181
FXUS63 KBIS 051445
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
945 AM CDT Sat Sep 5 2026

.KEY MESSAGES...

- Periods of isolated to scattered showers and thunderstorms
  are expected through the holiday weekend. A few storms may be
  strong to severe. The highest chances for showers and storms
  are from Sunday night into Labor Day.

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

- Windy on Tuesday, with near-critical fire weather conditions
  in southwest North Dakota.

&&

.UPDATE...
Issued at 945 AM CDT Sat Sep 5 2026

Strong to severe thunderstorms continue across the north
central. A lingering low level jet along perpendicular to an
elevated boundary has found abundant MUCAPE and modest shear,
continuing to produce these stronger storms. These could last
all morning in the northeast as little movement from this system
is noted. With high cape and modest shear large hail to perhaps
golf ball sized is possible, especially to any rotating storms.
DCAPE and lapse rates are limited with these storms, thus
strong winds are overall not expected to be a major threat
although a collapsing thunderstorm could produce a gust to 60
MPH. Hodographs showing some increased SRH in this area. Storms
would have to be surface based to have any sort of a tornado
threat this morning, and STP is low. This increased SRH will
more help with some elevated rotation, increasing the potential
hail size. Pwat values of 1.5 inches are also found in this
area, bringing a heavy rain threat. Radar estimates of 1 to 4
inches have already fallen in northern Bottineau county where a
Flood Advisory remains. Will have to continue to monitor this
and any other areas for heavy rain for flooding potential.
Overall those with outdoor plans today and through the Labor Day
weekend will want to stay weather aware as isolated severe
storms remain possible throughout the holiday weekend.

UPDATE
Issued at 637 AM CDT Sat Sep 5 2026

A cluster of strong thunderstorms continues to slowly propagate
eastward along the Canadian border in north central North
Dakota. The most prominent hazard with these storms is
torrential rainfall causing localized flooding of urban, low-
lying, and poor-drainage areas. Isolated instances of wind gusts
to around 40 mph and hail up to the size of nickels are also
possible. With stout low level warm air advection and moisture
transport forecast to remain in place, it seems likely these
storms will persist, departing our forecast area around mid
morning.

&&

.DISCUSSION...
Issued at 505 AM CDT Sat Sep 5 2026

A mid to upper level ridge axis resides over the Red River
Valley early this morning, with a lee surface trough/low
analyzed over far eastern Montana. A strong low level jet
underneath the mid/upper ridge supplying warm air advection and
moisture transport has produced a corridor of isolated to
scattered thunderstorms from northwest North Dakota through the
James River Valley. Storms in the northwest have been strong to
severe at times and are producing torrential rainfall. This
activity is likely to persist and continue drifting eastward
through the morning as low level flow remains southerly.

The lee trough/low is forecast to slide into the western
Dakotas today, accompanied by the thermal ridge. Highs in the
90s are forecast along and to the south and west of the
Missouri River, hottest in the southwest where lower dewpoints
are forecast. Areas along and to the north and east of the
river are likely to see daytime dewpoints in the mid 60s to
lower 70s, which could push maximum heat indices near 100 along
the Missouri River Valley, south of Lake Sakakawea.
Deterministic guidance notes persistent low level warm air
advection in the eastern half of the state throughout the day
today, with subtle shortwave impulses climbing the top of the
ridge. Because of this forcing, daytime convection aside from
what is ongoing this morning cannot be ruled out. But rising
mid level height tendencies along with very strong MLCIN seem
likely to suppress convective initiation through the afternoon,
which is the outcome that has the greatest support from CAMs.
If a surface or mixed-layer based storm were to develop, it
could have access to at least 3000 J/kg CAPE and deep layer
shear around 30-40 kts. This leaves a conditional risk for
isolated severe thunderstorms in the forecast for today. For
this evening into tonight, the low level jet could once again
initiate elevated convection. This seems most likely to occur
over the southern half of the state where warm air advection is
strongest, but the RAP also shows a strong plume of poleward
theta-e advection across the north. Most CAMs predict that any
nocturnal elevated convection is unlikely to tap into the
maximum potential buoyancy and shear, but the 06Z HRRR is a
noted outlier in bringing a cluster of strong to severe storms
through south central and southeast North Dakota this evening.

The lee surface low is forecast to sag southward and settle
over the Black Hills on Sunday, maintaining a broad southeast
to easterly surface flow reinforcing the moisture-rich air mass.
The NBM keeps highs in the 80s to lower 90s, but this seems a
little too warm conceptually. It will nevertheless be another
unseasonably warm and very humid day, with dewpoints remaining
in the mid 60s to lower 70s. The convective forecast during the
day Sunday is similar to that of today, with two exceptions
being 1) falling rather than rising mid level height tendencies
and 2) stronger deep layer shear. But the lack of a strong
forcing mechanism combined with strong CIN keeps thunderstorm
probabilities low until later in the day Sunday, although the
NBM does maintain a broad expanse of 30 to 40 percent PoPs
across all but southwest North Dakota. Heading into Sunday
night, shortwave energy ejecting from the base of a deep
western CONUS longwave trough will reach the Northern High
Plains, resulting in increased probabilities for showers and
thunderstorms. CAMs that reach this forecast time range do
support a risk for strong to severe thunderstorms, favoring
western North Dakota during the evening hours. These storms
would likely be rooted at the base of an elevated mixed layer
with strong MUCAPE and effective bulk shear.

On Monday, a closed upper low sitting at the base of the
western CONUS trough is forecast to lift northeastward and
interact with a shortwave ejecting northeast off the Northern
Rockies and a separate upper low over southern British Columbia.
Broad lee cyclogenesis combined with strong low level warm air
advection and mid level height falls will bring much higher
chances for showers and thunderstorms across the state late
Sunday night through Labor Day. Ensemble guidance still favors
the northern half of the state for higher rainfall amounts as
southwest and south central North Dakota remain favored to be
dry slotted. However, prior to the arrival of the dry slot,
deterministic guidance is beginning to hone in on a strong cold
frontal passage that could potentially support an all hazards
severe event if requisite ingredients can become spatially and
temporally aligned.

The strongest vort max is forecast to swing from Montana to
Manitoba Monday night into Tuesday, but the dry slot from the
preceding cyclone could keep all but far northern parts of our
forecast area rain free over this time period. Cooler high
temperatures ranging from the upper 60s northwest to near 80
south central and southeast are in the forecast for Tuesday, as
are strong westerly winds with the NBM and ECMWF EFI already
advertising medium probabilities for reaching advisory criteria.
The strong winds could also introduce fire weather concerns on
Tuesday, particularly in the southwest where the lowest RH is
forecast. After Tuesday, ensemble clusters show a variety of
synoptic-pattern solutions contingent on the existence and
placement of a southern US dome of high pressure. The NBM tends
to favor near normal temperatures over this timeframe with
little to no chances for rain.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 637 AM CDT Sat Sep 5 2026

Isolated to scattered showers and thunderstorms are possible
throughout the forecast period, mainly across the eastern two
thirds of North Dakota. Confidence in if and/or when a shower
or storm could move over a terminal remains very low. The
strongest storms could produce heavy rain reducing visibility
to IFR levels, erratic wind gusts, and small hail. VFR
conditions are otherwise expected to prevail through the
forecast period, although there is a low chance of MVFR/IFR
ceilings developing from KMOT to KJMS this morning. Winds will
primarily be southeasterly to easterly around 10-15 kts. Low
level wind shear is expected at KBIS until mid morning.

&&

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

&&

$$

UPDATE...Anglin
DISCUSSION...Hollan
AVIATION...Hollan