751
FXUS63 KFSD 171142
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
642 AM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Increasing southerly winds expected today and tonight,
  strongest west of the James River. Gusts 30 to 45 mph late
  this afternoon through tonight.

- Low (generally < 30%) rain chances through this afternoon.
  Confidence in timing and placement is low, but expect amounts
  to remain light, less than a quarter inch through this period
  at any location.

- Greater chances for rain (40-70+%) tonight through Wednesday,
  mainly focused late tonight west of I-29. Moderate (~ 50%)
  probability of rainfall exceeding 0.25", but low (< 30%)
  probability of exceeding 0.50".

- Moderate chances (40-60%) for showers/storms late week into
  the weekend, with highest chances currently focused Friday
  night through early Sunday. Exact timing and location remain
  uncertain.

&&

.DISCUSSION...
Issued at 353 AM CDT Tue Sep 17 2024

TODAY AND TONIGHT: Another morning of isolated to scattered showers
developing along the convergence of the LLJ and a weak short wave
moving across the area. Severe weather is not expected, but these
storms have been producing frequent lightning. A stronger wind gust
to 50 mph and small hail may occur in the strongest storms. Isolated
to scattered showers/storms will continue to develop, mainly east of
the James River, through daybreak.

Morning showers and storms move east and slowly dissipate through
the late morning and early afternoon as the subtle short wave moves
east and we begin to mix out, losing the forcing from the LLJ.
Meanwhile, the main low pressure/trough begins to move east into
the Rockies. Lee side cyclogenesis occurring through the day and
tonight leads to the compressing of the surface pressure gradient
(SPG), and with mixing into stronger winds aloft, a breezy day
and night are expected. Strongest winds are anticipated west of
the James River where the gradient is the tightest and LLJ is
the strongest late this evening. Gusts in this area may reach
up to 40 to 45 mph; however, given marginal conditions, held off
on a Wind Advisory for now. Ensemble probability of exceeding
45 mph gusts is less than 10%. Elsewhere, wind gusts through
tonight of 30-40 mph are expected. Sustained southerly winds of
15-25 mph prevail. Warm conditions continue, with highs today in
the 80s to near 90 in south central SD and mild lows overnight
in the 60s.

Expect a lull in precipitation this afternoon into the evening
hours, as the aforementioned trough begins to lift northeast into
ND, dragging an elevated boundary and short wave into the central
and eastern Dakotas overnight through Wednesday morning. Greatest
rain chances will be overnight west of I-29, where the best dynamics
line up (boundary, LLJ, upper level forcing). Not expecting any
severe weather with activity overnight and early Wednesday; however,
stronger storms may produce wind gusts to 50 mph and small hail in
addition to frequent lightning and locally heavy rain. Some
locations tonight could see a quarter inch or more, with probability
of over 0.25" between 30% and 60% west of I-29. Lower end chances
(< 30%) of more than a half inch.

WEDNESDAY AND WEDNESDAY NIGHT: On Wednesday, low pressure deepens as
it slides north from eastern MT and western ND. Initial boundary
from the overnight and early morning slides east, with another short
wave and lower/mid level front poised to slide through the area
during the evening and overnight hours. Surface low and attendant
cold front won`t be too far behind the mid/upper level support
Wednesday evening and night. Models show some variance in exact
timing of the front, and there is uncertainty in where any
convective outflows are. Depending on the timing of the boundary and
how much the atmosphere can recover from any early day convection, a
stronger storm or two may be possible in the late afternoon through
the evening hours. CAPE around 1200-1500 J/kg and shear around 30
knots may support wind gusts to 60 mph. With lapse rates less than 7
deg C/km, quarter sized hail appears to be a lesser threat.

Rainfall amounts of less than a quarter of an inch expected
Wednesday into Wednesday night. Slightly cooler with highs in the
lower/mid 80s and lows in the 60s. Southerly winds remain on the
breezier side, but not as strong as today.

THURSDAY ONWARD: Showers and storms linger into the day Thursday
east of I-29 as the front and surface low track east. Confidence is
low in the location and timing, but severe threat on Thursday is
very low. Weak/flat mid and upper ridge builds for the end of the
week as low pressure deepens across the southwestern US, leading to
continued above normal temperatures and mostly dry conditions.
Models diverge toward Saturday with the evolution of this low and
another across southern Canada, but all generally lean toward these
features moving east through the weekend. This brings a return to
the more active pattern with short waves and more defined
lows/troughs moving across the north central Plains into the early
part of next week. Ensembles indicate that a return to near or even
below average temperatures is on the way, with ensemble probabilities
of exceeding 80 degrees less than 30% on Saturday, and less than 20%
into early next week. With confidence low on the details and
periodic rain chances, did not make any changes to the NBM in the
extended.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 634 AM CDT Tue Sep 17 2024

Scattered showers and storms continue through late morning. Have
included mention at KHON and KFSD, but omitted from KSUX as
activity seems to be decaying as it moves off to the northeast,
but will keep an eye on trends.

Main concern through this TAF period will be in the increasing
southerly winds. Sustained winds around 15-20 knots, with gusts
20-30 knots. Gusts around 35 knots are expected west of the
James River. Winds remain strong into tonight. Marginal
directional LLWS is possible overnight, and could see some speed
shear if gusts taper off. Given the uncertainty, left mention
out.

Otherwise, additional showers and storms move from west to east
late tonight into Wednesday morning. Outside of convection, VFR
conditions are anticipated.

&&

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

&&

$$

DISCUSSION...SG
AVIATION...SG