Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 150554 AAC

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1254 AM CDT Thu Mar 15 2018

Issued at 940 PM CDT Wed Mar 14 2018

Overall, the forecast is on track. Made a few adjustments to the
coverage/timing of fog mention in the forecast for later tonight.
Also, monitoring high cirrus clouds that have slowly moved into
the west-river counties of this CWA. Timing and extent to which
those high clouds move east overnight will be a major determining
factor of where fog develops.

UPDATE Issued at 632 PM CDT Wed Mar 14 2018

See below for an aviation forecast discussion for the 00Z TAFs.


.SHORT TERM...(This Evening through Thursday Afternoon)
Issued at 325 PM CDT Wed Mar 14 2018

Quiet and mild conditions are expected in the short term period, as
an upper level ridge slides across the central part of the country.
Surface high pressure will dig southward from central Canada to the
eastern Dakotas/Minnesota tonight, then begins to get pushed east on
Thursday as low pressure intensifies over eastern Colorado/western
Kansas. With the warm temperatures today resulting in continued snow
melt, cannot rule out some fog development overnight, especially
across the eastern CWA which will be under stronger influence of the
surface high.

Temperatures will remain mild with overnight lows in the mid teens
to lower 20s, and highs on Thursday in the mid 30s to lower 40s.

.LONG TERM...(Thursday Evening through Wednesday)
Issued at 325 PM CDT Wed Mar 14 2018

Main forecast concern continues to be a negatively tilted trough
that ejects into the plains Friday, with a deepening surface low
that tracks from western Colorado, across Nebraska and into Iowa by
Friday night. There is still some differences between
timing/track/intensity but good agreement on the general trends,
lending some degree of confidence that parts of the area will be
impacted by this spring snowstorm.

The main question is timing and how far north to take precipitation.
Generally, the NAM remains an outlier, although closer to
expectations when these systems encounter a fetch of dry air from
high pressure anchored over the Manitoba/Ontario provinces.
Unfortunately, no updated GEFS plumes but SREF plumes still show
a wide array of outcomes across the area, which again places doubt
in the deterministic NAM output. Until these come into alignment
or some other form of guidance matches the NAM outcome, the
preference is to follow the EC/GFS/Canadian and SREF trends and
jump on these higher impact scenarios.

How impactful? SREF probability of an inch QPF in 24 hours is 30%
across south central South Dakota, and greater than 50% for a 12
hour QPF of 1/2 inch, suggesting the potential for quite a bit of
moisture in that area. Trend in BUFKIT profiles support deep
dendritic growth zone above 700mb, with lift in that layer, however
outside the period of peak dynamics, we lose saturation within the
dendritic growth zone and are probably left with drizzle/freezing
drizzle. Peak intensity across south central South Dakota is during
daylight hours which may cut down on ice so long as morning
temperatures don`t drop to precipitously. Currently the anticipation
is for clouds to move in overnight but guidance temperatures are
showing lows in the teens and 20s, thanks to a lack of warm
advection into the region. So the evolution is light ice, followed
by heavy/wet snow with the northern boundary still probably
suffering some loss due to evaporation, with light ice on the tail
end. Not much for high pressure to follow so clouds are expected to

Now, flash forward to late Sunday and we see another system that
could impact the region. Guidance has latched onto a surface track
across Kansas, though flow aloft becomes strongly diffluent across
South Dakota and Nebraska spreading precipitation this far north.
Still lots of time to sort this one out however.


.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
Issued at 1254 AM CDT Thu Mar 15 2018

With a light east-northeast wind continuing, expecting a couple of
areas of potential fog development over the next few hours where
said wind is a terrain-influenced upslope wind, with a gradual east
to west migration/expansion. There are some high cirrus clouds over
western and central South Dakota slowly working their way east.
The eastern-most edge of these clouds will end up being the
determining factor for whether or not fog will form. For the time
being, guidance suggests KABR (and perhaps maybe KATY) has the
greatest probability of seeing sub-VFR fog or low clouds between
now and appx 15/16Z on Thursday. Since the cirrus clouds are over
the KMBG/KPIR terminals now, have them staying VFR for the whole
TAF valid period.&&



LONG TERM...Connelly
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