453
FXUS63 KFSD 210853
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
353 AM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Rainfall amounts continue to trend down for today through
  Sunday, with only low chances (20-30%) for a few hundredths of
  an inch of rain through the Woodbury to Clay counties
  corridor.

- Northwesterly winds will increase today behind a cold front,
  gusting 30 to 40 mph into this evening.

- With the passage of a cold front, 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 350 AM CDT Sat Sep 21 2024

In a strong southerly flow with high pressure to our east and a
surface trough to our west, it has been a very mild night across the
area with current temperatures in the mid 60s to lower 70s. Not
expecting temperatures to fall a whole lot more through the night -
bottoming out in the 60s near daybreak. The main story for today
continues to be the passage of a cold front through the area -
presently located over central SD very early this morning. As has
been the trend over the past few model runs, the impacts from this
front continue to lessen - at least in terms of precipitation
chances. Latest models have the front entering the western portions
of our CWA between 09Z to 12Z, then sliding eastward through morning
and afternoon, exiting our area around 20Z. Current hi-res models
produce very little precipitation with the front - with the better
forcing well to our north in association with an upper level trough
moving across ND during the day. Related to this, current forecasted
precipitation amounts during the day are only a hundredth or two of
an inch in the Spencer to Storm Lake IA corridor. Latest HRRR and
RAP soundings indicate capping up to around 650 mb over that area in
the afternoon, which lend support to storm chances being very low.
Even so, SPC continues to hold onto a marginal risk (level 1 out of
5) for severe storms (large hail/damaging wind) - mainly affecting
Buena Vista and Ida counties, though again this looks to be an
unlikely scenario. The bigger impact may be the winds behind the
front. With strong cold air advection, northwesterly winds are
expected to increase with the passage of the front, with winds
gusting 30 to 40 mph at times into the evening. 850 mb temperatures
will be decreasing from northwest to southeast through the day, and
we are looking at highs in the lower 70s from east central SD to
south central SD, to the lower to mid 80s through northwestern IA.

Slightly better - albeit still low (20-30%) - chances may come
tonight over our far southeastern IA counties (Woodbury to Buena
Vista) with possible shower activity along an elevated boundary as
an upper level shortwave tracks through NE into southern IA. Even
so, deterministic models and ensembles support only a few hundredths
of an inch of precipitation at most. With cold air advection
continuing tonight, temperatures will be at or just below seasonal
normals, dropping into the mid 40s to lower 50s.

Strong surface high pressure then builds into the Northern and
Central Plains on Sunday. Although we currently carry a low chance
(20%) for precipitation south of Highway 20 as an upper level low
lifts into NE during the day, with the exception of a couple hi-res
models, most keep it dry. This is supported by ensembles which
indicate only around a 10% probability of receiving a hundredth of
an inch of rain over that area. With an 850 mb thermal trough (7-10
degrees C) overhead on Sunday, it will be cooler with highs in the
mid to upper 60s.

High pressure drifts to the east on Monday, and with warm air
advection in a returning southerly flow, temperatures will warm
slightly - into the upper 60s to lower 70s. For the remainder of
next week, temperatures will slowly warm back to normal to just
above normal levels through the week. This as upper level troughing
gives way to some ridging by the end of the week. Unfortunately,
current guidance gives little hope for any significant rainfall,
with current ensembles suggesting a less than 20% probability of our
area receiving .05" on any one day for Monday through Friday.

&&

.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...JM
AVIATION...Dux