208
FXUS63 KFSD 190352
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1052 PM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Shower and thunderstorm chances (30%-40%) increase overnight
  for areas mainly along and south of Interstate 90. The
  strongest storms may be capable of producing hail to the size
  of quarters.

- An active pattern aloft will lead to renewed shower and
  thunderstorm chances (40%-60%) this weekend with the highest
  chances occurring on Saturday and Saturday.

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

&&

.UPDATE...
Issued at 918 PM CDT Wed Sep 18 2024

Showers and thunderstorms have largely failed to materialize
this evening within broad area of increased instability ahead of
an approaching cold front - likely due the lack of substantial
forcing. That said, focus has shifted to the overnight hours
when some guidance suggests additional thunderstorms may develop
in response to an increasing 60 kt 500 hPa jet streak and 40-50
kt 850 hPa jet. In particular, multiple runs of the HRRR have
shown thunderstorms first developing near the central SD/NE
border region after midnight and then pushing east mainly along
and south of I-90 through the overnight hours. Additional storms
may develop across eastern NE and move north. With the loss of
daytime heating, the near surface layer is quite stable, but
model soundings do show roughly 1000-1500 J/kg of elevated CAPE.
This could allow for the strongest updrafts to produce some
isolated large hail up to the size of quarters. To account for
this possibility, introduced 30-40 percent POPs overnight, but
some adjustments are likely over the next several hours.

&&

.DISCUSSION...
Issued at 211 PM CDT Wed Sep 18 2024

REST OF THE EVENING/TONIGHT: Taking a look at satellite imagery,
areas of lingering stratus continue to lift northeastwards across
our eastern-most column of counties with a few pockets of light
showers. Expect this lingering activity to continue to progress
northeastwards before exiting our area around 18z. Looking
aloft, a vertically stacked ULL situated over Eastern Montana
and the far western Dakotas continues to rotate northeastwards
keeping our overall flow locked into a northeasterly regime
through tonight. Shifting gears to our precipitation chances,
isolated shower and thunderstorm could develop by as early as
21z as a subtle shortwave rotates around the ULL.

However, with rather weak forcing and a modestly stable airmass in
play due to this morning`s rain/lingering cloud cover; not too
confident in our overall severe weather chances this afternoon
and evening. Nonetheless, with up to 1500 J/kg of instability,
30-35 kts of speed shear, and a dry sub-cloud layer; can`t
completely rule out a stronger storm or two mostly between the
James River Valley and I-29 corridors with the main threats
being damaging winds up to 60 mph and quarter sized hail.
Otherwise, expect mostly quiet conditions to return overnight as
breezy southerly winds gradually decrease with the approaching
surface high. Lastly, lingering cloud cover and southerly
surface winds will keep temperatures elevated overnight as lows
gradually decrease into the low to upper 60s for the night.

THURSDAY: By Thursday morning, could potentially see a few isolated
pockets of light to moderate showers develop east of I-29 as the LLJ
strengthens ahead of the cold front. Expect this area of activity to
gradually scoot eastwards through parts of the morning with most of
the activity exiting our area by about 15z. From here, cloud cover
should begin to clear out from west to east with an abundance of
sunshine returning for the rest of the day. With this in mind,
expect our overall temperatures to stay above normal with highs
topping out in the upper 70s to upper 80s with the warmest
conditions expected across portions of the Missouri River Valley and
northwestern IA.

THE WEEKEND ONWARDS: Heading into the weekend, mostly quiet and
breezy conditions will continue as a surface high moves across the
region by Friday. A tightening SPG will lead to periodic breeziness
over the weekend with wind gusts up to 25 mph possible each
afternoon. From here, an active pattern returns aloft with a
northern stream trough progressing eastwards across Montana and an
ULL ejecting out of the four-corners region. Both these features
will likely influence the precipitation chances across our area from
Saturday through Sunday. However, to what degree is still in
question. Most long-range guidance continues to vary on the
strength of both systems which has result in some uncertainty.
Nonetheless, most ensemble guidance continues to show low to
medium confidence (30%-50%) chances of up to half an inch of QPF
with the focus being areas south of I-90 which could mean some
beneficial rain is on the way for parts of our area. Either way,
we`ll have to monitor these systems as they approach. Lastly,
the increasing precipitation chances will lead to temperatures
trending downwards heading into the new week with daily dropping
from the low to mid 80s on Friday to the low to upper 60s by
Monday.

&&

.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.

&&

$$

UPDATE...Rogers
DISCUSSION...Gumbs
AVIATION...Rogers