Conditions will be ripe along the northern periphery of an expanding dome of high pressure for severe thunderstorms in the upper Midwest Tuesday through the July 4 Holiday Wednesday. This high pressure system will encapsulate most of the contiguous U.S. by midweek with high hear and humidity. As it expands, upper-level perturbations will ride along it’s circulation, triggering rounds of severe thunderstorms. Damaging wind gusts will be the dominant threat with the storms, but there is also a slight risk for large hail and tornadoes for some areas of the threat region.
Remnants of overnight severe storms will remain in parts of the upper Midwest into Tuesday afternoon. Clearing from west to east will gradually destabilize the atmosphere in time for a shortwave trough and associated cold frontal passage in the evening.
Storms will first begin to pop up late Tuesday afternoon in the Dakota’s and the foothills of the Rocky Mountains after clearing. A few of these initial storms may produce strong wind gusts and/or hail but the storms of most concern will develop during the evening hours. Ascent from the shortwave and cold front will trigger clusters of storms and a few discrete supercells in the foothills of the Rockies early in the evening. These storms will intensity as they flow down the foothills, stretching and encountering an extremely unstable environment with modest wind shear in the high Plains of the western Dakota’s and northwestern Nebraska.
Rapid City, Bismarck and Fargo are all in the line of fire from this second and more intense round of storms. The storms will organize into clusters and linear segments the further east they spread late into the evening hours. The storms will be more discrete in Rapid City early in the evening, when the storms could produce large hail and possibly a tornado in addition to damaging wind gusts. By the time the storms reach Fargo, ND late in the evening, the storms will have joined into bowing segments, increasing the damaging wind threat but reducing the hail and tornado threat.
A similar evolution of severe storms will occur over central and northeastern Minnesota and western Wisconsin Wednesday. Morning storms and cloud debris will give way to the sun destabilizing the surface ahead of the cold front. The outflow of a few storms late in the afternoon will trigger organization into strong bowing segments, which will be capable of producing wide swaths of wind damage. Large hail may also be a possibility within these segments and initial storms. Unfortunately, some of these storms may interfere with fireworks displays and other July 4th festivities.
In the Northeast, scattered thunderstorms will develop in the afternoon ahead of a weak cold front. Scattered storms will first develop ahead of the front in the Hudson valley, the northern tier of Pennsylvania, and western Massachusetts early in the afternoon. Meanwhile, extreme instability will build in New Jersey, southeastern Pennsylvania, eastern Maryland and Delaware under the “cap” of the dome of high pressure responsible for the recent heat. By mid-afternoon, clusters of storms will quickly develop, bringing with them torrential downpours and frequent lightning by evening rush hour. Given the extreme instability, some of these storms will be capable of producing damaging wind gusts and small hail along the I-95 corridor. Fortunately, storms will clear with the loss of day time heating in time for pre-holiday fireworks.
The hot, humid air mass in the Northeast will moderate very little behind the cold front responsible for Tuesday’s storms, making for another hot day for the July 4th holiday. The Northeast will not experience real relief until a stronger cold front triggers widespread thunderstorms and downpours between Thursday and Friday.