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090
FXUS63 KUNR 200427
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1027 PM MDT Wed Jun 19 2024

.KEY MESSAGES...

- Below average temps continue through Thursday

- Unsettled conditions return Thursday and the late week period.
  Strong storms and heavy rain possible on Friday

- Becoming hot and mainly dry Sunday into early next week.

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 147 PM MDT Wed Jun 19 2024

Upper level troughing remains in place over the
western US with a ridge over the eastern US. The northern plains
sits under southwest flow. Cold airmass has dipped down from Canada
and sits over the western US. This airmass was responsible for our
record breaking lows last night with some stations across the area
reporting minimum temperatures at or below freezing. At the sfc,
high pressure sits centered over the northern plains, shifting off
to the east. As the high moves further to the east, southerly flow
returns to the region bringing moisture and warmer air back into the
northern plains.

For tonight, a weak shortwave will move through the region. With
ample moisture at the low to mid-levels we could see overnight
precip, especially over south central SD and northeastern WY into
northwestern SD. MUCAPE of 1000-1500 J/kg will certainly be enough
for thunderstorms, though effective shear will be low - generally
less than 20 kts - so severe weather isn`t expected. Better
instability moves into southern SD by Thursday afternoon with MUCAPE
values of 2000-2300 J/kg in south central to southwestern SD by 00z
Friday. Effective shear of around 30-40kts will be supportive of a
low severe threat, though high CIN of -50 to -100 J/kg will likely
limit any risk of widespread severe weather in southern SD.

Upper level high over the eastern US retrogrades into the southern
US as we move into the weekend, with upper level flow transitioning
from southwesterly to more zonal flow. Weak shortwave moves through
the region Friday with ample GoMEX moisture advecting into the
region. PWATs will increase to 1.2-1.6" (150-200% of normal) by
Friday night, meaning storms that do form will be capable of heavy
rainfall. South central South Dakota looks to have the best chances
for heavy rainfall on Friday with the 12z HREF grand ensemble run
painting medium-high (50-75%) probs of 24 hr QPF exceeding 0.25".
There is a risk of strong to isolated severe storms on Friday
afternoon as well, especially towards central SD. 1000-2000 J/kg MU
CAPE with 20-30kt 0-6km shear will be supportive of a few strong to
severe storms. Main threats right now look to be large hail and
damaging winds.

Ensemble and deterministic guidance in fairly good consensus of the
evolution of the upper level flow through the weekend. The upper
level high continues its westward trek across the southern US
through the weekend and then parks itself over the desert SW/Four
Corners region by early next week. Broad ridging develops over the
central US and hot air mass sets up over the Rockies into the desert
southwest. 850 temps by Monday will reach 30C with above average
highs through early next week. Ensembles paint 80-100% probs of high
temps exceeding 90+F by Tuesday.

&&

.AVIATION...
-- Changed Discussion --
(For the 06Z TAFS Through 06Z Thursday Night)
Issued At 1024 PM MDT Wed Jun 19 2024

Moisture will continue to stream across the forecast area...with
increasing and lowering cigs through 12z Thursday. Look for MVFR
ocnl IFR cigs across much of the fcst area aft 14z...with pockets
of MVFR vsbys in haze. There will also be scattered
showers/thunderstorms across southcentral south dakota through the
valid forecast period...with MVFR/IFR cigs/vsbys near some of the
heavier storms.
-- End Changed Discussion --

&&

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

$$

DISCUSSION/KEY MESSAGES...Wong
AVIATION...Hintz