Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 171727

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1127 AM MDT Tue Apr 17 2018

Issued at 944 AM MDT Tue Apr 17 2018

Sharp upper trof over Rockies shifting slowly eastward. Surface
low moving southeast into ern WY with inverted trof along WY- SD
border. Large band of precip over cntrl WY-MT will move into
forecast area late morning and through the afternoon. Rain will
change to snow as colder air moves into the area behind the trof.
An isolated thunderstorm is possible over the southern BH- extreme
eastern WY, but strong inversion keeps higher CIN values. Easterly
winds will shift to the northeast, with low clouds moving over the
foothills. Temperatures will remain cool where snow remains and adjusted
some high temps for increased cloud cover.


.DISCUSSION...(Today Through Monday)
Issued at 331 AM MDT Tue Apr 17 2018

Current surface analysis shows low pressure moving into northeast
WY, with cold front extending back to the southwest across WY
into UT. Warm front extends south-southeast of the low across
eastern WY and CO. Upper level analysis shows a large trough over
the western US, with a strong jet wrapping around the base of the
trough and northeastward across the Rockies and into the high
plains. Jet energy is producing showers ahead of the trough across
northwest WY into south central MT. Skies around here are clear
to partly cloudy at the moment, with a wide range of temps again
across the area. Temps are in the upper 20s and lower 30s to the
north and east of the Black Hills, with mid 30s to lower 50s
across northeast WY to the Black Hills and far southwest SD. Brisk
southeasterly winds are still noted in most areas on the plains.

The trough to the west will move relatively quickly east across the
region later today and tonight, with scattered to numerous showers
spreading east across much of the area late this morning and
afternoon. Low pressure and cold front will slide southeast into the
Central Plains today, with a modest cooling of temperatures aloft
behind it and breezy conditions. Highs will mostly be in the 40s.
Any pcpn this morning will mainly be in the form of rain, with
colder air aloft turning pcpn over to snow in the afternoon across
northeast WY, the Black Hills, and parts of northwest SD. Pcpn will
change over to snow east of the Black Hills this evening. The higher
elevations of the northern and central Black Hills, especially above
5000 ft, will see up to a few inches of snowfall this afternoon and
early evening.

Models continue to show the trough strengthening and developing a
mid level low over central SD as it moves east of the region
tonight, with the development a bit quicker than previously shown.
This would likely result in slower movement of the system out of the
region and the potential for a bit more snow on the back side
tonight for areas east of the Black Hills into central SD. The GFS
is the most aggressive with this possibility, but the heaviest
snowfall will still be east of the forecast area, from parts of
southeast SD into southern MN and northern IA. The forecast will
have 1 to 2 inches across eastern parts of northwest SD to south
central SD tonight, with local amounts of around 3 inches possible
across the higher ridges by early Wednesday. Temps will only drop
to around freezing in most areas on the plains by late tonight.

Upper level ridge behind exiting system will bring dry conditions
for much of Wednesday through at least Thursday evening, but
temperatures will stay below average. The next strong storm system
will dig southeast across the western states Thursday into Friday.
The trends continue to be further south with this system, especially
with the EC and Canadian. Shortwave energy lifting northeast ahead
of the system could bring some light rain, especially to southern
SD, Friday into Friday night. Some snow is possible across the Black
Hills, but looks to be light at this point.

As the low pressure system passes to the south of the region over
the weekend, dry weather and more seasonable temperatures should
return, especially by Sunday. Some unsettled weather looks likely
now for the first part of next week as a weak front crosses the
Northern Plains.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1123 AM MDT Tue Apr 17 2018

A cold front will continue to move east through northeast
Wyoming/western South Dakota today with MVFR/IFR cigs developing
and shifting east. Scattered rain/snow will spread southeast
through the day, with conditions deteriorating after 20z. Gusty
southeast winds will become northwest behind the cold and remain
gusty. The winds combined with rain and snow showers could lead to
greatly reduced VIS at times. Conditions will gradually improve to
MVFR/VFR west to east beginning late this evening after 00-01z
over NE WY, trending slowly east tonight over the western 2/3.

Expect IFR conditions across the region Wednesday.




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