698
FXUS63 KFSD 140336
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1036 PM CDT Sat Jul 13 2024

.KEY MESSAGES...

- High humidity brings dangerous heat index readings around 100
  to 107 degrees through early this evening.

- Low (<30%) chance of isolated showers and storms through this
  evening. Low confidence in exactly where/when storms will
  develop if they do. However, if a storm is able to develop,
  it may quickly become strong to severe. The main threats with
  the strongest storms will be large hail up to golf ball size
  and wind gusts up to 60 mph.

- Very slightly cooler on Sunday, but high humidity will likely
  bring high heat index readings of 95 to 105 degrees, hottest
  south of the I-90 corridor.

- Unsettled pattern continues into the middle of next week with
  periodic low shower and storm chances. Starting Tuesday,
  temperatures will be near to below normal for this time of
  year.

&&

.DISCUSSION...
Issued at 244 PM CDT Sat Jul 13 2024

TONIGHT: Early this afternoon, satellite images show a cluster of
mid level clouds and isolated showers in central SD tracking
eastward. Sparked by a mid level wave, this is the most likely
forcing for ascent for any isolated strong to severe Tstorms through
this evening. A hot, very humid atmosphere remains primed for strong
to severe multicell storms with plentiful instability, 7-8 C/km mid
level lapse rates, and 25-35 kts 0-6km deep layer shear. However, it
is important to note that confidence remains very low regarding
where, when, and even if storms could form due to strong capping and
lack of a more pronounced wave or jet streak aloft necessary to
initiate convection. This low confidence is clear in the convective
allowing/hi-res models which have been erratic in where/when/if
convection develops at all. If storms can develop, there is
potential for quick intensification to strong to severe elevated
storms with a main focus from 5 PM to 10 PM and main threats of
large hail up to golf ball size (or perhaps even larger if updrafts
can sustain) and damaging wind gusts up to 60 mph. Weak low to mid
level wind profiles and capping make tornado threat low, though
substantial moisture content and a STP of 1 east of I-29 suggest
that a brief spin up cannot be entirely ruled out.

Much of the evening and overnight are expected to remain dry for
much of the region. Beyond low evening storm threat, additional
rounds of storms may develop upstream in 1) eastern ND/northwest MN
and 2) western ND and northwest SD. A few models suggest the first
round may clip portions of southwest MN as it grows upscale and
treks southeast through MN late tonight, with a low threat of
remnants of the second round grazing the Hwy 14 corridor in the
early morning hours Sunday. Severe weather threat with this activity
is very low, but cannot entirely rule out potential for weaker
showers and storms.

Aside from marginal severe weather threat, the main impact continues
to be dangerous heat and humidity with heat index readings already
at 90 to 105 degrees as of 1 PM. Passing mid level clouds may
provide some relief from extremely hot air, capping temperatures in
the low to mid 90s, but stifling humidity persists with dew points
broadly in the mid to upper 70s east of the approaching inverted sfc
trough nosing into south central SD. Some locations may see a bit of
relief from heat if cloud debris from storms can develop early this
evening, but otherwise diurnal cooling brings low only to around 70
degrees overnight.

SUNDAY: The same muggy airmass remains in place across the region
through Sunday as dew points linger in the low to mid 70s. The
previously approaching inverted sfc trough/weak boundary washes out
in the region on Sunday, oriented west to east roughly along the
SD/NE border eastward to just south of the MN/IA border. Easterly
flow and remnant clouds and/or showers near and north of the
boundary will help make for a slightly cooler day. Areas north of
the boundary are likely to only reach the upper 80s to lower 90s
with lower to mid 90s more likely further south. Additionally,
wildfire smoke streaming through aloft may continue to inhibit
heating slightly. Coupled with high confidence in dew points in the
low to mid 70s, heat index readings of 95 to 105 degrees are very
likely near and south of the I-90 corridor. Thus have issued a Heat
Advisory for 1 PM to 8 PM again Sunday in the area most likely to
see 100+ degree heat index readings.

Sunday is poised to be mostly dry with sunny skies in quiet
northwesterly flow aloft as strong capping lingers with 850mb
temperatures at the top 10% of NAEFS climatology. The next ridge
riding disturbance treks into the Northern Plains Sunday late
evening or overnight with deterministic guidance increasingly
favoring a chance of showers and storms, at least for areas near and
north of the Hwy 14 corridor in SD/MN. Moderate instability and
enough deep layer shear to support a complex of storms with
multicells suggest isolated to scattered severe storms are possible
overnight into Monday morning if storms can persist that long.

MONDAY: Warm, humid conditions linger into Monday with heat index
readings in the upper 90s to near 100 degrees continuing south of I-
90 in the afternoon. Big question is how much of a role north winds,
cloud cover, and departing showers will play in heating so have
lower confidence on temperatures and any heat related headlines.

TUESDAY AND BEYOND: The upper wave stalled over the Canadian Prairie
starts to push east Monday night, bringing a cooldown to near to
below normal temperatures heading into Tuesday and beyond. Northwest
flow aloft thanks to the stalled building ridge over the West brings
spotty, low confidence rain chances, but no high impact or
widespread rains look to be on the horizon for the remain of the
week.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1036 PM CDT Sat Jul 13 2024

Quiet conditions begin the TAF period late this evening. A complex
of strong storms has crossed the Montana/South Dakota border and
will track southeastwards through the overnight hours. The storms
look to reach the Missouri River Valley after 08z but there is some
uncertainty how long storms will last past the MO River. However,
confidence is high enough to include mention of thunder in KHONs
TAF. Any chance for storms will come to an end during the earlier
morning hours tomorrow.

There could be another round of showers and storms during the
afternoon and evening hours but confidence is low in this
possibility. As such, have left all TAFs dry for the daylight hours
tomorrow but trends will be monitored.

Otherwise, light and variable winds tonight will southeasterly and
continue through the remainder of the TAF period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory from 1 PM to 8 PM CDT Sunday for SDZ050-057>071.
MN...None.
IA...Heat Advisory from 1 PM to 8 PM CDT Sunday for IAZ001>003-
     012>014-020>022-031-032.
NE...Heat Advisory from 1 PM to 8 PM CDT Sunday for NEZ013-014.

&&

$$

DISCUSSION...BP
AVIATION...Meyers