356
FXUS63 KUNR 150840
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
240 AM MDT Sun Sep 15 2024

.KEY MESSAGES...

- Very warm through Tuesday with near critical fire weather
  conditions
- Strong/severe thunderstorms possible late today/this evening
- Pattern change end of next week with cooler and wetter weather
  likely

&&

.DISCUSSION...(Today Through Saturday)
Issued at 233 AM MDT Sun Sep 15 2024

08z surface analysis had trough from central ND into western NE.
Dry line from MT/ND border through southwest SD. Water vapour loop
had upper trough dropping into the western CONUS with southwest
flow over the northern Plains. Next upstream shortwave of interest
over southern NV.

Today/tonight, NV shortwave moves into the CWA tonight. Weak low
develops along dry line with southeasterly boundary layer flow
promoting influx of richer moisture into western SD. Steep mid-
level lapse rates will contribute to 2KJ/kg MLCAPE by late
afternoon ahead of dry line. 0-6km bulk shear around 35kts. If
MLCIN erodes, should see convection in/near Black Hills by mid-
afternoon, expanding regardless as shortwave approaches this
evening. PWATs surge to 150-200% of normal tonight as moderate
MUCAPE persists. Most CAMS depict isolated strong/severe storms
late this afternoon/early this evening where marginal risk is
painted and then a blob of welcome precipitation overnight over
western SD on nose of low level jet. Even though the probability
of >0.25" QPF (HRRR) is low, suspect some folks will hit the
jackpot under the stronger thunderstorms. Temperatures will be
near guidance.

Monday, subsidence behind shortwave promotes mostly dry weather
as upper trough moves into the Rockies. Tight pressure gradient
creates southerly wind gusts in excess of 40 MPH over south-
central SD. Temperatures will be near guidance.

Tuesday, upper trough lifts into the region with Fall-like
synoptic forcing Tuesday night. Widespread showers/thunderstorms
will occur with a 50% chance of >0.10" QPF for much of the CWA.
Temperatures will be near guidance.

Wednesday through next weekend, high probability that upper
trough/moist cyclonic flow slowly settles into the northern
Plains. A welcome pattern change. Thermal profiles cool with highs
struggling to exceed the 60s next weekend.


&&

.AVIATION...(For the 06Z TAFS Through 06Z Sunday Night)
Issued At 930 PM MDT Sat Sep 14 2024

Some patchy fog with MVFR/IFR conditions may form across north
central SD early Sunday morning. Thunderstorms are expected over
SW SD late Sunday afternoon and evening with gusty winds and small
hail possible. Otherwise, VFR conditions are expected through the
TAF period.

&&

.FIRE WEATHER...
Issued At 233 AM MDT Sun Sep 15 2024

Elevated to near critical fire weather conditions expected
through Monday (possibly stretching into Tuesday), especially over
northeastern WY and far southwest SD where minimum relative
humidities will dip to 10-20%. The strongest winds today and
Monday look to be east of the Black Hills, so probabilities for
widespread critical fire weather conditions over northeast WY and
far southwest SD is low (joint probability of <15% RH and wind
gusts >25 MPH is less than 25% this afternoon). A wetter and
cooler pattern will develop mid to late next week.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...JC
FIRE WEATHER...Helgeson