Area Forecast Discussion
Issued by NWS Rapid City, SD

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519
FXUS63 KUNR 191638
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1038 AM MDT Tue Sep 19 2017

.UPDATE...
Issued at 1031 AM MDT Tue Sep 19 2017

16z surface analysis had low near KD07 with trough into western
NE and cold front into northeast WY. Water vapour had lead
shortwave over western ND, which is assisting elevated convection
ahead of it across central ND into north central SD. Upper trough
over the northern Rockies with main energy spinning into central
WY.

Main energy pushes across the CWA this afternoon/evening with
cold front moving southeast. Cold front will be over south
central SD by 21z with around 2KJ/kg MLCAPE ahead of it. MLCIN
quite notable, however. Timing of synoptic support coincident with
frontal forcing will determine when cap breaks. 0-6km bulk shear
strong. Right now given frontal timing and CAMS, strongest TSRA
activity will be east of the CWA, but will have to be closely
monitored if large mid/high cloud shield over western SD should
suddenly break loose and allow ample solar insolation.

Forecast updated for latest radar trends and temperatures tweaked
for temperatures per aforementioned cloud shield.

&&

.DISCUSSION...(Today Through Monday)
Issued at 233 AM MDT Tue Sep 19 2017

Deep upper trough will remain over the PAC NW for most of the
week, supporting local height undulation and associated periodic
unsettled conds. Decent impulse is in the process of advecting
east into the Northern Plains. This system will force a cold front
into the region today with associated LSA, forcing chances for
mainly post frontal showers. A few thunderstorms will also be
possible this afternoon. WAA will persist across the SE third
today, supporting temps into the 80s and few 90s with much cooler
conds post frontal across the NW. Shower chances will be more in
the scattered range given lack of sig moisture. However strong
forcing should support a decent chance of rain to NW areas closer
to the strongest lift. Large scale subsidence will spread over the
region Wed behind today`s impulse. However, given the anchored
upper level massive trough, SW flow will persist across the
region, supporting Rockies downsloping with very dry air spreading
into the region. This will support gust SW winds Wed afternoon
with fire wx concerns to the SW third. WAA will ensue over the SE
2/3 Wed and Thur supporting a warming trend there through mid week
ahead of the next round of impulses as the western trough pushes
east toward the end of the week. Precip chances will return toward
the weekend with decent chances for much of the area as better
moisture per gomex connection develops. Parts of the region could
see a decent rainfall where FGEN and moisture convergence
persists. CAA will also support a cooling trend this weekend with
much cooler more fall like weather returning to the region. Cooler
temps, wetbulb effects, nocturnal timing could support some
higher elevation snow esp Friday night and Sat night. Much cooler
weather looks to persist well into next week as mean troughing
lingers over the central CONUS.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Wednesday Morning)
Issued At 459 AM MDT Tue Sep 19 2017

LLWS will continue across the western SD plains early this
morning before dissipating by mid-morning. A cold front will
cross the area today, bringing gusty northwest winds and some
showers with isolated thunderstorms. MVFR conditions are expected
in showers/storms, with VFR conditions otherwise.

&&

.FIRE WEATHER...
Issued At 233 AM MDT Tue Sep 19 2017

Min RH`s around 20 percent are expected across portions of SW SD
and south central SD this afternoon ahead of a cold front.
Although critical fire weather conds are not expected, extreme
grassland fire danger is forecast. Winds will veer NW with rising
RH behind the cold front, which is expected sometime mid to late
afternoon. Dry gusty SW flow will develop Wed afternoon behind
today`s upper level impulse, potentially supporting critical fire
weather conds. RH looks to fall to or below 15% over much of NE WY
and SW SD with winds of 20 to 25 mph (higher gusts possible). The
gustiest winds look to be across NE WY and far SW SD attm, with
the potential for very low RH and breezy conds in and around the
Black Hills. Winds and RH tend to be higher and lower respectively
in this type of setup. Given current forecast numbers and a
decent potential for critical fire weather conds, have issued a
Fire Weather watch for NE WY and SW SD.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for SDZ262>264.

WY...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for WYZ259-297>299.

&&

$$

Update...Helgeson
DISCUSSION...JC
AVIATION...Pojorlie
FIRE WEATHER...JC



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