Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1059 PM CST Thursday Nov 27 2014
Short term...(this evening through friday)
issued at 401 PM CST Thursday Nov 27 2014
Isentropic lift on the 290-295 k surface will lead to the
development of a light band of snow across the northern half of the
forecast area late this afternoon into the evening hours. Not a lot
of moisture with this transient system...with the column only
saturating for a few hours in any given location. Given that this
the case...only expect a few tenths of an inch of snow or less in
most locations. Have lows early this evening...as Arctic air mass
retreats off to the east and continued warming is expected through
the night into Friday. Did not make a lot of changes to highs on
Friday...but did slow the warming slightly across southwest
Minnesota as the weak southeast flow holds on there. Where the
southerly and southwesterly winds prevail...temperatures should warm
nicely eating away at the snowpack.
Long term...(friday night through thursday)
issued at 401 PM CST Thursday Nov 27 2014
After a solid day of melting and building the inversion...the
pooled moisture and decreased gradient...especially to the
northeast of the warm frontal boundary...will be a good set up for
fog and stratus development. Fog will not wait until later in the
diurnal cycle...and will likely find seeds of fog already by middle
evening...and likely continuing into at least middle to late morning
on Saturday with flow fairly weak. Eventually...the increasing pre
frontal westerly flow and eastward progression to the warm frontal
boundary will flush out the shallow clouds/fog...which will open
the door for a mild start to the weekend. In terms of temperatures...
will be somewhat non diurnal Friday night as may find a greater
dip earlier in the night...then steadying or rising as temperatures
continue to warm aloft in the cloud layer. With increased
moisture...found it difficult to drop temperatures as much as most
Even with a relative lack of winds in the surface layer...would
not take much off the morning ksux soundings to reach the upper
50s with minimal mixing. Appears that recent snows adversely
impacting the near surface thermal fields in models...with raw
guidance fields holding 30s to lower 40s corrupting all the
blends. Have held on to very warm temperatures in the previous forecast
which have support in raw off surface temperature fields. With strong
frontal zone dropping southward through the County Warning Area during the
afternoon...will find temperatures varying from around 40 north to at
least the upper 50s in the Missouri Valley.
May end up with a few flurries in transition to the colder air by
later Saturday night and Sunday...as will have to wait until the
lower levels drop toward more favorable ice formation range. Middle
level boundary weakens and undergoes frontolysis coming
southward...and thus without any deeper support would expect
flurries at most. Will wait to add these in later forecasts. Temperatures
will do well to increase much more than a few degrees off early
day readings...with wind chills in the single digits above and
Arctic high settles into the northern plains through Monday.
However...will start to modify with lack of snow cover farther
south across the region. More below zero wind chills expected...
with lowest readings for early Monday morning. Return flow starts
on Monday afternoon...with strong southerly gradient on Monday
night. General pattern of 40 to 50 knot low level jet and
inversion suggests some enhanced ridge winds from east central South Dakota
into SW Minnesota...and have started hints of this in current grids.
By midweek...guidance solutions become so incongruous that seems a
lost cause to outguess the uncertainty. As an objective suite...
Canadian is buried in deep and cold northwest flow...the European model (ecmwf)
similar but less amplified...and the GFS is sporting a southern
stream system with fairly decent mixed precipitation scenario.
Have not strayed from general guidance beyond Tuesday.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1057 PM CST Thursday Nov 27 2014
Band of middle and high clouds continues to stream across the
forecast area but will shift east and become scattered through the
early morning hours. A few remaining flurries near the Buffalo
Ridge should also end within the next few hours. VFR conditions
are expected through the taf period.