Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 201708

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1108 AM MDT Tue Mar 20 2018

.DISCUSSION...(Today Through Monday)
Issued at 247 AM MDT Tue Mar 20 2018

A drying and warming trend is on tap through Thursday. For today,
a weak short-wave trough -- currently in eastern MT -- will move
across the CWA. Combined with relatively steep lapse rates of 7-8
C/km, this will produce a few snow/rain showers. Highs will be
above yesterday`s values as 850-mb temperatures rise (lowest over
northwestern SD where snow cover is deepest). Some low clouds and
areas of fog this morning will gradually give way to clearer
skies as the day progresses. Also, as the boundary layer mixes
out, breezy northwest winds of 10-20 mph will be felt over the
open plains.

For tonight through Thursday, upper-level ridging and low-level
warming will ensue. Given the potential for snowmelt today, along
with relatively light winds tonight, expect some fog to redevelop
across parts of the CWA late this evening through early Wednesday
morning. Otherwise, return southeast flow will develop on
Wednesday, becoming stronger on Thursday, especially across
northeastern WY. On Thursday night, a short-wave trough will break
down the ridge as it crosses the CWA. A very strong (50 kt)
southerly low-level jet stream will foster warm/theta-E advection,
especially across western SD into ND. This should lead to
rain/snow showers, with the best chance across northwestern SD.
The SREF precip-type probabilities point to a small potential for
freezing rain in far northeastern Perkins County, which is a trend
to watch for in subsequent runs.

On Friday, a large upper low will move to the Pacific Northwest,
while one short-wave trough moves away to our northeast and yet
another moves just to our south. This should lead to a relative
minimum in PoPs across the CWA, with another mild and breezy day.
On Friday night, the big upper low should be well established
across the West, with a weak short-wave ridge over the CWA. Some
evening thunderstorms are possible across south-central SD as a
wave passes to our south and MUCAPE reaches 100-500 J/kg. A weak
FROPA also is expected. Then on Saturday, return southeast surface
flow develops quickly in response to another approaching short-
wave trough, which will result in another late-day FROPA. It
should be mostly dry during the day, but some rain/snow showers
will be possible in the evening (with even a few thunderstorms
over parts of western SD).

Sunday through Tuesday suggests that troughiness will dominate the
weather pattern, bringing periodic chances of precipitation along
with generally below-average temperatures.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1108 AM MDT Tue Mar 20 2018

VFR conditions are generally expected at KGCC/KRAP through the
TAF period. Brief SN/RA showers cannot be ruled out this
afternoon at KRAP and over the surrounding SD plains, which could
temporarily lower cigs/vsbys to MVFR/IFR. An isolated lightning
threat may also develop in deeper showers.

Tonight, patchy BR/FG may again develop, particularly in low-lying
areas of the SD plains with relatively fresh snowpack. If so,
MVFR/IFR and localized LIFR conditions will be possible.




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