Heat in parts of the central Plains and upper Mississippi River Valley will be broken today and will be signaled by a period of natural fireworks. As a cold front associated with a low pressure system in southern Canada drapes eastward, clusters of thunderstorms will develop late this afternoon and into early evening. Besides interrupting manmade fireworks displays, damaging winds and large hail may also impeded on holiday festivities.
Heavy morning thunderstorms will gradually dissipate as they move eastward Wednesday, leaving cloud debris to cap a warm, moist environment in their wake. As the sun breaks through the clouds, extreme instability will build at the surface. Scattered thunderstorms will develop starting midday in southeastern South Dakota and western Minnesota. The storms will move southeastward ahead of the cold front and rapidly intensify. The storms will organize into linear and semi-linear segments by late afternoon over northeastern Nebraska, northern Iowa, and southeastern Minnesota.
Meager wind shear over most of the thunderstorm threat area will hinder widespread development of severe storms. Shear will be just sufficient enough over northern Nebraska, northern Iowa and southern Minnesota for the formation of bowing segments of storms. These bowing segments will be capable of producing large areas of wind damage in their path. Some of the discrete cells ahead of the linear and bowing segments could also be capable of producing large hail.
Cities at the greatest risk of experiencing severe storms during their Fourth of July festivities include Sioux Falls, Omaha, Minneapolis, and Duluth. Large hail will be a more significant threat in Nebraska, northern Iowa and southwestern Minnesota than elsewhere in the threat zone. Damaging straight-line winds will be the primary threat with bowing segments in eastern Minnesota and western Wisconsin.
The timing of the storms could not be any more inconvenient for many areas. Storms will approach northeastern Nebraska, southeastern South Dakota, central Minnesota and northwestern Wisconsin near sunset. Lightning, rather than fireworks, could light up the sky during the festivities in cities like Omaha, Sioux Falls, and Minneapolis. Storms will likely clear northern Minnesota just in time for fireworks, leaving Duluth in the clear.
Strong storms will continue to threaten Iowa, southern Minnesota, northern Wisconsin, and the eastern two-thirds of Nebraska into the overnight hours as the front slows its advance. The loss of daytime heating and a weak low-level jet stream will cause the storms to decay into the overnight hours, lowering the severe threat for cities like Des Moines and Lincoln.
Further west, upsloping along the foothills of the Rocky Mountains from a lee cyclone will spark scattered thunderstorms in Colorado and the western Plains from Nebraska to the Texas Panhandle. These thunderstorms will not be as widespread as those in the Upper Midwest but they may still interfere with evening fireworks displays, bringing with them a slight chance for damaging winds and large hail.
Thursday, the severe threat will shift to the Northeast, where scattered thunderstorms are expected to develop ahead of a strong upper-level low set to break the extended heatwave. Well ahead of the leading cold front, a pre-frontal trough of low pressure at the surface will serve as the trigger Thursday afternoon along and west of the Appalachian mountains in Pennsylvania, New York and northwestern New England. Storms may organized into linear segments, elevating the threat for wind damage. Southeast of the Appalachian and Adirondack Mountains, storms will hold off until overnight Thursday or late Friday morning, when slow moving, heavy downpours may result in flash flooding.