Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
645 am CDT Thursday Aug 28 2014
Short term...(today and tonight)
issued at 444 am CDT Thursday Aug 28 2014
Pattern continues to unfold notoriously slower than models suggest.
Short term hi-res models are not a bad starting point...
precipitation is mainly driven by 850 hpa warm air advection/Theta-E advection.
Mainly followed this for pop placement...however... forcing
appears to move fairly quick and slowed progression of the probability of precipitation
compared to the models especially through the afternoon hours. As
the cold front moves through that should effectively shut down the
precipitation...however...with upper level wave shearing off to
the northeast still a fair amount of forcing across eastern half
of the forecast area and expect the potential for light to
moderate shower activity to remain across the eastern County Warning Area through
the overnight. Have some Middle Range probability of precipitation for now...but would not be
surprised if these probability of precipitation will need to be raised in future updates.
With weak forcing along the front and clouds lingering through
much of the night...have sided closer to the milder side of
guidance for lows.
Long term...(friday through wednesday)
issued at 340 am CDT Thursday Aug 28 2014
Upper level trough will be main feature working slowly eastward
across the area. Configuration begins to take on more of a
positive tilt as it works eastward in solutions...perhaps too much
so...but even despite a weakening convergence signature in the
middle levels...there remains enough qg forcing along and ahead of
the axis along with pv advection and a neutral to weakly unstable
lapse rate to suggest a decent chance for showers and embedded
thunderstorms on Friday across the southeast...especially by later
morning and early afternoon as the limited diurnal cycle aids in
further destabilization. Decent warm cloud depths as well so would
expect efficient precipitation producers...and while perhaps a
half inch or so on the average...some amounts of up to an inch or
a bit more possible southeast of a Spencer to Sioux City line.
With such a weak shear profile and instability hedging more toward
the moist neutral side than greatly unstable...do not see severe
storms as a large threat...perhaps in isolated wind or brief
spinup in the far southeast...depending fully on whether can get
any destabilizing of the lowest layer.
Further to the west...heating of the day a bit more favored and
will actually work to destabilize somewhat more after a little
more hostility toward lift earlier in the day. By later
afternoon...potential for developing a Narrow Cape profile and
secondary convergence axis with northern stream wave to perhaps
generate a couple of rogue late day storms...with the threat
traversing mainly the northern County Warning Area along with the weak secondary
wave overnight and very early on Saturday. Remainder of Saturday
should be quiet with short wave ridging.
With little push to Flushing out moisture at lower levels...
temperatures will struggle somewhat on Friday in the east...likely
only reaching the middle to upper 70s...while partial clearing
in the west and location ahead of the secondary convergence pushes
into the middle 80s. By Saturday...should end up a bit warmer most
areas...even with the slightly east of south winds at the
Saturday night will get another boost to lower to middle level Theta
east advection ahead of deep trough digging through the northern
rockies. Leading weak pv impulse/divergence q swings east/northeast
across the area during the night...but hard to push much past a
modest chance pop for South Dakota County Warning Area...and highest in the far northwest...
with clear lack of a boundary.
Sunday is probably the most interesting day of the longer range
period with next strong jet diving through the central rockies...
making a sharp turn across the northern plains. Model development
of outflow type jet to north...enhancing the divergence
signature...probably more of a result of convective development
than driver of development. Nonetheless...is a very good setup for
Depending on the wave timing...which varies from GFS a bit faster
to European model (ecmwf) a bit slower...may time out to coincide with diurnal
cycle...as deeply convergent boundary pushes across the County Warning Area. Much
stronger winds for more significant and distributed deep layer
shear profile...but instability may be somewhat less than desired
for a widespread severe threat. Therefore...anticipate that there
will be a mentionable severe threat for Sunday afternoon and
evening...at this point looking from I 29 and eastward.
Precipitation should pick up the pace with organized system
developing and be well clear by daybreak Monday...leaving a quiet
and slightly cooler than normal day.
Monday night and Tuesday could see a little return flow activity
as another subtle wave zips through. Overall...will be in a
transition back toward western trough the Central Plains ridge by
middle to late week...making for stronger return flow by Wednesday
along with a small chance for a shower or storm just beyond the
forecast period. Temperatures likely to start just below normal early
week...and moderate to slightly above normal by Wednesday.
Aviation...(for the 12z tafs through 12z Friday morning)
issued at 638 am CDT Thursday Aug 28 2014
Area of warm air advection rain is expected to lift through the
region this morning leading to IFR conditions. Visibilities and
ceilings should gradually improve through the late morning
hours...however...additional instability showers should develop
and continue through the afternoon ahead of the cold front working
though the area. Cold front becomes orphaned...and may not make it
to Sioux Falls/Sioux City before stalling out. May have the
potential for stratus and/or fog to develop across the area
tonight. Hinted at the potential...but only put a ceiling at ksux
where better low level moisture will linger.