614
FXUS63 KFSD 190851
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
351 AM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Shower and thunderstorm chances (30 to 40%) continue this
  morning, mainly south of I-90. Additional shower and storm
  chances (less than 30%) this afternoon and early evening east
  of IA/MN State Hwy 60. Severe risk remains low, but hail to
  quarter size and 60 mph wind gusts are possible in the
  strongest storms.

- An active pattern aloft leads to renewed shower and
  thunderstorm chances (30 to 60%) this weekend into early next
  week, with the greatest chances Saturday and Sunday.

- Confidence continues to increase in cooler stretch of
  temperatures starting as early as Sunday.


&&

.DISCUSSION...
Issued at 346 AM CDT Thu Sep 19 2024

TODAY THROUGH TONIGHT: Elevated convection early this morning has
continued to develop along the LLJ and elevated boundary from south
central MN stretching southwest into southeastern NE. Meanwhile,
further west, surface cold front is draped from near Aberdeen
southwest to east of Huron, into northern NE west of O`Niell.
Convection has struggled along the surface front due to capping in
place and lack of upper level support. We may begin to see this
upper level support (mainly with the 500 mb jet streak) aid in
convection through daybreak. If storms can develop and strengthen,
can`t rule out hail to quarter size.

Aforementioned surface cold front continues to move east during the
day. It is possible to see some storms develop along the front this
afternoon into early this evening across portions of northwestern IA
into southwestern MN; however, coverage may be limited in our area
depending on how quickly we can destabilize and how far east the
front is by mid afternoon. If convection can initialize, strong to
severe storms are possible. Dew points in the 60s lead to
instability around 1500 J/kg, and shear increases to around 35
knots. Mid level lapse rates are a bit marginal, around 7 deg C/km.
If a stronger storm could develop near the boundary, there is a non-
zero tornado risk, but with 0-1 km shear around 10 knots and LCL
heights around 4000 ft, tornadoes are unlikely. Main hazards with
any stronger storm would be 60 mph wind gusts and hail to the size
of quarters. Rainfall amounts through this evening generally a tenth
of an inch or less, although locations which see thunderstorms could
see rainfall totals around 0.25" or more.

Highs today climb into the 80s once again. Lows tonight fall into
the 50s, with clearing skies. Winds may become light enough
overnight thanks to weak surface high pressure that patchy fog may
develop in low lying areas, near river valleys, and in areas which
receive precipitation today. Confidence is too low for mention in
the weather grids at this time, mainly due to uncertainty in
where/when precip occurs today.

FRIDAY: Dry and warm to end the week, as mid/upper level ridge
builds in. WAA and the return of southerly flow at the surface aid
in warming temperatures again into the 80s. Compressed surface
pressure gradient as well as efficient mixing leads to another
breezy day, with gusts up to 30 mph possible west of the James
River. This, along with the warmer temperatures and lower dew
points, will lead to RH values in the 25-35% range areawide.
Although fuels are still largely green, localized areas of elevated
fire danger are possible in areas with more cured fuels. Greatest
concern will be areas West River, where lowest humidity and
strongest winds align.

FRIDAY NIGHT INTO THE WEEKEND: Ridge begins to break down late week
into the weekend as a couple of low pressure systems/troughs aloft
strengthen - one in the southwestern CONUS and one across southern
Canada. 19.00z model runs have come into a bit more agreement with a
couple of weak shortwaves moving across the north central Plains, as
we are stuck in the middle of the spilt flow. By Sunday afternoon,
the northern low is moving northward across southern Canada, while
the southern low as tracked into western NE/KS. This active flow
keeps rain and storm chances (30-60%) in place through the weekend,
highest for areas south of I-90 and east of I-29. As finer details
are resolved, expect pops to be refined. Areas with highest pops
have a 30-70% chance of seeing more than 0.25" of rainfall in a 24
hour period, and a 30-50% chance of more than 0.50". This will
change with any shift in track of the low, but those with outdoor
plans this weekend may want to keep an eye on the forecast. Not
expecting any day to be a total washout, though.

Near to above average temps continue for the first half of the
weekend as WAA prevails. Some uncertainty with highs on Saturday due
to the timing of the surface cold front, but 70s to lower 80s are
expected. Cooler temperatures move in for Sunday, with highs in the
60s. Confidence is increasing in these cooler temperatures for the
second half of the weekend, as ensembles show a less than 10% chance
of temperatures exceeding 70 degrees Sunday. Breezy winds expected
on Saturday.

EARLY TO MID NEXT WEEK: By Monday, models are struggling with the
evolution of the weekend low pressure as is meanders east, as well
as the next mid/upper low/trough progged to move through the
northern and central Plains sometime Tuesday. Could see renewed
precipitation chances during this time. Northerly flow should keep
temperatures cooler than average for at least the early part of the
week, with warmer temperatures mid week as ridging returns. Highs in
the mid 60s to lower 70s with lows in the mid 40s to near 50.


&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1052 PM CDT Wed Sep 18 2024

Primary challenge will be timing and coverage of any nocturnal
thunderstorms. Recent guidance suggests there may be additional
shower and thunderstorm development overnight, especially for
areas along and south of I-90. Best chances will be at KFSD from
19.10-19.12Z, when a TEMPO for -TSRA was introduced. Less
confidence at KSUX, so will keep dry for now. KHON is expected
to remain dry.

Ceilings are expected to remain VFR through the period. If a
thunderstorm moves directly overhead a TAF airfield, could see
brief reductions down to 3500-4000 ft agl.

Still looking at low-level wind shear at KSUX overnight given
strengthening jet. While surface winds will be rather light,
expect an increase to around 40 kts at 2000 ft agl. Some
southerly breezes are expected overnight, but winds will veer
to the west-southwest through the day Thursday with an
approaching cold front. KHON in particular could see some
stronger wind gusts in the afternoon...topping out near 25 kts.

&&

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

&&

$$

DISCUSSION...SG
AVIATION...Rogers