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000
FXUS63 KUNR 312336
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
536 PM MDT Tue Mar 31 2020

.DISCUSSION...(This Evening Through Tuesday)
Issued at 229 PM MDT Tue Mar 31 2020

Latest analysis shows the broad upper low complex is still in
place over southwestern Canada and the northwestern CONUS. The
main upper low is currently centered over Alberta with another
upper low/trough dropping southward into the Pacific Northwest on
the west side of the broad upper circulation. A shortwave rotating
around the east side of that system is currently approaching the
forecast area from the west and with additional lift provided by a
surface cold front is producing a line of rain showers in eastern
Montana extending southward into north central Wyoming. Diurnal
heating and the instability aloft has produced some showers
currently over parts of northeastern Wyoming and the Black Hills
area. The afternoon relative humidity is very dry in those areas,
around 20 percent, so expect that much of the showers will
evaporate before reaching the ground, resulting in some local
brief gusty winds. There may be enough instability for a couple of
lightning strikes as well.

A surface low/trough will develop over the forecast area tonight
and as it moves slowly eastward on Wednesday, colder air will
flow into the region. The chances for precipitation will increase
during the day on Wednesday. Strong low and mid-level
frontogenesis will develop in an SW-NE oriented band extending
from northeastern Wyoming across the Black Hills and into
northwestern South Dakota, as the relatively warm and moist air
flows in above the colder air near the surface. As the system
develops, rain will change over to snow over most of the area on
Wednesday night, with a wintry mix expected over south central
South Dakota late Wednesday into Thursday. The areas that end up
under the band of strongest frontogenesis could potentially see
heavy snow of 6 inches or more so have added much of northeastern
Wyoming and the northwestern South Dakota plains to the Winter
Storm Watch that was already issued for the Black Hills. Storm
total snow accumulations will be ultimately tempered by the
somewhat warm ground but periods of locally intense snowfall will
aid accumulation, especially during the nighttime hours. The snow
will end from west to east on Thursday night.

Mainly zonal flow will set up for the rest of the forecast.
Chilly conditions continue on Friday with a modest warm up for the
weekend and early next week. Dry weather is expected on Friday
and Saturday, but embedded waves in the flow aloft may lead to
some light precip at times from Saturday night through early next
week.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Wednesday Evening)
Issued At 530 PM MDT Tue Mar 31 2020

Mainly VFR conditions are expected through 06z, though brief,
isolated MVFR conditions will be possible in heavier showers this
evening. Breezy NW winds will diminish later this evening.
Lower ceilings will move into northern portions of the area after
06z as colder air arrives behind the front. Rain and snow will
begin across far northern and western areas late Wednesday
afternoon with widespread IFR/LIFR conditions expected to
overspread much of the area Wednesday evening.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from Wednesday evening through late Thursday
     night for SDZ024-028-029.

     Winter Weather Advisory from midnight Wednesday night to 6 AM
     MDT Friday for SDZ027-041>043.

     Winter Weather Advisory from 9 PM Wednesday to midnight MDT
     Thursday night for SDZ001.

     Winter Storm Watch from Wednesday evening through late Thursday
     night for SDZ002-012>014-025-072-073.

     Winter Weather Advisory from 10 PM Wednesday to 6 AM MDT Friday
     for SDZ030>032.

     Winter Weather Advisory from 9 PM Wednesday to 6 AM MDT Friday
     for SDZ026-074.

WY...Winter Storm Watch from Wednesday evening through late Thursday
     night for WYZ057.

     Winter Storm Watch from Wednesday evening through Thursday
     evening for WYZ054>056-058-071.

&&

$$

DISCUSSION...10
AVIATION...15