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000
FXUS63 KUNR 211956
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
156 PM MDT Sun Apr 21 2024

.KEY MESSAGES...

-Gusty northwest Monday behind the cold front.

-Chances for showers and thunderstorms later this week.

-Windy at times this week.

&&

.DISCUSSION...(This Evening Through Sunday)
Issued at 152 PM MDT Sun Apr 21 2024

Water vapor and upper level analysis shows a deep upper low over
eastern Ontario with a long-wave trough extending down through the
Ohio River Valley and another low over Alberta. In between these two
features, weak upper ridging and zone flow is present over the
western portions of the forecast area, with northwest flow
continuing over the eastern CWA. At the surface, high pressure
remains over the central Great Plains, extending into the Northern
Plains, as a cold front extends southward through the northern
Rockies from the Alberta low. Latest surface observations show
temperatures in the upper 50s to mid 60s across most of the area
with gusty south to southwest winds of 15 to 20 mph across much
of the area.

The main concern for tonight and monday is wind speeds along and
behind the cold front. Most hi-res models agree the front will begin
moving through the forecast area around 06Z tonight before exiting
the CWA before 17Z Monday. Pressure rises behind the front will
create gusty northwest winds, especially on the western South Dakota
Plains. Short range ensembles give a 60% chance for wind gusts to
exceed 40 mph as the front passes, but only 10% chance of gusts to
exceed 50 mph. At wind advisory criteria (45 mph), there are a few
areas just east of the Black Hills that may reach criteria for an
hour or two as probs are around 40%. Hi-res models also show the
strongest winds with the frontal passage, but with a well-mixed
regime, afternoon wind gusts of 35 to 50 mph will be common across
northwestern South Dakota. With pressure rises at 3 mb, maybe 4 mb,
have decided to hold off on issuing an advisory for now and will let
future shifts monitor trends to see if an advisory is needed;
especially considering the winds will drop off between the front
moving through and the midday/afternoon mixing. A few light showers
are also possible late tonight with the frontal passage; however,
coverage will be limited with the lack of appreciable moisture.

Semi-progressive northwest flow will dominate the weather through
the first part of the week, with the pattern trending toward
southwest flow late Thursday and through the weekend. The northwest
flow pattern will result in more windy days. With the transition to
southwest flow later in the week, will see moisture levels increase,
with greater chances for showers and storms. Additionally, the
models look to bring a pretty deep trough and closed low through the
forecast area next weekend and the following week. Right now it
appears temperatures will be warm enough for most locations to
receive a widespread, much needed rainfall event - although night
time snow at the higher elevations in the Black Hills is still
possible.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Monday)
Issued At 1124 AM MDT Sun Apr 21 2024

VFR conditions will continue through 06z Monday. South-southwest
winds will gust to 30-35kts at times on the open plains through
this evening. A sharp cold front will move through the area 06-15z
Monday with a few gusts to 40-45kts over northwest SD into Monday.
A brief period of local MVFR CIGS are possible behind the front
over northeastern WY/higher Black Hills after 06z Monday.

&&

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

$$

DISCUSSION/KEY MESSAGES...MLS
AVIATION...Helgeson