Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 150915

Area Forecast Discussion
National Weather Service Sioux Falls SD
415 AM CDT Thu Mar 15 2018

.SHORT TERM...(Today through Friday night)
Issued at 415 AM CDT Thu Mar 15 2018

Not much of concern through this evening. Easterly flow will
increase a bit through the day, and this along with areas of high
clouds will keep temperatures cooler than yesterday, with highs from
the mid 30s in southwest MN to near 50 around Sioux City.

Late tonight through Friday night will be the period of greatest
concern with this forecast. Models have come into better agreement
on the overall timing/track of the system set to impact the region
during this time, though still minor variability in exact location
of the circulation center, and the associated thermal profiles which
could greatly impact precipitation type through the period.

In general, mid-upper trough and associated surface low will swing
out of the central Rockies into western Kansas/Nebraska by late
tonight, then slide east near the Kansas/Nebraska border through
the day Friday. Ample moisture and streaming into the Missouri River
Valley ahead of the low by late tonight/early Friday, gradually
expanding northeast to about a KHON-KFSD-KSPW line by midday, and
over most of the CWA by late afternoon. Fairly narrow ribbon of very
strong lift along the mid-level front will likewise lift northeast
through the day, followed by drying aloft which could transition any
substantial snow and/or freezing rain threat over freezing drizzle.

Some questions remain regarding precip type and potential impacts:
1. Surface air temperatures, especially south of I-90, could be near
or even slightly above freezing, limiting impact of potential icing.
2. Likewise, road temperatures across our southern areas are above
freezing based on current road sensor observations. With another
mild day today and air temperatures remaining near freezing tonight,
see no reason why these areas would see road temperatures drop below
freezing. This, too, could limit the impact of potential icing, at
least for travel concerns. Could still see some ice accretion on
elevated objects/trees/power lines, though.
3. Forecast soundings from both GFS and NAM maintain a warm layer
around 825-850mb in areas east of the James River Valley through
much of the day Friday. Location and just how warm this layer is
will ultimately impact precip type and amounts of snow/sleet/ice.

Despite the above questions, concern for travel impacts is high
enough this morning to issue headlines for parts of the area. Opted
for a Winter Storm Watch from south central SD, eastward along the
Missouri River Valley to Dixon County Nebraska. This area appears
most likely to see significant icing threat, though air and road
temperatures could impact eventual ice accumulations as mentioned
above. Northeast of this area, confidence in a band of moderate snow
(2-4") and/or light ice accumulations warrants a Winter Weather
Advisory from Huron/Mitchell, eastward toward Sioux Falls and

Far northeast parts of the forecast area (including Brookings,
Marshall and Windom) not likely to see much more than light snow,
or possibly a light glaze of ice if snow turns over to freezing
drizzle before ending Friday night.

Southeast parts of the forecast area (including Sioux City, Spencer
and Storm Lake) could see some icing, but impacts will be dependent
on those road and air temperatures, especially since much of the
precipitation will be occurring during daylight hours when the
increasing sun angle can also have a melting effect, even through
thick cloud cover. Thus, opted to not issue any headlines for that
area at this time, despite having a mention of ice accumulations in
the forecast.

Thanks to all of our surrounding offices and WPC for the extensive
collaboration on a difficult forecast this morning!

.LONG TERM...(Saturday through Wednesday)
Issued at 415 AM CDT Thu Mar 15 2018

System expected to be east of the forecast area by early
Saturday, and weak ridging will provide warmer, but still below
normal temperatures across the region through the weekend.

Next trough will slide out of the Rockies into the Plains late
Sunday into the early part of the work week. Focus on Friday did not
allow for in-depth interrogation of this next system, but despite
some timing differences, models generally agree on this system
bringing another shot of rain changing to snow across the region
early next week.

Temperatures show a slight upward trend again toward the middle of
next week with dry conditions by the end of this forecast period,
though extent of warming will initially depend on how much snow is
received over the next several days.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1052 PM CDT Wed Mar 14 2018

Models continue to show a small chance for patchy fog along the
Buffalo Ridge in southwest Minnesota overnight; however, they have
decreased the intensity and coverage of fog. This seems reasonable
given the strength of the east winds coming in during the early
morning hours: These winds should be strong enough to mix down
some drier air into the shallow moist surface layer, thus
decreasing the threat of fog. Winds become breezy from the east
tomorrow afternoon, gusting 20 to 25 kt. This is in response to an
approaching weather system that will bring mixed winter
precipitation to much of the region early Friday morning.


SD...Winter Weather Advisory from 7 AM Friday to 1 AM CDT Saturday
     for SDZ038-039-052>056-058>062-066-067.

     Winter Storm Watch from late tonight through late Friday night
     for SDZ050-057-063>065-068>070.

MN...Winter Weather Advisory from 7 AM Friday to 1 AM CDT Saturday
     for MNZ089-098.

IA...Winter Weather Advisory from 7 AM Friday to 1 AM CDT Saturday
     for IAZ001-002.

NE...Winter Storm Watch from late tonight through late Friday night
     for NEZ013.



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