Scientific Forecaster Discussion
fxus63 kfsd 200908
Area forecast discussion
National Weather Service Sioux Falls South Dakota
308 am CST Wed Nov 20 2019
Short term...(today and tonight)
issued at 307 am CST Wed Nov 20 2019
Focus in the near term is on wave currently sliding into northwest
Mexico, slated to lift northeast across the plains tonight, and into
the Great Lakes on Thursday. Relatively moist southerly flow ahead
of the wave will result in another seasonably mild day today, with
increasing low level moisture expected to build northward through
the day. Could see some spotty late afternoon showers in response to
Theta-E advection along/east of I-29, but main lift with the wave
will move in by late evening/overnight, and should see coverage of
rain increase accordingly.
Given track of the wave/surface low, heaviest precipitation should
be across southeast portions of the forecast area, where 1/3-2/3
inch of rainfall is likely. Consensus timing on the cold air pushing
in behind the wave still results in mainly rain across northeast
Nebraska/northwest Iowa, possibly transitioning to some very light
snow as the system is exiting early Thursday morning.
Farther to the northwest, still a good chance at seeing some light
precipitation tonight, but northern stream wave looks to be tracking
too far north to have much influence in developing a secondary band
of moderate precipitation north of I-90. Models in decent agreement
in showing a decrease in precipitation potential, and thus have
trimmed back our snowfall amounts to around a half inch or less.
Although snowfall rates should be rather light, could still see some
decent visibility restrictions due to increasing northwest winds,
gusting at times to near 35 mph across our west and north after
midnight tonight. Cannot rule out some mixed precip types (freezing
rain or sleet) if near surface cold air undercuts the departing warm
layer aloft during the transition, but appears this would be brief
and spotty, more likely in higher elevations which could cool faster.
As far as temperatures, highs ahead of the wave today are expected
to reach the upper 40s and 50s. The 40s should hang on across the
southeast until at least midnight, while readings drop back toward
freezing in our western counties. By daybreak Thursday, look for
temperatures below freezing most areas, with some mid 30s lingering
ksux-kspw and points southeast.
Long term...(thursday through tuesday)
issued at 307 am CST Wed Nov 20 2019
After near term system exits off to the east Thursday morning, we
will be looking at a relatively quiet stretch of weather through
this weekend and into early next week. Temperatures will be the main
focus, briefly below normal Thursday/Friday, but then moderating
back above normal Saturday-Monday. The coldest day will be Thursday
in the wake of the departing system, with temperatures in the 20s
and 30s accompanied by strong northerly winds 20 to 35 mph. Much
lighter winds Friday as a surface ridge moves across the region.
Increased diversity in details for the weekend, with some solutions
depicting a stronger wave sliding southeast across the region on
Saturday. Moisture limited, so expected to pass through dry, but
could delay the warm-up until the latter half of the weekend.
With the Thanksgiving Holiday coming up next week, we are closely
monitoring model trends for the pre-Holiday travel period. A few
solutions showing the potential for a more impactful system during
this period, but very little consistency among various operational
and ensemble solutions, so confidence in the forecast from Tuesday
Onward is quite low at this point.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1045 PM CST Tue Nov 19 2019
Tonight: VFR conditions are expected at all sites.
Wednesday: VFR conditions are expected to continue through the
day, before rain showers move through. MVFR conditions will begin
to impact the region by late Wednesday afternoon as precipitation
chances increase. Rain may mix with or change over to snow by
late Wednesday night. However, did not mention -snra/-sn in tafs
due to low confidence in timing. IFR conditions are possible
towards the end of the taf period.
Additionally, some low level wind shear of 40 kts between 1400-2000 ft will be
possible during the morning at kfsd and ksux.