Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
657 am CDT Thursday Jul 2 2015
Short term...(this afternoon through Friday afternoon)
issued at 651 am CDT Thursday Jul 2 2015
Most important player to the short term weather is compact upper
wave moving slowly southeast across North Dakota and northern South
Dakota in the wee hours this morning. Large mesoscale convective system across western
Nebraska working southward...and if lower level flow was critical to
development would expect this to have a significant impact.
However...lift forcing primarily a result of the strong upper wave
pushing southeast...and have a fairly distinct instability gradient
in place across the western half of the County Warning Area...somewhat west of
earlier model guidance...as well as a modest lower to middle level
convergence axis across central South Dakota. Have seen a rapid increase in
scattered convection coverage back along these boundaries in central
South Dakota in the last hour or two...which leads to a bit higher confidence
in the high resolution solutions of encroachment in the western County Warning Area
between 09z and 12z...and a much slower and less significant
progression into areas east of i29 during the morning. Trends in
both high resolution and other short range operational solutions is
to keep a fairly strong buffer of stability over the eastern
County Warning Area...and either stop or at least limit the potential precipitation
intrusion into far eastern portions of the County Warning Area through the day. Will
focus higher probability of precipitation through the James Valley this morning...and
gradually settle toward the lower Missouri Valley and northeast
Nebraska later morning and afternoon. Lingering moisture/convergence
axis will keep a lower coverage threat for precipitation later day
and into the evening across the west and south as subtle secondary
wave swings southward through the Western Plains...before fading
with loss of diabatic support and meager upper forcing.
Again...not looking at a tremendous amount of instability to start
the day as the greater dynamic lift slips across the area...with
elevated cape perhaps 1000 j/kg or a bit more far west...and
tapering off to a couple hundred j/kg east of Interstate 29. With 40
knots of effective shear across the southwestern County Warning Area corresponding
to greater instability...a couple of storms in the early morning
west of the James could be capable of marginally large hail...but
neither deep layer shear or instability suggests any greater threat.
May see a little patchy fog across mainly areas near/west of i29
late tonight as the clouds break and gradient GOES light. Eastern
areas should be free of fog with a little more dry trajectory.
Otherwise...a seasonally cool day with highs in the 70s...coolest in
the precipitation axis. For tonight...should again return mainly to upper
50s to around 60.
Long term...(friday night through wednesday)
issued at 651 am CDT Thursday Jul 2 2015
Friday looking like a very nice slightly humid day with winds less
than 20 knots up to about 200mb. While the majority of the area will
remain dry a weak boundary will leak into the far eastern County Warning Area by
late afternoon and at peak heating with a minor amount of cape a few
thunderstorms could be possible. Will only carry a 20 pop for now.
Highs mainly upper 70s to middle 80s.
Friday night into Saturday will continue the fairly quiet weather as
high pressure gradually works through. Return flow will set up over
central South Dakota by Saturday afternoon and will need to have a
small chance for thunderstorms Saturday afternoon and Saturday night
but at this time not anticipating any severe weather.
In the outer periods(sunday through wednesday)...a large area of low
pressure over central Canada will keep westerly flow in place across
the northern plains. It also keeps ridging from developing aloft so
no major heating expected through this period. In fact temperatures
will more than likely be below normal during this time as this flow
favors locking in cooler air from the north and keeping heat to the
south and west of the area. The only decent chance for thunderstorms
during this time should be on Sunday/Sunday night as a jet maximum moves
through North Dakota and swings a cold front through. Still looking
like a threat for severe so will need to continue to monitor
potential. The next better chance will more than likely be
Aviation...(for the 12z tafs through 12z Friday morning)
issued at 656 am CDT Thursday Jul 2 2015
Primary focus for convection today will be west of Interstate 29
through the day...and most likely to get brief drop into MVFR
conditions or any thunder for khon through midday. Should get a
bit more stratocumulus at MVFR levels to develop Post rainfall for
the early afternoon...before deepening into a VFR layer.
Otherwise...areas remaining in a bit more moist low level
environment and closer to the low level convergence axis west of
Interstate 29 may find a bit of patchy fog develop late
tonight...and for now have only mentioned MVFR visibilities and no
ceilings...but clearing skies and lighter winds could present some
more dire conditions at low elevation locations.