936
FXUS63 KABR 171121 AAB
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
621 AM CDT Thu Sep 17 2026

.KEY MESSAGES...

-Heavy rain is expected beginning this afternoon and continuing
through Friday evening. The highest accumulations are expected over
eastern and central South Dakota. The probability to see two inches
of rain or more reaches 40-60% over eastern and south central South
Dakota (decreasing elsewhere as you move north and west).

-There is a Slight Risk (level 2 of 4) for excessive rainfall
leading to flooding across across the forecast area today. The
heaviest rainfall is expected overnight tonight through early Friday
morning.

- Near-normal temperatures continue today, with highs in the low to
mid 70s across much of the forecast area. Friday and through the
weekend will have slightly cooler temperatures, with highs generally
in the upper 60s to low 70s.

&&

.UPDATE...
Issued at 615 AM CDT Thu Sep 17 2026

Updated discussion for the 12Z TAFs below.

UPDATE Issued at 515 AM CDT Thu Sep 17 2026

Per webcams and surface obs, issued a Fog Advisory for portions
of central and northeastern SD until 14Z with visibility down to
a quarter mile or less at times, especially over the higher
elevations of the Leola and Sisseton Hills.

&&

.DISCUSSION...
Issued at 210 AM CDT Thu Sep 17 2026

Early this morning, there are some chances for fog over the area.
Light winds and nearly full humidity recovery over the entire
forecast area will provide prime conditions for potentially
widespread development early this morning. Cloud cover, particularly
over parts of northeastern South Dakota and western Minnesota, may
limit some of the radiational cooling this morning. However, surface
observations in that area indicated dew point depressions of 3
degrees or less as of 06Z, so cloud cover may not pose much of a
hindrance to fog development after all. Fog that does develop may
restrict visibility to a mile or lower.

With the onset of a heavy rainfall event nearly upon us, models
still appear to be coming a bit more into consensus on the location
of the axis of heaviest rainfall. The NBM in particular had pushed
northward over the past 24 hours, until the latest run (01Z) has it
shifting back to the south. The center of the axis now passes just
south of the Watertown area and stretches west to be just south of
Pierre as well. Euro ensemble guidance is in a decent level of
agreement with this solution, as are many of the high-resolution
models that are able to capture the full event at this point. As for
rainfall totals, those have trended down over the past 24 hours as
well. There is still around an inch of spread within ensembles, but
in general median values sit between 1.5"-2.25" over central and
eastern South Dakota. Probability to see 1" or more sits at around
70-90% over those areas, while probability to see 2" or more has
fallen to 40-60%. At this point, expectations are still that the
heaviest rainfall will occur overnight Thursday into Friday morning.

Despite the downward trend in rainfall totals, there is still a
flooding threat, particularly with the convective element to some of
the storms. NBM 90th percentiles, a proxy for a "reasonable worst
case scenario" for an extended period of heavy rain, sit at around
3"-3.5". Should this level of rain be realized, it would very likely
exceed flash flood guidance. Antecedent conditions over eastern and
central South Dakota will vary somewhat, but after the recent
rainfall yesterday, much of the area sits at roughly 10-30% soil
moisture. When looking at model soundings, many of the
meteorological ingredients for flooding will be in place. Namely, a
fully saturated profile with PWATs well above the 90th percentile
(widespread PWATs around 1.75" for this event) prove favorable, as
does the deep warm cloud layer of 11-12k feet. There remain some
question as to how quickly storms will be, as models give anywhere
from 10-20 knots (which is a somewhat large variance when
considering flooding potential). There is also the question of
instability. The best instability will be over central South Dakota,
up to 1000 J/kg at times. Therefore, that is the area of highest
concern when it comes to flooding with this event, despite not
strictly being the highest level of QPF in the forecast area.
Particularly if storm motion is slow, central South Dakota has the
best potential to see their rain come down over a shorter duration,
as opposed to eastern South Dakota where rain will linger for
longer, but likely with lighter rainfall rates.

After this round of rain moves out, the upper-level low currently
over the western CONUS will progress eastward. Additional rain
showers will be possible over the weekend as the low moves overhead.
A cooler airmass will take hold of the region due to the pattern
shift, subsequently bringing below normal temperatures (highs in the
60s) through the weekend and into the start of next week. Models
trend towards a warming trend back to near-normal by mid-week next
week.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 615 AM CDT Thu Sep 17 2026

Terminals KABR,KATY,KPIR,KMBG

Patchy to areas of dense fog and low stratus continue across
much of the CWA this morning with IFR/LIFR conditions at KATY
and KMBG. A Dense Fog Advisory remains in effect for portions
central and northeastern SD until 14Z as visibilities could
drop to a fourth mile or less. Fog will burn off a few hours
after sunset with cigs improving to VFR at all TAF sites by
middle to late morning. A system will bring widespread rain to
the area starting over KPIR this evening with rain spreading
north and northeastward to all TAF sites through the overnight
hours. Widespread half inch to potentially 2 inches of rain
possible with the highest amounts from south central through
east central SD.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 9 AM CDT /8 AM MDT/ this morning for
     SDZ003>005-008>010-015>017-020>023-034>037-051.
MN...None.

&&

$$

UPDATE...MMM
DISCUSSION...BC
AVIATION...MMM