021
FXUS63 KFSD 181720
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1220 PM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Periods of showers and storms (30-60% chances) continue into
  Thursday afternoon, with the greatest chances this morning
  west of I-29 and chances this afternoon into tonight near and
  east of I-29.

- Although unlikely, a strong to severe storm is possible this
  afternoon and evening. Wind gusts to 60 mph and quarter sized
  hail are the main threats. Locally heavy rain is possible,
  although there is only about a 45% chance of 24 hour rainfall
  exceeding 0.50 inches.

- Breezy southerly winds again today into tonight with gusts
  around 30 to 35 mph.

- Unsettled pattern continues. Moderate chances (40-60%) for
  showers/storms late this week into early next week. Highest
  chances currently focused late Friday night through Sunday.
  Exact timing and location remain uncertain.

&&

.DISCUSSION...
Issued at 402 AM CDT Wed Sep 18 2024

TODAY THROUGH THURSDAY NIGHT: Another morning with elevated
convection developing along the eastern edge of the LLJ and WAA;
expect these showers and storms to continue to develop just west of
the I-29 corridor toward the James River Valley and slowly move off
to the northeast through the late morning hours. Severe weather is
not expected with this activity, but stronger storms may produce
small hail and wind gusts to 50 mph in addition to lightning and
locally heavy rainfall.

Shifting further to the west, watching another area of convection
across south central SD along the better mid level wave and forcing
ahead of the main upper level low pressure centered this morning on
the MT/WY border. This area shifts east as the upper level low does,
leading to periods of showers and thunderstorms through the day as
we`ll continue to see pieces of vorticity and WAA move across the
area into tonight. Meanwhile, at the surface, dry line in south
central MN retreats west as cold front in western SD moves to the
east during the day. Boundaries in place, at the surface and aloft,
may aid in focusing convection. Although risk is low, can`t entirely
rule out a strong to severe storm or two - particularly if the
atmosphere can recover between morning and late afternoon/evening
convection. Soundings show a cap in place, but if we can break the
cap, instability around 1500 J/kg and shear around 30 knots with dry
sub cloud layer could be enough for wind gusts to 60 mph. Quarter
sized hail is a lower end risk, given mid level lapse rates less
than 7 deg C/km. Isolated strong to severe storms are possible into
the late evening hours. Keep an eye on the forecast, especially if
you have outdoor evening plans. Greatest severe risk is along I-29
corridor into northeastern NE.

With the timing of the cold front today, there is a tight moisture
gradient across south central SD, with critical fire conditions for
western and central portions of NE and SD. Given our less cured
fuels and location on the eastern edge of this gradient (leading to
RH values well above criteria), did not issue a Red Flag Warning for
any of our counties. However, should the gradient be much further to
the east, conditions should be monitored for possible elevated fire
concerns in western Gregory/Brule counties.

Showers and storms become more isolated to scattered in nature
overnight and into Thursday as the system moves off to the east and
better dynamics lift north. May see some lingering showers/storms
near the surface cold front during the day on Thursday east of I-29.
Again, a very isolated stronger to severe storm cannot be ruled out,
but confidence is low in convective development.

Rainfall totals through Thursday night look to remain light, with
chances of seeing more than half an inch less than 45% across the
ensembles. Generally expect a quarter of an inch or less of
rainfall, but higher amounts are possible in thunderstorms. Highs
today and tomorrow in the 80s with lows in the 50s and 60s.

FRIDAY ONWARD: A reprieve from our more active weather pattern
comes to end the work week as flat, weak ridging builds into the
north central Plains. Therefore, another warmer than average
day is expected Friday. Models diverge into the weekend as a
couple of low pressures track eastward - one focused in southern
Canada and one across the southwestern CONUS. Given the
variance in timing and location of these features, have left the
NBM as is. Expect periodic rain and storm chances by late this
week into the early part of next week as these lows track
eastward. Below average temperatures move into the region by
early next week, with ensemble probability of temperatures above
70 degrees less than 40% (average slightly inflated by the warm
biased GEFS).

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1215 PM CDT Wed Sep 18 2024

VFR conditions with occasional MVFR ceilings and vsby will be
possible this TAF period mostly due to developing showers and
thunderstorms. Taking a look at satellite imagery, areas of
stratus along with pockets of light showers continue to push
eastwards this afternoon. While it shouldn`t take long for this
activity to exit our region, there could be some isolated
redevelopment along and east of I-29 by the late afternoon to
early evening hours potentially bringing occasional MVFR cigs
and vsbys. Lastly, breezy southerly winds will continue through
this evening before gradually decreasing after sunset.

&&

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

&&

$$

DISCUSSION...SG
AVIATION...Gumbs