000
FXUS63 KUNR 212104
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
304 PM MDT Fri Jul 21 2017

.DISCUSSION...(This Evening Through Friday)
Issued at 232 PM MDT Fri Jul 21 2017

Upper analysis shows mainly zonal flow across the northern CONUS
as an upper low flattens the broad upper ridge as it crosses the
Canadian prairie provinces and swings and weak upper trough
across the northern plains. A sfc low moving southeastward across
the region has been followed by a weak "cold" front, mainly
resulting in drier air flowing in from the northwest. A tightening
of the sfc pressure gradient has created breezy northwest winds
over much of the forecast area this afternoon. The northwesterly
sfc flow has allowed smoke from the fires in central Montana to
move into northeastern Wyoming and western South Dakota,
reducing the visibility at times. The smoke is expected to
continue to affect the forecast area tonight and on Saturday.
Ahead of the surface boundary, over south central South Dakota,
very warm temperatures with dewpoints in the 60s to lower 70s have
driven the heat index to 100-104F so will continue the Heat
Advisory that was previously issued for those areas.

On Saturday, deep mixing is possible again which may create breezy
to windy conditions, especially over northwestern South Dakota.
While temperatures are not expected to be as warm as today, low
relative humidity will increase the fire danger. A Fire Weather
Watch has been issued for Saturday for part of northwestern South
Dakota. However, the smoke flowing into the region from Montana,
may limit high temperatures on Saturday, which would diminish the
extent of the boundary layer mixing, and could lead to slightly
lower wind speeds.

On Sunday the upper ridge will rebuild over the western half of
the CONUS leading to northwesterly flow aloft over the northern
plains. Dry air will still be in place, so PoPs not warranted.

Next week, the upper ridge will shift eastward over the great
plains. A series of shortwaves riding the US/CAN border will push
a few frontal boundaries into the forecast are, which may be the
focus for the development of showers and thunderstorms.
Temperatures will peak early in the week and then cool to near
average for the rest of the week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Saturday)
Issued At 232 PM MDT Fri Jul 21 2017

Smoke from fires in central Montana are reducing visibility
across portions of northeastern Wyoming and far western South
Dakota and may result in occasional periods of MVFR VSBY.
Otherwise VFR conditions are expected.

&&

.FIRE WEATHER...
Issued at 232 PM MDT Fri Jul 21 2017

Drier air is moving into the region on breezy to windy
northwest winds this afternoon. The combination of gusty winds, warm
temperatures and low relative humidity is producing critical fire
weather conditions this afternoon. Conditions will improve early this
evening.

Minimum relative humidities will drop to 15-25 percent Saturday
afternoon with breezy winds. If it looks drier or winds become
stronger, critical fire weather conditions may develop. A Fire Weather
Watch has been issued for northwestern South Dakota on Saturday.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Red Flag Warning until 6 PM MDT this evening for SDZ261.

     Heat Advisory until 7 PM MDT /8 PM CDT/ this evening for SDZ032-
     043-046-047-049.

     Fire Weather Watch from Saturday morning through Saturday
     evening for SDZ261-266.

WY...Red Flag Warning until 6 PM MDT this evening for WYZ297-298.

&&

$$

DISCUSSION...10
AVIATION...13
FIRE WEATHER...10