Winter Storm and Winter Weather in Wyoming, April 11 to 12, 2020

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1117 AM MDT Sat Apr 11 2020

.UPDATE...
Issued at 1041 AM MDT Sat Apr 11 2020

Rapidly changing situation as many areas are changing over to snow
a bit quicker this morning than expected. The melting of
precipitation as it falls, cold air surging southward, and broad
ascent has resulted in colder thermal profiles and subsequent surface
temperatures cooling rather quickly this morning. Have updated
the forecast to try to capture the quicker change over from rain
to snow, especially across the Black Hills and adjacent foothills.
Model guidance has struggled to accurately capture these trends this
morning with some of the morning runs of the HRRR/RAP the worst
offenders. With the earlier onset of snow, likely see a bit more
snow possible, although still expect heaviest snow later by early
evening into the overnight hours.

&&

.DISCUSSION...(Today Through Friday)
Issued at 231 AM MDT Sat Apr 11 2020

Sig pattern change about to commence locally as a lead NW flow
shortwave merges with the ejecting SW CONUS upper trough, bringing
heavy snow to the region. Primary cold front continues to advance
into the region tonight, as it expected to arrive in the FA later
this morning. The main change in the evolution of this system is
the northward bias of the main FGEN zone tonight. This is due to a
slower/weaker sfc front surge, which supports warmer thermal
fields and later rain to snow change over. With the main FGEN zone
now further north, much more lift and moisture will available,
supporting higher snow totals, esp from the lee of the Black Hills
southeast into scentral SD. Lead shortwave will continue to
support scattered showers early this morning across the area.
However as the bulk of LSA shifts SE, a decent FGEN band will
develop across NW SD later this morning, persisting into the
afternoon. Dynamic cooling and wetbulb effect will allow rain to
change to snow there with some accums expected, esp on grassy
sfc’s. If precip is heavy enough, accumulation on the roads
would be possible later today. As stated earlier, warmer thermal
fields will support a later change over across the rest of the
area, esp NE WY where weak WAA will develop in response to the
ejecting SW CONUS upper trough. This has resulted in headline
tweaks to account for this, with the most sig timing change being
the adv area over NE WY where a 12 hour shift was implemented. As
the SW CONUS upper trough begins to shift east and phase, lift
will increase across the region, maximizing this evening locally
as the upper jet left exit region times over the FA, supporting a
strong FGEN band given expected steep ll lapse rates which happen
to be centered in the DGZ. End result will be a period of moderate
to heavy snow tonight, in a banded pattern over southern SD.
Current model trends are for this FGEN band to setup from SW to
scentral SD. This shift north will also support much more
available moisture in the northern and eastern Black Hills where a
period of strong upslope lift will develop, indicated well in
model soundings with deep lift will be center in the DGZ. Still
expecting the highest snow totals there with over a foot expected
in parts of the northern Black Hills. Snow will be heaviest
overnight, quickly tapering off Sunday morning. As stated above,
did make numerous headline tweaks/changes. Extended the warning
north to account for the northward shift in the storm track. Also
decided to add NW SD to an adv. Not sure how much snow will
accumulate there during the day today with concerns on strong
April insolation, but with some additional accums tonight, expect
deteriorating road conds with temps below freezing and increasing
winds. Hence felt an adv was warranted. Some blowing and drifting
snow can be expected esp in the lee of the Black Hills. Very cold
temps (esp morning lows) expected through mid next week, with
temps below normal at least through then. Record to near record
lows can be expected Sunday night. Continued unsettled NW flow
will allow snow shower chances nearly every day, esp over NE WY
and the Black Hills.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1115 AM MDT Sat Apr 11 2020

Mixed rain/snow will become all snow across the area by this
evening. Conditions will deteriorate this afternoon with
widespread IFR conditions due to snow/blowing snow tonight. LIFR
conditions likely from the Black Hills into southern SD due to
heavy snow. Conditions will start to improve Sunday morning.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning until noon MDT Sunday for SDZ024-028.

     Winter Storm Warning from 6 PM MDT /7 PM CDT/ this evening to
     noon MDT /1 PM CDT/ Sunday for SDZ027-030-031-041>044-046-
     047-049.

     Winter Weather Advisory until 6 AM MDT Sunday for SDZ001-002-
     012>014.

     Winter Weather Advisory from 6 PM this evening to noon MDT
     Sunday for SDZ032-073.

     Winter Storm Warning until noon MDT Sunday for SDZ025-072.

     Winter Storm Warning from 3 PM this afternoon to noon MDT Sunday
     for SDZ026-029-074.

WY...Winter Storm Warning until noon MDT Sunday for WYZ057.

     Winter Weather Advisory from 6 PM this evening to noon MDT
     Sunday for WYZ054>056-058-071.

&&

Update...Eagan
DISCUSSION...JC
AVIATION...Helgeson

Hazardous Weather Outlook
National Weather Service Rapid City SD
404 AM MDT Sat Apr 11 2020

SDZ001-002-012>014-032-073-WYZ054>056-058-071-121015-
Harding-Perkins-Butte-Northern Meade Co Plains-Ziebach-Haakon-
Southern Meade Co Plains-Northern Campbell-Southern Campbell-
Western Crook-Weston-Northeastern Crook-
404 AM MDT Sat Apr 11 2020

This Hazardous Weather Outlook is for portions of northwestern
South Dakota...southwestern South Dakota...and northeast Wyoming.

.DAY ONE...Today and Tonight

A spring snowstorm will bring moderate snowfall and breezy winds
to the area this weekend. For details, see the latest Winter
Weather Advisory.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Very cold air will move into the region this weekend into early
next week. Well below average temperatures, possibly record-
breaking, will be possible.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today.

&&

Stay tuned to NOAA weather radio for further updates...or check
our web site at weather.gov/rapidcity

SDZ024>031-041>044-046-047-049-072-074-WYZ057-121015-
Northern Black Hills-Northern Foot Hills-Rapid City-
Southern Foot Hills-Central Black Hills-Southern Black Hills-
Custer Co Plains-Pennington Co Plains-Fall River-Oglala Lakota-
Jackson-Bennett-Mellette-Todd-Tripp-Sturgis/Piedmont Foot Hills-
Hermosa Foot Hills-Wyoming Black Hills-
404 AM MDT Sat Apr 11 2020 /504 AM CDT Sat Apr 11 2020/

This Hazardous Weather Outlook is for portions of northwestern
South Dakota...south central South Dakota...southwestern South
Dakota...the Black Hills of South Dakota...and the Black Hills of
Wyoming.

.DAY ONE...Today and Tonight

A spring snowstorm will bring heavy snowfall and breezy winds to
the area this weekend. For details, see the latest Winter Storm
Warning.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Very cold air will move into the region this weekend into early
next week. Well below average temperatures, possibly record-
breaking, will be possible.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today.

&&

Stay tuned to NOAA weather radio for further updates...or check
our web site at weather.gov/rapidcity