A violent afternoon and evening is in store for much of the Upper Midwest, and the northeastern Rocky Mountains due to the widespread development of supercells and intense thunderstorm clusters. Hail up to baseball size in diameter paired with strong tornadoes will wreck havoc, likely bringing devastation to population centers in these storms’ path. Other severe storms with much less intensity are also expected across the South and parts of the Northeast this afternoon.
Multiple rounds of thunderstorms will develop over the Upper Midwest this afternoon through tonight. An initial round of thunderstorms will develop over North Dakota, joining clusters of other storms over northern Minnesota. Later in the afternoon, scattered supercells will be triggered by a cold front in eastern Montana, spreading into North Dakota. These storms will be the most dangerous, capable of producing gigantic hail, violent tornadoes, and strong wind gusts. Around sunset, the primary mode of storms will be clusters, and eventually a strong linear system which will race eastward into Montana with damaging winds early overnight.
Every ideal condition will be satisfied for rapid development of supercell thunderstorms and tornadoes this afternoon over eastern Montana, North Dakota, and northern South Dakota. Southeasterly flow behind a warm front will deliver an abundance of moisture, with dew points in the upper 60s and low 70s. Wind flowing down the Rocky Mountains will force dry air to ride over it, resulting in an extremely unstable atmosphere. Meanwhile, a robust upper-level low will be crossing the northern Rockies, dragging a cold front along with it and providing a substantial source of lift.
Southeastern Montana and western North Dakota will be in the vicinity of the right entrance to a vigorous jet streak. Winds will accelerate and diverge here. This is where lift will be greatest, where severe thunderstorm development will be the most rapid, and where tornadoes will be most likely. The location on the lee side of the mountains will also help encourage tornado development. When air flows down a mountain, it stretches vertically. Just as a figure skater spins faster when they draw their arms and legs inward, air spins faster when it is stretched in such a manner. This process can encourage the development of tornadoes in an unstable environment.
Eastern Montana and the western half of North Dakota have the highest risk of tornadoes and large hail, threatening Bismarck, ND and Dickinson, ND. Hail and tornadoes will be scattered, but the powerful blasts of wind from these storms is unavoidable. All residents of these areas are encouraged to pay close attention to weather updates and heed emergency warnings.
Further north, west, and south, storms will be less widespread, but tornadoes could still threaten cities like Billings, MT. The domain will shift to multicellular systems east of Bismarck, where wind and smaller hail will be the primary threats for cities including Fargo, ND and Moorhead, MN. A few of the storms in these systems will be capable of producing a tornado, altogether they will not be as intense as those further west. Overnight, the linear systems will race through the northern half of Minnesota. These storms will mainly produce strong wind gusts, but hail is also possible.
In the Middle Mississippi Valley, impulses riding around an upper-level high will trigger a few thunderstorms Thursday afternoon. The warm, highly unstable air mass will support a few scattered severe storms. Some will be capable of producing strong winds and large hail, over an inch in diameter before clustering into the evening.
Further south and east, a more substantial shortwave will trigger scattered thunderstorms over a larger area in the Deep South over much of Georgia and Alabama. With shear lacking over most of this region, damaging winds will be the primary threats, with only the mountains of northern Alabama escaping it. Storms will weaken through the evening as they migrate toward the Florida Panhandle.
In the Northeast, modest shear and instability beneath the right entrance region of a jet streak and ahead of a cold frontal boundary will continue to support scattered thunderstorms into the afternoon. A few of these storms may produce strong wind gusts, but the severe potential will not be substantial given abundant cloud cover and the cold front’s delay behind the jet streak. Storms will end from west to east between late afternoon over New York and Pennsylvania, and late evening over eastern New England.