310
FXUS63 KFSD 240922
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
422 AM CDT Mon Jun 24 2024

.KEY MESSAGES...

- River crests have already occurred or will occur within the
  next 24-48 hours. While no significant amounts of rainfall
  will occur, areal flooding caused by the prior excessive
  rainfall continues across portions of the area.

- Hot and humid today, with heat indices above 100 degrees. The
  previously issued Heat Advisory has been expanded to include
  the entire forecast area.

- Highly conditional threat for severe storms this afternoon and
  evening. While we could see some showers and weak storms, if
  storms are able to tap into more of the instability, they may
  quickly become severe with 2 inch hail and wind gusts of 70
  mph.

- The pattern through mid week does not favor any long duration
  rain, with another chance for isolated storms Tuesday
  afternoon. Conditional risk of severe weather exists on
  Tuesday afternoon and into the evening. Additional scattered
  rain chances return later this week into next weekend.

&&

.DISCUSSION...
Issued at 418 AM CDT Mon Jun 24 2024

CURRENTS: A warm and muggy morning is on-going, with 3 AM CDT
observations showing most with temperatures in the upper 60s to mid
70s. Dew points remain in the 60s and lower 70s, so with increasing
WAA, not expecting temperatures to fall more than 5 degrees or so
from where they currently sit. Have started to see isolated showers
and storms develop in the last hour over northwestern IA thanks to
the aforementioned WAA and the increasing LLJ. Stout dry layer
should help limit precip reaching the ground, but can`t rule out
amounts around a tenth or quarter of an inch of rainfall (especially
if showers/storms develop over the same area).

TODAY/TONIGHT: Robust WAA continues today as mid level ridge and
wave move east. We`ll mix into this anomalously warm (both 850 and
700mb in the top 1% of climatology) air aloft through the day,
leading to highs in the 90s. Increased temperatures a degree or two
in the east with increasing probability of exceeding 90 degrees.
Paired with the incredibly high dew points in the lower to mid 70s,
heat indices climb into the 100-110 degree range. Have expanded the
Heat Advisory to include the rest of the forecast area (adding
southwestern MN and Brookings county). Pondered issuing an Excessive
Heat Warning for the Hwy 20 corridor; however, given the uncertainty
with mid cloud development and subsequent impacts to highs, left the
Advisory in place. Regardless, the heat and humidity will lead to
unpleasant to dangerous conditions to those working outdoors today
or without air conditioning. If you are spending time outdoors,
please take frequent breaks in the shade or air conditioned areas,
stay hydrated, and know the signs of heat illness. Breezy southerly
winds will not provide much in the way of relief, even with gusts at
times 25 to 30 mph.

Additional showers and thunderstorms are possible this afternoon and
evening with the approach of a weak mid level wave and surface cold
front. With the robust layer of warm air aloft, atmosphere is
strongly capped. Although we`ll be mixing into the cap, not sure
we`ll mix deep enough to erode it entirely to allow convection to
develop. Timing of this relatively weak wave and uncertainty in the
location of the cold front relative to this leads to some
uncertainty in if we`ll even see much more than elevated ACCAS type
showers and isolated storms. However, if we can break the cap,
showers and storms could quickly become strong to severe. Bulk shear
values are the strongest off to the northeast in central MN, but
despite this, we`ll still have over 3500 J/kg of CAPE (instability)
to work with if we can break the cap. Steep mid level lapse rates
over 8 deg C/km and sounding profiles (as well as downdraft CAPE
near/above 1000 J/kg) are supportive of severe weather. Threats if
we can break the cap are large hail of 2+ inches in diameter and
wind gusts to 70 mph. Given the expected elevated nature of storms,
tornadoes are not anticipated. Although widespread heavy rain is not
expected, any thunderstorm could produce a quick inch to 1.5" of
rain.

Isolated showers and storms are possible near/west of the James
River into the Hwy 14 corridor overnight as the front stalls out and
the next weak wave/WAA push swings through. Lows tonight remain in
the upper 60s to lower 70s.

TUESDAY: With the cold front washed/stalled out over the area
(somewhere near I-90), another day of very conditional severe
weather threat exists Tuesday. Similar atmospheric conditions,
although temperatures in the upper 80s/lower 90s and highest dew
points will generally be confined to areas near Hwy 20. Same severe
weather hazards are in play, with uncertainty to due forcing
mechanisms, although the cap looks a little weaker during the
afternoon and evening Tuesday. Again, widespread heavy rain is not
expected but any thunderstorm could produce a quick inch or more of
rainfall. Highs Tuesday in the 80s to lower 90s and with lower dew
points, heat indices will be a bit more bearable (into the mid 90s
across northwestern IA). Lows in the upper 50s to lower 60s, with
rain chances ending late evening/early overnight.

WEDNESDAY: Dry conditions prevail on Wednesday as surface high
pressure moves over the region and mid level ridging develops to the
west. Slightly cooler (upper 70s/lower 80s) with northeast to east
flow.

WEDNESDAY NIGHT ONWARD: Next wave moves in late Wednesday night,
with rain chances largely focused west of I-29 overnight, moving
east through Thursday. More active/unsettled pattern continues into
the weekend with a few mid level waves and at least one surface low
swinging across the northern/central Plains. Models do vary on the
timing and strength of each wave, but generally have a stronger one
Thursday night. Thursday into Friday would be the period of concern
for more widespread heavy rainfall potential (totals of an inch or
more). Guidance shows increased moisture advection, as well as PWATs
near 2" (in the top 10% of climatology). Temperatures should be
within 5 to 10 degrees of normal.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1036 PM CDT Sun Jun 23 2024

VFR conditions are expected for the TAF period. Light southeasterly
winds will persist through the overnight hours. The low level jet
(LLJ) is on track to strengthen tonight which will bring some low
level wind shear (LLWS) to KSUX. Some showers and thunderstorms look
to develop on the nose of the LLJ but latest guidance keeps any
chance for rain confined to parts of northwest Iowa, east of KSUX.
As such, have kept out any mention of rain in at KSUX but trends
will be monitored. Winds will turn southerly during the morning
hours while gusts strengthen up to around 20-25 knots. A cold front
will sweep through the area during the afternoon and evening hours
tomorrow, turning the winds to out of the northwest. The northwest
winds will finish out the TAF period.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for SDZ038>040-050-052>071.
MN...Heat Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for MNZ071-072-080-081-089-090-097-098.
IA...Heat Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for IAZ001>003-012>014-020>022-031-032.
NE...Heat Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for NEZ013-014.

&&

$$

DISCUSSION...SG
AVIATION...Meyers