Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 201700

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1100 AM MDT Wed Sep 20 2017

.DISCUSSION...(Today Through Tuesday)
Issued at 228 AM MDT Wed Sep 20 2017

Deep upper trough will remain over the PAC NW through most of the
work week, with this system finally pushing east by the weekend.
Strong Fujiwara type trough on the southern periphery is in the
process of rotating south into the west coast. This system will
support local height rises with WAA ensuing later today with sfc
lee side toughing expected over the FA. Near stalled height
structure aloft will support a temp contrast across the fa, with
warmest temps over the SE half the next few days. Dry weather can
be expected today under subsident flow, although diurnal mixing
and linked flow aloft will support a breezy day across the region
with gusty SW winds expected, strongest far west. Precip chances
will return toward Friday and into the weekend with decent chances
for much of the area as better moisture per gomex connection
develops with the rotating front side impulse. Parts of the region
could see a decent rainfall where FGEN and moisture convergence
persists given advection of the front side wave and then the main
upper trough into the region. CAA will also support a strong
cooling trend this weekend with much cooler more fall like weather
returning to the region. Many locations will struggle out of the
40s both Sat and Sunday. The cooler temps, wetbulb effects, and
nocturnal timing will support some higher elevation snow this
weekend, esp Sat night where some light accums will be possible
over parts of the Black Hills. The cool weather will persist into
next week as mean troughing lingers over the central CONUS. Warmer
weather looks to return by the middle to end of next week as
another round of western CONUS ridging is favored in GFS/CMC/ECMWF


.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1058 AM MDT Wed Sep 20 2017

VFR conditions are expected through the period. However, LLWS is
expected to be prevalent tonight, especially from 06z-12z, over
southwestern into west-central SD as a strong (50-60 kt) low-
level jet stream develops.


Issued At 228 AM MDT Wed Sep 20 2017

Dry gusty SW flow will develop this afternoon, supporting
critical fire weather conds for parts of NE WY and far SW SD. RH
is forecast to fall to or below 15% with winds of 15 to 25 mph
(with gusts to around 40 mph) for a majority of the affected fire
weather zones (298, 299, 262, 263, and 264). Hence, a Red Flag
warning has been issued for this afternoon there. Have trended to
the lower end of the RH spectrum in model solutions, given SW flow
setup. There is some question on the degree of warming that will
occur across this area today. Given mixing potentials and full
sun, warmer temps could materialize further lowering RH`s this
afternoon. Have left out Campbell county given concerns on min
RH`s which look to remain in the upper teens to around 20 percent.
However, it will be very windy there. The remainder of the area
will see less wind and slightly higher min RH`s, with extreme
grassland fire danger index numbers on most of the western SD
plains. Much less wind forecast Thur with dry air remaining in
place, esp across far SW SD where extreme grassland fire danger
index numbers are forecast.


SD...Red Flag Warning until 7 PM MDT this evening for SDZ262>264.

WY...Red Flag Warning until 7 PM MDT this evening for WYZ298-299.



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