874
FXUS63 KFSD 131738
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1238 PM CDT Sat Jul 13 2024

.KEY MESSAGES...

- With high humidity bringing dangerous heat index readings of
  95 to 105 degrees, a Heat Advisory remains in effect through
  early evening.

- Wildfire smoke continues to stream overhead through the day.
  Surface impacts are currently not expected.

- Low chance of isolated showers and storms through this
  evening. Low confidence in exactly where/when storms will
  develop. 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.

- Not quite as hot on Sunday, though there remains a moderate to
  high (50%-70%) chance of exceeding heat advisory criteria with
  continued high humidity levels.

- Unsettled pattern continues into the middle of next week, with
  periodic low shower and storm chances. Temperatures will be
  near to below climatological averages.

&&

.DISCUSSION...
Issued at 347 AM CDT Sat Jul 13 2024

Watching a couple areas of convection very early this morning - one
weakening band over central SD, and another more vigorous band
pushing through northeastern SD. Most hi-res models are not
resolving either areas very well, but on their current path may see
the central SD storms enter our far western CWA around 09Z, and the
other band push through portions of southwestern MN toward 11 or
12Z. Soundings indicate a rather deep cap in place below 650 to 700
mb, so any of the activity would be extremely elevated. While there
is plenty of elevated instability in place, effective shear is on
the weaker side - around 25 kts, so think that the severe potential
will remain on the low side. With the departure of this activity by
mid morning, expect it to remain dry through most of the day.

Of course the biggest concern for the day time hours will be the
increasing heat as 850 mb temperatures soar into the mid to upper
20s C by this afternoon. This will play out to high temperatures
well into the 90s, and with dewpoints in the lower to mid 70s, the
heat index will top 100 degrees for most of the area. With that in
mind, the current Heat Advisory remains in place for this afternoon
and evening.

As has been the trend of late, hi-res models continue to struggle
with agreement on convective chances for tonight into Sunday. By
this evening, a frontal boundary settles into the Highway 14
corridor - this as a shortwave tracks across northern SD into MN.
Some of the CAMs - most notably the RAP/NAMNest/ARW want to initiate
thunderstorms, primarily over southwestern MN, during the late
evening where the cap is somewhat weaker. While plenty of
instability will again be present, bulk shear remains on the lower
side - 25 to 30 kts depending on model of choice. Even so, with
steep mid level lapse rates and robust CAPE values cannot rule out a
strong to severe storm with large hail and damaging wind - though
again, confidence is low due to model differences. Another chance of
convection may come to the Highway 14 corridor later in the night as
an MCS develops over western ND then tracks to the southeast during
the night. The latest hi-res models are differing on the evolution
of this system however, some not developing it at all, or having it
weaken before it reaches our area. It is this scenario that
initiated an SPC Day 1 Slight risk across our far northern CWA,
though much uncertainty remains.

The aforementioned frontal boundary settles southward across the
area on Sunday, and with weak cold air advection there is some
uncertainty on high temperatures - with most recent models coming in
a little cooler. Even so, the current forecast is for highs in the
lower to mid 90s, with dewpoints in the lower 70s. This should still
be able to produce afternoon heat indices around 100 degrees, though
confidence is slightly lower than today. Of note, the most recent
HREF ensembles indicate a 50 to 70% of heat indices of 100 or
higher. In terms of headlines, with the slightly lower confidence and
a current advisory already out, it was decided to hold off on
issuing another heat advisory for Sunday. This will be evaluated more
today. Regardless, it will still be uncomfortably hot and folks
should still take the appropriate precautions for outdoor plans.

Into next week, an upper level trough begins to deepen over south
central Canada on Monday, with the surface reflection low pulling a
more significant cold front through our area. This will bring
additional chances of storms for Sunday night into early Monday. It
will still be quite warm on Monday, with cold air advection delayed
until later in the day. Highs will be in the upper 80s to lower 90s.

For the remainder of next week, our area remains under the influence
of an upper level trough which deepens over the MS Valley and
eastward through the period. In a northwesterly upper level flow,
temperatures will drop back to near or just below seasonable
normals. There may be a couple shots for precipitation through at
least midweek, though probabilities are low (20-30%).

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1233 PM CDT Sat Jul 13 2024

Aside from near storms, VFR conditions and southerly winds at
or under 15 kts will prevail through the period. Isolated high
based showers and storms remain possible this afternoon and
evening. If storms can develop early this evening, they have
potential to quickly become strong to severe with frequent
lightning, gusty erratic winds, and large hail possible. Due to
low confidence in if, where, and when storms may develop, have
not included this in TAFs at this time.


&&

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

&&

$$

DISCUSSION...JM
AVIATION...BP