628
FXUS63 KABR 212315 AAA
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
615 PM CDT Thu May 21 2026

.KEY MESSAGES...

- There is a 30-70 percent chance of 0.75in or more rainfall
  occurring from early Friday morning into late Friday evening.

- After a cloudy and rainy cool Friday in the 50s, things turn
  much warmer Sunday through next week, around 10 to 20 degrees
  above normal, with highs expected in the 70s and 80s.

- An unsettled weather pattern may be developing by next
  Thursday. This could mean additional chances for much needed
  moisture heading into the last full weekend in May.

&&

.UPDATE...
Issued at 609 PM CDT Thu May 21 2026

Made a few tweaks to hourly pops to account for some isolated
showers north of Aberdeen. Otherwise, the forecast remains on
track.

&&

.DISCUSSION...
Issued at 120 PM CDT Thu May 21 2026

At 1 PM CDT, skies are a mix of sun and clouds, but most of the
iso`d/sct`d sprinkles have dissipated. Temperatures are warming
through the 60s on south-southeast winds 15 to 25 mph with gusts up
to 35 mph.

The low pressure system over Wyoming is still scheduled to make an
appearance over the northern plains late tonight through Friday
evening. The latest probabilistic guidance suggests that there is a
30-70 percent chance the CWA receives 0.75in or more of rainfall
with this system. This evening, convection that is expected to
develop out over the central/northern high plains may reach as far
as a portion of this CWA`s west-river counties. There is a little
bit of CAPE around within the lee-side surface trof that convection
develops in/moves east in, so some thunder is possible out across
said west-river counties. Otherwise, the main impetus for rainfall
becomes focused on more than adequate low/mid-level forcing and
moisture/warm air advection processes, as well as favorable upper
level diffluence (as mid/upper level trof becomes negatively
tilted).

One thing that`s being watched closely is the cloud-cover Friday
afternoon. RAP13 model data showcases a surface low moving north-
northeastward across the forecast area. At this point, the most
likely scenario is that it is raining and cool and cloudy all the
way through daytime heating and nothing happens (per the latest
available sounding data in BUFKIT). But, if the rain-footprint moves
north through the CWA faster than forecast on Friday, leaving the
eastern zones in a partially mixed environment where some
sunshine/heating can bolster the low level instability/lapse rates a
bit, then there could end up being a genuine low-topped supercell
tornado threat during the afternoon/early evening across the James
River valley over into west central Minnesota. Advection of CAPE,
more than adequate deep layer and 0-1km shear and 0-1km CAPE top out
the "tor is possible" side of the fence. But, there are a few things
weighing against this scenario/threat; namely cloud-cover/stable-cool
boundary layer and resulting weak low level lapse rates and 0-3km
CAPE. Some Meso-A scrutiny will have to be applied to the
radar/surface map Friday afternoon. If the worst case scenario were
to unfold, it would be another one of those cases where there will
probably not be much in the way of lightning, hail signatures or
even notable cooling cloud tops. "Meso" circulations could also be
tough to discern if heat-of-the-day convective cells were to start
popping up.

Once this precipitation event ends, the vast majority of the
forecast reverts back to being dry until, perhaps, the second half
of next week, when the flow pattern (that this CWA is most concerned
about) becomes highly amplified/blocky and potentially a split-flow
situation, where upper level troughing sets up west of the CWA and
upper level ridging becomes stalled out to the east/northeast of the
CWA. This is an unsettled weather pattern where the Gulf opens up
and plentiful low level moisture can be transported up into this
region for diurnally-driven thunderstorms (since deep layer shear is
not expected to be able to support much more than ordinary-cell to
multi-cell type convection.

The precipitation on Friday probably has low stratus attached to it,
making sunshine/heating during the day a tenuous proposition.
Probably high temperatures on Friday revert back down out of the 60s
into the 50s, but rebound nicely on Saturday into the 60s and 70s.
As (next week`s) upper level ridging builds across the nation`s mid-
section, warmer temperatures climbing into 70s and 80s can be
expected. The green-up may preclude typically hot areas of the CWA
from reaching/exceeding 90 degrees during the warmest periods next
week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 609 PM CDT Thu May 21 2026

Terminals KABR,KATY,KPIR,KMBG

VFR conditions are expected to deteriorate Friday morning as a
system brings rain to the region. Vsby and cigs will both fall
to IFR before the system exits after 21z.

&&

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

&&

$$

UPDATE...20
DISCUSSION...10
AVIATION...20