543
FXUS63 KFSD 222307
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
607 PM CDT Sun Sep 22 2024

.KEY MESSAGES...

- Temperatures remain normal to below normal through Monday.

- Chances for scattered light rain showers return Monday evening into
  the early hours on Tuesday as a cold front moves through,
  with 90% chance rainfall amounts remain below a tenth of an
  inch.

- Temperatures trend back to normal and above normal levels for
  next week, with limited chances for precipitation through the
  period.

&&

.DISCUSSION...
Issued at 233 PM CDT Sun Sep 22 2024

A rather pleasant Autumn day out there, with temperatures largely in
the mid to upper 60s, though a few have climbed into the lower 70s
given the less than expected cloud coverage for today. North-
northwest winds will continue to slowly decrease throughout the
afternoon hours, and becoming light and variable overnight as a
surface high pressure slides off to our east. With mostly clear
skies and light winds overnight, radiational cooling should bring us
down into the upper 30s to mid 40s, with no frost expected. NBM
Probabilities for temperatures below 40 degrees are highest near the
Brookings area and onto the Buffalo Ridge in SW MN, in the 40-60%
range. Not impossible to see some patchy river/valley fog given
water temperatures in the mid 60s to lower 70s, though models
suggest it would be very isolated.

Southerly return flow sets up again on Monday as the surface high
slides further off to our east, and a front begins to move towards
the area from the west as the parent low pressure slides eastwards
in southern Canada. With the region being in the developing warm
sector, a warmer day with highs in the lower to upper 70s are
expected along and west of I-29, with lower to mid 70s expected east
of I-29. The aforementioned cold front will be moving through the
area overnight and into the early morning hours on Tuesday, with 12Z
guidance trending wetter as both global and high resolution
convection allowing models (CAMs) showing light scattered showers
moving across the region. The Euro ensemble remains the most
convinced (50-70% chance for more than a trace), while the GEFS and
GEPS are more reserved in the 20-30% range, and the HREF in the 30-
40% range. Given the scattered nature have capped pops at 24%, with
the ensembles showing a 0-10% chance for amounts greater than a
tenth of an inch of rain. Expect similar temperatures on Tuesday as
we rebound back into the lower to mid 70s behind the morning cold
frontal passage.

Behind the upper level trough that moved through, upper level
heights rise Wednesday into Thursday as a ridge of high pressure
aloft builds back into the central/northern plains. Ensemble
situational awareness tables continue to show that this ridge will
bring temperatures and geopotential heights at or above the 99th
percentile of climatology for this time of year for layers above
700mb, while 850mb temperatures don`t quite reach the 90th
percentile. Because of that, expect warmer than normal temperatures
up into the upper 70s to lower 80s for Wednesday into Friday, with
mostly dry conditions expected.

Uncertainty returns into the extended/weekend as the aforementioned
upper level trough turns into a closed off upper level low moving
into the central/southern plains, which interacts with the remnants
of the tropical cyclone creating a Fujiwhara effect. This effect is
where two cyclonic vortices, in this case one being a tropical storm
and the other an upper level low, rotate around a common point and
is notoriously difficult to forecast. Have stuck with the NBM, which
shows normal temperatures continuing, with periodic low
(20-30%) chances for rain from Friday evening through Sunday
east of the James River Valley.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 605 PM CDT Sun Sep 22 2024

VFR conditions expected through the night as winds turn very
light and variable. Should temperatures drop into the upper 30s
to lower 40s some valley, river, stream, lake fog will be
possible. Will include a shallow fog mention in all the TAFs for
a couple hours.

Winds increase slightly from the south on Monday, with another
VFR day expected.


&&

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

&&

$$

DISCUSSION...APT
AVIATION...Dux