756
FXUS63 KFSD 111727
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1227 PM CDT Thu Jul 11 2024

.KEY MESSAGES...

-  Wildfire smoke aloft continues to advect into the region.
   Smoke should track further east as the upper ridge builds in
   over the next few days.

-  Isolated showers and storms are possible late this week, with
   additional chances early next week. Confidence is very low in
   the details.

-  Temperatures and dew points increase late week and into the
   weekend. Moderate to high (> 70%) chances of seeing heat
   indices above 100 degrees by Sunday afternoon.

&&

.DISCUSSION...
Issued at 334 AM CDT Thu Jul 11 2024

Widespread areas of patchy, dense fog this morning, especially in
low laying areas and along river and creek beds. Any fog is
expected to burn off quickly after sunrise.

Water vapor imagery shows the upper ridge pushing east this morning.
Winds at the surface have begun to turn to the south as surface high
pressure builds in. Expect light and variable winds to increase to
10-15 mph. Aloft, winds continue to be north-northwesterly bringing
smoke south from the Canadian wildfires. Skies will be hazy with
some late morning/early afternoon cumulus forming. Highs will be
limited a bit by the smoke, in the low-mid 80s. Smoke should track
further east as the ridge builds in during the next few days.
Looking into the overnight, there is some indication a weak mid-
level wave will trigger showers early in the morning Friday. These
look to form roughly over central South Dakota and track sharply
southeastward, taking them out of the region within a few
hours. There is risk for some of these storms to become strong
to severe, with lapse rates approaching 7 deg C, roughly
500-1200 J/kg MUCAPE, and 25-30 kts bulk shear. Model agreement
is low on timing and placement, so will leave PoPs at Isolated
chances (15-24%). Location most likely to see showers is a line
from Chamberlain south into Wagner. Showers should clear out
later Friday morning. 500 mb heights continue to rise into
Friday afternoon, coupled with strong WAA this will boost
surface temperatures into the mid 80s to low 90s. Strong
moisture influx from the south will begin to tick our dew points
up into the upper 60s to low 70s. A low pressure system moves
into the region Friday night into Saturday morning, but guidance
is in low agreement on track. The GFS keeps rain east of our
area of responsibility, while the Canadian has isolated showers
sweeping across from northwest to southeast. With this
uncertainty in mind, left what the NBM loaded for PoPs.

The trend of increasing dew points and high temperatures continues
into Saturday and especially for Sunday. WAA brings 850 mb
temperatures into the mid to upper 20 deg C range. Good mixing to
the surface will translate into widespread highs in the low to mid
90s Saturday. Dew points increase to the low to mid 70s, especially
over northwestern Iowa. The combination of heat and humidity will
result in the heat index approaching 100+ degrees along the James
and Missouri Rivers. Sunday is a very similar story with slightly
warmer high temperatures: low 90s to low 100s. Heat index looks to
be 100-105 along the southern Missouri counties. There remains some
uncertainty in the heat forecast, as the ensemble guidance has
backed off somewhat on the probability of widespread greater than 90
degree highs. In addition, evapotranspiration may boost dew points
higher, but to what extent is uncertain. Considering this, have
opted to forego headlines at this time. Even so, those with outside
plans this weekend should continue to be weather and heat aware.

As noted in the previous discussion, a strong cap remains in
place for the weekend. A few weak waves are expected to move
through, and if they can break through it there is plentiful
instability for storms to tap into. But given the strength of
the cap, confidence is low. Sunday night/Monday morning a cold
front moves through turning winds at the surface northwesterly.
Increasing CAA will offer us a respite from the heat, dropping
highs Monday into the upper 80s to low 90s, and then into the
low to mid 80s for Tuesday. By midweek a more organized trough
looks to bring our next chances at widespread rain.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1217 PM CDT Thu Jul 11 2024

Through the period, winds generally will be southerly to
southeasterly with gusts 15-20 knots. Gusts drop off after
sunset, but should pick up fairly quickly after sunrise Friday.
Winds may be light enough along/east of US Hwy 75 into early
Friday morning for some patchy fog development in low lying
areas, but confidence is low.

Diurnal cumulus clouds are expected each afternoon/evening, with
wildfire smoke aloft creating some hazy skies. Isolated showers
and storms are possible west of the James River overnight, with
additional chances after daybreak Friday in southwestern MN.
Confidence in timing and coverage is low (especially for later
chances). Outside of any fog development and convection, expect
VFR conditions to prevail.

&&

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

&&

$$

DISCUSSION...AJP
AVIATION...SG