104
FXUS63 KBIS 271003
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
503 AM CDT Sun Sep 27 2026

.KEY MESSAGES...

- Widespread showers and scattered thunderstorms for most areas
  late tonight through Monday.

- High chances (70 to 90 percent) for at least 1 inch of rain
  from south central to northeast North Dakota tonight through
  Monday night.

- A few thunderstorms may be strong late tonight through Monday
  evening, and there is a conditional risk for a severe storm
  or two in south central North Dakota Monday afternoon.
  Thunderstorms could also produce locally excessive rainfall.

- Temperatures are favored to remain near to slightly above
  normal through the upcoming work week.

&&

.DISCUSSION...
Issued at 502 AM CDT Sun Sep 27 2026

Benign southwest flow aloft over the Northern Plains this
morning, with an occluded cyclone over central Canada and an
upper trough digging into the Pacific coast. A cold front
attendant to the Canadian occluded cyclone has nearly stalled
from the northern Red River Valley to near Lake Oahe. Light
winds and high RH just ahead of the front had allowed patchy
dense fog to develop from around Lake Oahe to the southern
James River Valley, but the fog has been consistently clearing
from north to south since the middle of the night. Radar imagery
also shows scattered showers over southwest North Dakota, with
a few thunderstorms in northwest South Dakota. Little to no rain
appears to be reaching the ground on the North Dakota side of
the border though, as observed cloud bases have mostly remained
above 10 kft.

Surface high pressure building off the Northern Rockies should
eventually push the cold front further east today. A secondary
cold front is forecast to sag into northern parts of the state
today, but will likely stall before reaching I-94. This gives a
high temperature forecast today of around 60 degrees northwest
to 70 degrees southeast. The forcing mechanism responsible for
the high- based showers in southwest North Dakota early this
morning appears to be a plume of ~700 mb theta-e advection.
This moisture transport is forecast to slowly translate
eastward through the day, which could maintain the scattered
showers. Any rainfall amounts through the afternoon are
expected to be minimal.

The Pacific coast trough will begin inducing low level warm air
advection and moisture transport over the Northern High Plains
later tonight, while also ejecting a shortwave from the Great
Basin to the Northern Plains. Chances for showers steadily
increase this evening through tonight, and by early Monday
morning, widespread showers with scattered embedded
thunderstorms are expected across much of western and central
North Dakota. This initial round of showers and storms may begin
to clear the forecast area from west to east by midday, but
there is uncertainty in how quickly a second round of showers
and storms could develop, and how widespread it could be, over
central North Dakota Monday afternoon as the left exit region
of an upper level jet streak approaches from the southwest. The
upper level divergence could spread over a quasi-stationary
frontal boundary that is attendant to both the origins of the
cold front moving through eastern parts of the region this
morning and lee cyclogenesis near the Black Hills, but there is
latitudinal uncertainty on where this frontal boundary could
settle. From Monday evening through Monday night, lingering
showers and storms should be confined to the warm/moist sector
of the lee cyclone from south central into eastern North Dakota
where the low level jet could also assist with maintaining
convection.

There is a wide array of potential outcomes in the intensity
and coverage of thunderstorms late tonight through Monday night.
For the initial round late tonight through Monday morning,
thunderstorms should remain elevated above a shallow but stable
boundary layer with access to around 500-1000 J/kg MUCAPE and
modest mid level lapse rates around 7 C/km. Deep layer shear
will be strong, but that is driven by easterly to southerly
flow in the lowest 1 km AGL underneath a prevailing southwest
flow in the mid to upper levels. It is therefore likely that
any updrafts will experience a lower amount of effective bulk
shear, but it could still approach 30 kts. UH tracks in both the
HREF and REFS are short and barely touch the 75 m^2/s^2 mark
that tends to be an indicator of severe convection. This leads
to a most likely outcome of scattered strong thunderstorms of a
pulse nature across southwest and central North Dakota through
Monday morning, with hail being the main hazard (aside from
heavy rain and lightning).

The Monday afternoon through Monday night time period carries
much greater uncertainty. First, there is the question of where
the quasi- stationary front will end up during the afternoon,
which deterministic models indicate could be as far south as
between I-94 and the SD border or as far north as along Highway
2. This may determine the most likely corridor for afternoon
convection. Another question is whether there will be a capping
inversion above a moist but conditionally unstable boundary
layer. The degree of CIN could vary over both space and time
and may ultimately be driven by breaks in cloud cover. Forecast
CAPE has generally remained in the 500-1000 J/kg range, but
there is meridional uncertainty in where a plume of locally
higher buoyancy could set up in southern North Dakota. There is
also uncertainty in deep layer shear Monday afternoon. Latest
guidance has trended lower (around 30 kts), but there are still
outliers as high as 50 kts. Forecast confidence in strong low to
mid level shear is higher. Taking all this into account yields
a spectrum of potential outcomes ranging from little to no
thunderstorms at all beyond mid afternoon (e.g., NAMnest and
ARW) and a long duration of strong to severe thunderstorms
capable of all hazards (including tornadoes) from south central
to southeast North Dakota from the afternoon through the night
(e.g., HRRR and Fv3). Perhaps the most likely outcome, which is
generally advertised by most members of the REFS, is for one or
two strong to severe storms between the Missouri and James
River Valleys Monday afternoon and early evening.

The final aspect of this event to discuss is rainfall and the
potential for locally excessive amounts. The broader QPF
footprint has not changed much, and the axis of maximum ensemble
mean QPF and threshold probabilities is still generally located
from around Bismarck to Grand Forks, where there are high
chances (70 to 90 percent) for exceeding one inch of rain.
Convection will likely cause amounts to be highly variable
however, and HREF guidance indicates local rain amounts as high
as 3 to 5 inches cannot be ruled out due to training convection.
The threat for locally excessive rain will be present
throughout the duration of the event.

After a cooler but humid Monday on account of the showers and
storms (highs mostly in the 60s), a brief warmup is now favored
for Tuesday and Wednesday as flow aloft behind the departing
system turns zonal. A stronger cold front could then move
through the region late Wednesday into Thursday as a trough digs
into eastern Canada. Ensemble clusters show uncertainty in the
synoptic pattern later in the work week until some consensus
emerges for a weekend shortwave that could bring a clipper
through the region. The NBM continues to favor cooler
temperatures for the first weekend of October, but not too far
below normal.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 502 AM CDT Sun Sep 27 2026

VFR conditions are expected through this evening. A few showers
are possible across southern parts of the state from time to
time throughout the day, with much higher chances for showers
and thunderstorms developing after 06Z Monday. Light and
variable winds are expected for most areas through this
afternoon.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Hollan
AVIATION...Hollan