Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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781
FXUS63 KFSD 220930
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
430 AM CDT Fri Sep 22 2017

.SHORT TERM...(Today and Tonight)
Issued at 429 AM CDT Fri Sep 22 2017

Entrenched pattern featuring large ridge over the Great Lakes with a
trough over the inter mountain west will begin to ever so slowly
break down as the trough amplifies, allowing energy from upper jet
to begin to focus over the northern plains. This, along with a
series of small upper impulses, will induce the development of a
broad area of lower pressure at the surface that will eventually
stretch from eastern Colorado northeast through western Ontario.
Focus for sensible weather in our CWA will be a surface cyclone that
lifts northeast through South Dakota on Friday and the attendant
boundary that will eventually drape roughly along a Wagner SD -
Ivanhoe MN line by Friday evening. Tight pressure gradient during
the afternoon will lead to brisk southerly winds during that time
period.

Eventually, heating of the day combined with ample warm advection
will lead to destabilization along and just on the warm side of the
boundary. Another shortwave impulse approaches the SD/NE border
close to 00z Friday, providing just enough forcing for convection.
Significant capping on the eastern side of the boundary will leave
most convection on or just to the east of the boundary. Organized
convection will be possible given MUCAPE of 1000 J/kg and 0-6 bulk
shear greater than 35 knots around the boundary. 0-6 shear
orientation is along boundary which may prevent storms from staying
isolated - marginal is certainly an apt word for the severe threat
because of this. However, Bunker`s method suggests that a right
moving supercell could get off the boundary for a short time before
running into the significant capping to the east. With DCAPE greater
than 1000 J/kg, damaging winds will be the top threat, followed by
severe hail. 0-1 helicity will near to 100, so cannot completely
rule out a tornado either.


.LONG TERM...(Saturday through Thursday)
Issued at 429 AM CDT Fri Sep 22 2017

Potential for heavy rain will remain the focus through the weekend
and into Monday. Moisture will certainly be in place, with forecast
precipitable water amounts topping 1.5 inches across eastern South
Dakota from Saturday afternoon through Saturday night. These amounts
don`t seem overly impressive...for mid-summer. However, now that we
are in early autumn, climatology shows this to be over 200 percent
of normal. NAEFS climatology also continues to show these values are
in the 99th percentile, with strong moisture transport above the
90th percentile of the ensemble climatology. Warm cloud depth peaks
Saturday evening above 13kft, with soundings showing nearly moist
adiabatic lapse rates, suggesting potential for very efficient rain
production. Broad scale lift will be provided by right entrance
region of unseasonably strong upper level jet lifting through the
western Dakotas, holding the greatest threat of precipitation near
and west of a nearly stationary frontal boundary wavering across
the central portions of the forecast area. Given the above scenario,
have no problems with the continuation of a slight risk of excessive
rainfall across our western forecast area Saturday afternoon/night.
Models point to some modest instability closer to the front, near a
KYKN-KFSD-KMML line by late Saturday afternoon. Deep layer shear in
excess of 40kt right along the boundary decreases rapidly as one
heads east of the front. The shear could support stronger storms if
they are able to tap into the better instability east of the front.
Though with marginal lapse rates, severe threat should be isolated
at best with the greater focus on the heavy rain threat west of the
boundary. Anticipate a strong temperature gradient across the front,
with highs from the mid 60s in the far western CWA, to the upper 80s
in northwest Iowa where rain chances are minimal.

All of this shifts slightly eastward for Sunday/Sunday night, with
only a slight reduction in precipitable water values and warm cloud
depth by that time. Thus, the potential for locally heavy rain will
likewise shift a bit to the east, encompassing the I-29 corridor
into more of southwest Minnesota, especially Sunday night. Also see
weaker instability through this time, which should allow for more
widespread efficient rainfall with a isolated-scattered embedded
thunder.

The western trough over the Rockies begins to make a greater push to
the east Monday, shifting the upper jet into the central/eastern
Dakotas and the low level boundary into northwest Iowa and southern
Minnesota. This should shift the greater rain chances into eastern
portions of our forecast area for Monday/Monday night, though rain
chances will linger west of I-29 at least into Monday evening. When
all is said and done, some areas could see 3-day rainfall totals in
excess of 2-3 inches, with pockets of higher amounts where stronger
convection occurs Saturday afternoon/night.

Latter part of the forecast period, from Tuesday onward, should be
much quieter, as cooler air settles into the region. Generally
looking at highs in the 60s and lows in the 40s through the middle
of the week, with little if any rain.


&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1121 PM CDT Thu Sep 21 2017

Breezy winds and low level wind shear are the primary hindrances
to aviation operations expected during the TAF period. Nocturnal
low level jet will lead to widespread low level wind shear
including the FSD and SUX terminals. Gusty winds, generally out of
the south, continue through the day on Friday. Some patchy fog is
also possible tonight but the breezy conditions will prevent any
operationally relevant visibility reductions.


&&

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

$$

SHORT TERM...Ferguson
LONG TERM...JH
AVIATION...Ferguson



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