Thursday marks the peak of a multiday stretch of severe weather that’s featured hail and picturesque tornadoes, including an elegant “landspout” near Denver on Sunday that captivated onlookers as much as 30 miles away.
Monday, Tuesday and Wednesday brought additional instances of severe weather in Montana and the Dakotas. A remarkable flying saucer-like storm known as a “mother ship supercell” spun up near Highway 191 in central Montana on Wednesday, dazzling storm chasers as it churned overhead.
Some described the storm as exhibiting the most remarkable structure observed all year. Fortunately, it didn’t produce any tornadoes.
Thursday may be different, however, as a mid-level disturbance kicks out of the northern Rockies and Columbia River Basin, sparking potentially dangerous storms over the northern Plains.
Ahead of the disturbance, tropical moisture streaming north from the Gulf of Mexico is overlapping with anomalously hot temperatures approaching record values in spots. Mid- to even a few upper 90s are possible in western North Dakota and adjacent parts of Montana. That will energize the atmosphere, leaving it replete with instability or the proclivity for air near the surface to rise. That’s the fuel for storms.
A change of wind speed and/or direction with height, known as wind shear, will also foster rotation within storms and boost the chance of a few tornadoes, especially in northwestern North Dakota.
In this area, storms will initially develop by about 5 p.m. local time, remaining somewhat discrete apart from one another as rotating supercells. That will present the risk of large hail, perhaps to baseball size and winds to 70 mph. That first batch of storms may move into southeastern Saskatchewan and transition to mainly hail-producers as they move north of the warm front during the late evening.
Simultaneously, new cells will fire along the dryline, or leading edge of dry and comparatively cool air from the west, down into western South Dakota and northwestern Nebraska. Those storms may quickly congeal into a line with damaging winds becoming the main threat. “[V]ery damaging gusts to hurricane force may be observed through the evening and possibly the early overnight hours,” the Storm Prediction Center wrote.
Those storms will plow east during the overnight, taking advantage of the toasty air mass ahead of them. A few gusts in the James River Valley could top 80 mph. The line may weaken by the time it reaches the Minnesota border, potentially arcing south with remnant gusty winds even into northeast Nebraska during the early morning hours Friday.
Where storms become established Friday is predicated on what happens Thursday evening, but severe weather chances look to diminish markedly into the weekend.