Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 150351

Area Forecast Discussion
National Weather Service Sioux Falls SD
1051 PM CDT Fri Oct 14 2016

.SHORT TERM...(This evening through Saturday)
Issued at 323 PM CDT Fri Oct 14 2016

A couple of concerns for the overnight period, both really focused
near and east of I-29 corridor, and both associated with a stout
southerly low level jet.

First will be low level moisture and stratus across the central
Plains. Stratus has been eroding with mixing to our south somewhat
this afternoon, as was expected. However, as mixing diminishes this
evening, stratus should redevelop and expand into northwest Iowa,
continuing north into areas of southwest Minnesota and the I-29
corridor toward Sioux Falls and Brookings. Questions with this are:
1) How far west will the stratus spread, and 2) Will the moisture
layer become thick enough to support some patchy drizzle as warm
advection/shear near the top of the stratus increases with the
strong low level jet? Greatest consensus of the high-res models take
the lowest stratus to somewhere between I-29 and the James River
Valley, and have generally followed this idea. Most of these models
also produce spotty trace precip amounts late tonight into early
Saturday, so continued with the mention of patchy drizzle for areas
along and east of I-29. Exception to both of these may be in eastern
downslope areas of the Buffalo Ridge in southwest Minnesota, which
leads to concern #2 for tonight.

Aforementioned low level jet increases to around/just over 50kt
tonight, and southwesterly direction favors enhancement to gusts
atop/downslope of the Buffalo Ridge. Several of the high-res models
indicate a potential for 45-50kt gusts in these areas during the
overnight hours, and cannot completely discount this in localized
areas. Confidence not high enough to issue wind headlines at this
time, but evening shift will want to monitor the trends in these
models to see if a small advisory area may be warranted.

Winds will diminish early Saturday with slackening gradient ahead of
an approaching cool front. Stratus and some drizzle potential will
linger ahead of the boundary, but a shift to more westerly flow as
the front drops southeast should help mix out/erode the stratus
throughout the afternoon. Thus should still be looking at highs in
the lower-mid 70s for most of the forecast area.

.LONG TERM...(Saturday night through Friday)
Issued at 323 PM CDT Fri Oct 14 2016

The weather looks pretty straightforward Saturday night. Winds are
light and they begin to turn back to the east and southeast as a
surface ridge of high pressure moves away from the area. But because
of the much above normal temperatures, lows will not be too chilly
despite the light winds, with 45 to 50 expected. Sunday and Sunday
evening are not as straightforward. Strong warm air advection
returns as the south to southeast flow of air becomes rather windy.
However the wind speeds will not be as strong as they are today,
with about 25 knots noted in the 925-900mb mixed layer, compared to
today where 30 to 40 knots are common. All models show a decent
700mb short wave moving northeastward into this area, which is the
southern extension of mid level low pressure churning in southwest
Canada. In the low levels, stratus is a threat once again moving in
from the southeast, and generally encompassing areas along and east
of the I 29 corridor late Sunday and Sunday evening. There is ample
mid and upper level moisture due to the wave, but what is missing,
is moisture in the crucial 850-700mb layer. Therefore only small
pops for rainfall are warranted in our eastern zones Sunday
afternoon and evening. It is a bit unstable, especially by Sunday
evening, with negative lifted index values noted from 925-800mb, and
ML CAPE values approaching 1000 J/kg. So would not rule out some
isolated TSRA embedded in the showers especially Sunday evening east
of I 29. Concerning temperatures, despite the abundant mid and high
level clouds, and stratus potential in our east, highs should be
quite mild given the 925mb temperatures, with a lot of 70s in store,
and probably around 80 near Chamberlain where low clouds will not

The surface winds temporarily shift to a light north or northeast
direction Sunday night behind the departing mid level wave. However
after that there is quite a bit of model discrepancy in how fast to
turn the winds back around to the southeast on Monday. For now,
hedged toward the faster southeasterly solutions but at any rate,
Monday will be another very warm day with 925mb temperatures
averaging near +17C to +19C. This will produce highs once again in
the 70s to around 80 degrees.

Temperatures will then begin a cooling trend after Monday. Mid level
short wave scoots eastward across the northern plains which will
turn the winds around to the northwest and usher in temperatures
closer to normal. The moisture profile is partially saturated enough
to warrant small pops generally north of I 90 Monday night, but
amounts still look very light. The bigger story is the huge amount
of model discrepancy which exists by mid week. The GEM global and
ECMWF are in one camp in bringing an open upper trough through this
area on Wednesday reinforcing the chilly air. The GFS builds a
positively tilted trough near the four corners area. With a
northerly flow at the lower levels, the GFS would give an over
running scenario and light rainfall throughout our southern zones.
But for now, was forced to compromised between the solutions,
keeping only low pops from I 90 southward during the mid week in
case the ECMWF and GEM Global pan out which they very well may. But
again, the main emphasis is cooler air and we are looking at 50s for
highs by next Thursday and Friday.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1045 PM CDT Fri Oct 14 2016

MVFR to IFR stratus continues to expand northward, and will
affect areas from the I-29 corridor and eastward through Saturday
morning. Spotty drizzle will also be possible with these clouds
toward 09Z. With a developing low level jet, there will be a
period of low level wind shear around KHON through 09Z, but KFSD
and KSUX will likely keep strong enough surface winds to preclude
a consistent LLWS threat.

IFR Stratus/drizzle conditions will gradually improve beginning late
Saturday morning, but MVFR ceilings will likely persist over parts
of northwest Iowa and southwest Minnesota into the afternoon.




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