Scientific Forecaster Discussion
fxus63 kfsd 222256
Area forecast discussion
National Weather Service Sioux Falls South Dakota
556 PM CDT Sat Jul 22 2017
Short term...(this evening through sunday)
issued at 221 PM CDT Sat Jul 22 2017
A weak short wave is moving across our southern zones at this time.
It is largely responsible for creating a lot of mid and upper level
clouds extending from central Nebraska into our southern zones which
has modified our temperatures cooler a bit this afternoon for many
areas south of I 90. Put in a low isolated pop for our zones in
south central South Dakota late this afternoon due to some very skittish
activity in southwest South Dakota at this time. There is a strong pv gradient
along the 1.5 pressure surface in that area through 00z denoting the
short wave, with an exiting jet streak further east in southern
Minnesota placing this area in the right rear quad of the jet.
Therefore large scale ascent is enhanced. Ml cape is very thin in
that area this afternoon however, with 30 knots of 0-6km shear and
unidirectional soundings out of the northwest. So not even sure if
any activity occurs at all, if it will generate much in the way of
lightning/thunder. The dry dew points have been slow to enter our
forecast area, and one can see the affects of recent rainfall in
Huron and Brookings the past week or two, as their temperatures are
still only in the lower 80s despite being mostly sunny all day.
As the short wave in our southern zones, and upper trough to our
northeast move away from this region, the drier air will finally
advect into our forecast area, which should produce lows in the
upper 50s to lower 60s. Sunday will feature a northerly surface wind
and finally drier air. Temperatures mix to highs ranging from the
upper 70s in parts of southwest Minnesota, to near 90 in south central South Dakota.
Most locations will be in the seasonal 80s. There could very likely
be some stratus moving southward into our northeast zones very late
tonight and Sunday morning behind the departing upper trough.
However they will scour out not severely impacting highs for
Long term...(sunday night through saturday)
issued at 221 PM CDT Sat Jul 22 2017
Surface ridge slides across the forecast area Sunday night, and with
drier air mass in place, should be able to see lows fall into the
upper 50s-lower 60s most areas. Southerly flow behind the ridge will
begin to draw low level moisture back into the region Monday, though
will not yet be to The Levels we have seen of late, with dew points
in the lower-mid 60s expected by late Monday. Thermal ridge will
also work into our far western forecast area, pushing temperatures
back into the mid-upper 90s west of the James River. Subtle wave
could bring some spotty showers/storms to the area Monday night, but
most areas will remain dry.
Upper jet pushes into the northern plains, with the axis north of
our forecast area, on Tuesday, with the associated wave dragging a
cool front into eastern South Dakota by late in the day. This could
become a focus for storm development late Tuesday/Tuesday night,
with better forcing focused in northern parts of the area. Upper jet
placement is somewhat uncertain, but consensus places axis north of
the North Dakota/South Dakota border. This places greatest deep
layer shear north of Highway 14, with fairly weak farther south
toward I-90 and points southward. Cannot rule out an isolated strong
to severe storm during this period, though greater threat looks to
be to our north at this time.
Model consistency seen at this time yesterday regarding precip
chances Wednesday night is, as anticipated, no longer present, with
GFS/European model (ecmwf) both trending drier for this period as boundary mentioned
above pushes south of the area by late Wednesday. Will continue to
carry some chance pops for now, as earlier European model (ecmwf) and latest Canadian
are slower in moving the front through the area, and thus still a
little more boisterous with their chances.
Late next week, surface high begins to dominate across the upper
Mississippi Valley, with any significant low level moisture cutoff
by a boundary pushing through the central/Southern Plains. This
should lead to a fairly dry period Thursday-Saturday, with daytime
temperatures near to a few degrees below normal.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 555 PM CDT Sat Jul 22 2017
VFR conditions expected at taf sites through the period.