Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 212018

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
218 PM MDT TUE JUN 21 2016

.SHORT TERM...(This Evening Through Wednesday Night)
Issued at 215 PM MDT Tue Jun 21 2016

Current surface analysis shows low pressure over far
southeast MT, with warm front extending southeast into western SD
and a dryline across far northeast WY. Cold front extends
southwestward from central MT to eastern ID. Southerly winds ahead
of the dryline continue to bring moisture northward, with dewpoints
in the 50s and lower 60s. Humidities behind the dryline have dropped
near or below 15 percent across portions of the northeast WY plains
into far southwest SD. Upper level analysis shows high pressure
remaining centered over the Four Corners area, with ridge axis
extending north-northeast into the northern high plains. Shortwave
energy noted on water vapor imagery over south central MT into
northwest and north central WY. Skies are mostly sunny across much
of the area, though isolated showers and storms are now developing
over portions of northeast WY. Temps are in the upper 80s to near
100 across the forecast area.

The potential for active weather is still there for portions of the
area late this afternoon and early evening as low pressure slowly
moves east into northwest SD/southwest ND and approaching shortwave
crosses the western Dakotas. Isolated to scattered thunderstorms
will continue to develop late this afternoon as cap erodes
sufficiently enough, especially across far northeast WY into far
western SD. Storms will move into more unstable atmosphere, where
cape values will exceed 3000 j/kg in some areas to the north and
east of the Black Hills through the early evening. Shear will be
most favorable for supercells across northwest into central SD this
evening. HRRR has been persistent with showing a potential line of
storms across northwest SD by early evening, with an isolated
supercell or two also possible from northwest into central SD. Large
hail and strong wind gusts will be the main threats. Activity should
taper off from west to east through the evening, with severe threat
greatly diminished by late evening.

Cold front will move through northeast WY early this evening and
across the rest of the cwa overnight. A period of gusty northwest
winds can be expected in many areas behind the front. Cold advection
will drop temps into the 50s across northeast WY, the Black hills,
and portions of northwest SD by morning, with low to mid 60s

Subsidence and drier air in place across the area on Wednesday will
bring rather pleasant conditions. Highs will be in the mid 70s to
mid 80s under mainly sunny skies. Northwesterly winds will be brisk
across much of the SD plains, especially northwest SD. Relatively
cool conditions in store for Wednesday night, with lows in the 50s
and light winds.

.LONG TERM...(Thursday Through Tuesday)
Issued at 215 PM MDT Tue Jun 21 2016

The upper ridge will remain over the region on Thursday. At the
surface, southerly return flow will develop as the surface high
moves off to the east and the lee side trough begins to deepen as
the next low pressure system moves into the Pacific Northwest. The
ridge will begin to shift eastward on Friday as the upper low
approaches from the west. A cold front will swing through the
forecast area late friday night or early Saturday as the upper low
crosses southern Saskatchewan. This will lead to cooler
temperatures on Saturday with highs remaining near average through
the weekend. Isolated thunderstorms are possible on Thursday and
Friday with a slightly better chance of thunderstorms on Friday
night. Mainly dry weather is expected on Saturday and Sunday with
increasing thunderstorm chances again Monday night and Tuesday.


.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 215 PM MDT Tue Jun 21 2016

VFR conditions will continue through the period. Isolated to
scattered thunderstorms will develop this afternoon and evening.
Strong and variable wind gusts are possible near any
thunderstorms. A cold front will push through the region late this
afternoon and this evening followed by gusty northwest winds.
Windy conditions are expected on the South Dakota plains on


Issued At 215 PM MDT Tue Jun 21 2016

Over northeastern Wyoming, very dry conditions have developed
with relative Humidity values ranging from 10 to 15 percent.
isolated thunderstorms with variably gusty winds are expected.
Gusty southwest winds will continue into early this evening, then
a cold front will cross northeastern Wyoming, shifting winds to
the northwest. A period of gusty northwest winds is expected
behind the cold front. A Red Flag Warning continues for
northeastern Wyoming until Midnight tonight.

Isolated to scattered thunderstorms are expected over parts of
western South Dakota this evening with variably gusty winds. A cold
front will cross the area later this evening followed by a period of
gusty northwest winds. Windy conditions will occur on the South
Dakota plains on Wednesday.


.UNR Watches/Warnings/Advisories...
WY...Red Flag Warning until midnight MDT tonight for WYZ259-297-298.



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