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370
FXUS63 KUNR 181921
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
121 PM MDT Wed Sep 18 2024

.KEY MESSAGES...

- Upper low over southeast MT will create dry gusty winds today
  resulting in critical fire weather conditions over portions of
  the SD plains
- Elevated fire weather conditions Friday
- Low confidence long term forecast with cooler weather for the
  weekend

&&

.UPDATE...
-- Changed Discussion --
Issued at 112 PM MDT Wed Sep 18 2024

Windy Advisory conditions over northwestern South Dakota are
expected to continue tonight into early Thursday afternoon due to
the tight surface pressure gradient with a cold front crossing
that area. Have expanded the area of the Wind Advisory to include
zones that will still be affected by very windy conditions after
the Red Flag Warning expires this evening.

UPDATE Issued at 918 AM MDT Wed Sep 18 2024

Winds have decreased over the northern foothills area, so have
updated the forecast to cancel the High Wind Warning.

UPDATE Issued at 532 AM MDT Wed Sep 18 2024

Some terrain enhanced wind gusts are affected the
Spearfish/northern foothills area early this morning, so have
hoisted a small High Wind Warning to account. Not sure how far off
the terrain the highest winds will expand per preliminary look at
12z KUNR sounding (50kt around 500m AGL) and latest RAP. Will
continue to monitor.
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&&



.DISCUSSION...(Today Through Tuesday)
Issued at 230 AM MDT Wed Sep 18 2024

08z surface analysis had low over KJDN with upper low stacked on
top. Surface trough extended into central NE with cold front into
northeastern WY. Waning band of showers/TS ahead of surface trough
slowly moving north/northeast over central SD per regional radar
loop. Main short term concern revolves around disposition of upper
low.

Today/tonight, upper/surface low spins into southwest SK pushing
a cold front into the CWA this afternoon. Tight pressure gradient
as well as well-mixed boundary layer will promote gusty
west/southwest winds ahead of the cold front and gusty northwest
winds behind the front. Bufkit momentum transfer suggests gusts to
50mph over northwest SD this afternoon/tonight and 45mph over
southern SD today/this evening. HREF supports with >50% chance of
45+ mph for these locations. Will hoist Wind Advisory to account
over northwest SD, but leave out where fire weather headlines are
hoisted on the western SD plains. Backwash moisture will slide
into the WY/MT/SD/ND border areas with showers this
afternoon/tonight. Temperatures will be near guidance.

Thursday, gradient slowly decreases as stacked low pulls away
with slightly cooler temperatures. Friday, warm air advection will
support summery MaxTs with a few spots approaching 90F. Over the
weekend into early next week, progressive west/northwest flow
supports a series of waves and fronts. First front moves through
Friday night with noticeably cooler temperatures for the weekend.
Moisture chances low with main moisture-laden upper low passing
south of the CWA. Confidence is low with substantial ensemble
guidance uncertainty. Chance of >0.25" QPF over the weekend <40%
over southeast half of the CWA and <20% over the northwest half.
Thus, lowish PoPs scattered about for the long term forecast.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1210 PM MDT Wed Sep 18 2024

Showers are possible this afternoon into tonight across
northeastern Wyoming and northwestern South Dakota and may
produce MVFR/IFR ceilings overnight. Otherwise, VFR conditions
are expected through the forecast period. Gusty southwest winds
will continue through this afternoon into this evening before a
frontal passage shifts winds to the northwest for the overnight
into Thursday.

&&

.FIRE WEATHER...
Issued At 230 AM MDT Wed Sep 18 2024

Upper low over eastern MT will promote subsidence, deep boundary
layer mixing, and a tongue of very dry air (HREF >70% chance <20%
RH) over southwest into central SD this afternoon. Gusty
west/southwest winds foreseen, sufficient to hoist a Red Flag
Warning this afternoon and early evening. Northeastern
WY/northwestern SD will see elevated fire weather conditions per
gusty winds and MinRH dipping to 20-30%. Some relief expected
Thursday, but elevated fire weather conditions return Friday with
MinRH 15-30%.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
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SD...Wind Advisory until 3 PM MDT Thursday for SDZ001-002-012>014-025-
     072-073-078.
     Red Flag Warning until 8 PM MDT /9 PM CDT/ this evening for
     SDZ322-324>326-329>335.
WY...None.
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&&

$$

UPDATE...10
DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...13
FIRE WEATHER...Helgeson