Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202106
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
406 PM CDT Fri Oct 20 2017

.SHORT TERM...(This evening through Saturday)
Issued at 406 PM CDT Fri Oct 20 2017

Main feature of the short term forecast will be the likely showers
and thunderstorms overnight tonight through Saturday evening.
Western upper level trough currently located over the Pacific
Northwest will continue to translate eastward. This will drive a
broad area of lee cyclogenesis and the associated development of a
robust cold front stretching from Lake Winnipeg all the way south
into Texas. As this occurs, a mid level shortwave will eject from
the Rockies over the central plains and translate northeastward
tonight. This shortwave will provide enough forcing for
precipitation as early as midnight tonight, taking advantage of the
moisture brought in by the robust southerly flow of the past couple
days. As the shortwave slips northeastward of our forecast area by
12z, the front will already be in our western zones, leading to an
additional period of precipitation.

As for the thunderstorm threat tomorrow, MUCAPE values will get up
to around 1000 J/kg just ahead of the front, enough for some
potentially robust convection. However, with the shortwave
associated precipitation ahead of the front occurring only a few
hours before the front swings through, current thinking is that the
short amount of time for destabilization will put a damper on the
chances for severe weather in all but our eastern most zones in
northwest Iowa - east of a Correctionville to Spirit Lake line. Main
severe threat will depend highly on where storms develop. Nearer to
the cold front, storm mode will be decidedly linear, with 0-6 bulk
shear almost directly along the boundary. In this scenario, the main
threat will be severe winds. If something can get going ahead of the
front, there will be enough low level shear for a non-zero threat of
a tornado as well. Reiterating the point from earlier, current
thinking is that the closer to the boundary scenario will occur
owing to the lack of time for destabilization tomorrow.

.LONG TERM...(Saturday night through Friday)
Issued at 406 PM CDT Fri Oct 20 2017

A few lingering showers will continue east of Windom to Sioux City
early Saturday evening as upper trough axis pulls eastward, with
some brisk northwest post-frontal winds coming to an end with
weakening of low-level cold advection and progression of weak ridge
axis expanding from southwest NE into southeast SD by late night.
With drying of low-level airmass and weakening winds, especially at
lower elevations, could see a few readings into the lower to mid
30s, with warmest temps along elevation and downstream across
southwest MN.

Return flow develops rapidly on Sunday, with southwest flow
increasing through the day ahead of strong jet digging over flat
western ridging. Should be readily set up for near maximum mixing
potential, suggesting 60s to around 70 around the region. Frontal
boundary will begin to push across the area Sunday night into early
Monday, shifting winds to northwest, and a fairly significant
northwest gradient will again mix out easily on Monday.  Somewhat
surprised not to find at least some lower end PoPs accepted given
the concentration of moisture and lift forcing along mid-level
frontal boundary as trough digs into the Mississippi valley by
Monday afternoon, perhaps a bit too amplified on the ECMWF compared
to the larger ensemble.

Stronger cold push settles across the area Monday night into early
Tuesday with some winds likely to gust 25 to 35 mph, knocking temps
back a solid 10-15F for Tuesday from readings Monday. A little
stratocumulus likely to cycle through, mainly earlier in the day,
which will give things an even more autumn feeling.

Remainder of next week looks to be a bit chaotic when it comes to
temperatures with several amplifying waves moving along active jet
through the northern tier.  Quick moving ridging settles in Tuesday
night to shut down winds, and also bring a quick shot of radiative
cooling around spotty cirrus.  Next trough digging toward the
northern Rockies will again induce development of return flow on
Wednesday, which while less obvious than Sunday, will again promote
some decent mixing, especially toward south central SD. This will
yield another mild day as warm plume spreads in across the western
to northern plains ahead of yet another frontal boundary.  This
front will aggressively push southeast Wednesday night into Thursday.
Thursday likely to see warmest temps earlier in the day, with advent
of strong cold advection behind the frontal boundary along with
increasing potential for precipitation through the day. There remain
some significant timing differences in the progression of
postfrontal precipitation band, as a high degree of spread in how
much energy in the trough splits between heading toward southern
Canada and the southern plains, impacting the progression of the mid-
level trough as well as the rapidity of cooling. For the time, have
settled on initial blend timing which is close to a operational
GFS/ECMWF blend, only with a bit more frontogenetic forcing along
the advancing axis.  While not explicitly in the forecast, the
dewpoint trends do suggest an increasing potential for wet bulbing
toward a non-liquid precip type Thursday into Thursday evening,
provided dynamics supporting precip continue to be indicated.
Uncertainty increased with the impacts of Typhoon Lin on downstream
pattern yet to be determined.  Temperatures Thursday into Friday
certainly cooler, with ensemble mean trends shading support toward
even colder than going forecast, which would be doubly true with
precipitation.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1143 AM CDT Fri Oct 20 2017

VFR through the evening and much of the overnight. Rain, scattered
thunderstorms and MVFR ceilings will become more likely very late
tonight into Saturday morning. The better chances for rain,
thunderstorms and MVFR ceilings will be east of the James River.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Ferguson
LONG TERM...Chapman
AVIATION...08



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