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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
1028 PM CDT Wednesday Sep 2 2015

Short term...(this evening through thursday)
issued at 415 PM CDT Wednesday Sep 2 2015

Warm and humid conditions today will continue into Thursday as upper
level ridging continues to strengthen over the central US. Mostly
clear skies and south to southeast winds around 5 to 15 miles per hour will
continue overnight. Pressure gradient tightens up late Thursday
morning as surface low pressure deepens slightly in the Western
Plains. This will allow for breezy southerly winds to develop and
toasty middle level temperatures to mix down to the surface. 850
temperatures reach 24 to 28 c...and should easily reach the upper
80s to middle 90s at the surface. With dewpoints once again likely in
the 60s west and the lower 70s will feel muggy. With the
lowest dewpoints located where the hottest temperatures are
expected...maximum heat index values will largely remain just below
heat index criteria of 100 degrees.

Long term...(thursday night through wednesday)
issued at 415 PM CDT Wednesday Sep 2 2015

Broad southerly flow will maintain the warmth and humidity at the
start of the longer range period. Maintaining a good southerly
gradient through the night should keep lows in the lower 70s...with
perhaps a few upper 60s towards the ridge to the east. First of
several subtropical waves lifting north northeast on the upstream
side of upper ridge axis expected to lift into the western Dakotas
by early Friday. Both NAM and GFS forecast soundings indicate a
little high based instability up to a few hundred j/kg based around
650-700 hpa...and watching weak signal of divergence q lifting northward
with strengthening pulse of Theta-E advection spurred by system in
the Western Plains. Likely any high based convection would be quite
isolated even if something would for now have pushed
probability of precipitation up to The Brink of mentionable. Have maintained slightly better
lower chance probability of precipitation west of the James River Valley later at night
closer to the wave dynamics. Best chance of any shower/thunder
action should be weighted earlier in the day on Friday as wave
shears past...and especially across the far north/West County warning area. As weak
ridging attempts to reestablish over the area later Friday and
Friday evening...still maintains a mixed signal with continued
moisture convergence/advections. In fact...NAM is most aggressive in
breaking down ridge with various waves...especially the first...and
thus allows the low level boundary to push into the County Warning Area on Friday
night...and actually through most of the County Warning Area on Saturday as it
maintains convection to its north to aid in the push against the
mean flow tendencies. This seems much too far to the aggressive
side...but would not be shocked to see a drift down toward i90 on
Friday night or early Saturday. May decrease the strength of the
southerly flow for a time...but still plenty of humidity in place
with potential exception of the far far west Friday afternoon...
with cloud/inversion restrained highs in the middle to upper 80s east...
and some lower 90s west.

Model suite seems to not be paying enough credence to impacts from
the next subtropical wave on Saturday...tracking perhaps a bit east
of the Friday wave path. Expect that convection may be a bit more
problematic for a period during the morning and early afternoon on
Saturday...again especially the west and northern County Warning Area. There will be
a bit more impact of middle level subtropical moisture by this time as
well...and a more responsive middle level lapse rate in place. Have
trimmed back the chances further lack of focus toward
the upper ridge. Should be humid highs in the 80s shared by all...
with the warmer readings from earlier days eliminated with degree of

Large scale trough will dig into the central rockies and lift toward
the northwest plains by Sunday afternoon. Large scale lift forcing
will increase through the day...with a capped cold front pushing
eastward across the County Warning Area. Some timing differences remain with
synoptic features...but more in the Camp having boundary approach
the i29 corridor later afternoon...pushing across SW Minnesota and northwest Iowa
during the evening. Elevated lapse rates and subtropical moisture
plume may yield some elevated showers/thunder ahead of the front
through the day...but only a low chance at this point. More vigorous
coverage/development should commence later day as boundary moves
into far eastern South Dakota...and especially evening pushing into
SW Minnesota and northwest Iowa.

Overall...severe chances would again appear to be fairly meager for
the late week convection. Potential is just a bit better along
advancing cold front on Sunday afternoon and evening...but shear is
fairly weak ahead/along boundary...better trailing...and there is
potential for a lot of cloud matter ahead to impact destabilization
potential. For now...the risk of some locally heavy rainfall would
likely be of greater concern...given lapse rates a bit to the
unstable side of moist neutral...and precipitable water values of an
inch and a half to inch and two thirds.

Labor Day will bring relief from the high humidity along with cooler the middle 70s to around 80. Mixing will bring some breezier
conditions...especially toward the Highway 14 corridor closer to
deeper upper low across southern Manitoba/Ontario.

Tuesday has the potential to be a huge change from our current
weather...even more so than currently indicated. Dry air at lower
levels along with threat for right entrance region development of
precipitation could start wet bulbing process fairly quickly to
compound the impact of cloudiness...making for a chilly day that may
struggle to get out of the 60s in some locations. Other than closer
to the southern tier where lower level baroclinicity in place...not
much of even elevated instability to work with...and have minimized
overall thunder mention...especially by Tuesday evening.


Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1027 PM CDT Wednesday Sep 2 2015

VFR through the period. Some patchy light fog possible early
Thursday morning. Low level wind shear also possible late tonight and early
Thursday morning...especially near ksux.


Fsd watches/warnings/advisories...
South Dakota...none.


Short term...
long term...Chapman

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