730
FXUS63 KUNR 181140
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
540 AM 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...
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.

&&

.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 12Z TAFS Through 12Z Thursday Morning)
Issued At 447 AM MDT Wed Sep 18 2024

VFR conditions are expected through the day with gusty southwest
winds late this morning through this evening. A few showers are
possible this afternoon into tonight across northeastern Wyoming
and northwestern South Dakota. MVFR/IFR ceilings could spread
into far northeastern Wyoming/northwestern South Dakota tonight.

&&

.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...
SD...Wind Advisory from noon today to noon MDT Thursday for SDZ001-
     002-012-078.
     Red Flag Warning from noon MDT /1 PM CDT/ today to 8 PM MDT /9
     PM CDT/ this evening for SDZ322-324>326-329>335.
     High Wind Warning until 10 AM MDT this morning for SDZ025.
WY...None.
&&

$$

UPDATE...Helgeson
DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...7
FIRE WEATHER...Helgeson