Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 171953
AFDABR

Area Forecast Discussion
National Weather Service Aberdeen SD
153 PM CST Sun Dec 17 2017

.SHORT TERM...(This Evening through Monday Afternoon)
Issued at 153 PM CST Sun Dec 17 2017

A weak high pressure ridge is departing the area and that has
allowed surface winds to start to turn to the southwest. This
departure and an upper trough moving east through western SD has
led to the clouds that covered much of the area this morning to
start shifting to the east. Where the clouds have cleared out,
temperatures have risen into the upper 30s and 40 degrees, while
the clouds have held temps down around 5 degrees over eastern SD
and west central MN. Expect the clearing to slowly take hold
over the rest of the area through the evening hours, but will
need to watch the cirrus approaching from Montana for arrival
late this evening or during the overnight hours. One other thing
to mention for tonight is the NAM going all in on low clouds
settling in over the eastern half of of SD and western MN tonight
into tomorrow morning. With winds increasing just above the
surface under the warm air advection, thinking there will likely
be enough mixing into the dry air aloft to keep that from
occurring.

For tomorrow, highs and wind gusts are highly dependent on the
amount of mixing that occurs during the afternoon. Seems like the
models have trended the mid clouds to be a little more scattered
(more into ND), which may be enough to increase mixing and tap
into some of the stronger winds at the base of the warm air
advection. But the cirrus currently in MT looks to move over the
area, so confidence is too low on mixing depth to stray too much
from MOS and model blends for highs.  As for winds, did trend the
forecast up some, with gusty WSW winds ahead of the weak cold
front that will move through Monday evening. At this point, with
the low sun angle and no real good isallobaric component, the wind
gusts will be highly dependent on mixing and may only last for an
hour or two over north central SD (gusts to 30-40 mph). Farther
east, the direction is favorable for downslope winds on the east
side of the Coteau and the high resolution models are picking up
on that idea. Decided to follow that thought and show gusts of
30-40 mph for much of the afternoon in that localized area.
Finally, tomorrow will be the warmest day for the rest of the
year, with temperatures rising well into the 40s and even some 50s
over central SD. If we get enough mixing, could even see some
areas in northeast SD nearing or hitting 50 degrees. The
combination of the winds and lower RH will produce a brief period
 of elevated fire danger tomorrow afternoon.


.LONG TERM...(Monday Evening through Sunday)
Issued at 153 PM CST Sun Dec 17 2017

Main concern for the long term portion of the forecast is the
pattern change and associated system brushing the area around mid
week.

Cold front will slide southeast through the area Monday evening,
but should only bring a brief period of clouds and a shift of the
wind to the northwest. Along with the wind shift, expect a couple
hour period of gusts around 30 mph before winds settle back down.
Models do vary on the amount of low level cooling, so there is
some uncertainty on how widespread the stronger winds will be.

For Tuesday, expect dry and generally sunny conditions under a
weak surface and low level ridge. That will be short lived as
warm air advection takes hold ahead of the developing low to the
east of the Northern Rockies. Expect a brief period of clouds as
this pushes to the northeast, then much of the area looks to be
in the dry slot. It does look like far northeast SD and west
central MN will be on the edge of the WAA/Moisture, so do think
they will see some light show showers Wednesday afternoon.

Now for the main snow chances in the long term. Models have slowed
the system down considerably compared to 24hrs ago, with the main
snow chances occurring late Wednesday night into Thursday. There
has also been a trend up in the QPF forecast due to the right
entrance of the west-east upper jet that stretches east into
the New England states lingering back over southern MN. Meanwhile,
the jet rounding the shortwave will meet up with the surface low
in the Central Plains and lift it northeast late Thursday
afternoon into Friday. Before that occurs, should see snow develop
to the northwest of the inverted 850-700mb trough that develops as
the 850mb low merges with the intensifying low farther to the
south. Starting to see increased consistency in the GFS/GEM/ECMWF
for this system and greater potential for light snowfall on
Thursday. Since it`s still 4-5 days out, will follow a general
model blend at this point and will fine tune over the coming days.
One other factor to consider are the winds on the back side of the
system. They will be highly dependent on the timing of the
surface low as it moves northeast and the surface high sliding
east across ND on Thursday. With still varying solutions, will
hold off on trending too far from the blends. Since it looks like
a greater snow potential from yesterday, but a little lighter on
the winds, will continue to highlight a basic outlook for reduced
visibilities in the HWO.

Behind the system, colder air will move into the area as the
upper trough axis moves over head and continues through next
weekend and likely through the end of the month. 850mb temps fall
into the -10s C by Saturday (around 10th percentile based off
KABR/KHON sounding climo) so expect temperatures to be well above
normal. With the area under the upper trough, will see occasional
shortwaves rotating through the area, potentially producing an
isolated light snow shower. These waves and weak clipper systems
will also produce periods of stronger winds and with the cold
temperatures in place, could see some low wind chill concerns,
especially heading into next week. Speaking of early next week,
models continue to indicate another colder surge of 850mb temps
arriving for Sun/Mon. 12Z GEFS indicates around a 40-50%
probability of experiencing 850mb temps below -20C, while the GFS
continues to run on the cold side of the ensemble mean. Either
way, will be much colder than what we`ve grown accustomed to so
far this month.


&&

.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
Issued at 1140 AM CST Sun Dec 17 2017

Expect VFR conditions to occur through the period. Still some
lingering low end VFR ceilings along the KPIR to KATY line that
are pushing east, but expect them to slowly raise and largely be
south of the TAF sites.  Otherwise, the mid clouds over the area
should depart from west to east this afternoon into evening
before high clouds approach late tonight into Monday. Winds will
become gusty late in the period and expect the strongest winds of
around 25kts near KMBG to occur after the TAF valid period.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
&&

$$

SHORT TERM...SRF
LONG TERM...SRF
AVIATION...SRF



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