Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 110415
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1015 PM CST Wed Jan 10 2018

.UPDATE...
Issued at 1014 PM CST Wed Jan 10 2018

Some fairly strong trends continuation in model solutions this
evening, with southern portion of the system in TX remaining well
south for a longer time, and the more eastward track of this
latter wave will likely impact areas east of KSLB more toward
central Iowa later tonight and Thursday morning. As a result,
have cut back on snowfall to the east. We will be left with the
northern stream trough to fulfill our dreams of snow, and will be
watching the progression of the mid-level trough eastward across
the area overnight. Band nicely formed from Gregory county toward
De Smet area, with eastward shift rather sluggish.

High resolution solutions of RAP/HRRR have been much more
consistent and have reduced amounts further eastward from present
location of the band. May be a bit overdone with some of the
earlier runs totally eliminating most of the band east of I-29 as
mid level trough works through, with frontogenesis and some
limited instability remaining present, so have generally kept
about a half inch at the least. Held on to the advisory not as
much for the snow amounts, but for limited time of reduced
visibility in snow and blowing snow as well as the threat of a
period of freezing drizzle prior to onset of snow, and then the
flash freezing of water on roadways with temp drops of 6-9 degrees
per hour as northerly winds increase, freezing what is in place
on roadways.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 318 PM CST Wed Jan 10 2018

Obviously the main focus in the near term through Thursday is the
upper trough and associated arctic air mass moving through the
plains. Currently, the southern stream upper trough is located in
New Mexico, while the northern stream trough is moving through the
central and northern Rockies. The surface front has moved into our
northwest zones including Huron, and extends southwestward into
central Nebraska.

The southern stream upper trough moves northeast toward the Kansas
City area by 12Z Thursday. Initially the PV surge with this wave is
quite strong tonight, but then weakens as it moves eastward on
Thursday through the MS valley. Therefore the northern stream takes
over as it moves into the plains on Thursday. Dynamically, the
various models continue to show a trend of very strong mid level
frontogenesis tonight in our western zones, maximized in the 750-
650mb layer, coupled with this area being in the right entrance
region of a jet streak in eastern ND. Deep saturation is noted
through this layer, albeit the thermal profile is not very
dendritic. With this frontogenesis, many of the CAMs are picking up
on a narrow band of heavier snowfall along an axis from near Burke
SD to Huron SD. These areas could see 3 to 3.5 inches of snowfall.
The frontogenesis then weakens severely late tonight as it tries to
approach I 29, producing a relative weakness in the QPF field right
in the center of our forecast area. Dynamically the most favorable
item in the middle portion of the CWA is being placed in the right
entrance region of the aforementioned jet streak late tonight and
early Thursday aiding broad ascent.

Further to the east, the southeast corner of our northwest IA zones
could see near 3 inches of snowfall associated with a brief
encounter with the southern stream wave. However, this is very
problematic. The arw and nmm show stripes of heavier QPF in
northwest IA late tonight and early Thursday, actually right into
the southeast sections of southwest MN. This was not followed
however, because the 700mb streamlines are getting rapidly cutoff
with the northern stream wave. Therefore the mid levels are pretty
dry in our southeast zones until a brief saturation period arrives
for a few hours Thursday morning. Therefore if anything, the QPF
amounts, and thus snowfall, may have an error of being too high for
the Storm Lake area and this will be monitored.

One thing for certain will be the wind and rapidly falling
temperatures, staying steady on Thursday if not perhaps continuing
to fall slightly. Mixed layer winds through 925mb point toward 25 to
45 mph winds. For these reasons, have a winter weather advisory out
for the entire forecast area as visibilities will be significantly
reduced during falling snow. In fact, would not rule out brief white
out conditions at times during the worst of the snowfall. Where more
snowfall is projected in our far western zones, that area will need
to be watched for a change to a possible blizzard warning for part
of tonight. The Storm Lake and Spencer Iowa areas are also tricky in
that if a few inches of snow does pan out, they also could touch
blizzard criteria late tonight and Thursday morning. But am hesitant
to issue a blizzard warning for our southeast zones right now in
case less snow falls then forecast, and that is a real concern. All
told, there will be a several hour window of very unpleasant
conditions. Lastly, there could likely be a light wintry mix of
precip ahead of the snowfall in about a 1 or 2 hour window.
Therefore some locations may receive a light glaze, but admittedly
flash freezing of wetness already on the ground is probably a bigger
concern.

.LONG TERM...(Thursday night through Wednesday)
Issued at 318 PM CST Wed Jan 10 2018

The remainder of the forecast looks like below normal temperatures,
with some moderation by mid week next week. There is still an upper
low moving through this area on Friday and Friday evening, with a
corresponding surface low southwest of our CWA. The thermal profile
is quite efficient, with dendricity over a thick layer, coupled with
isentropic lift in the 280-290K layer. There is even some subtle
warm air advection Friday morning. Therefore would not rule out
another inch or so of snow generally along and west of I 29. Behind
this wave, the arctic flow will be reinforced on Friday night, with
another arctic push coming down Sunday night behind another wave.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 553 PM CST Wed Jan 10 2018

An unpleasant next 18 to 24 hours for aviation concerns with
wintry system working through the area. Main precip band will
develop across central to northeast SD this evening, progressing
eastward overnight with secondary band developing across eastern
portions of northwest Iowa late tonight and Thursday morning.
Some lingering warm temperatures aloft could yield a brief period
of light freezing rain or sleet before precipitation changes to
snow. Main snow band should stay west of KFSD and KSUX, but will
will likely get a period of 1-2sm visibility due to just snowfall.
Winds will be but one other concern, shifting northerly behind
boundary this evening with frequent gusts of 30 to 40 knots into
Thursday. This will create areas of blowing snow once
accumulations develop and further act to reduce visibility.
Likely will have to drop visibilities down toward 1/4 to 1/2sm for
a short period once timing of more significant snowfall can be
determined.

Ceilings will drop to LIFR/IFR in snow band before drier air helps
to raise into MVFR range. Fair amount of MVFR ceilings likely to
persist well into Thursday.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory until 6 PM CST Thursday for SDZ067-070-
     071.

     Winter Weather Advisory until 8 AM CST Thursday for SDZ038-050-
     052-053-057>059-063-064.

     Winter Weather Advisory until noon CST Thursday for SDZ039-040-
     054>056-060>062-065-066-068-069.

MN...Winter Weather Advisory until 6 PM CST Thursday for MNZ071-072-
     080-081-089-090-097-098.

IA...Winter Weather Advisory until 6 PM CST Thursday for IAZ001>003-
     012>014-020>022-031-032.

NE...Winter Weather Advisory until 6 PM CST Thursday for NEZ013-014.

&&

$$

UPDATE...Chapman
SHORT TERM...MJ
LONG TERM...MJ
AVIATION...Chapman



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