Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 170520 AAC

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1120 PM CST Sat Dec 16 2017

Issued at 543 PM CST Sat Dec 16 2017

Removed mention of precipitation across south central SD this
evening. No rain or snow is currently reaching the surface and
hi-res guidance has backed off from any mention. Cannot completely
rule out a sprinkle or a flurry, but the thinking right now is
that it will remain dry.


.SHORT TERM...(This Evening through Sunday Afternoon)
Issued at 310 PM CST Sat Dec 16 2017

Cloud cover is the main forecast challenge tonight, along with fog
potential for eastern areas. The degree of cloud cover will also
play a part in how cold temperatures will get overnight. Any
sufficient clearing for a period of time may allow temps to fall
below forecast lows. Although, going with mostly cloudy to cloudy
skies overnight as low clouds begin to fill back in. Plus, there
should be quite a bit of mid-level cloudiness as well. Low clouds
look to affect the eastern CWA the most, with lower levels saturated
enough to perhaps produce some fog as well. Several of the models
are indicating this possibility, so have inserted this into the
grids. Don`t have real high confidence in how the lower sub-VFR
clouds will pan out during the overnight hours, as they have been
hit and miss for most areas today. Seems as if the lower level
moisture axis ends up splaying out from the eastern CWA, down
through FSD CWA and into south central SD.

For Sunday, surface high pressure moves off to the east with winds
remaining light from the southwest by afternoon. Will likely still
be dealing with cloud cover and potentially fog in the morning, but
there seems to be a trend of clearing skies from the northwest
throughout the day. Temperatures will remain above normal with highs
in the upper 30s to lower 40s once again.

.LONG TERM...(Sunday Evening through Saturday)
Issued at 310 PM CST Sat Dec 16 2017

The long term forecast is a tale of two halves, with the first
half of the work week above normal before a system moves through
the region on Wednesday night and ushers in significantly colder
temperatures that will likely remain in place through the rest of
the month.

For the first half of the week, we`ll start with an elongated
upper trough stretching from the southwest CONUS through the
Upper Mississippi Valley which will be quickly cutoff by a 125kt
jet moving through the Pacific NW and British Columbia. The jet
and associated shortwave will move a low east across northern
Manitoba and it will sweep a dry cold front across the area late
Monday afternoon.  Ahead of the front, could briefly see winds
get a little gusty out of the southwest (to 25-30kts) but weak
warm air advection aloft and the potential for some mid clouds
may limit mixing from reaching the stronger winds around
850mb. That mixing will also determine highs and have generally
followed a blend of the non raw model guidance (highs into the 40s
and lower 50s). Behind the front, winds will likely be gusty out
of the northwest, but with limited cold air to keeping lapse rates
from steepening too much, only expect brief or occasional gusts to
the 30kt range. High pressure will build over the region on
Tuesday and temps will be a little cooler (upper 30s and lower
40s) with 850mb temps falling into the single digits below 0C.

Will see a significant shift in the pattern on Wednesday night as
an upper trough drops southeast through the western CONUS. This
will lead to the development of a surface low east of the
Northern Rockies, which will dive southeast through the Central
Plains by Thursday afternoon. The evolution of this low will
determine our precip/snow chances for the long term. As the low
drops southeast through wrn SD/NE and northeast Colorado
Wednesday night (tied to the PV anomaly and better energy rounding
the trough), a inverted surface trough will stretch northeast
into eastern SD and southern MN (due to the right entrance of the
upper jet). That surface trough and 850-700mb low/trough will
likely produce some snow across North Dakota and Western SD, but
there is some question on how much of it will affect our CWA due
to dry slot on the se side of the 850-700mb trough/low moving
through south central ND and northeast SD. Models continue to show
much of the precip splitting the area as the upper dynamics split
as the system moves through this area, so thinking at this point
there will be some light snow accumulations (generally a dusting
to 2", with the 2" dependent on the location of the warm air
advection precip that will be somewhere around ne SD) and gusty
winds on the back side. While the system splits, a strong high
building in behind the low will aid in a producing a quick 6-12hr
period of strong winds over the CWA. Max wind gusts will be highly
dependent on the timing/strength of the inverted trough, but will
likely see some gusts around 35kts. If this ties in with any snow
occurring, visibilities will be reduced during snow. With no snow
currently on the ground, not thinking we`ll see enough snow at
this point to see blowing snow concerns after the snow ends.

For the rest of the period, expect below normal temperatures and
occasional periods of light snow as we sit under the upper trough
axis. Still a lot of variability in how cold temps will be aloft
in the GEFS/ECMWF-EPS, but mean values in the lower teens below
would fall between the 10th and 25th percentile based off
KABR/KHON sounding climo. This will lead to highs ending up in the
teens (around 10 degrees below normal), which will feel quite
cold compared to what we`ve been used to this month.


.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
Issued at 1118 PM CST Sat Dec 16 2017

VFR conditions will prevail at most taf sites. Cigs and vsby will
fall to IFR at KATY by early morning in stratus and fog.
Conditions will improve by 17z.




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