Area Forecast Discussion
Issued by NWS Aberdeen, SD

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000
FXUS63 KABR 240150 AAB
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
850 PM CDT Fri Oct 23 2020

.UPDATE...
Issued at 850 PM CDT Fri Oct 23 2020

00Z KABR RAOB is very dry (0.15 inch Pwat value). Take away the
extremely dry air over the Rocky Mountains, and the northern
plains has the coldest/driest air (-16C to -23C at 850hpa) over
the CONUS this evening, where a 1032+hpa surface high pressure has
taken up residence. The only thing keeping overnight lows from
falling to zero or colder will be these thick cirrus clouds
streaming into the region.

With models continuing to shift coverage/amounts of snow further
south and west in the region, won`t be a bit surprised if 00Z
models start streaming into AWIPS with virtually no snow
accumulating (too much dry air to overcome and not enough
forcing/lift) over northeast South Dakota prior to 00Z Sunday.
Looks as though the current headlines in place should suffice.
Where the biggest question mark exists is with the
Clark/Codington/Hamlin/Deuel county grouping. It`s still possible,
that between 00Z and 18Z Sunday, 2 or 3 inches of snow could
accumulate there.

No major changes planned to the tonight period of the forecast.

UPDATE Issued at 620 PM CDT Fri Oct 23 2020

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

&&

.SHORT TERM...(This Evening through Saturday Night)
Issued at 238 PM CDT Fri Oct 23 2020

Cold air remains trapped in place, with temperatures 20-30 degrees
below normal for this time of year. Quickly falling temperatures
tonight will be a few degrees away from record lows by daybreak. At
this point, Sisseton has the best chance of breaking their record of
15 degrees in 1955. For Aberdeen, the record is 6 degrees in 1917.
We should be close.

A more southern track to the incoming weather system seems to be
continuing with a majority of the 12Z model runs today. If this
continues, it will result in a moderate to heavy snow over our
southwestern counties, with little to no snow in the northeast
(northern Roberts and Traverse Counties).

Taking a look at the details, the 500mb trough over much of the
Canada and the northern half of the U.S. will continue to be a
dominant feature through the period. An west to east oriented
embedded trough currently over southwestern Canada will slowly sink
across ID/MT Saturday morning. We should start to see snow falling
in Corson and Dewey counties prior to daybreak Saturday. Some
uncertainty remains, not only on how far south the bulk of the
precipitation will go, but also with respect to snow to liquid
ratios (SLRs). Higher ratios with lighter/fluffier snow could be
possible if the incoming moisture has enough residence time in the
deep dendritic growth zone (DGZ). Have gone with SLRs of
13-16:1. Jet dynamics will also be in play, with the potential
for enhanced banded snowfall over central SD. With east winds
increasing into midday Saturday, expect patchy blowing snow during
mainly the daytime hours.

Have gone ahead and upgraded a few counties to Winter Storm Warnings
for snow amounts of over 6 inches in 12 hours, and added a few
more. Winter Weather Advisories for accumulations of 3 to 5
inches have been added bordering the Watch, and along our southern
border. While Hyde and Hand Counties were borderline given the
current forecast for snowfall, I`m not as confident that we will
get to the 6 inch mark there. As a result, have gone with an
Advisory, with the idea that those counties may need to be
upgraded as we get closer to the event.

.LONG TERM...(Sunday through Friday)
Issued at 238 PM CDT Fri Oct 23 2020

Once the snow is away the cold will steal the show. While 850mb
temperatures are already a standard deviation or two below climo, we
will also see a 1040mb high move into the high plains. There will be
a weak gradient across the region Monday morning with light west to
northwest winds but with all the deep/fresh snow cover and an
already cold start (limited clearing during the day Sunday to
suppress temperatures) it wont take much to encroach on records (or
smash records for that matter, with current forecasts already 5 to
10 degrees below record lows). As the high moves south, Tuesday will
see weak sustained westerly flow which will help with a slow,
gradual dispersal of this cold airmass. Thereafter, were back to a
dry forecast with moderating temperatures that will still be
influenced by residual snow cover.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday Evening)
Issued at 620 PM CDT Fri Oct 23 2020

Terminals KABR,KATY,KPIR,KMBG

Good VFR will likely stick around at all four TAF sites through
12Z Saturday, even longer than that for the KABR/KATY terminals.
Between 12Z and 18Z Saturday, snow and lower ceilings/visbies
should begin impacting the KMBG/KPIR terminals. It`s possible that
by 00Z, all four terminals could be in sub-VFR conditions and
snow. Easterly component surface winds will increase during the
day on Saturday to 15 to 20 knots with gusts potentially as high
as 30 knots across the Missouri River valley, creating areas of
blowing snow at the KMBG/KPIR terminals.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory from 1 PM Saturday to 7 AM CDT Sunday
     for SDZ004-009-010.

     Winter Storm Warning from 1 PM Saturday to 7 PM CDT Sunday for
     SDZ045-048.

     Winter Storm Warning from 7 AM CDT /6 AM MDT/ Saturday to 7 AM
     CDT /6 AM MDT/ Sunday for SDZ003-015-033.

     Winter Weather Advisory from 4 PM Saturday to 4 PM CDT Sunday
     for SDZ018>020-022-023.

     Winter Weather Advisory from 1 PM Saturday to 4 PM CDT Sunday
     for SDZ017-036-037-051.

     Winter Storm Warning from 10 AM Saturday to 1 PM CDT Sunday for
     SDZ016-034-035.

MN...None.
&&

$$

UPDATE...Dorn
SHORT TERM...KF
LONG TERM...Connelly
AVIATION...Dorn


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