Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 170358
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1058 PM CDT Mon Apr 16 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 418 PM CDT Mon Apr 16 2018

Starting out quiet tonight in the wake of our recent snow, as ridge
of high pressure drops through the area tonight. With mainly clear
skies and light winds, would expect temps to really drop, especially
in deeper snow covered areas at lower elevation near and east of I-
29.  Could see an increase in higher clouds later in the night from
the west/northwest to slow up things, along with winds turning
toward the southeast.  Will have to watch to see if we can get some
fog to develop in favored easterly upslope areas west of the James
River. For those areas further east, should be cool enough for much
of this melt moisture to actually frost out.

Next system progressing through the Rockies on Tuesday is likely to
throw some higher clouds across the area, with some models
suggesting a stratus surge developing in the easterly flow, but
seems fairly unlikely. Depending on the extent of sunshine tomorrow,
could see locally several degrees higher along lower portions of I-
29 corridor where most of not all of the snow has melted today, or
will tomorrow. Just a little milder tomorrow for most, with highs in
the mid 30s to mid 40s, some upper 40s toward the lower I-29
corridor. By late day, could see a couple of very light showers
encroach on south central SD.

.LONG TERM...(Tuesday night through Monday)
Issued at 418 PM CDT Mon Apr 16 2018

Sounds like a broken record, but the main attention in the medium
range will again focus on a strong weather system and the
likelihood for significant wintry weather across the area. Trends
been fairly inconsistent, but there has mostly been a shift
northward with the heavier snowfall threat, most notably with the
last few runs of the NAM, with consensus axis mainly northern
parts of northwest IA, toward I-90 in far eastern SD, and through
the southern half of southwest MN. ECMWF remains a southern
stalwart, taking maximum precipitation through KSUX and across
northwest IA later Tuesday night and early Wednesday. GFS
ensembles are fairly broad in potential amounts yet, especially
further north. While can put some limited faith in a northward
split, the presence of strong dynamics to the south along with
being on the northern edge of some marginal instability aloft
places a desire to keep higher precip amounts a bit further
southward.

Precipitation will begin to blossom across central SD during the
evening in the warm advective/diffluent zone ahead of trough
starting to take on more of a negative tilt across the western
Plains. Temps surface/aloft start warm enough for generally rain,
but soon get a few issues with some areas on the leading edge
failing to saturate deeply enough for a couple hours with temps
hovering near freezing. While there could be a few areas of light
freezing rain, not expecting enough impact with the brief period of
unsaturated regions aloft along warm frontal lift, before deepening
mid-level low and cooling force precipitation over toward mainly
snow. Wrapped up precipitation remains from east central SD into
northwest IA on Wednesday as trough wraps up and pulls gradually
east of the area through the day. Snowfall will continue in
frontogenetic zone which settles back southward through the region,
but weakens fairly quickly through the afternoon.  While most of the
snowfall will likely remain in the 2-6 inch range, there may be some
locally heavier amounts somewhere toward the Iowa Great Lakes and
south central MN.  Winds will also increase into the 20 to 30 mph
and gusty range during the day as wave winds up to the
east/southeast.

After looking things over, have decided to keep watch going as is,
but did consider expanding a few counties northward toward the Sioux
Falls area and over toward Slayton with amounts closer to 4-6" in
the forecast.  Seems a bit counterintuitive to have the watch
continuing around the KSUX area, but the reasoning is that at this
point, the track of the main dynamics through northwest Iowa
suggests that focus could again shift southward which is typical of
a deepening and closing off system with jet energy as far south as
it will be, while areas north of the existing watch are more likely
to remain in an amount range that would likely be an advisory in the
end.

Ridging builds back in for Thursday leading to a dry and continued
cool period. End of week system looks largely to impact central
plains with mainly rainfall, with northern edge perhaps glancing
areas south of I-90.  More likely that there would not be
precipitation, and if there is, temps would seem to be just a bit
too warm at this point to expect snow.  Otherwise, will be working
to rid ourselves of whatever snow is around from the recent
snowstorm and what falls Tuesday night into Wednesday, so temps will
remain well below normal but moderate gradually as we migrate toward
a more split upper flow with northern stream repositioning north of
the area!

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1053 PM CDT Mon Apr 16 2018

Primarily VFR conditions through the TAF period. The exception
will be east-central SD and portions of southwest MN where patchy
shallow fog may develop after 06z tonight through 15z with light
winds and recent snowmelt. Will leave out of TAFs for now, but
will need to monitor for development. Mid and high clouds increase
from the west tonight, with MVFR ceilings arriving late in the TAF
period ahead of the next weather system. Rain and a wintry mix
will begin moving into the Missouri Valley after 00Z.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from late Tuesday night through Wednesday
     evening for SDZ071.

MN...Winter Storm Watch from late Tuesday night through Wednesday
     evening for MNZ081-089-090.

IA...Winter Storm Watch from late Tuesday night through Wednesday
     evening for IAZ001>003-012>014-020>022-031.

NE...Winter Storm Watch from late Tuesday night through Wednesday
     evening for NEZ014.

&&

$$

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



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