Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 082027

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
127 PM MST Mon Mar 8 2021

.DISCUSSION...(This Evening Through Monday)
Issued at 103 PM MST Mon Mar 8 2021

Water vapor shows large upper trof covering the western CONUS with
main low off the Pac NW coast. Abundant cirrus streaming overhead
the Northern Plains in the southwest flow aloft. At the surface,
trof is deepening along the northern/central Rocky Mountain front,
with breezy southeasterly winds across our region.

For tonight, pressure gradient strengthens ahead of an approaching
cold front, with 3-5mb difference across the Black Hills and
winds towards the top of the boundary layer near 40kts. There is
the potential for downslope winds to develop, especially in the
northern Black Hills, which may be close to advisory speeds after
midnight until shortly after sunrise.

Cold front moves through early in the day, with good mixing both
ahead of and behind the cold front resulting in some gusty
northwest winds and temps rising into the 60s east/south of the
Black Hills and 70s in south central SD. Have gone a bit above
guidance for high temps given dry ground and expected low RH`s.

Cold advection continues Tuesday night into Wednesday, with robust
jet streak ejecting from the western CONUS trof. Band of
frontogenesis sets up across our southern half of zones, and
models are in quite good agreement with placement of heavier
precip generally south of I-90. Rain will change to snow Tuesday
night, with mostly snow on Wednesday, before precip tapers off
Wednesday evening. This will be a wet snow, with some melting
expected especially during the day on Wednesday given highs in the
30s. This will limit overall accumulations. The current Watch
looks good with highest accumulations from southern portions of
northeast WY into far southwest SD, and there may be some need
for advisories a bit further to the east and will make that
decision late tonight or tomorrow. Wind should not be much of an
issue with this system.

Split flow pattern through the weekend will keep our area mainly
dry with temps gradually warming back to above average by the end
of the week and weekend. There is some indication that a cut-off
low over the Desert SW may bring some additional precip towards
the end of the weekend and early next week (if it`s able to track
this far north).


.AVIATION...(For the 18Z TAFS Through 18Z Tuesday)
Issued At 1029 AM MST Mon Mar 8 2021

VFR conditions will continue across the area through the forecast
period. Breezy south to southeast winds will develop over a good
portion of the plains this afternoon, with gusty southwesterly
winds expected from northeast WY to the Black Hills tonight.


Issued at 103 PM MST Mon Mar 8 2021

Downslope winds are expected to develop after midnight tonight,
especially in the northern Black Hills, followed by a cold front
passage early Tuesday. Very dry air both ahead of and just behind
the cold front Tuesday will result widespread RH`s in the 20s,
and possibly some teens, along with some gusty northwest winds.
This will lead to elevated fire wx conditions. Colder temps with
rain and snow will develop late Tuesday night and Wednesday,
especially south of I-90.


SD...Winter Storm Watch from Tuesday evening through Wednesday
     afternoon for SDZ024-027>029-041.

WY...Winter Storm Watch from Tuesday evening through Wednesday
     afternoon for WYZ055-057-058.



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