The heat dome over the southern Plains will continue to act as a highway for upper-level disturbances, generating severe weather over the Plains and the Midwest Friday. The targeted area for this weather will be the northern Plains, between central Nebraska and central South Dakota, where the storms will be the most damaging and the most widespread. A secondary risk for severe storms exists between southern Minnesota and Iowa, eastward into the Ohio River Valley. A third area at risk for severe storms will set up along an axis over the central and southern Plains, between southwestern New Mexico and western Texas northward into western Nebraska.
A very dangerous environment favorable for destructive supercell thunderstorms is setting up next to the daytime heating in the northern Plains. Hot temperatures with a rich flow of moisture from the southeast are contributing to an impressive amount of deep instability. Once surface heating breaks the “cap” on convection late this afternoon, storms are expected to quickly develop, towering up to 9 miles into the stratosphere.
Storms will begin developing along the leeward side of the Rocky Mountains this afternoon, as surface winds will be forcing air up along the mountains. The presence of a cyclone spawned by lee cyclogenesis over Colorado and a passing shortwave trough of low pressure will further contribute to the development of storms this evening.
The storms will migrate into South Dakota and Nebraska, spawning more storms as their outflow forces air upwards, resulting in cascading development of strong thunderstorms. A marginal clockwise change in wind speed and direction. bringing increasing height (shear), will keep storms that develop late this afternoon continuing for several hours. Many of these storms will become supercellular, capable of supporting very large hail with diameters of up to 2 inches or more, powerful wind gusts in their forward flanks ahead of the rain, and even a few tornadoes.
Throughout the evening, supercells will merge into multicellular clusters and squall lines as they cross the plains. The tornado threat will diminish over the eastern halves of South Dakota and Nebraska as these multicellular systems form. The primary threat will shift to damaging wind gusts ahead of the storms and large hail up to 1 inch in diameter. The hail threat will be smaller here than over the western threat area. Overnight, the storms will reach the southern half of Minnesota. The only severe threat here will be a slight chance for damaging wind gusts.
Further south and east in the Midwest, a shortwave associated with thunderstorms Thursday night will generate thunderstorms Friday morning and afternoon between Iowa and western Ohio. Clouds may curtail widespread convection, but strong thunderstorms capable of producing damaging winds and small hail are possible, especially over central Indiana and west central Ohio.
In the southern plains, a stalled dry line (a boundary between dry and moist air masses) will continue to trigger isolated thunderstorms Friday afternoon into Friday evening. Since it is so hot, with the presence of strong wind shear, large hail is possible with storms that do develop in addition to strong wind gusts.
Saturday, a brief reprieve from widespread severe thunderstorms will grace the country, but isolated storms are still expected. The environment across most of the central US and the southern Mid-Atlantic will actually be ripe for thunderstorm develoment, but cloud debris from Friday’s storms and a cold front passing through the Great Lakes and Northeast will act as a “cap” on widespread convection. As has been the pattern this week, passing shortwaves may spawn thunderstorms Saturday across the Plains and the Midwest. Storms that do develop may produce locally damaging wind gusts. In the Mid-Atlantic, a frontal boundary will produce scattered thunderstorms from western Pennsylvania to the Delmarva Penninsula. Moderate instability and marginally strong wind shear may support locally damaging wind gusts.
A better chance for severe storms will develop over the Northern Rocky Mountains. Surface dew points will be marginal for severe storms, but an incoming amplified trough of low pressure will encourage large scale ascent, so some of the scattered storms that develop may be capable of producing damaging winds and large hail up to 1″ in diameter.
Many residents of the Central US are wondering why severe weather has been targeting the region almost every evening this week. In a “ring of fire” pattern in which a dome of high pressure resides over Texas and the southern Plains, any passing upper-level disturbances easily generate thunderstorms as they ride along the fringe of the high pressure system.
These upper-level disturbances are local regions of lower pressure at the surface, aloft, or both, and are the exclusive result of wind flowing over the Rocky Mountains. Air is vertically compressed going upward, but vertically stretched going downward. Going up the mountains, air loses its moisture to condensation. Going down, vertical stretching results in widespread upward motion lee of the mountains, spawning cyclones in a process called lee cyclogenesis. These cyclones draw hot and very humid air from the Gulf of Mexico at the surface. With hot, humid air at the surface, but dry air aloft from the Rocky Mountains, rising air will cool at a much slower rate than the environment does with height, resulting in massive thunderstorms. With a stagnant ridge of high pressure forcing winds to blow over the Rocky Mountains, cyclones and upper-level disturbances repeatedly form over and over again.
As discussed yesterday, the persistent ring-of-fire pattern is expected the change by mid-June, potentially resulting in a lower frequency of thunderstorms for the areas that have lately been targeted. Until then, we are expecting severe thundertorms to threaten areas of the Central US nearly every day.