Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 162316

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
516 PM MDT Mon Apr 16 2018

.DISCUSSION...(This Evening Through Monday)
Issued at 153 PM MDT Mon Apr 16 2018

The latest water vapor images and upper-air analysis had a long-
wave ridge across the central CONUS with a large upper low over WA
that will be affecting the CWA Tuesday through Wednesday. The
latest surface observations indicated borderline wind advisory
conditions over Harding County with southeast winds at K2WX of
35g45 mph. Given that this is a localized windy spot will not
issue an advisory.

For tonight the upper ridge moves east of the CWA as the western
CONUS trough moves further onshore. A jet streak will move to UT/CO
as a lead short-wave trough reaches central WY by 12z. Increasing
clouds and lift (Qconv) may lead to a few showers over northeastern
WY after 06z. Otherwise, some low clouds and patchy fog may develop
around the BH. However, a strong southerly low-level jet will keep
breezy winds across the SD plains most of the night, and thus fog
would not be widespread.

On Tuesday the LFQ of an upper-jet streak moves across the CWA and
should provide sufficient lift for widespread showers. Thermal
profiles favor a mix of rain/show, transitioning to all snow from
west to east as colder air aloft moves in behind a cold frontal
passage. Northwest winds will become breezy as the front moves
through. Some hint at favorable lift in the dendritic growth zone,
along with lapse rates of 7-8.5 C/km, suggest there will be
pockets of heavier snow. Nevertheless, accumulations should only
be a trace to 1 at best for most places, but some 1-4 inch amounts
may fall in the highest BH.

For Tuesday night the jet moves across KS/NE as the upper trough
closes off near KSUX. The main moisture and lift will shift to the
eastern CWA (and places eastward). The cold frontal passage will
finish in the evening over the remaining/eastern CWA, with gusty
northwest winds through at least the evening.

Short-wave ridging then is forecast Wednesday and Wednesday night,
with perhaps a few diurnal showers Wednesday on the back side of
the departing system. Breezy northwest winds likely will continue.
Then on Thursday an upper ridge will be over the CWA with return
southeast flow.

The next system will affect the area Thursday night into Saturday.
This looks to be a relatively warm system with the best chance of
snow over the higher BH. In addition, the bulk of the moisture and
lift will be south of the CWA as the deformation zone lies near the
SD/NE border. The best chance of rain/snow appears to be Thursday
night through early Friday with the initial short-wave trough that
ejects out of the system. Then, a relatively nice/seasonal Sunday
is in store as high pressure aloft builds over the CWA.


.AVIATION...(For the 00Z TAFS Through 00Z Tuesday Evening)
Issued At 513 PM MDT Mon Apr 16 2018

VFR conditions will continue through this evening with gusty
southeast winds across much of the area. Stratus/patchy fog may
develop across the plains of western South Dakota tonight. Low
confidence in this stratus affecting the KRAP terminal...but did
throw some lower scattered clouds overnight. Cold front moves into
northeast Wyoming/western South Dakota Tuesday morning with
MVFR/IFR conditions with scattered rain/snow lasting through the
day at many locations. Gusty northwest winds behind the cold




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