Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 222104

Area Forecast Discussion
National Weather Service Sioux Falls SD
404 PM CDT Thu Mar 22 2018

.SHORT TERM...(This evening through Friday)
Issued at 356 PM CDT Thu Mar 22 2018

Current upper air analysis shows a large trough extending along the
Pacific coast. At the surface, high pressure sprawls from central
and eastern Canada, down the MS valley and into the gulf. The
weather is quite right now, however a leading short wave will move
up the southwest flow aloft into our CWA by late tonight, and move
across the area on Friday before exiting our eastern zones overnight
Friday night. This feature is best shown with the mid and upper
level QG forcing fields. Following closely will be a strong short
wave ejecting off of the parent upper trough, moving across our CWA
Friday night and Saturday morning coupled with fairly robust PV at
the 1.5 PVU surface. So there is actually a couple of things going
on here making this event somewhat prolonged.

There is strong model consensus in showing strong warm air advection
with veering soundings, coupled with strong mid level frontogenesis
Friday and Friday evening with the initial wave. This will help aid
robust ascent and very rapid saturation, especially for locations
along and north of the I 90 corridor, but even lighter precip is
quite probable in our southern zones. What is most interesting is
southwest MN Friday evening. It is this area where things come
together including the aforementioned initial short wave and its mid
level frontogenesis, coupled with the advancing second strong short
wave over east central SD. Strong 850mb frontogenesis and a very
strong baroclinic zone from 850-700mb is maximized over southwest
MN from 00Z-06Z Saturday. Soundings in southwest MN show a deep
saturated layer Friday evening with warm air advection still
occurring as the warm conveyor belt streams northward from the
gulf and into Minnesota. In addition, the lapse rates are pseudo
adiabatically unstable from 700mb upward, albeit EPV* is not
overly unstable. So to summarize, warning potential is there for
the eastern sections of southwest MN, but not enough confidence
yet to pull the trigger.

With all this said, worried about a window of very heavy snowfall
over the eastern half of southwest MN Friday evening. But this
highly depends on how cool the surface to 850mb wet bulb
temperatures will be, which will also have a bearing on surface
temperatures. Many of the guidance values are quite warm, in the mid
to upper 30s all day Friday and Friday evening in southwest MN. But
with their elevation, do not believe that is going to happen, so
opted to use consensus raw model hourly temperatures Friday and
Friday night throughout the forecast area, as these numbers should
pick up the dynamical cooling better than MOS based guidance
values. Therefore in southwest MN, raw model hourly values were
cooler, right around 32 to 34 degrees. But there is just enough
question on temperatures to not issue a winter storm warning quite
yet. For instance, if temperatures from the surface to 925mb warm
up just 1 or 2 degrees C from projections, then southwest MN will
likely see advisory type snows, so its that close. Noting the
elevation in southwest MN, went the more pessimistic route for
snowfall totals, but again, not enough confidence yet to go warning.
What we did however which appears a bit safer is make a three county
winter weather advisory from Brookings SD to Worthington MN, on
the backside of the heavier snow. This headline starts Friday
night as that area may be battling a rainy mix through the day on

Elsewhere, snowfall totals are projected to rapidly drop off due to
warmer temperatures from the surface to 925mb which in large part
are above freezing. Concerning temperatures and winds, due to a
brisk easterly wind setting up tonight and Friday, used consensus
mos values to boost speeds. Coupled with cloud cover, tried to
keep diurnal temperature spreads tonight through Friday night to a
minimum. Again, hedged toward raw model values for the first few
periods which kept lows warmer, and Fridays highs cooler than
standard MOS readings. Superblend wind speeds were too light,
noting 30 to 40 knots across the forecast area at 925mb.

.LONG TERM...(Friday night through Thursday)
Issued at 356 PM CDT Thu Mar 22 2018

Admittedly further out, did not spend a lot of time perusing over
the forecast due to the system Friday and Friday night. In general
the models are still showing some light precip along a warm air
advection band late Saturday night and Sunday. The precip type is a
little tricky late Saturday night and Sunday morning, with warm
temperatures aloft potentially causing some light freezing rain or
freezing drizzle, so this will need watched. But warming
temperatures Sunday afternoon will melt any ice on roadways. This
will be followed Sunday night and Monday with an overrunning setup
as an upper trough of low pressure in southern Canada advects chilly
air into the northern plains, and a developing surface low over the
southwestern plains extends a warm front front into Iowa late Sunday
night and Monday. So high pops are warranted late Sunday night and
Monday for rain or snow.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1212 PM CDT Thu Mar 22 2018

Quiet weather and VFR conditions are expected to continue for the
first half of the TAF period. Mid and high level clouds will be on
the increase by the evening hours ahead of an incoming storm
system. Rain and MVFR/IFR ceilings will spread into the region
from west to east through the morning hours. Rain could mix with,
or change to, snow over southwest Minnesota, especially into the
afternoon hours after this TAF period.


SD...Winter Weather Advisory from 7 PM Friday to 7 AM CDT Saturday
     for SDZ040.

MN...Winter Weather Advisory from 7 PM Friday to 7 AM CDT Saturday
     for MNZ089-097.

     Winter Storm Watch from Friday morning through Saturday morning
     for MNZ071-072-080-081-090.



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