Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 212125

Area Forecast Discussion
National Weather Service Aberdeen SD
Issued by National Weather Service SIOUX FALLS SD
425 PM CDT Wed Mar 21 2018

.SHORT TERM...(Tonight and Thursday)
Issued at 420 PM CDT Wed Mar 21 2018

Upper ridge of high pressure will nudge eastward across the front
range of the Rockies and into the western plains tonight and
Thursday. At the surface, high pressure will strengthen tonight
over northern MN and western Ontario ushering in a flow which will
become increasingly easterly. Concerned about patchy to areas of
fog development tonight and Thursday morning primarily from the
James Valley westward with a moist upslope flow prevailing. In
addition, current surface dew points across the area are not real
low and as temperatures fall tonight, surface dew point
depressions should become quite small. The NAM was very aggressive
in developing stratus throughout central SD and points westward,
with the GFS less robust. Did not buy the NAM aggressiveness at
this time with the stratus development, as some of it may be an
artifact of cold moist ground, etc. That said, there is some
evidence that mid level clouds may move into our far western and
southern zones late tonight. Further east, stratus may linger in
our northeast corner as even if it moves out this evening which is
questionable, it will will then advect back in from the east
along the low level easterly flow.

Concerning temperatures, lowered readings tonight in the James
River valley with light winds, and this area may be in that sweet
spot between stratus to the east, and mid clouds ushering in to
the west. Overall Thursday should be a pleasant day as easterly
winds continue with highs of 40 to 50.

.LONG TERM...(Thursday night through Wednesday)
Issued at 420 PM CDT Wed Mar 21 2018

The pattern in the extended portion of the forecast looks pretty
active at this time.

In the intermediate term, strong upper troughing will move into
the intermountain west Thursday night with a leading short wave
extending from western ND to north central SD by 12Z Friday. This
feature is shown well on the 700-500mb QG forcing. Soundings and
humidity time sections show a rapid saturation Thursday night
across our western zones, with likely to categorical pops
warranted late at night west of the James River. The QG forcing
exits the forecast area Friday evening with this initial short
wave, however a second short wave then takes over late Friday and
Friday night keeping saturation robust through 3 or 4 km.
Frontogenesis with both of these waves is quite deep, extending
from 850-600mb coincident with the saturation, and a warm conveyor
belt coming up provides a lot of warm air advection with strongly
veering soundings.

The most concerning area where a winter storm watch has been
issued is for locations near the ND border in north central SD,
extending southeastward through northeast SD. This area is along a
strong 850mb baroclinic zone Friday and Friday evening. Concerning
the dynamical and wet bulb cooling potential, much of the
precipitation should fall as snow in the headlined area, with
rain further west in central SD as the primary precip type
although that will need watched too. Confidence is high that less
precip will fall in our western zones, so if they unexpectedly
switchover to snow, amounts will be less than warning criteria.
But at this time, the 925-850mb temperatures in central SD are
above zero celsius. Not the case in the watch area, where readings
are colder, and saturation is deep enough to produce ice crystals
at the upper portions of the cloud depth. Lastly, there is a bit
of unstable air per the 600mb EPV* in far northeast SD Friday
afternoon over the mid level frontogenesis. This may enhance snow
rates for a while, especially in the higher terrained area.

High temperatures are quite a challenge on Friday. They could be a
bit warm in the winter storm watch area, especially noting that
cooler raw model hourly temperatures looked better then the
superblend hourly readings when precipitation is falling.
Therefore consensus raw model temperatures were primarily used for
the hourly weather. At this time, we are looking at a potential
for 6 to 8 inches of snow in the watch area, with the heaviest
amounts hugging the ND border near Aberdeen, then extending
eastward and southeastward through northeast SD and west central

Further out, a very large area of high pressure will filter into
the northern plains from eastern Canada this weekend after the
aforementioned wave exits. Light rain or snow is possible late
Saturday night along a band of warm air advection. This will be
followed by high pops late Sunday night and Monday for more rain
or snow with an overrunning potential. A southwest flow aloft
should exist coupled with surface low pressure developing in
response over the southwestern plains, extending an inverted
trough northeastward.


.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
Issued at 1255 PM CDT Wed Mar 21 2018

MVFR to locally IFR ceiling band persists from the northernmost
James River valley through the Sisseton Hills at midday. While a
slow eastward erosion to the main area indicated by satellite
trend, look from above also reveals a tendency for the trailing
clearing to succumb for a time to some self destructive sunshine
with additional upper IFR to low-end MVFR ceilings redeveloping.
Will be more of a struggle to get clearing to push toward KATY
before terrain-supported clouds try to redevelop tonight. Fairly
high near- surface moisture in place as generally weak surface
flow this evening turns toward an easterly upslope component.
Would expect this should yield redevelopment of some patchy to
areas of fog tonight, most significant where supported by the
upslope component. Shallow nature suggests that widespread
ceilings may not accompany, and have only hinted at through
mention of scattered layer around or below 500 ft AGL. While
surface flow will take some time to respond, will get increased
gradient into central SD by late tonight and Thursday morning
which should reduce the threat for widespread lower visibility, as
well as tendency for some warm advective mid-level cloud to
develop toward central SD by late tonight and Thursday morning.


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.



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