034
FXUS63 KFSD 210321
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1021 PM CDT Fri Sep 20 2024

.KEY MESSAGES...

- Rainfall amounts continue to trend down for the weekend,
  though moderate chances (20-30%) for a tenth of an inch of
  rain remain across portions of NW Iowa and NE Nebraska.

- With the passage of a cold front on Saturday, temperatures trend
  downward to below normal levels for Sunday and Monday.

- Temperatures trend back to near normal levels for next week,
  with a limited chance of precipitation through the period.

&&

.DISCUSSION...
Issued at 249 PM CDT Fri Sep 20 2024

With a weak high pressure system now well off to our southeast and a
cold front beginning to move into far western North Dakota, our warm
and semi-breezy conditions continue into the evening hours.
Observations show current temperatures in the lower to mid 80s,
possibly cresting the upper 80s in south-central South Dakota, with
breezy winds west of I-29 gusting into the 20s. While it does make
for rather pleasant conditions outside, given the low relative
humidity values in the 20s and 30s there is an elevated fire danger
risk. Heading into the overnight hours, a small quick moving surface
low pressure begins to moves through from west to east. Global
models and ensembles suggest scattered showers and tstorms may be
possible, though hi-resolution guidance is not convinced...so have
kept the overnight period dry, though a non-zero chance for
an isolated shower does exist.

As a closed off upper level low remains in the desert SW, an upper
level trough off to our north with the aforementioned cold front
will be moving through the area from northwest to southeast during
the day. The front will abruptly be swinging winds around to become
out of the north-northwest, with a few hours of stronger wind gusts
into the 30s and lower 40s possible behind the front, especially
along and west of I-29 into the mid-morning hours. While the timing
and location still needs to be better agreed upon, high-resolution
convection allowing models (CAMs) do show the potential for post-
frontal convection developing along the slightly lagging elevated
cold front. The HRRR is most aggressive with this possibility,
but it does have support from both the NAMnest and ARW, all of
which put down between a tenth and a quarter of an inch
sporadically. With 500- 1000J/kg of CAPE, depending which model
and layer one lifts from, along with 30-40 knots of wind shear,
marginally severe thunderstorms can`t be ruled out. However,
will need to monitor the convection that is expected to form
well south of the region (Nebraska/Iowa/Missouri/Kansas border),
as that could keep moisture from reaching up into the
region...SPC carries a marginal risk (level 1 out of 5) for
portions of the area along and east of a line from Sioux City to
Spencer, IA.

As the front continues to slide southeastwards, wind gusts into the
20s are expected throughout the afternoon hours. Given the cold
frontal passage throughout the day, daytime highs range from lower
70s near Huron, SD to lower/mid 80s by Storm Lake, IA. Heading into
the evening hours with the front well south/east of the area, models
continue to hold onto low (20-30%) chances for precipitation
generally along and south of the highway 20 corridor. Overall, for
the 24 hour period starting Saturday morning into Sunday morning,
the NBM and ensemble systems give a roughly 20-40% chance for a
tenth of an inch, highest in our southernmost counties
(Woodbury/Ida/Buena Vista) in Iowa.

Sunday will see the arrival of a surface high pressure, which will
continue to advect cooler and drier air across the region, keeping
precipitation chances low. NBM continues to hold onto 20-30% chances
for light rain south of I-90, but given the trends to push QPF
further south I would imagine pops will continue to lower over the
next model runs. However, Sunday will actually feel like fall across
the region with afternoon highs in the mid to upper 60s, along with
decreasing sky coverage throughout the day. Probabilities for
afternoon highs greater than 70 deg F are rather low in the 10-20%
range, outside of the warm-biased GFS, so these cooler temperatures
do have good ensemble support leading to higher confidence in these
cooler than normal temperatures.

While Monday warms up slightly into the upper 60s to lower 70s, the
below normal temperatures continue with mostly dry conditions. Model
uncertainty rises into the middle of the week as they disagree on
how the upper level pattern evolves, though they do all agree on
keeping the forecast mostly dry into the end of the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1018 PM CDT Fri Sep 20 2024

Mid-lvl moisture continues to slowly arrive late this evening.
As the LLJ and a weak wave approach from the southwest, some
isolated elevated convection will be possible over eastern
SODAK after 3am. This aforementioned LLJ may also bring a period
of LLWS through daybreak.

At the same time, a surface front will plunge southeast in the
morning, likely reaching KSUX/KOTG by mid-day, and then exiting
the CWA by 3pm. Behind the front, lingering mid-lvl clouds may
be possible, but more impactful is the gusty northwest winds
over 25 mph at times.

Most of the area will stay dry on Saturday, with some elevated
post frontal shower development near Highway 20 after dark.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.

&&

$$

DISCUSSION...APT
AVIATION...Dux