Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 272307

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
507 PM MDT Tue Sep 27 2022

.DISCUSSION...(This Evening Through Tuesday)
Issued at 149 PM MDT Tue Sep 27 2022

Regional satellite imagery this aftn shows terrain-driven convection
across CO and WY ahead of a weak mid to upper-level disturbance.
However, this convection is exhibiting diminishing depth and vigor
with newd extent, with only shallow, fair weather cu evident over
portions of the Bighorns and mostly clear skies across our area.
Modestly strengthening sfc ridging from s cntrl CN swd into the MO
Valley coupled with deepening lee trof are combining to support
steady east to southeast flow across the area this aftn. Temps this
hour are seasonable to a few degrees above average, generally
ranging from the mid 60s to mid 70s.

Sfc ridge continues to strengthen modestly tonight into early
tomorrow as sfc trof slides slowly ewd. Resulting tightening
pressure/height gradient should support the development of a
southerly LLJ over the High Plains late this evening into tonight.
Regionwide pressure falls tomorrow will promote continued tightening
of the pressure gradient over the area, particularly over the
wrn/cntrl SD plains, supporting a broad area of boundary layer winds
25-30 kt. Further strengthening of this LLJ is anticipated tomorrow
night into early Thursday. As such, could see some gusts approaching
advisory criteria from tomorrow AM thru Thursday aftn, mainly over s
cntrl SD. HREF is particularly bullish on this potential tomorrow
night, with mean gusts exceeding 45 mph from the Pine Ridge ewd into
s cntrl SD beginning around 03z. NAM/GFS fcst soundings also appear
favorable for some strong gusts, with 40-50+ kt winds in the 1-2 kft
above ground layer. However, EPS probabilities of gusts exceeding 34
kt remain low, and 12z Euro deterministic gusts remain under 45 mph.
Given only moderate confidence and timing, will hold off on any
headlines for now. Sfc low evolves into an inverted trof by Friday,
extending from SW-NE thru wrn/cntrl SD, with strong southerly winds
generally shifting east of our area.

A mid to upper-level trof currently near the Pac NW coast will
gradually shift ewd toward the area over the next 48-72 hrs. Its
progression and evolution will depend on impacts to the large-scale
pattern from Hurricane Ian as it approaches FL. Ian will likely
pinch existing ridging across the ern two-thirds of the CONUS and
could amplify this ridge over the Plains, slowing ewd progression of
the upper low. Amplification of the ridge will also play a big role
in the swd progression of the upper low with it being forecast to
close off, and potentially even cut off, from the nrn stream late in
the work week. Regardless of the system`s track and ultimate
evolution, it will be accompanied by a broad area of well above
average PWATs (175-225% of normal per the EPS mean) and cooler
temperatures Friday into Saturday. As such, confidence is fairly
high in a transition to unsettled conditions and below average
temps from late Thursday through at least Saturday.


.AVIATION...(For the 00Z TAFS Through 00Z Wednesday Evening)
Issued At 504 PM MDT Tue Sep 27 2022

High clouds will move across the area Wed. South-southeast winds
will become gusty Wed morning, with brief LLWS possible early in
the day as winds aloft increase before surface winds do.


Issued At 149 PM MDT Tue Sep 27 2022

As a trough approaches the region over the next few days, south to
southwest winds will increase across portions of northeastern WY,
southwestern SD, and south central SD. Downslope flow and a warm,
dry air mass will lead to well above average temperatures (highs in
the 80s to near 90) and low relative humidity values (minimum
afternoon values 10-20%) in these areas. For now, the strongest
winds (as high as 20-30 mph sustained with gusts approaching 45 mph
Wednesday night into Thursday) and lowest relative humidity values
are not expected to coincide in time and location. However, the
potential exists for localized elevated to near-critical fire
weather conditions tomorrow through Thursday.




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