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FXUS63 KFSD 070430

National Weather Service Sioux Falls SD
1030 PM CST Fri Dec 6 2019

.SHORT TERM...(This Evening through Saturday)
Issued at 401 PM CST Fri Dec 6 2019

Clouds have been holding tough, but are steadily losing hold north 
of I-90, eroding from the south and west as the southerly flow has 
been increasing through the afternoon on the back side of the 
exiting ridge.  Still weak enough gradient where temp falls could be 
fairly quick tonight mainly across northwest IA, but even there the 
winds will increase enough to set up a minor non-diurnal trend to 
temps.  The big question for tonight will be whether the enhancing 
inversion overnight will set up redevelopment of some lower clouds.
Models are fairly decent in establishing a shallow fairly saturated 
layer, especially over snow covered areas. It is possible that if 
winds can fully decouple despite a gradual increase in surface 
gradient toward morning, some low areas could see a bit of fog, and 
there may be some patches of stratus that develop in southwesterly 
upslope areas in the James River valley and Buffalo Ridge. Will 
leave this to monitoring, as probability of occurrence at this time 
is a bit low.

Saturday still appears to be the best day of the forecast cycle in 
terms of temperature, although will become a bit breezy many 
locations through the day. The bane of our potential warming 
tomorrow would be the extent to any low clouds, as any limiting of 
the sad insolation would have a big impact.  Most of the raw models 
seem to be overreacting to the inversion, and not allowing much near 
surface warming. Truth will likely be one similar to other southerly 
flow warmer days of late, where the warming has been underestimated 
along with the winds, in this case at least for areas without snow 
cover.  Should see those areas and buffering locations reach the 
40s, with mainly upper 30s elsewhere.  

.LONG TERM...(Saturday Night through Friday)
Issued at 401 PM CST Fri Dec 6 2019

Should continue to be quiet on Saturday night, as a leading but 
dry mid-level trough scrapes past just to the north during the 
evening. This should help a leading wind shift to likely sag into 
at least the far northern CWA by Sunday morning, with the warmer 
air remaining in place near the surface and aloft across the area.
The decreasing gradient toward the boundary will open the window 
for a quick drop late in lower-elevation snow-covered locations 
across mainly southeast SD, where a few lower to mid 20s could 
disrupt the general upper 20s to lower 30s expected.

Say goodbye to the warmth as the weekend works toward an end, but 
could still shake out a relatively mild day as the main cold air 
push will hold off until later into the afternoon and especially the 
evening. Could still see some readings into the lower 40s toward 
Highway 20, but likely to not see much more than 30 with temps 
falling during the afternoon north of I-90. As the near surface 
layer moistens behind the front, the saturation remains shallow and 
lacks ice bearing temps later in the afternoon.  Weak shallow lift 
in a front-relative sense, so could be enough to get a few drizzle 
drops going within the moist layer.  Best chance for any deeper 
saturation holds off until very late toward Highway 14. For this 
reason, have included a mention of some freezing drizzle before the 
further cooling and/or deepening saturation allows introduction to 
ice as the post frontal zone works southward during the evening and 
early overnight.  In general, the dynamics supporting the 
frontogenetic banding have shifted just a bit northward in the 
ensemble set (the Canadian and several relatives remain the holdout 
to keeping the band of 0.25" QPF further south closer to I-90), with 
generally one to two tenths of an inch in the max band toward 
northern portions of southwest MN. Onset a bit slower as well, 
especially heading to the southeast toward northwest IA. There 
remains a favorable overlap of the lift and the increasing depth of 
the dendritic growth zone as the band evolves Sunday night into 
early Monday, so hard to picture there not being measurable snow for 
most areas.  The wave sharpens moving across South Dakota Sunday 
night, which will also aid to enhancing the low-level gradient, 
opening for some spots of blowing/drifting snow as well. 
Precipitation should be winding down from west to east during the 
morning hours Monday, with temps not likely to recover more than a 
token amount during the day with fresh snow and the invading cold 
air mass.

The other facet for the week will be the colder air. Still some 
greater uncertainty in timing of features by Wednesday onward for 
the week, which could throw some challenge to timing the coldest 
potential.  For now, the coldest period should be contained in the 
Tuesday-Wednesday night time frame, when the apparent temperatures 
will likely be from -10 to -25 for most, and for much of the time. 
Could end up being a brief period or two where there is a risk for a 
few flurries or a brief period of light snow, but timing of the 
individual waves/brief pulses of warm advection, have little 
consistency.  Gradual warming trend does appear toward the end of 
the week, but should be slowed.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1028 PM CST Fri Dec 6 2019

Some limited potential for fog to develop yet tonight, but
probabilities remain too low to include in TAF. Will need to
monitor areas near HON for low stratus development, but elsewhere
VFR conditions are anticipated into Saturday morning.

Winds will increase through the day on Saturday, with 25 mph gusts
likely in the afternoon. 




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