tonight, with lows likely occurring during the evening while winds
are still fairly light. Likely will see a minor elevation wind
enhancement tonight across southwest MN and east central SD, but
inversion fairly broad and gradient not greatly strong.

A temporary and very misleading excursion toward mild conditions
will exist for a good chunk of Sunday, before a stronger wave begins
to dig into the Northern Plains.  This feature will drive an arctic
front southward toward the area by mid afternoon, sweeping into
northwest IA by early evening. The main PV advection will glance far
eastern SD, southwest MN and northwest IA during the afternoon and
early evening, suggesting the highest probability for precipitation.
Should be a fairly distinct area of precip near/east of I-29, and
more spotty to the west, until a bit more favored again with another
area of Div Q/frontogenesis slides toward south central SD. Snow
should be a fairly brief period of moderate snowfall at any
location, which should limit amounts to less than an inch.
The PV advection suggests that the snow could be somewhat ahead of
the main increase in winds which should hold off until evening, but
shift and increase in winds around the band could produce some
difficult conditions for a brief time.

Pre-frontal warming should allow mainly 20s for highs, and 30s
through the Missouri River valley.

.LONG TERM...(Sunday night through Saturday)
Issued at 421 PM CST Sat Jan 13 2018

Heading into Sunday evening, the boundary and associated organized
snow band should be near the Missouri Valley arcing into far
southern portions of northwest Iowa.  As cold advection increases
during the evening, coupling of 25 to 35 kt winds will increase,
supported by an isallobaric gradient of +5/-3 hPa/3h right down the
wind direction.  Thermodynamic profiles appear to develop a small
shallow CAPE Sunday night behind the frontal boundary, so would no
be surprised to see development of shallow snow shower or flurry
bands through the evening hours while temps are very much in the
dendritic range within clouds.

Probably the most certain aspect of the forecast will also be the
most dangerous - extreme cold.  By midnight Sunday, the wind and
falling temps will introduce the first -20 wind chills toward the
highway 14 corridor, and by Monday morning, -20 to -35 will be
common across the area. Little improvement expected through Monday,
and by Monday night, a more widespread area of -35 to -45 wind
chills may appear from east central SD through southwest MN toward
the IA Great Lakes area.  Winds will diminish during the day Tuesday
as core of arctic ridge settles over the area, but actual temps will
struggle in the single digits above and below zero.

Return flow developing Tuesday night into Wednesday, but should see
a quick drop in temps across northwest IA and adjacent areas before
rising later in the night. Could be a marginal wind chill advisory
again Tuesday night in the return flow, but nowhere as cold as
earlier in the week.  The rising temp trend will largely continue
right into Thursday, with increases winds limiting any fall Thursday
night. Mild weather looks to remain through the end of the week,
with many spots above freezing on Thursday, everywhere on Friday and
almost everywhere Saturday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1122 AM CST Sat Jan 13 2018

A large area of 2000 to 6000 ft ceilings currently extends from
western South Dakota into most of Nebraska and southern Iowa. As
the upper wave move southeast through the day, winds in this layer
will gradually shift to southwest. The combination of advection
of these clouds and lift associated with warm air advection will
move these clouds eastward into HON and SUX first and then into
FSD. At this point, have continued with MVFR ceilings for a
portion of late tonight and tomorrow morning given expected low
level lift near the inversion. But there are signs that model
output is overestimating low level moisture so will need to
monitor ceilings to see if forecast can be made more optimistic
in future. A strong arctic front with low ceilings, strong winds,
and snow will move across the region after 18Z Sunday and impact
all forecast locations.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...Chapman
AVIATION...Schumacher



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