Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 212353

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
553 PM MDT Tue Mar 21 2017

Issued at 500 PM MDT Tue Mar 21 2017

Have added the srn Black Hills zone to the Dense Fog Advisory.

UPDATE Issued at 448 PM MDT Tue Mar 21 2017

Issued a Dense Fog Advisory for this evening for many of the areas
circling the Black Hills with a mention of areas of light
freezing drizzle. Some of the areas included in the advisory area
are currently above freezing, some below and some of the areas
currently below freezing may warm above the freezing mark this
evening. Therefore decided to simplify the headlines to a Dense
Fog Advisory with mention of the possibility of freezing precip
with icy areas developing.


.SHORT TERM...(This Evening Through Wednesday Night)
Issued at 218 PM MDT Tue Mar 21 2017

20z surface analysis had 1042mb high over southern MB with east
to southeast flow over the CWA. Upslope flow sustaining dreary
day over much of the CWA. Frontal boundary on southwest edge of
high from southeast WY into western MT. Latest water vapour loop
had upper ridge heading into the plains with a couple of weak
shortwaves over ID and NV. These waves heading in our direction
for tonight.

Tonight, upslope flow continues with strengthening warm air advection
from 850-700mb in response to shortwave energy passing through. At
least through this evening, expect upslope fog mixed with drizzle or
freezing drizzle (depending on ground temperatures) to continue,
especially on the SD plains and eastern slopes of the Black Hills.
Gradient tightens considerably with low level jet developing. This
will cause gusty winds over northeast WY and the higher Black Hills
as well as the frontal boundary to move northeast as a warm front.
Synoptic support for precipitation is there with forecast soundings
suggesting a wintry mix at times. Ice accumulations should be light,
but will need to be watched if sustained -FZRA develops.

Wednesday, upper ridge builds into the CWA with much warmer
temperatures and precipitation ending early. Temperatures will be
near guidance.

Wednesday night, upper low pulls into the southwest US with impulse
laden southwest flow across the CWA which will support a chance of
mainly rain, possibly mixed with snow over the higher elevations.
Lows will be near guidance.

.LONG TERM...(Thursday Through Tuesday)
Issued at 218 PM MDT Tue Mar 21 2017

Upper low moves from the four corners area Thursday to the
southern plains by Saturday morning with good model consistency.
Several pulses of energy will move through the CWA with decent
waves for the far northwest Thursday and then the southwest
Thursday night, where some convection may occur. Thermal profiles
show mostly a rain event at this point, but some snow could sneak
in over the higher elevations at night and possibly near the NE
border if dynamic cooling extends far enough north from main upper
low. Temperatures will be seasonal.

Sunday and Monday, progressive flow expected with PoPs warranted at
times, although timing confidence low given fast flow. Temperatures
will remain seasonal with cold air bottled up north of the CWA.


.AVIATION...(For the 00Z TAFS Through 00Z Wednesday Evening)
Issued At 551 PM MDT Tue Mar 21 2017

Widespread LIFR conditions will continue near the Black Hills with
widespread IFR conditions across much of the rest of western south
Dakota through this evening with some improvement late, and
especially Wednesday morning. Areas of -FZDZ/FZFG/BR will be
present. Later tonight, some -FZRASN are expected over western SD.
Areas IFR/local LIFR conditions due to stratus/BR over northeast
WY will slowly improve this evening with rapid improvement


SD...Dense Fog Advisory until midnight MDT tonight for SDZ012-025>027-

WY...Dense Fog Advisory until midnight MDT tonight for WYZ057-071.



SHORT TERM...Helgeson
LONG TERM...Helgeson
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