614
FXUS63 KBIS 061035
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
535 AM CDT Sun Sep 6 2026

.KEY MESSAGES...

- Increasing chances for showers and thunderstorms today through
  Labor Day. A few storms may become strong to severe.

- Unseasonably warm and humid through the holiday weekend, then
  trending cooler.

- Windy on Tuesday, with near-critical fire weather conditions
  across southwest and south central North Dakota.

&&

.DISCUSSION...
Issued at 530 AM CDT Sun Sep 6 2026

An omega pattern is in place across North America early this
morning, with deep upper lows spinning over the Pacific
Northwest and Canadian maritime provinces and a ridge axis over
the US Plains. The local area is just upstream of the ridge
axis, and the resulting southwest flow aloft with broad
isentropic ascent and underlying warm air advection has
initiated scattered showers and isolated thunderstorms across
southwest and south central North Dakota. At the surface, low
pressure is analyzed over the Black Hills with an inverted
trough extending north along the MT/ND border. A broad easterly
surface flow over much of the state has led to patchy dense fog
and very low stratus against the sloping terrain along the
Highway 52 corridor from northern parts of the state to the
James River Valley.

As has been the case the past few days, the convective forecast
through this evening is highly uncertain. Each run of each CAM
seemingly offers a solution different from all others, and none
of them have properly simulated what is being observed early
this morning. Broad synoptic scale lift is forecast to be
present over western and central North Dakota throughout the
day. But if, where, and when storms develop and/or intensify
will likely be determined by the evolution of mesoscale
features that carry low predictability beyond a forecast time
range of a few hours. Mixed layer capping also contributes
uncertainty to the convective forecast this afternoon, with low
level wind trajectories mostly unfavorable for strong boundary
layer mixing. One potential feature to keep an eye on that
recent deterministic and high-resolution ensemble guidance has
consolidated on is a west-to-east oriented surface warm front
that is forecast to sharpen across central North Dakota this
afternoon. Ignoring convective inhibition for surface and mixed-
layer based parcels, the forecast environment is favorable for
severe storms with CAPE on the order of 2500-3500 J/kg and deep
layer shear ranging from around 30 kts south to near 50 kts
north. Storm mode may be mixed, but any convection initiating
along the warm front could quickly become linear with a
damaging wind threat given shear and deep layer wind vectors
being parallel to the front. Regardless of mode, any storm or
complex thereof along the warm front could be in a favorable
environment for tornadogenesis, although modeled hodographs
tend to favor crosswise vorticity over streamwise.

From this evening into tonight, the low level jet will
reintensify over the western Dakotas ahead of much stronger
shortwave energy ejecting from the base of the upstream
longwave trough. This could initiate another round of strong to
severe storms, but any effects from daytime convection could
alter this potential. By Labor Day morning, strong DCVA and lee
cyclogenesis are forecast to reach the Northern High Plains,
which is expected to yield widespread showers and embedded
thunderstorms over western North Dakota, with east/northeastward
propagation through the afternoon. There is uncertainty in
potential severe hazards and magnitudes thereof on Labor Day.
First, the morning convection is likely to remain elevated and
could modify the mesoscale environment for the rest of the day.
Then in the afternoon, the phasing of forcing mechanisms
becomes highly uncertain. The most likely outcome appears to be
a forward-propagating mid level shortwave moving through
central North Dakota during the afternoon, leaving behind the
lee cyclogenesis that some guidance shows could evolve into a
double-barrel surface low from southern Saskatchewan to
northwest South Dakota. CAMs generally indicate at least low
probabilities for storms associated with both of these features
in the afternoon, but the arrival of a dry slot could mitigate
convective coverage (although latest guidance appears to have
slowed the progression and/or dampened the influence of the dry
slot). If the mid level shortwave stays ahead of the lee
cyclone, any convection that initiates from it could be in a
volatile environment with strong shear at all layers keeping
all severe hazards in play, including tornadoes. But in turn,
there could be enough residual capping to lower the threat
ceiling. Farther west, any storms that initiate near the lee
cyclone would more likely be high-based with a predominant
damaging wind threat. Another aspect of the system as whole
that will need to be monitored is the potential for excessive
rainfall. Of note, 00Z HREF members show potential for local
rain amounts as high as 2-3 inches across already rain-soaked
northern parts of the state.

Forecast temperatures today through Labor Day remain above
normal north (upper 70s to mid 80s) to well above normal south
(upper 80s to mid 90s), with unseasonably high dewpoints in the
60s to lower 70s. Chances for showers and storms quickly
diminish Monday night as a potent upper low at the base of the
upstream trough kicks through the Northern Rockies and forces
the lee cyclone into southern Manitoba. A few showers could
return to northwest parts of the state on Tuesday as the upper
low passes by. The main concern for Tuesday though is strong
winds. Both the NBM and ECMWF EFI support advisory criteria
winds of sustained speeds between 30 and 40 mph and gusts of 45
to 55 mph. Tuesday will be much cooler with highs from the
upper 60s northwest to lower 80s southeast, but also drier with
minimum humidity falling to around 30 to 35 percent. The
combination of strong winds and lower RH will likely bring
near- critical fire weather conditions to all of southwest and
south central North Dakota, with critical conditions possible
in the far southwest where RH could fall closer to 25 percent.
Cooler temperatures remain in the forecast on Wednesday, but
with winds not nearly as strong. Ensembles have now trended
toward a quick warmup on Thursday due to a transient ridge,
followed by a distinct cooling trend heading into next weekend,
but with increasing spread due to synoptic scale wave phasing
uncertainty.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 530 AM CDT Sun Sep 6 2026

Chances for showers and thunderstorms will gradually increase
through the forecast period. A few strong to severe storms are
possible this afternoon and evening, capable of large hail and
strong, erratic wind gusts. IFR visibility from heavy rain and
brief MVFR ceilings are also possible with heavier showers and
any thunderstorm.

LIFR ceilings and patchy dense fog have developed from around
KMOT to KJMS. Conditions should improve by late morning. Aside
from this and any showers or storms, VFR conditions are
otherwise likely to prevail. Winds will primarily be easterly
to southerly, increasing to around 10-15 kts on Sunday.

&&

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

&&

$$

DISCUSSION...Hollan
AVIATION...Hollan