470
FXUS63 KFSD 230907
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
407 AM CDT Sun Jun 23 2024

.KEY MESSAGES...

- Rivers in some areas continue to rise today as runoff enters
  basins and water moves downstream. Record flooding is on-going
  and expected on several basins.

- Dry and seasonal conditions are expected Sunday.

- Hot and humid conditions Monday, with heat indices near to
  above 100 degrees. A Heat Advisory has been issued for most of
  the area outside of southwestern MN.

- The pattern through mid week doesn`t favor any long duration
  rain, but several brief risks for passing convection exist. If
  storms can develop Monday and Tuesday, a couple could be
  severe. Additional scattered rain chances return late this
  week into next weekend.

&&

.DISCUSSION...
Issued at 404 AM CDT Sun Jun 23 2024

CURRENTS: Light winds thanks to high pressure off to the southeast
in addition to ample low level moisture is supporting some fog
development across northwestern IA. For now, development has been
very sparse, but will keep an eye on trends. Dense fog is possible.
Also keeping an eye on some stratus/fog streaming across the
northeastern portion of the CWA with a weak mid level wave. Lows
tonight in the 60s.

TODAY/TONIGHT: High pressure continues to slide east today. Warmer
conditions prevail today as WAA starts to expand from the southwest
over the area. With the elevated warm front lifting north expect
winds to shift back southerly later today and through tonight. Highs
today in the mid 80s with lows in the mid/upper 60s. Some hi-res
guidance brings some very light and very isolated showers to the
area this afternoon/evening and again overnight; however, have
maintained a dry forecast given low confidence in development.
Better (~15%) chances would be overnight near/south of Hwy 20. If
anything can develop, new rainfall amounts should be less than a
tenth of an inch.

MONDAY: Weak low pressure ejects into the western Dakotas, with a
more defined low in Canada. WAA strengthens and lifts north through
the day with increasing southerly flow. This brings much warmer
temperatures into the region as well as higher dew points. This
combination of highs in the upper 80s to upper 90s and dew points in
the 70s creates heat index values in the 100-105+ degree range over
much of the area. In collaboration with neighbors, and given that
high temperatures aren`t even toward the higher end of guidance at
this time, have gone ahead and hoisted a Heat Advisory for almost
all of our counties outside of southwestern MN for the afternoon and
evening hours. Those responding to recent flooding and anyone with
outdoor plans Monday will need to prepare for the heat (plan
frequent breaks in the shade or air conditioning, stay hydrated, and
know the signs of heat illness). Unfortunately, breezy southerly
winds likely won`t provide much relief.

Mid level wave slides through the ridge with an upper level jet
streak over the central Plains by the evening hours. With our hot
and moist air mass, instability ahead of the attendant cold front
climbs to 2000+ J/kg of CAPE (high confidence in this over nearly
all of the CWA), with some deterministic models bringing over 3000
J/kg of CAPE. Shear values are around 20-30 knots, with steep mid
level lapse rates over 8 deg C/km would be supportive of large hail
and DCAPE over 500 J/kg would be supportive of damaging winds.
However - the incredibly large caveat to storm development is the
cap. With such strong WAA, confidence is low that forcing near the
front and with the weak mid level wave will be enough for storms to
develop in our area. Will keep an eye on trends, and any storm that
could break the cap could become severe. PWATs around 1.5 inches
would support some locally heavy rainfall, which would create issues
for already hard hit areas. Lows Monday night won`t provide much
relief, only falling near 70 degrees.

TUESDAY: Cold front gets washed out early Tuesday in the eastern
forecast area, which keeps severe weather threat and rain chances in
place as the warm front moves south. A couple of weak mid level
short waves track through the area Tuesday as well. Greatest
rain/severe threat looks to be east of I-29 during the afternoon and
evening. As with Monday, strong cap in place leads to lower
confidence in development, but storms could become strong to severe.
Locally heavy rain will again be a threat, with PWATs 1.5-2". Highs
will be slightly cooler in the 80s to lower 90s and lows in lower
60s.

WEDNESDAY: Northwesterly flow aloft and a quick moving surface high
pressure keeps dry conditions in place through the day. Highs in the
70s and 80s with northeasterly winds.

WEDNESDAY NIGHT ONWARD: Ridging aloft builds for mid week, although
a series of short waves keeps scattered shower/storm chances in
place through Thursday. More defined waves/troughs move through late
week into next week, although guidance varies on the timing and
location of each of these. Unfortunately, this keeps the active
pattern in place. At the time of this forecast, period of greatest
concern for heavy rain (1" or more) is Thursday night into Friday
morning. Active pattern may linger through at least the first
half of the weekend before ridging builds back over the Plains.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1044 PM CDT Sat Jun 22 2024

Potential for fog is the main aviation concern tonight. With winds
already calm and variable, as we cool towards our crossover
temperatures patchy to areas of fog are expected to form during the
overnight period into Sunday morning. This appears most likely for
areas along and east of the James River Valley, and given the wet
antecedent conditions, patchy to areas of fog are expected with
visibilities below 1 mile possible. Given spatial uncertainties,
kept mention of fog limited to tempo groups for the TAF sites. VFR
conditions return after any remaining fog dissipates Sunday
morning.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory from 1 PM to 8 PM CDT Monday for SDZ038-039-050-
     052>071.
MN...Heat Advisory from 1 PM to 8 PM CDT Monday for MNZ098.
IA...Heat Advisory from 1 PM to 8 PM CDT Monday for IAZ001>003-
     012>014-020>022-031-032.
NE...Heat Advisory from 1 PM to 8 PM CDT Monday for NEZ013-014.

&&

$$

DISCUSSION...SG
AVIATION...APT