Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 121653

Area Forecast Discussion
National Weather Service Sioux Falls SD
1153 AM CDT Sat Aug 12 2017

.SHORT TERM...(Today and Tonight)
Issued at 350 AM CDT Sat Aug 12 2017

Moisture transport between 700-800 mb leading to scattered showers
mainly along and west of the James Valley this morning.  By mid-
morning, this forcing is expected to break down resulting in a
decrease of convective activity.  Difficult forecast today in terms
of precipitation forecast.  Not a lot to latch onto in terms of
dynamics, but atmosphere becomes increasingly unstable through the
afternoon hours with surface heating.  There is a minimal cap in
place, and as shortwave energy approaches from the west have the
highest pops concentrated late this afternoon into the overnight
time frame. Instability is fairly limited, but the wind shear is
fairly good with strong veering profiles.  Therefore, support the
marginal risk west of James River.  Further to the east, instability
is extremely limited.

As for temperatures today, there will likely be a fair amount of
clouds around today keeping highs mainly in the 70s. With clouds,
southerly flow and increasing moisture across the area, temperatures
will remain a bit warmer tonight than the last few nights with lows
near 60.

.LONG TERM...(Sunday through Friday)
Issued at 350 AM CDT Sat Aug 12 2017

The main forecast concern from Sunday through Tuesday night will be
thunderstorms chances primarily through Sunday night and again on
Monday night.

A fairly strong upper level wave will move from eastern South Dakota
into western Minnesota during the day on Sunday. Ahead of this wave,
ongoing rain with embedded thunderstorms, primarily along and north
of I90, will continue to move east through southwestern Minnesota
through the morning. There could also be scattered showers and
storms in far southeastern South Dakota and northwestern Iowa but
coverage will be much less. Then the question is if new showers and
storms will redevelop west of I-29 during the afternoon. The NAM and
GFS/Canadian have real differences as the NAM keeps skies overcast
throughout the day which really limits the instability while the GFS
hints that there will be at least some sun to warm temperatures into
the mid 70s. With rain ending and the upper wave moving east, do
expect at last enough thinning of clouds and even perhaps a few
breaks to warm temperatures into the mid 70s. This will result in
MLCAPEs up to 1000 J/kg. There is also some deep layer shear of
around 35-40 kts. While sounding profiles do not favor large hail
due to moist adiabatic lapse which limits CAPE between -10 and -30
C, the potential for at least some organized storms due to the deep
layer shear would yield a low threat of severe weather during the
afternoon if storms develop west of I-29. If there is more sun such
that temperatures approach 80, then the combination of shear and
higher instability would increase the severe threat. At this time,
this appears unlikely and will only highlight a marginal threat for
1 inch hail in HWO for Sunday afternoon.

As these storms continue to move east Sunday evening, an increasing
low level jet of 20-30 kts will develop. This will enhance theta-E
advection and should help storms to expand in coverage as they move
east of I-29. It is likely as the night progresses storms on the
southern flank of the complex will become favored as new cell
development will occur. This means storms should persist longer
across northwestern Iowa. So have raised PoPs overnight along and
east of I-29 with higher PoPs lasting after midnight in northwestern
Iowa. Farther west, skies should gradually clear. Lows overnight
will be in the upper 50s.

On Monday, there may be lingering showers and thunderstorms around
Spencer and Storm Lake in northwestern Iowa with an isolated shower
or thunderstorm possible over southwestern Minnesota. However, most
of the day should be dry across the area. With more sun, highs will
range from he mid 70s in southwestern Minnesota to the mid 80s in
south central South Dakota.

On Monday night, a strengthening warm front will begin to move north
from the Missouri River toward Hwy 14. How fast this front moves
north will determine where convection develops. The GFS, ECMWF, and
Canadian push the 850 mb front to Hwy 14 by 06Z while the NAM keeps
the front closer to I90. As the front moves north in the evening
there could be an isolated storm along or south of I90 but the best
chance will be after midnight where the LLJ cross the 850 mb
boundary. Have generally sided with the GFS/ECMWF/Canadian and have
highest chances around KHON but with the boundary extending
southeastward, storm may move southeastward along the boundary so
have a chance over most of southeastern South Dakota.

For the extended, Tuesday through Friday, the pattern remains active
as a couple of short wave troughs move across the area. Model
solution diverge by Tuesday. So while confidence is high there will
be precipitation, especially with the first wave, the timing of
these waves differences are 12 to 24 h apart. At this point, there
is not a strong signal to favor one model over another in terms of
timing. But given the strength of the upper wave Tuesday and
Wednesday, the ECMWF appears to underforecast the precipitation
coverage and have favored the GFS/Canadian in terms of coverage of
precipitation with this system. The high uncertainty in timing has
resulted in a much longer time period with rain chances. So as
confidence increases as to when the wave will move through the area,
the time period for rain chances at any one location will likely
decrease. A second wave is then expected to move through Thursday
night into Friday. The ECMWF is again much faster with this wave and
as a result has very limited moisture return and almost no
precipitation. The GFS is slowest with this wave and has most
locations get 0.1-0.5" of rain. With even greater uncertainty on
timing, have kept PoPs closer to 30 percent. Temperatures through
the period will near or just below seasonal normals with highs
between 75 and 85 each day.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1148 AM CDT Sat Aug 12 2017

VFR conditions through most of the afternoon and evening. Showers
and thunderstorms should expand in coverage in central SD late
this afternoon, then work east overnight into Sunday morning. As
the showers move through late tonight into Sunday morning an MVFR
ceiling will become likely.




LONG TERM...Schumacher
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