626
FXUS63 KUNR 170428
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1028 PM MDT Sun Jun 16 2024

.KEY MESSAGES...

- Nocturnal storms expected tonight into Monday morning. Some
  storms could be severe over southwestern and especially south
  central South Dakota.

- Locally heavy rain is possible with the thunderstorms tonight.

- More storms possible later Monday, with some severe potential
  again.

&&

.DISCUSSION...(This Evening Through Sunday)
Issued at 133 PM MDT Sun Jun 16 2024

Looking at the big picture this afternoon the cold front is
located well off to the southeast with a weak area of relatively
cooler high pressure over the forecast area. An upper low is
located over central Saskatchewan/Manitoba with a trough extending
southwestward to another low over the Pacific Northwest. This is
resulting in fast southwesterly flow aloft with a 90+kt jet streak
currently crossing the forecast area.

A surface low over eastern Colorado will expand into southwestern
Nebraska tonight causing the SFC-850mb winds to become easterly.
A strengthening 50+kt southwesterly 700mb jet is expected to lead
the development of a line of elevated thunderstorms to the north
of the surface boundary from southwestern into south central South
Dakota later this evening. With a period of MUCAPE greater than
2000J/kg and ample shear, it appears that large hail will be
possible, along with locally heavy rain. Due to the elevated
nature of the storms, strong wind gusts are less likely, but still
locally possible with the stronger storms. The system will tend
to become less organized as the line pushes north overnight into
early Monday.

An even cooler day is expected for Monday, with low level
easterly flow continuing over the region. Due to cloud cover,
highs are expected to remain in the 60s for much of the South
Dakota side of the forecast area. Northeastern Wyoming, and areas
near the SD/Neb border are expected to have less cloud cover
during the day, so high temperatures in those areas may be in the
mid to upper 70s on Monday afternoon.

Enhanced lift ahead of the approaching PacNW trough may lead to
more thunderstorms on Monday afternoon and evening, especially
over the warmer parts of northeastern Wyoming and south central
South Dakota. Development of showers and storms over the rest of
western South Dakota will be conditional and highly dependent of
the amount and extent of lingering low stratus limiting daytime
heating. Severe storms will be possible if the stratus is able to
break up earlier in the day. If storms are able to develop, the
main threats with the later Monday disturbance will be both hail
and strong winds, however there will also be a chance for a
tornado due to the increased low level helicity near the surface
boundary over south central South Dakota.

The upper level PacNW trough that had been lingering to the west
will finally swing across the northern plains later Tuesday. Some
showers/storms may be possible. There is then a wide spread in the
ensemble precipitation chances through the rest of the week.
However, it appears that the will be chances for showers/storms
over parts of the forecast area nearly every day.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Monday Night)
Issued At 1024 PM MDT Sun Jun 16 2024

Scattered to numerous thunderstorms will lift northeast across
southwest and south-central SD overnight, mainly near and south of
I-90, then diminish around sunrise. IFR conditions expected with
the strongest storms. Around sunrise, low stratus will develop
near and south of I-90 across western SD, with lower probability
of the stratus expanding into northeast WY. Widespread MVFR and
areas of IFR ceilings expected with the stratus. The low stratus
may linger through much of the day Monday, especially south and
east of the Black Hills.

&&

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

$$

DISCUSSION/KEY MESSAGES...10
AVIATION...Johnson