Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191755
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1255 PM CDT Tue Sep 19 2017

.SHORT TERM...(Today and Tonight)
Issued at 419 AM CDT Tue Sep 19 2017

Most significant concern within the next 24 hours is the threat for
severe storms across the area tonight.

Early this morning, however, is a different story with widespread
stable low-level conditions leading to expansive low clouds with
some pockets of fog from the western slopes of the James River
valley all the way eastward through southern MN and central IA.
Visibilities which had been pretty poor during the evening hours
near the Buffalo Ridge have generally improved through the
overnight.  With the modest southerly surface winds, would expect
that fog should not get too dense overall, concentrating again
through early morning near southerly/southeasterly upslope areas,
especially around the Buffalo Ridge.  With the widespread stratus
below a strong inversion, feel the pessimistic route to clearing
would be best road traveled. Advection from the south should work
clearing into the Missouri River corridor toward midday, but some
areas toward southwest MN/east central SD may struggle much of the
afternoon to shake the bulk of clouds. As a result, highs today have
been shaded most areas just a bit lower, but still mid 70s to mid
80s, and quite humid. No question that near-surface moisture will be
fairly considerable for much of the area, with the best chance to
mix out the inversion likely to occur by late afternoon toward Lower
Brule locations. Most model solutions indicate this mixing which
reduces the MLCAPE toward 1000-1500 J/kg at late day and generates a
remarkable DCAPE of 1500-2000 J/kg, while areas moving east toward
the James Valley likely to upward 2000-2500 J/kg, but very stoutly
capped. This cap will likely limit accessible surface-based
instability for much of the area, and provide somewhat a stabilizing
influence as convection pushes progressively eastward through the
night.

Water vapor imagery early this morning indicates the main feature
for the upcoming 24 hours - a very strong wave pushing into the
northern Great Basin early this morning which will crash into the
Northern Plains ridge going through tonight. Lift by late day will
be courtesy of a strong jet of 100-120 kts which slings across the
region by early evening, and a strong frontal zone will will move
into the far western CWA around 00z. Deep-layer shear of 45 to 55
knots and higher CAPE profiles are in the wheelhouse for severe
convection. Perhaps most concerning is the orientation of the lower
shear vectors, greatly normal to the boundary, which suggest that
mesovortices will be probable with transition to linear structures
after initial more discrete cells.  With such potent parameters,
have continued to mention severe storms in the grids, mainly in a
couple or three hour period containing the boundary passage at any
location.

But, there`s the cap.  Big question remains how active convection
will be the farther southward along the front you go, as initial
development streams northeast along the boundary until organizing
more linear - favoring a more eastward motion. Have kept highest
PoPs to the north of I-90, especially KHON-KMWM and north.

The message here - while convection is not a certainty, and even
less so for areas south of I-90 - any convection that does develop
by early evening will be highly capable of producing a gamut of
severe weather types. Very high threat for damaging winds for storm
north of I-90, with initial discrete storms showing potential for
large hail up to golf ball size and even a tornado during the
evening. As any linear structures push east, winds will continue to
be a concern, but the stronger cap heading eastward will begin to
reduce the threat of more extreme types, but still a small tornado
threat heading into southwest MN after midnight along with the
potential mesovortices.

In addition, even if convective modes fail further south, hi-res
models indicate that there will be a very strong pressure gradient
behind the front, so may end up with some advisory level or stronger
wind gusts for an hour or so immediately behind the frontal passage.

.LONG TERM...(Wednesday through Monday)
Issued at 419 AM CDT Tue Sep 19 2017

Medium and extended forecast concerns remain focused on a large
scale warmup and the potential for heavy rainfall by the end of the
week and next weekend.

High pressure will sweep into the region on Wednesday dropping
temperatures to near normal levels. As troughing begins to deepen
over the Western CONUS again on Thursday, strong return flow
develops and temperatures will start their journey back well above
normal to finish the week. One area of the forecast to monitor would
be Thursday evening/overnight as a sneaky shortwave and strong theta-
e advection could help produce scattered convection in the evening.
GFS is the only model really picking up on this, but from a
parameter perspective, it could bring a risk for strong elevated
convection if it forms.

The CWA remains firmly in the warm sector for Friday, which will
allow both moisture and temperatures to pool south of a frontal
boundary over northern SD/MN. Convection may develop along this
boundary late Friday into Saturday, and may bring a risk of strong
storms into Saturday morning.

Next weekend looks VERY unsettled as the aforementioned sfc boundary
and elevated baroclinic zone remain planted overhead. Pattern
analogs from CIPS guidance provide increased confidence in elevated
risks for an elongated corridor of heavy rain potential especially
Saturday evening through Sunday morning. NAEFS ensemble and
climatology percentiles suggest PWAT values and moisture transport
approach the 99th percentile by late Friday and these values may
continue into Sunday. So it`s possible that there would be
afternoon/evening strong convection Friday and Saturday south of the
boundary, which evolves into a couple of nights of moderate to heavy
rains in the Tri-State region.  The biggest questions are just where
the boundary will become established (or will it be more progressive
as the ECMWF suggests) and will it move each day based on overnight
convection.

The synoptic pattern breaks by Monday, which should force the front
through the area and eventually a trend towards cooler temperatures
the following week.

Forecast Change Summary:

1. Increased overnight low temperatures Friday/Saturday nights.
Increased max temperatures Friday/Saturday. There could be record
overnight minimum temperatures on Friday/Saturday.

2. Increased PoPs over the Friday-Sunday timeframe towards
categorical.

3. Increased thunder probabilities over populated guidance.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1247 PM CDT Tue Sep 19 2017

Active weather expected along and north of I-90 this evening into
the overnight. Stratus should gradually lift through the afternoon
hours. A few isolated storms are possible across central South
Dakota after 22Z. As a cold front approaches from the west, expect
a line of thunderstorms to develop along the front sweeping east.
Gusty winds and hail are possible with these storms. This may
affect both the KHON and KFSD terminals. KSUX is likely too far
south to be affected by the thunderstorms, but strong low level
jet feeding into the line of thunderstorms will impact KSUX this
evening ahead of the front.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...BT


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