Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 272219

Area Forecast Discussion
National Weather Service Sioux Falls SD
419 PM CST Mon Feb 27 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 419 PM CST Mon Feb 27 2017

Tricky forecast for tonight into Tuesday, regarding precipitation
type and temperatures. Plenty of warm air advection working into the
region with southerly winds also tapping into increased low level
moisture. Expect clouds to continue to increase this evening, which
along with the southerly winds, will help hold temperatures above
freezing near and east of I-29. Along with the low level moisture
advection, expect areas of fog to also develop this evening and
overnight, mainly east of the James River. There`s a chance for some
locally dense fog across northwest Iowa late tonight into mid
morning Tuesday as the surface winds go light and remain from the
southeast in this region. Lows will range from the upper 20s to mid

Weak surface low and subtle shortwave tracks through the forecast
area tonight. While low level moisture remains, there will be a
significant dry layer aloft to really put a cap on precipitation
chances. Still, forecast soundings suggest that the low level
moisture remains borderline for drizzle or freezing drizzle chances.
The freezing drizzle potential is not expected to amount to enough
impact roadways and surfaces, so will not issue any headlines for
this event. Nearer to the highway 14 corridor the low level moisture
is deeper, so will keep the highest chances for precipitation will
be north of I-90. Areas to the west of I-29 will also have a more
favorable potential for some flurries or light snow in addition to
drizzle, with the low level moisture more likely to extend into the
dendritic growth zone. Still, overall chances for measurable
precipitation remain quite low. Expect the fog, drizzle, and light
snow to diminish in the afternoon. Despite clouds and overall dreary
conditions, highs should still manage to reach the upper 30s and
lower 40s.

.LONG TERM...(Tuesday night through Monday)
Issued at 419 PM CST Mon Feb 27 2017

Depth of saturation will continue to be an issue going into the
evening on Tuesday. Split jet structure suggests there could be two
areas of more coherent precipitation to start the evening, one
brushing up against the far southeast CWA associated with southern
stream energy, and mid-level frontal band settling into the northern
CWA. Between does not appear likely to see any mutual lift support,
and drying through the mid level will keep any moisture fairly
shallow.  Generally weak and somewhat shallow lift profiles would
keep drizzle in the equation, with temperatures gradually sagging
beneath freezing for differentiation between light drizzle and light
freezing drizzle.  Eventually, lift with approach of the PV
advection/mid-level front will allow for saturation into ice-bearing
depths, and should see precipitation transition over toward patches
of light snowfall later evening into the early overnight, but at
this point with little accumulation, and generally toward southwest

Lingering low level baroclinicity may be enough to interact with
terrain to see a low-end continuation of light snowfall threat
mainly through southwest Minnesota early Wednesday morning, but will
again see moisture profiles shallow out and lead to generally a
flurry/sprinkle threat.

A clipper-type wave will follow quick on the heels of this system on
Wednesday night and early Thursday. Great diversity in track of
wave, but much more support to a more southern track which could
spread some light snowfall again toward east central SD into
southwest MN.  The farther south the wave tracks, the more
intrusive the precipitation could be, and enough uncertainty to
take a low chance down to at least I-90, but would not have minded
seeing some more mid-range pops toward KMML/KMWM areas. Would not be
impossible for this to end up around an inch or so snowfall in
parts of SW MN if some of the marginal instability suggested by GFS
or displaced in the NAM occurs, and as long as the drying at lowest
levels does not overwhelm.  Fast-moving system will not stick around
long, and will become breezier and cooler with trend down toward
flurries through the day along southward sinking baroclinicity.
Should see some non-diurnal temps south of the surface wave track,
especially toward the Missouri and James valleys, and would be more
impressive in not for modification due to lingering snowcover.

Thursday night should be the coldest night on the average with
surface ridge slipping across Minnesota and Iowa, but a strong
return flow will develop later in the night across South Dakota.
Strong band of elevated warm advection will interact with wave
dropping southeast, and could be a close call to get a brief period
of early day snow going across southwest MN.  The main story through
for most of the remainder of the forecast will be the warmer
temperatures.  Friday will feature a strong south wind with clouds
thinning some through the day. Flat ridging through the weekend
likely to keep some degree of higher clouds in the mix, but also
will find mild temperatures continuing, with snow melt by the
weekend allowing return to at least some near 60 degree readings.

Very little agreement in what orientation of the larger scale flow
may be by Monday.  GFS ensemble is a real mess, and even the
operational run is a relative outlier to the warm side. ECMWF has
shifted somewhat that warmer direction at least through late Sunday,
but still suggestive of a strong wave deepening as it digs in
northwest flow toward the mid-Mississippi valley, contrary to the
southwest flow aloft in the 12z GFS.  For now, have not strayed off
the initial blend, but this would be one period to watch as the days
pass with potential for a little more significant of a system.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1136 AM CST Mon Feb 27 2017

VFR conditions expected through 00Z Tuesday. Thereafter, low level
moisture will increase resulting in developing IFR or lower
conditions. Areas of fog will likely develop, especially along and
east of I-29. Could also be a little bit of drizzle/freezing
drizzle north of I-90, but confidence remains too low to place in
the TAFs for now.




LONG TERM...Chapman
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