Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 182321

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
521 PM MDT Sun Mar 18 2018

.DISCUSSION...(This Evening Through Sunday)
Issued at 238 PM MDT Sun Mar 18 2018

Mean western CONUS trough slowly shifting east with a well formed
baroclinic leaf over the western FA. This has supported rain,
snow, and freezing rain invof the WY/SD border. As the warm nose
breaks down aloft, precip has been trending to snow, esp over WY.
A blend of the 12Z GFS/CMC/and ECMWF has latched on well to the
area of FGEN banding and have sided with this solution through
Monday for QPF amounts. This equates to a 2-4 inch band of snow
over far NW SD into the northern Black Hills, with a couple inches
either side of this band. Strong March insolation is
counteracting accums on roads in many areas, but this will change
as the transition to evening occurs. FGEN band will slowly shift
east and wane across NW SD, with a new area of ll forcing
supporting another concentrated area of FGEN lift over scentral SD
this evening and overnight, lasting into Monday. Precip will
likely start out as rain there and transition to snow. It is still
looking like 1 to 3 inches of accums, maybe 4 there. However,
this includes snow Monday. Diurnal warming will again limit snow
accums on roads given general low snowfall rates. Hence, will
defer any headlines to the night shift. Otherwise, there is a
decent potential for freezing drizzle across portions of NE WY and
NW SD after snow ends and the mid levels dry. Moist to near
saturated BL with steep ll lapse rates and some weak upslope
component presents a good setup for drizzle. Kept most of the
previous headlines intact. Did trim out the central hills given
much lower forecast snowfall with a slight shift to the FGEN band.
Extended the adv thorough 6 AM Mon given freezing drizzle

Snow will slowly shift east Monday, gradually ending over
scentral SD Monday night. Diurnal heating may support secondary
snow showers over the Black Hills and to the southeast Monday
afternoon and again Tues with steep lapse rates and a cold pool

Western CONUS trough re-loading will occur toward the end of the
week with local WAA and height rises. This may support WAA showers
Thur afternoon and night (with chances shifting NE in this
period). Deep western CONUS trough will eventually shift east
late in the weekend and early next week, possibly supporting a
decent storm system. Something to watch in this period.

Temperatures will remain below seasonal norms for the next few
days, heavily influenced by the snow pack in place on the SD
plains. Decent melting should occur mid week, with spring like
temps expected by Thur-Fri (50s and 60s).


.AVIATION...(For the 00Z TAFS Through 00Z Monday Evening)
Issued At 517 PM MDT Sun Mar 18 2018

A storm system with two impulses will move through the area
through Monday afternoon. The first impulse is taking a band of
moderate snow across northeast WY/Black Hills/northwest SD
northward this evening into ND. A secondary impulse will bring
wintry precipitation to the SD plains later this evening through
Monday afternoon. Areas of fog will also occur given T/Td spreads
and abundant boundary layer moisture. Result will be widespread
IFR conditions through Monday afternoon with local LIFR conditions
at times. Some improvement may occur across northeast WY/southwest
SD by Monday afternoon.


SD...Winter Weather Advisory until 6 AM MDT Monday for SDZ001-002-012-

WY...Winter Weather Advisory until 6 AM MDT Monday for WYZ057-071.



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