Scientific Forecaster Discussion
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
406 PM MST Thursday Dec 18 2014
Short term...tonight and tomorrow
issued at 238 PM MST Thursday Dec 18 2014
Upper level trough continues to move across the northern
plains...with the trough axis sliding into the eastern Dakotas.
Eroding of the low clouds continues to spread across western S
Dakota...with most of the County Warning Area now seeing mostly sunny skies and
temperatures above freezing. With mostly clear skies continuing into the
evening hours temperatures will fall quickly into the 20s after sunset.
Temperatures bottom out in the teens to lower 20s. Tomorrow a week
disturbance will move across the area. Soundings are fairly dry in
the lower levels and expect only some passing middle/high clouds. This
should not impede much on warming with highs in the 30s to lower 40s.
Long term...Friday night through Thursday
issued at 238 PM MST Thursday Dec 18 2014
Primary forecast concerns in the Middle Range of the forecast deal with
light snow chances over northeast Wyoming Saturday morning and with
a slight warming trend in temperatures through the weekend. Focus
then turns to the pattern shift for next week as it turns active for
much of next week.
A shortwave trough drops southeast across South Dakota Friday night
with another shortwave trough from the Pacific crossing the northern
rockies and into northeast Wyoming by 12z Saturday. The first system
will move through dry as there is a large lack of moisture to work
with...but there may be enough moisture still with the second system
after crossing the mountains to bring some light precipitation to
the northeast corner of Wyoming Saturday morning. The GFS has a
clear dry layer just off the surface so may not be enough moisture
to make it through the boundary layer for measurable precipitation.
The NAM is not as robust with the dry air but would indicate there
may be enough lift and moisture to at least include a slight mention
of snow Saturday morning.
Temperatures will see a slight moderation through the weekend.
Behind the weak Pacific front that moves though Saturday morning
warmer air will begin to push north as longwave ridging builds over
the eastern Pacific. By Sunday...a large area of low pressure at the
surface develops over southwest Canada with a trough extending south
along the Front Range of the Rocky Mountains. Southerly flow in the
lower levels will bring warmer air northward...with highs expected
to rise into the low and middle 40s across portions of the forecast
area on both Saturday and Sunday. A question with temperatures will
be if and/or how much of an impact the snow cover will impact
temperatures. Kept highs a bit cooler where snowpack exists...while
other areas should easily rise into the 30s and 40s.
By Monday...northwest flow aloft has been established across the
north central part of the Continental U.S. As the upper level pattern changes
with a longwave ridge over the eastern Pacific and a fairly deep
trough centered across the plains and Mississippi River valley. The
models are in good agreement with this pattern change which looks to
stay in place through the week. This pattern will keep the weather
active through the remainder of the period as multiple weak
shortwave troughs push over the ridge and pass across the local
area. Timing of the individual systems is problematic as is
typically seen in these patterns so it is quite difficult trying to
pinpoint when the higher chances for precipitation will be. This
leads to precipitation chances through most days Monday through
Thursday. With the northwest flow though...will get upslope flow
over the Black Hills much of the period so as long as there is some
moisture to work with will get snow to develop and these areas will
maintain the more consistent and higher precipitation chances.
Amounts Don/T look significant with any of these quick hitting
systems at this point so the current forecast just has light amounts
currently. By Wednesday night and Thursday the models are digging a
slightly more significant shortwave down through the central
rockies. If this holds out...the precipitation will be mostly across
the southwest part of the forecast area.
One thing that is fairly certain is as the northwest flow is
established that temperatures will be cooling off back to near
normal levels...and colder by late next week with highs likely in
the low to middle 20s by Thursday.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 403 PM MST Thursday Dec 18 2014
VFR conditions for both krap and kgcc expected the next 24 hours.
MVFR ceilings across parts of central South Dakota will shift east of the area by
early this evening.