Area Forecast Discussion
Issued by NWS Aberdeen, SD

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000
FXUS63 KABR 240919
AFDABR

Area Forecast Discussion
National Weather Service Aberdeen SD
319 AM CST Sat Feb 24 2018

.SHORT TERM...(Today and Tonight)
Issued at 303 AM CST Sat Feb 24 2018

Fog continues across the James valley, Coteau, and into western
Minnesota. This will likely remain in place until snow starts to
fall in the area later today.

The main question is how much snow will the area receive today.
While the upper trough is positively tilted currently, it becomes
more negatively tilted as it heads into the Dakotas/Minnesota
later today. This is the impetus for rapid mid level frontogenesis
and thus generation of snow. GEFS and SREF are both on the order
of .2 to .3 inches QPF for the Watertown area, .1 to .2 for
Aberdeen, and around .1 for Pierre area, so that kind of gives us
a range of values assuming a 15:1 to 20:1 ratio of 1 to 5 inches
across the CWA. The deterministic NAM looks like the high end of
guidance while the GFS deterministic is a bit more modest. Omega
values as the trough transitions increases to 10 to 20 microbars
between 15 and 21Z with the GFS being a tad faster which is also
the reason for the lower QPF. Again, this is mainly at the
600-500mb levels with less lift in the low levels, but it still
appears that this will occur over a deep dendritic growth zone,
which may push up the snow ratios some. CAMS suggest there also
may be more convective/banded type elements, so there will
probably be a tighter gradient to the western edge of the
snowfall, and probably more variation in totals which sets it
apart from the last 2 events which were much more uniform.

The rapidly deepening surface low is placed well to the east and
will generally have limited effect on the pressure gradient. With
only around a 6-8mb gradient from southwest to northeast, and a lack
of strong thermal advection on the backside, winds are only going to
average around 10 to 20mph range. While that may be enough to cause
some minor blowing and drifting of the very light snow in place,
as well as what is expected to fall, its probably not all that
significant to this event.

Overall, only made some minor adjustments in snow totals, again
on the 2 to 5 inch range for counties in the Advisory. Did adjust
timing on the Advisory however to better match short term
CAM/ensemble guidance.


.LONG TERM...(Sunday through Friday)
Issued at 303 AM CST Sat Feb 24 2018

The extended period starts off with surface high pressure to the
south of the region, and low pressure to the northwest. The pattern
will be dominant until Tuesday, when high pressure slides across the
area. Around mid week, a low pressure system will slide somewhere
across the central part of the country, but the models are currently
in terrible agreement with where that will be. The ECMWF takes it
across the Southern Plains, the GFS across northern
Oklahoma/southern Kansas to the central Great Lakes, and the
Canadian across Kansas to Iowa/southern Minnesota/southern
Wisconsin. The Canadian is the only solution at this time to have
precipitation reach the Aberdeen CWA, but the GFS is close.
Therefore, will stick with small pops for the Wednesday afternoon
through Thursday afternoon time frame for the eastern half of the
CWA. High pressure brings a return to dry conditions on Friday.

Temperatures will be fairly mild, with highs mainly in the upper 20s
to upper 30s, and overnight lows in the teens.


&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday Night)
Issued at 1149 PM CST Fri Feb 23 2018

IFR/LIFR fog and stratus will remain through the night into Saturday
morning at ABR and ATY. Otherwise, another upper level low pressure
trough will spread snow across the region for Saturday with IFR/MVFR
conditions with ceilings and vsbys. One to 5 inches of snowfall is
expected.


&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory from 9 AM this morning to 9 PM CST this
     evening for SDZ006>008-011-017>023.

     Dense Fog Advisory until 8 AM CST this morning for SDZ006>008-
     011-018>023.

     Winter Weather Advisory until 6 PM CST this evening for SDZ036-
     037-048-051.

MN...Winter Weather Advisory from 9 AM this morning to 9 PM CST this
     evening for MNZ039-046.

&&

$$

SHORT TERM...Connelly
LONG TERM...Parkin
AVIATION...Mohr



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