311
FXUS63 KABR 260840
AFDABR

Area Forecast Discussion
National Weather Service Aberdeen SD
340 AM CDT Wed Jun 26 2024

.KEY MESSAGES...

- Dry and pleasantly mild conditions will be in store for today as
high pressure gradually takes hold. This will be short lived as
showers and storms will be on the increase late tonight in parts of
central South Dakota before spreading east through the day on
Thursday.

- A second round of storms is expected for late Thursday through mid-
day Friday (40-80% chance for moisture). A slight risk (2 out of 5)
for severe storms extends from western to central South Dakota for
mainly Thursday evening. A marginal risk (1 out of 5) covers east of
the Missouri valley all the way to western Minnesota during the
Friday morning through about mid-day hours.

- Temperatures will be 5-15 degrees below normal this weekend, with
highs in the 70s and lows around 50 Sunday morning.

&&

.SHORT TERM /TODAY THROUGH THURSDAY AFTERNOON/...
Issued at 338 AM CDT Wed Jun 26 2024

The main forecast challenge in this period will be precipitation
chances that will be introduced back into the area by late in the
period. In the meantime, there are a few scattered showers dotting
our eastern zones(Watertown area) early this morning in response to a
weak sfc cold front and mid level s/w trough moving through the
eastern Dakotas and western MN. Guidance progs this activity should
be all but clear of the CWA by daybreak. This will lead us into a
dry and pleasantly mild day as a sfc high pressure ridge noses into
central and eastern SD. Northerly winds will eventually turn east to
southeasterly late today into tonight as the sfc ridge axis shifts
east of our area.

An upper ridge will be in place across the region later today into
tonight. Embedded s/w energy will try and work its way over the top
of the ridge into western SD late tonight into Thursday morning. Mid
level induced elevated showers and storms (30-60 percent chance)
will be possible across our western zones overnight as a low level
jet strengthens across the western half of SD. Model guidance progs
this rainfall to shift eastward into the James Valley and points
east during the day Thursday, perhaps waning in strength and coverage
in our east by Thursday afternoon. The mid level warm advection will
help to boost 700mb temps to +13C during the day, essentially
capping the atmosphere with time. So, a dry period will be possible
across central SD into portions of the James Valley the latter half
of the day Thursday. Probabilities of seeing rainfall reach or
exceed a quarter of an inch in 24 hrs ending early Thursday evening
is the highest (40-60 percent) across the Missouri Valley, where the
better chances exist for rainfall the first half of Thursday.

&&

.LONG TERM /THURSDAY EVENING THROUGH TUESDAY/...
Issued at 338 AM CDT Wed Jun 26 2024

Open the long term with surface lows developing in western South
Dakota and southeast Alberta Canada connected by an elongated
trough, and associated with an upper low over northern Montana. As
the upper wave progresses east, we see a 100kt jet develop over the
Dakotas, an elevated mixed layer lift into the area (+12 to +14C at
700mb), and passage of a surface trough/weak surface low. The
surface tough/low comes with some weak cold advection and dry air
advection. At 12Z Friday it will be situated north south in the
vicinity of the James valley.

As we proceed through Friday, the surface trough and its delineation
between the more humid airmass and drier air continues to get pushed
into far eastern SD/western MN, along with the elevated mixed layer.
Farther west, we see the cold advection portion of the system begin
to wrap around and into the western Dakotas. There is still a
confidence issue on timing with the degree/timing of cold advection
as deterministic guidance lacks consistency... with GEFS plumes
containing about a 12 hour window between the first and last (minus
outliers) ensemble members. The range in lowest 850mb temperatures
is +9 to +3C...so there is also a high degree of spread still in how
much cold air moves into the region.

As for moisture potential with the system, profiles and the presence
of an elevated mixed layer point to elevated convection as the main
mo for thunderstorm activity - though it should be noted that to the
west there is greater potential for surface based convection so our
western CWA will be in the transition region. NAM MUCAPE values are
up in the 1500-2000j/kg range, with GEFS mean and deterministic GFS
around 1000-1500j/kg (higher/earlier for KMBG/KPIR compared to
later/lower for KABR/KATY). As mentioned previously, shear runs at
around 35kts overnight.

Other features associated with the system are the initial low level
jet, with 850mb winds of 40-50kts (max over the coteau), with some
speed convergence noted over the northeast which may help provide a
focus for convection. 1/2km winds are also about 40kts at the top of
the Coteau, so look for windy conditions up that way along with a
brief window for potential downslope. On the backside of the system,
with cold advection, a 10-12mb gradient across the state, and 6 hour
pressure rises around 4mb, mixed winds in BUFKIT end up around
30kts.

A 1025mb high moves directly overhead for Sunday morning and with
ideal radiational conditions. NBM continues to trend downwards
slightly, with fairly high confidence of a low around 50F.

More impressive will be the return flow on the backside of the high,
in which we develop a 20mb gradient across the state. Another
shortwave traverses the region for early Monday, ahead of another
progressive upper trough which will help prolong precipitation
potential into Monday night.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1237 AM CDT Wed Jun 26 2024

Terminals KABR,KATY,KPIR,KMBG

VFR conditions will prevail at all 4 terminals for most of this
forecast period. There will a periodic exception to this quiet
pattern in the form of VCTS at KABR at the early onset of this
forecast early this morning. The scattered convection movg across
northeast SD early this morning could clip KATY, but at this point
left mention of VCTS out of their forecast.

&&

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

$$

SHORT TERM...Vipond
LONG TERM...Connelly
AVIATION...Vipond