Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 182315
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
515 PM MDT Thu May 18 2017

.DISCUSSION...(This Evening Through Thursday)
Issued at 246 PM MDT Thu May 18 2017

Deep closed low and associated low pressure will will continue to
support increasing chances for precip across the region as a lobe
of LSA shifts north tonight. This system will slowly shift east
the next couple days, supporting well below normal temps with
chances for rain and snow through at least Sat. Mean troughing is
then forecast with NW flow aloft into early next week with weak
impulses expected in the flow to bring chances for showers.

Main focus is tonight`s system esp given expected snow accums at
some locations. Initial surge of FGEN precip will continue to
fight dry NE ll flow this afternoon and evening, NE half. This
will slow the northward extent of the precip band. However, over
the southern NE WY zones, the snow is already falling at Pine Tree
junction and will likely remain all snow or a light mix with rain
through the evening. The heaviest precip should be late this
afternoon into the evening there and given thermal fields and
precip intensity, expecting snow accums of up to 3 inches, esp
from Wright south. The other area that will see snow will be the
higher elevations of the Black Hills, esp above 5.5 kft, where 1-3
inches will be possible. All other areas will see a mix of rain
and snow or all rain. Decided to go ahead and issue an adv for
southern Campbell county given snow accums and potential icy roads
as temps are expected to hover around freezing for a long period
of time there. Although precip will shift north with the FGEN band
tonight, chances won`t be that great over the northern third
given dry air fight. A break in precip can be expected early Fri
morning before more sct activity is expected through afternoon,
best chance south central SD as dry air begins to wrap into the
system from the west. Precip chances will diminish through Sat
with mainly dry conds expected Sunday, perhaps an afternoon shower
far west. Temps will be well below normal Fri with many areas
barely rising out of the 40s. Locations on the WY plains will see
freezing temps tonight, with some locations on the SD plains
possibly seeing a freeze or frost Sat morning.

After Sat, unsettled NW flow will persist through next week, with
more seasonal temps. Periodic impulses may bring sct-isold
shra/ts, esp Monday-Monday night, when a stronger NW flow impulse
advects through the region.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 510 PM MDT Thu May 18 2017

VFR conditions will prevail over northwest SD through Friday
afternoon. Areas MVFR conditions over northeast WY through south
central SD will deteriorate this evening to a good deal of IFR
conditions due to lower CIGS and -SHRA. Some LIFR conditions are
expected near the Black Hills and over northeast WY south of KGCC
where rain will mix with or change to snow. Precipitation will
taper off overnight, but redevelop by Friday afternoon across
southern SD.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...Winter Weather Advisory until 6 AM MDT Friday for WYZ055.

&&

$$

DISCUSSION...JC
AVIATION...Helgeson


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