485
FXUS63 KUNR 050414
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1014 PM MDT Fri Oct 4 2024

.KEY MESSAGES...

- Near critical to critical fire conditions possible tonight through
Saturday afternoon

- Strong northwest winds expected to develop across portions of the
western South Dakota plains Saturday

- Dry and generally above normal temperatures continue through early
next week with periods of elevated to critical fire weather
conditions

&&

.DISCUSSION...(This Evening Through Friday)
Issued at 225 PM MDT Fri Oct 4 2024

Broad upper level ridge sits over the northern tier of the US
this afternoon with strong shortwave moving onshore into the PNW.
Lee cyclone has begun to develop over southern AB while a sfc high
sits off to our east. Tight east-west pressure gradient has
formed over western SD which is facilitating the development of
breezy southerly winds over much of the area, especially over
south central SD where sfc stations are reporting winds of 25 to
30 with gusts of up to 40 mph. Warm and dry air has advected into
the region with temperatures in the upper 60s to mid 80s with RHs
dropping to below 20% in many areas.

Sfc low and upper level trough will continue to slide east
through the evening hours with attendant cold front approaching
the region from the northwest by late this evening. West to
southwesterly flow ahead of front will contribute to downslope
winds across northeastern WY into the Black Hills and northern
foothills. Compressional warming from these downsloping winds will
keep temperatures warm through the overnight hours which will
keep RHs from recovering over northeastern WY into extreme western
SD until the front passes by 12z Saturday morning. Winds
overnight will also continue to be quite breezy which will result
in near critical fire wx conditions persisting into the overnight
hours (see fire wx discussion).

3 hr pressure rises of 4 to 7 mb and CAA will support a period of
strong winds as the front crosses the region. The strongest winds
will generally be across the western SD plains north of I-90.
With the front and strongest pressure rises/CAA arriving during
the overnight/early morning hours, the strongest winds may not
make it down to the surface. Short range ensemble guidance has
been rather conservative on wind gusts with the grand ensemble
painting a few pockets of 70-80% probs for gusts exceeding 45mph
overnight. Still, with the robust pressure rises and modest CAA
behind the front, the winds could outperform guidance - especially
in our windier spots over northwestern WY and near the Black
Hills. Confidence in high wind warning criteria being met is very
low but have enough confidence in advsy level winds to upgrade the
high wind watch to a wind advisory.

After a cooler, more seasonable day on Sunday, ridging builds
back into the western US. Above average temperatures will return
to the region with the potential for highs in the upper 80s to low
90s by late next week. Our dry spell, unfortunately, will
continue with long range ensembles showing very low probs (around
0%) for total QPF over the next week exceeding 0.1".

&&

.AVIATION...(For the 06Z TAFS Through 06Z Saturday Night)
Issued At 1012 PM MDT Fri Oct 4 2024

VFR conditions will persist through the period. A period of low
level wind shear will develop tonight on the SD plains, including
the KRAP terminal, as a low level jet develops ahead of an
approaching cold front. The cold front will cross northeast WY and
far western SD after 09Z and through central SD by 15Z. Gusty
northwest are expected behind the front, especially across western
SD where gusts over 40kts possible. Winds will gradually diminish
through the afternoon, becoming noticeably calmer by evening.

&&

.FIRE WEATHER...
Issued At 225 PM MDT Fri Oct 4 2024

Strong cold front will cross the region late tonight/early
Saturday morning. Ahead of the front, breezy south to
southwesterly winds will continue through the evening hours. RHs
will struggle to recover overnight, especially over northeastern
WY into the Black Hills and extreme western SD where downslope
winds will keep conditions warm and dry through most of tonight.
In fact, overnight RHs over much of northeastern WY into western
SD will barely make it over 20 to 25% until the front crosses by
around 10z to 12z (4am-6am MDT). With the breezy southerly winds
continuing overnight we could see elevated to near critical fire
wx conditions persisting well into the overnight hours.

RHs will rise slightly Saturday morning as the front passes but
will drop to below 20% by Saturday afternoon. Combined with strong
northwest winds of 25 to 35 mph, gusting to 55 mph and the very
dry conditions over the past few weeks - have medium to high
confidence in critical fire weather conditions redeveloping
Saturday afternoon.

Slightly cooler temperatures and lighter winds will move into the
region by Sunday which should limit fire wx concerns. However,
above average temperatures will return to the region by Monday and
persist into the week. With the continuing dry weather, we could
see periods of elevated to near-critical fire wx conditions
through the upcoming week.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...     Wind Advisory from 3 AM to 6 PM MDT Saturday for SDZ001-002-
     012>014-026-031-032-072-073-078.
     Red Flag Warning until 6 PM MDT /7 PM CDT/ Saturday for
     SDZ319>335.
WY...Red Flag Warning until 6 PM MDT Saturday for WYZ314>318.
&&

$$

DISCUSSION/KEY MESSAGES...Wong
AVIATION...Pojorlie
FIRE WEATHER...Wong