Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 220945 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Aberdeen SD
445 AM CDT Thu Mar 22 2018

.SHORT TERM...(Today through Saturday Afternoon)
Issued at 422 AM CDT Thu Mar 22 2018

Fog is fairly local along the western edge of the James valley this
morning. Otherwise a mostly clear day is expected ahead of the next
storm system.

It appears that the main impetus for lift with this system is strong
mid level warm advection/frontogenesis, an extension of the
atmospheric river that has plowed into the western CONUS. Upper flow
is mainly diffluent with only a subtle/short lived jet steak of
around 60kts, and the lack of dynamics does leave me with some
concern as to how well guidance is handling this situation in both
duration and intensity.

We can expect the first high clouds to move into the forecast area
from the southwest this evening, so temperatures tonight should
remain relatively mild with some heat left in the ground. NAM/GFS
bufkit profiles do maintain a relatively dry subcloud layer before
strong warm advection enhances lift, saturating to the surface and
allowing for precipitation. Doesn`t look like a stratus layer is
evident before this happens so we shoudln`t see any pre-event
drizzle. CAMs and deterministic guidance are fairly similar in the
track/timing and intensity of returns across the CWA.

The overall evolution is fairly similar between the NAM/GFS
in BUFKIT profiles as well, and in sampling the main areas on the
edge of rain/snow (ie. Aberdeen/Watertown) we see once we
saturate there is good lift in the dendritic growth zone. Below
this is a the warm layer which is quite deep, though the degree to
which will be the kicker. As an example of how the deep warm
layer will influence the forecast, we look to the Aberdeen NAM
profile. Warm air extends from between 7-2kft and is right around
0C. Any warmer in that layer means all rain and no snow. A
slower/cooler progression means all heavy wet snow to several
inches accumulation - this is the conundrum as both these cities
sit on this precipice. Also of note, the NAM is the warmer set of
guidance in this layer, though it has the usual bias of a colder
boundary layer compared to the GFS.

As the day progresses, profiles lose ice in the dendritic growth
zone though retain a deep super cooled saturated layer. Thus we
can expect drizzle for a few hours after the best forcing has
moved on. Don`t expect significant icing at this point but is
something to watch with ground/air temperatures expected to be
around the freezing mark early Saturday.

As we mentioned atmospheric rivers, PWATs increase to upwards of 0.9
inches in the GFS, which is almost 3 standard deviations above

Will also add in that both the GFS/NAM support some CAPE, and in a
few sampled hours NAM BUFKIT does show some steep lapse rates atop
the dendritic growth zone, however EPV* never does go negative, so
opted to neglect any mention of thunder, though CAMS do suggest some
convective elements in simulated reflectivity and thus would not be
surprised given CAPE that a few convective elements develop with
higher snowfall rates/ratios.

I`ll also note that 1/2 km winds are upwards of 45kts, though under
less than efficient mixing under easterly low level flow and pressure
falls of 6mb/6hrs. There is an 18mb gradient across the state
though. BUFKIT mixed winds peak around 30kts, so windy regardless.

Given the complexity of the scenario, and the fact that snow amounts
are on a razor thin transition line somewhere close to most of
our major cities, will leave headlines in place.

.LONG TERM...(Saturday Evening through Wednesday)
Issued at 422 AM CDT Thu Mar 22 2018

The long term models show good agreement this time around with the
upper level flow and individual short waves moving through it. The
models continue to show an upper level low pressure area just off
the west coast Saturday night slowly dropping southeast and
intensifying into the southwest U.S. into early in the week.
Indications are that at least two more short waves will come off of
this western trough and across our region Saturday night through
Monday night. Rain or snow are mainly expected with each system with
possibly some mixed precipitation. Otherwise, Tuesday and Wednesday
look to be dry with mostly clear skies with west to northwest flow
aloft dominating the region. Highs through the period are expected
to be from near to 10 degrees below normal ranging from the upper
30s to around 50 degrees from Sunday through Wednesday.


.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
Issued at 1247 AM CDT Thu Mar 22 2018

VFR conditions will prevail through most of the period. MVFR/IFR
fog/stratus should develop overnight at KMBG, KATY, and KABR. They
should return to VFR by mid to late morning Thursday as easterly
winds increase to 10 to 20 kts through the day.


SD...Winter Storm Watch from Friday morning through Saturday morning
     for SDZ007-008-011-020-021-023.

     Winter Storm Watch from Friday morning through Saturday morning
     for SDZ005-006.

MN...Winter Storm Watch from Friday morning through Saturday morning
     for MNZ039-046.



SHORT TERM...Connelly
AVIATION...Mohr is the U.S. government's official web portal to all federal, state and local government web resources and services.