Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 200006
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
606 PM CST Mon Feb 19 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 256 PM CST Mon Feb 19 2018

Very light snows to a moderate flurry continue this afternoon as
bottom of soundings remain just cold enough to produce snow as
opposed to widespread freezing drizzle. That said, soundings are a
bit warmer over the eastern zones along the Buffalo Ridge where
very light freezing drizzle continues. Accumulating snows
continue to increase over central South Dakota this afternoon as
broad warm advection continues to focus further west.

This evening and tonight: Models in reasonable agreement now,
suggesting that as a PV anomaly rotates northeast out of the
Central Rockies, mid-level frontogenesis will begin to intensify
within a 700 mb baroclinic zone over central SD later this
evening. This enhanced area of lift,  will begin to enhance
snowfall rates over the western CWA later tonight. Models also
showing favorable upper divergence as this wave moves through
thanks to a developing upper jet streak over northern Minnesota
which should promote expansion of precipitation over much of the
region

To the south and southeast, there remains consistency in the
advection of a very warm and relatively unstable layer of air
between 750:700 mb.  Soundings suggest upward of 300 J/KG MUCAPE
above this layer, which would support the development of convective
sleet over northern Nebraska this evening, advecting through NW Iowa
by and after midnight.  While temperatures within the warm nose peak
around +2.5-3C, the rather thick refreeze layer bottoms out near -
9C, so favoring more sleet as opposed to freezing rain as this
activity moves through. These signals are also noticed in rather
high model simulated reflectivity values within the cellular
convection, suggesting the potential for sleet. Due to the enhanced
mid-level lapse rates above the warm nose, have introduced thunder
into the southeastern half of the CWA for a few hours.

Moving back northwest, anticipating a fairly broad area of 2-5" of
snow, however, given signs of banding within the larger baroclinic
zone, some of the higher snowfall amounts could be more localized in
nature. Snow amounts may be fairly minimal in northwest Iowa, with a
mixture of sleet and snow accumulations up to a half inch.  The
only concern I have tonight is the potential for convection to
really disrupt the developing frontal band over east central SD,
that could serve to lower amounts.

This system will move northeast quickly Tuesday, but lingering
flurries may continue through mid-morning. The surface pressure
gradient does remain increased through the day, so have bumped up
wind slightly with causes minor blowing snow to be introduced into
the grids.

Will not make any changes to the ongoing advisory given potential
uncertainty.

.LONG TERM...(Tuesday night through Monday)
Issued at 256 PM CST Mon Feb 19 2018

Tuesday Night: A cold night is expected as high pressure moves
southeast into the region.  Clear skies, fresh snow, and light winds
should promote double digit below zero temperatures. Have lowered
populated temperatures slightly.

Wednesday: High pressure moves across the area, but with fresh snow,
I question how warm temperature will climb.  Most areas should rise
into teens, but have lowered temperatures slightly.

Thursday-Friday: Southwesterly flow will bring the next shortwave
through the Central and Northern Plains late Thursday and into
Friday. Medium range models remain in good agreement providing the
potential for a widespread 1-3" snowfall accumulation for the CWA.
Bumped up to categorical PoPs in some areas, and increased QPF
slightly.

Next weekend: Reasonable agreement in medium and long range models
continue, leading to minor changes to the extended portions of the
forecast.  Temperatures continue to lean towards the below normal
range.  There remains slight differences in the progression of a mid-
level wave through the Central Plains so will not change populated
PoPs for Saturday into Sunday, but as of today, highest risks for
snow would be slightly southeast.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 545 PM CST Mon Feb 19 2018

MVFR/IFR conditions will continue tonight as the next round of
accumulating snow approaches the area from the southwest. IFR to
LIFR are likely with the heaviest snow during the overnight hours
through around 11Z Tuesday. Light freezing rain/drizzle and/or
light sleet are possible over KSUX terminal, and areas across
northwest Iowa after midnight. In fact, isolated thunder with
sleet cannot be ruled out during the nighttime. Ceiling/visibility
conditions are expected to improve after 12Z at all terminals,
with very light snow/flurries tapering off by late Tuesday
morning.

Gusty northwesterly winds of 20 to 25 knots are possible on
Tuesday from mid-morning into the afternoon hours.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until noon CST Tuesday for SDZ062-066-
     067-070-071.

     Winter Weather Advisory until noon CST Tuesday for SDZ038>040-
     050-052>061-063>065-068-069.

MN...Winter Weather Advisory until noon CST Tuesday for MNZ080-081-
     089-090-098.

     Winter Weather Advisory until noon CST Tuesday for MNZ071-072-
     097.

IA...Winter Weather Advisory until noon CST Tuesday for IAZ001>003-
     012>014-020>022-031-032.

NE...Winter Weather Advisory until noon CST Tuesday for NEZ013-014.

&&

$$

SHORT TERM...Dux
LONG TERM...Dux
AVIATION...05



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