391
FXUS63 KABR 201059
AFDABR

Area Forecast Discussion
National Weather Service Aberdeen SD
559 AM CDT Sun Jul 20 2025

.KEY MESSAGES...

- There is a Slight Risk, level 2 out of 5, for severe weather this
afternoon into tonight. However, there is a high level of
uncertainty on if and where storms will form, especially over
central South Dakota. There is a higher confidence in storms moving
over northeastern South Dakota this evening.

- With storms over central South Dakota today, supercells could
produce golf ball sized hail, wind gusts of 60 miles per hour, and
possibly a tornado. Elsewhere, quarter sized hail and wind gusts of
60 miles per hour are the main threats.

- Active pattern continues Monday and Tuesday, where a Marginal
Risk, level 1 of 5, is in effect. Quarter sized hail and winds of 60
miles per hour are the main threats.

- Heat index values will be in the upper 90s to near 100 degrees
Monday afternoon and again on Tuesday afternoon, across central SD
primarily south of Pierre.

&&

.UPDATE...
Issued at 558 AM CDT Sun Jul 20 2025

Aviation discussion has been updated in accordance with the 12Z
TAF issuance.

&&

.SHORT TERM /TODAY THROUGH MONDAY AFTERNOON/...
Issued at 321 AM CDT Sun Jul 20 2025

A weak upper-level trough based over the Pacific Northwest will
continue to eject shortwaves into the region and maintain upper-
level support of a nearly stationary low pressure center for the
next couple of days. The boundaries, mainly surface troughs, off
this low and the weak upper level support from the shortwaves
combine to help create persistent chances for showers and storms
over the next couple of days.

Currently monitoring some showers and storms moving into the area
along the North Dakota border at the time of this writing (~07Z).
These showers are expected to ride the border over the next couple
hours, remaining sub-severe. This complex and other lingering light
showers moving into the forecast area tonight are expected to decay
as they progress eastward, dissipating by sunrise or shortly
thereafter and providing a few hours break from precipitation across
the CWA.

The main focus for the forecast is today`s convection and potential
for severe weather this afternoon and evening. There is still a high
uncertainty on if and when storms will develop, but the CAMs can
effectively be broken down into two main regions. First, there is a
signal for an area of development over central South Dakota in the
evening hours. The presence of a cap (in the form of 18-20 degrees C
temperatures at 800mb) presents a conditional threat for these
storms to even go up in the first place. The area could very well
see no storm development at all this evening. On the other side of
the pendulum, a reasonable worst case scenario for this area would
seem to be a couple of isolated supercells. Hail and wind would be a
threats in this case, and an isolated tornado also could not be
ruled out in the environment. On the hail side, the potential is
supported by MUCAPE of 1500-2500 J/kg and 0-6km shear of 40-50
knots, and mid-level lapse rates of 7-8 C/km. Similarly, models are
showing DCAPE over 1000 J/kg, and while current model soundings do
not show strong low-level lapse rates, this is deceptive due to
models including the cap within the lowest 3 kilometers and the
calculation of said lapse rates. Should the cap break the mixing
potential to the surface will become much greater. Last but
certainly not least, the tornado potential in the area is somewhat
marginal, but if things come together in the right area the
potential is there. Curved hodographs near the surface, 0-1km shear
of 20-25 knots, low-level helicity >150 m^2/s^2 and LCLs below 500m
all support tornadic potential. While all of that sounds very
favorable, it is important to note that at this point there is not
very good spatial convergence with all of these variables. This
means that the tornado threat likely will remain in a state of high
uncertainty until a couple hours before the actual storm formation.

The second main area of storms potentially impacting the Aberdeen
CWA comes from the development of storms over North Dakota and
moving southeast into northeastern South Dakota by the evening. In
this area, 800mb temps are lower and the cap is much weaker, which
in turn is reflected by a much highest consistency in the
development of storms in the CAMs. Both MUCAPE ranging from 1000-
2000 J/kg and 0-6km shear upwards of 40 knots mean that the
environment is favorable for storm continuation into South Dakota.
The tornado risk remains to the south, but wind and hail will still
be a threat. Lapse rates again are in the 7-8 C/km at best and DCAPE
approaches 1000 J/kg in the area. Storms in this area will linger
through Sunday before moving into Minnesota early Monday morning.

NBM holds on to chances for showers and storms Monday morning and
afternoon. Development overhead looks unlikely in the morning and
afternoon due to the strengthening cap (particularly at the 850mb
level), but an upper-level ridge beginning to build into the area
may allow for just enough warm air advection into the region to once
again break the cap. Hail, wind and an isolated tornado are all once
again threats. SPC has a Marginal Risk out for the forecast area for
Monday afternoon, although the modeled signal is much more
pessimistic than for today`s threat.

&&

.LONG TERM /MONDAY EVENING THROUGH SATURDAY/...
Issued at 321 AM CDT Sun Jul 20 2025

Long term period starts out with an upper trough over the PacNW and
western CONUS and an upper ridge axis over the central CONUS (axis
over central/northern Plains). This setup will continue to bring
opportunities for showers/thunderstorms as pieces of shortwave
energy eject out of the upper trough. The primary question is if the
upper ridge will be far enough north to keep the better shear to
the north of the area, along with precipitation chances.

Still fairly subtle forcing for Monday afternoon/night, with the
surface low slowly shifting east and into the area. May be some low
level convergence over portions of central SD under the general
southeasterly flow, but picking out locations of best convergence
for development is difficult at this point. Confidence isn`t helped
by the increasing 700mb temps aloft, approaching 14-15C over much of
the CWA, which may help cap a good portion of the area. The low
level jet does seem to be focused a little farther to the north and
into ND as we go through the night, so would expect any nighttime
development to be in that area. That`s where the best overlap of
CAPE/Shear is located and storm motions would tend to point towards
convection lifting to the NE or ENE through the night, except for
any right moving supercells (to the ESE). So overall, seems like a
conditional severe threat, if storms can develop. Do want to mention
that there`s not a ton of low level shear (winds generally below
20kts through 3km). Hodograph does show some weak curvature in the
lower levels, but also backing winds at the mid levels, which may
help suppress updraft strength if that look continues in future runs.
So seems like the primary threats would be hail (up to golf balls)
if any supercells can form during the late afternoon/evening hours
in south central SD along the sfc trough / stationary front, then
wind if anything can get organized coming out of western SD as we go
into the nighttime hours (assuming storms that may develop on low
level jet stay north of the CWA).

Upper ridge begins to shift east on Tuesday and continues into
Wednesday, with it settling over the Great Lakes region. That puts
the area under more of the upper trough influence, while also
allowing the surface low over WY to shift east across SD. Expect
shower/thunderstorms to be possible as this occurs before the
surface low/trough slides far enough east through Wednesday night to
finally bring a break to the precipitation chances. Still a
favorable overlap of instability/shear for Tuesday, so any storms
that develop along the surface low/trough do have the potential to
be severe, which generally aligns with CSU/NSSL ML probabilities.
SPC Day 3 outlook continues the Marginal Risk trend for our area,
like seen the last few days. Like Monday night, biggest question is
if capping will be enough to suppress storm development, as 700mb
temps are still around 14-15C, although start to see some weakening
of that from the northwest to southeast with time heading into
Tuesday night. Finally, expect slightly above normal temps on
Tuesday (upper 80s to upper 90s), and with dewpoints in the upper
60s to mid 70s, heat indicies will be in the 90s to low 100s. Thus,
may need a heat advisory for south central SD on Tuesday afternoon.

Once we get into Wednesday and Wed night, the location of the
surface low/trough will likely limit the severe threat either to the
southeast of the CWA or just capturing a portion of the southeast
part of the CWA and even some of the precipitation may be post
low/trough. But with additional upper level energy, do expect
continued shower/thunderstorm concern during this time. There will
be a cool down behind that surface low/trough for Wednesday, but
then as the upper ridge over the Great Lakes on Thursday gradually
shifts westward through the weekend and early next week, expect a
trend back to normal to above normal temperatures by next weekend.
Will need to monitor heat indicies for next weekend, as experimental
NBM v5.0 percentiles are showing values in the mid 90s to around 100
at the 75th percentile (or 25% chance of occurrence) for Saturday
afternoon and a couple degrees warmer on Sunday afternoon.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 558 AM CDT Sun Jul 20 2025

Terminals KABR,KATY,KPIR,KMBG

Main aviation concern during the TAF period will be thunderstorms
throughout the period. There has been some thunderstorm development
west of the Aberdeen CWA over the past couple of hours that is
tracking into the area. There are a handful of scattered showers
around the KPIR airspace, and while the front end has not produced
lightning yet, some of the storms on the back end have shown some
flashes. At this point thunder has been left out of the KPIR TAF,
but may need to be added in depending on how the developing storm
tracks and the consistency of the lightning. A larger thunderstorm
consistently producing lightning is expected to reach KMBG within
the next few hours as well, and has been included in a TEMPO for the
time being.

Another round of storms may develop this afternoon into tonight,
although there is still a high level of uncertainty in the location
and exact timing. Due to the uncertainty and timing still being on
the back end of the TAF period, these have been indicated in PROB30
groups at this time. The greatest certainty for storms to develop
this evening into tonight is over northeastern South Dakota
(40-60%) while central South Dakota has less confidence in storm
development (20- 40%).

Beyond showers and storms, a low cloud deck supporting MVFR to IFR
ceilings is currently situated over central South Dakota. These
ceilings are expected to persist over the next few hours before
briefly lifting to VFR over most of the area ahead of storm
development. Storms will drop ceilings and visibilities to MVFR,
perhaps down to IFR in the heaviest downpours.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
&&

$$

UPDATE...BC
SHORT TERM...BC
LONG TERM...SRF
AVIATION...BC