textproduct: Bismarck

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.