As a dome of high pressure remains mostly idle over the Southern Plains this week, several disturbances aloft will ride around its circulation, contributing to several more rounds of severe thunderstorms. The Northern Plains will experience the brunt of the storms Tuesday, with a scattered risk for damaging wind gusts, large hail, and a small chance for tornadoes. Storms continue throughout the week as the primary threat shifts to the central Plains and upper Midwest Wednesday, the northern Rockies Thursday, and potentially parts of the Mississippi River Valley Friday.
Late Tuesday afternoon, scattered thunderstorms will develop over the Northern Plains ahead of a southeastward moving cold front exiting Canada. The primary threat will be large hail and damaging wind over the central Dakotas and possibly northwestern Minnesota. Isolated tornadoes may also develop with supercells over central North Dakota.
Storms over Northern Plains will be fueled by the boundary of the heat dome and by an abundance of moisture. Temperatures will climb into the upper 80s and low 90s in an environment in which dew points will reach the mid-60s. Lift south of the cold front will be enhanced by a strong jet stream passing over North Dakota and embedded shortwaves in the flow around the dome of high pressure. The strongest supercell thunderstorms will develop early this evening in the vicinity of the jet stream over central North Dakota. This is where the greatest threat for tornadoes will be. Further east, supercells will merge into the evening, possibly forming short linear structures over southeastern North Dakota, northern South Dakota, and northwestern Minnesota. Upon merging, the primary threat will shift to damaging wind gusts ahead of the storms. With sufficient wind shear and a moist inflow, these storms may persist late into the night.
In addition to the Northern Plains, isolated thunderstorms will develop ahead of a dry line in Texas. A dry line is a boundary between a moist and a dry air mass. Some of these storms could be capable of producing damaging winds and up to nickel sized hail.
By Wednesday the cold front will have reached the Central Plains, sparking more rounds of severe thunderstorms primarily in Nebraska, southern Minnesota and northern Iowa. Storms are expected to be more widespread in these regions than in the Dakotas Tuesday due to a more defined line of thunderstorms developing in the evening. The primary threat will be scattered to perhaps widespread damaging wind gusts. There may be localized embedded hail within this line of storms. If clearing occurs early enough, some storms may develop ahead of this line that and become supercelluar, capable of producing large hail and damaging wind gusts.
These storms will be supported by a hot and moist environment, with temperatures near 90 and dew points in the mid-60s. Weak leftover thunderstorms in the morning will leave a residual outflow boundary that will assist the cold front in triggering thunderstorm development in the afternoon. Further aid will be supplied by lift associated with small shortwaves in the flow around the dome of high pressure.
Further west, a weak cyclone will develop along and east of the Rocky mountains due to a process called lee cyclogenesis. Rising motion to the east of this cyclone will spawn scattered thunderstorms in southern Montana, eastern Wyoming, and most of the central Plains as the cyclone treks northeastward. Some of these storms may produce damaging wind gusts and pea to nickel sized hail, but the greatest chance for this to occur will be over western Nebraska, southeastern Wyoming, and northeastern Colorado, where convergence and wind shear will be most favorable for thunderstorm development.
Thursday, scattered thunderstorms will develop over the Northern Rockies. With a warm, moist inflow over the mountains, it won’t take much additional lift for some of these storms to produce large hail and damaging winds. Additionally, the pesky dry line will continue to trigger isolated storms over the southern Plains, despite the presence of the dome of high pressure. Any storms that do form will provide some brief but much needed relief from the heat as temperatures soar into the upper 90s to low 100s.
Friday, a shortwave and associated cold front may develop around the ridge of high pressure in the Mississippi Valley. Details are uncertain at this point, but with an unstable airmass in place and an abundance of shear, the threat for at least isolated severe weather exists.
The severe weather this week is unusually widespread and persistent. We will continue to provide new information about each threat as each event nears.