Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 180943

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
343 AM MDT Sun Mar 18 2018

Issued at 342 AM MDT Sun Mar 18 2018

Made a last-minute decision to include Perkins County in the
winter weather advisory -- mainly for the northwestern portion
where 4-inch snowfall may occur.


.DISCUSSION...(Today Through Saturday)
Issued at 231 AM MDT Sun Mar 18 2018

The latest water vapor images revealed a large, complex upper
low/trough over the western CONUS, with multiple circulation
centers. The main center/vort max was near Las Vegas, with a
smaller/weaker center over western MT. A weak short-wave trough
was evident over UT, and this will eject northeast today and
tonight across/over the CWA. The circulation over western MT also
eventually will move across the CWA. Otherwise, the main vort max
should remain to our south as it moves toward OK/TX by Monday.

For today, as the short-wave trough approaches, an area of lift
will develop (in conjunction with concentrated frontogenesis and
low-level isentropic lift), focused mainly from the central Black
Hills to far northwestern SD. An initially warm lower-troposphere
will give way to cooling as the thermal ridge moves southeast.
This will allow for an initial wintry mix to transition to all
snow over northwestern areas early this afternoon. Given the
strong localized lift and moderately steep midlevel lapse rates,
advisory snow of 3-6 inches is expected from the central Black
Hills up through Buffalo, SD. Given uncertainty in the exact
location of this eventual snowband, it is possible that it could
affect parts of Perkins, western Custer, southeastern Weston, and
the Crook County plains. For now have kept these areas out of the
advisory, but will have to watch as the snowband sets up to see if
adjustments are needed. Also, given a good dentritic growth
signal, there could be some local 6-8 inch amounts. Current
thinking is that this will be localized enough to warrant just an
advisory at this time. In addition, the winds are not expected to
be strong in the advisory area, precluding blowing snow. Else-
where, a mix of rain and snow is expected, with some concern for a
little freezing rain and sleet given the low-level warm layer.
This will be something to watch for if freezing rain becomes the
more dominant precipitation type. Otherwise, patchy areas of fog
(some locally dense) over west-central and northwestern SD should
slowly mix out during the morning. Do not think dense fog is
widespread enough for an advisory at this time.

For tonight, precipitation should transition to all snow from
northwest to southeast. Will let the winter weather advisory for the
northwest expire at 06z as the expected heavy band of snow should be
over by that time with the frontogenesis weakening and moving east.
Snowfall amounts elsewhere should be below advisory criteria. On
Monday, another weak short-wave trough will cross the CWA, with the
best moisture and lift shifting towards the eastern half of the CWA.
A few inches of snow will be possible there. It also will continue
to be relatively cool as northerly low-level flow brings cooling 850-
mb temperatures.

On Tuesday, the large-scale trough will start edging into the
eastern CONUS, with yet another weak short-wave trough crossing
the CWA in northwest flow. Moisture and lift will be more limited
with this system, thus only expected isolated snow/rain showers.
With a ridge building over the west, this will bring drying and
warming for later Tuesday and Wednesday. The ridge will be well
established by Thursday with mild temperatures; however, a weak
short-wave trough will break through the ridge and may produce
some showers (and even isolated thunder) over parts of the CWA.
Temperatures should be near to above average Wednesday through
Friday, which is dependent on the amount of remaining snow cover.

A large trough will transition from the western into the central
CONUS next Friday-Sunday. This portends unsettled weather with
chances of showers, and perhaps a few thunderstorms. As is typical
this time of year, the potential also exists for another


.AVIATION...(For the 06Z TAFS Through 06Z Sunday Night)
Issued At 1113 PM MDT Sat Mar 17 2018

A storm system will approach the area overnight and spread over
the region Sunday. Ahead of it across western SD through 15z,
areas IFR fog/stratus expected with local LIFR VSBYS, especially
near the higher terrain. Ahead of it across northeast WY through
Sunday morning, mainly VFR CIGS expected with local LIFR
conditions near the Wyoming Black Hills due to stratus/fog.
Conditions will then deteriorate Sunday afternoon and evening as
wintry mix transitions to all snow with widespread IFR conditions.


SD...Winter Weather Advisory from noon today to midnight MDT tonight
     for SDZ001-002-012-024-025-028-072.

WY...Winter Weather Advisory from noon today to midnight MDT tonight
     for WYZ057-071.



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