Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 191706

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1106 AM MDT Mon Mar 19 2018

Issued at 415 AM MDT Mon Mar 19 2018

Conditions are favorable for some light freezing drizzle over
northwestern parts of the area, including the northern Black
Hills, so have updated the forecast to include that through this


.DISCUSSION...(Today Through Sunday)
Issued at 253 AM MDT Mon Mar 19 2018

The latest water vapor images revealed a continuing complex
upper-level pattern, with the main low over KS/OK. A rather
diffuse upper flow was over the CWA, with a few weak short-wave
troughs in the vicinity. Forcing from these waves will affect
mainly the eastern CWA today where the models do show modest
isentropic lift, frontogenesis, and Q-vector convergence.
Precipitation has transitioned to snow across most of the CWA,
although there was some spotty freezing drizzle. Expect that snow
will be the dominant type today. One to four inches of snow are
possible over south-central SD. Have cancelled the winter weather
advisory for the northwest as things are winding down there.
Otherwise, it will be seasonally cold today with brisk northwest
winds over the SD plains. Clouds will be widespread, with some
areas of fog as well.

A few more weak short-wave troughs will continue to affect the CWA
tonight and Tuesday, but the result will only be isolated to
scattered snow/rain showers. Low clouds and some fog also are likely
again tonight. Tuesday will see some warming at 850-mb, which should
translate to slightly warmer weather than today (although still
tempered by snow cover). It will be dry Tuesday night and Wednesday
as upper riding continues. Areas of fog may again develop Tuesday
night and Wednesday night, especially over parts of northwestern
into central SD. 850-mb temperatures on Wednesday rise to 2-10C,
warmest over northeastern WY. There should be a fairly large
contrast in highs from snow-free parts of northeastern WY to snow-
covered areas of northwestern SD.

The upper ridge will still be affecting the CWA Thursday, and breezy
southerly winds will help mix down some of the warmer air aloft. A
large trough over the western CONUS will send a weak short-wave
trough across the CWA Thursday night, producing low-level theta-E
advection and fostering scattered showers. Additional short-wave
troughs will move across the CWA Friday and Saturday as a large
upper low moves over the Pacific Northwest. Some Pacific FROPAs will
occur both Friday and Saturday, bringing slightly cooler air and
breezy conditions. Unsettled conditions will persist Sunday and
Monday as troughiness dominates the weather pattern for the CWA. The
GFS is notably warmer than the ECMWF, and given the pattern would
expect the cooler ECMWF solution to be more realistic.


.AVIATION...(For the 18Z TAFS Through 18Z Tuesday)
Issued At 1106 AM MDT Mon Mar 19 2018

Conditions will gradually improve from west to east through the
afternoon and evening hours as cigs rise and snow tapers off.
However, far northern and eastern portions of our area may remain
MVFR/IFR through tonight, with lingering low stratus expected. VFR
conditions should prevail at KGCC/KRAP by late this afternoon.

Tonight, some FG/BR may develop, particularly within low-lying
areas. For now, it does not appear that this will significantly impact
TAF sites, though minor reductions to vsbys are possible overnight
into Tuesday morning.




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