Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201742
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1242 PM CDT Wed Sep 20 2017

.SHORT TERM...(Today and Tonight)
Issued at 422 AM CDT Wed Sep 20 2017

Looking at a much quieter day today, with lower humidity levels and
clearing skies in the wake of the line of storms which moved across
the region overnight. Along with the lower dew points will come
lower temperatures, with highs in the lower-mid 70s for most areas,
which is pretty close to our seasonal normals. Winds will be on the
lighter side, gradually shifting from west-northwest early, to the
southwest in the late morning and afternoon as a surface ridge moves
across the area.

Tonight expect mostly clear skies to continue, with light southeast
winds as the surface ridge shifts east of the area. Overnight lows
will be seasonable to slightly above normal, with readings in the
lower-mid 50s by daybreak Thursday.

.LONG TERM...(Thursday through Tuesday)
Issued at 422 AM CDT Wed Sep 20 2017

As the large scale flow begins to take on a more meridional
turn late week, Thursday will be characterized by a strong increase
in southerly winds, with the drying of today quickly replaced by
that familiar humid feeling. Should be a few more flat cumulus
streaming northward through the day, but likely still enough
sunshine to drive temperatures into the 80s, around 10-15 degrees
above normal.

Low-level jet continues to increase Thursday night to 45 to 55
knots, with stout pressure gradient remaining at surface ahead of
troughing through the central Dakotas.  Modestly steep mid-level
lapse rates are in place, and the moisture flux into the region gets
much greater. As a result, elevated CAPE reaches as high as 600-1000
J/kg, and cap to an elevated parcel is fairly weak especially toward
the KBKX-KMML areas which is close to the terminus/focus of the low-
level jet.  Still have an outside chance to see elevated convective
development on Thursday night, but have only gone with an isolated
level PoP with otherwise large scale lift forcing a bit negative
given the slightly rising heights.  With the higher moisture content
and the strong southerly winds, there will be a good chance of
having record high minimum temperatures for the date closer to and
east of I-29, as long as readings do not fall too much through
midnight which is unlikely given the expected frontal boundary
location, which will wander into southeast South Dakota by Friday
afternoon, and take up residence across the areas for a couple of
days.

Could again find a few low clouds by later Thursday night and Friday
streaming in along the low-level jet, but sunshine should prevail
again Friday. Temps ahead of the front are quite warm, and mixing
even a modest depth would favor well into the 80s, and maybe even a
few lower 90s toward south central SD just ahead of the boundary.
This will create a moderately unstable atmosphere in the afternoon,
still capped in the warm sector, but weaker in vicinity of the
boundary in the James River valley toward northeast SD.  Would
probably take until late day after max heating can work on cap, and
start to feel impact of jet pushing northward through the far
western plains.  Deep layer shear will likely reach 35 to 45 knots,
largely with increase in unidirectional flow, enhanced locally near
the boundary.  Shear and instability are supportive of organized
convection, and a risk for severe storms seems warranted in the
very late afternoon and evening hours.

The story from Friday night through at least Monday will then be how
the large scale pattern locks in with the tropical pair of Jose and
Maria providing the blockage to the east, and how this impacts the
boundary location as several waves swing through the highly
amplified pattern, not to mention the complexity of the daily rounds
of convection. It appears there will be a number of potentially
heavy rainfall threats over the weekend, and will highlight the
rainfall threat in the HWO as well as social media this morning.
After Friday severe, there could be additional severe storms through
the weekend, but location and timing uncertainty would preclude
mention at this point.  Likely that temperatures will feature a
greater gradient than forecast advertises, but uncertainty in
boundary location keeps from getting too extreme at this point. Some
blending of the CONSRaw data sharpened up the differences a bit on
Saturday and Sunday, keeping some upper 50s and 60s through parts of
east central through south central SD, while allowing 80s in
northwest Iowa.

Stronger northern stream influence by Monday night looks to finally
shove the boundary clear of the area by Tuesday, sharing the cooler
readings with the rest of the area.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1239 PM CDT Wed Sep 20 2017

VFR will prevail for this afternoon and much of the night. However
late tonight, there are indications that MVFR stratus will wrap
back in toward Sioux City IA toward day break from the southeast,
and eventually work into the KFSD TAF site just before 18Z
Thursday. This scenario is certainly plausible, given that there
is plenty of lower end VFR strato cumulus currently in southeast
Nebraska and southern Iowa ready to come back northward once our
light winds shift directions.

&&

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

$$

SHORT TERM...JH
LONG TERM...Chapman
AVIATION...MJ


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