970
FXUS63 KFSD 112317
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
617 PM CDT Thu Jul 11 2024

.KEY MESSAGES...

-  Mostly dry tonight, with very isolated storms possible west
   of the James River. Winds may be light enough tonight
   along/east of US Hwy 75 for patchy fog development.

- Wildfire smoke continues to stream overhead. Smoke should
  track east as the upper ridge builds in through the weekend.

- Can`t rule out showers and storms this weekend, some of which
  could be strong to severe if they develop. Additional showers
  and storms possible early next week. Confidence is very low
  the details.

- Temperatures and dew points increase through the weekend. High
  (> 75%) chance of seeing heat indices above 100 degrees by
  Sunday afternoon.

&&

.DISCUSSION...
Issued at 251 PM CDT Thu Jul 11 2024

CURRENTS THROUGH TONIGHT: Diurnal cumulus clouds continue to
develop, with wildfire smoke contributing to hazy skies aloft. Not
expecting any smoke to reach the surface today. Highs today in the
mid 80s. South to southeast winds gust to around 20 mph through
sunset, with sustained winds 5-10 mph overnight. These winds might
be enough to keep fog from developing toward daybreak, with the only
area of concern for fog development presently along/east of US Hwy
75. Any fog would be shallow, patchy, and likely in/near river
valleys and other low lying areas. Lows tonight fall into the 60s.
Confidence is low, but could see some isolated showers and
thunderstorms for areas west of the James River (more likely in
south central SD) as a mid level wave and upper jet streak move into
the area. Soundings are very capped from the surface, so expect any
showers/storms to be extremely elevated. Soundings also show a lack
of saturation, so storms could struggle in our environment. If a
storm could develop or survive into our area, can`t rule out a
stronger storm with up to quarter sized hail given lapse rates near
7 deg C/km, 0-6 bulk shear around 25-30 knots, and CAPE around 1250
J/kg. Could also see some stronger winds with any storm as well.

FRIDAY AND FRIDAY NIGHT: Another mid level wave dives southeast
through the day Friday as mid/upper level high pressure begins
to build across the southwestern US. Friday will likely be dry
during the day and warmer, particularly for areas west of the
James as WAA increases. Highs tomorrow warm into the lower/mid
80s to lower 90s. Southerly winds will gust around 20 mph
through the day. This southerly flow and increasing
evapotranspiration will aid in increasing dew points as well
with heat indices tomorrow in the upper 80s to upper 90s. Skies
will likely still have a milky appearance with some wildfire
smoke remaining overhead.

Heading into Friday night, models are struggling with resolving the
timing and track one to two additional waves/vorticity maximums.
Thus, very low confidence in pops after Friday evening. Taking the
11.12z CAM runs as an example, the ARW brings an area of elevated
convection into southwestern MN by 13.11z (6 AM CDT Saturday), with
this same area of convection from the NAMNest not reaching
southwestern MN until mid Saturday afternoon. A third solution (FV3)
keeps these storms in central to eastern MN and northwestern WI,
trailing further in time from the NAMNest. Given this, have
generally gone with a blend of pops and leaned on the drier side
until later in the overnight hours, keeping pops isolated. Lows
Friday night into early Saturday remain quite warm only falling into
the upper 60s to near 70.

THIS WEEKEND: As mentioned in the previous paragraph, confidence in
pops remains quite low. Not only are models struggling with the
timing and strength of waves which continue to track through our
west-northwest flow regime, but increasing WAA and temperatures
aloft in the top 10% of climatology keeps our atmosphere extremely
capped both Saturday and Sunday. However, with continued southerly
surface flow and crops adding moisture via evapotranspiration, any
storm that can break that cap will have ample instability (over 70%
chances of exceeding 2000 J/kg per ensemble guidance). Increasing
shear and steepening mid level lapse rates would favor strong to
severe storms - again, *if* they can break the cap.

Despite storm chances, of higher concern and higher confidence is
the increasing heat and humidity. As with yesterday, did not make
any changes to the temperatures for the weekend as we are already
toward the 75th percentile of NBM guidance. That said, did increase
dew points using some of the 90th percentile NBM and CONSAll. This
puts heat indices in the upper 90s to near 105, with the hottest day
on Sunday when temperatures are a couple degrees warmer. Did ponder
headlines, but with some uncertainty on areal component each day and
in collaboration with neighbors decided to wait on any headline
decisions. Heat Advisories will likely be needed for at least
portions of the area, with over 75% probability in the ensembles of
reaching criteria. One small concern is that we might mix more than
anticipated this weekend (lowering dew points and increasing
temperatures). Even if this is the case, we`ll likely end up with
heat indices near 100 degrees.

Regardless of headlines, folks with outdoor and travel plans should
keep an eye on the forecast for the latest information. Begin
planning now for possible heat impacts and have a plan in case
storms develop.

MONDAY ONWARD: High pressure well to our southwest breaks down
toward the beginning of the work week as a more defined wave
(Monday) and a stronger mid/upper trough (Tuesday) swing through the
northern Plains. Storm chances Monday (including how strong any
storms would be) will depend on the timing of the wave Monday and
how far south the instability is pushed. Periodic shower and storm
chances continue through early next week with the unsettled pattern.
Monday should be the warmest day, with temperatures returning near
to slightly below average into mid next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 615 PM CDT Thu Jul 11 2024

VFR conditions are likely into the overnight hours. Some patchy
river fog is once again possible, but should be lower in
coverage than Thursday morning.

Most guidance indicates a mid-lvl ACCAS field developing after
midnight west of the James River valley. Eventually some showers
or even an isolated thunderstorm may develop. Some guidance does
indicate this activity may try to reach as far east as I-29 by
mid-day. Otherwise a diurnal CU field may again develop in the
afternoon.

Through out it all, high level smoke, albeit a more narrow
corridor, will slide through during the day.

&&

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

&&

$$

DISCUSSION...SG
AVIATION...Dux