An upper-level low pressure system and a strong associated cold front will drop temperatures up to 15 degrees within a matter of minutes in parts of the Midwest today. The relief from the heat will come with a steep cost, however. Tornadoes, large hail and damaging straight-line winds are threats that many areas will need to confront before experiencing relief. A lower risk for severe storms exists in northern New England and the lee side of the central Rocky Mountains.
A wide slew of triggers will serve to initiate convection by early afternoon over Iowa, southern Minnesota, eastern Nebraska and eastern Kansas. Ahead of the cold front, temperatures will be in the low to mid 90s, with robust southerly flow providing abundant moisture. Aloft, dry air will be flowing to the northeast over the Rocky Mountains. The dichotomy between the moist surface layer and dry upper layers of the troposphere will result in extreme instability. When storms are triggered, they will develop rapidly.
Regardless of the timing of the cold front, residual outflow from early morning thunderstorms will force scattered thunderstorm development from Kansas to the upper Mississippi River Valley. In this environment and with strong speed and directional wind shear, some of these initial storms may become severe.
Later in the afternoon, the cold front will further trigger and organize thunderstorms. These storms will be the most likely to become severe because wind shear and lift will be greatest in its vicinity. Ahead of the front, winds will be from the southeast. Behind it, winds will be from the northeast. These converging winds of vastly different temperatures will force air to accelerate upwards. Passing kinks in the jet stream will further enhance upward ascent. These kinks in the jet stream will serve to further stretch air columns and possibly generate rotating supercells, which could support a few tornadoes.
Supercells will merge throughout the evening, joining to form bowing segments. These bowing clusters of storms will produce very strong wind gusts just ahead of them. Large hail and tornado chances will fall upon clustering, but wind damage will become more widespread. These bowing segments will initially form over Iowa and southern Minnesota early in the evening and spread into Wisconsin, producing wind damage in their path northeastward.
As the upper-level trough propagates eastward, a strong low-level jet will continue ahead of it through the overnight hours. With the cold front outpacing its parent low, sources of lift from the trough will cause the redevelopment and spreading of storms to the northeast. The jet will help organize the storms into segments. The storms may produce severe wind damage over Nebraska, but they will weaken upon reaching South Dakota and Iowa.
The tornado threat will be highest in east-central Nebraska and in the northwestern half of Iowa, where instability, shear and lift from the front and jet stream kinks will be greatest. There is also a significant risk for large hail at least one inch in diameter. Cities most at risk for tornadoes include Lincoln, NE and Omaho, NE. Widespread wind damage is likely where these storms initially start to bow. Des Moines, IA and Council Bluffs, IA may be in these storms’ bath. Wind damage will be more scattered in nature further northeast in Minnesota and Wisconsin.
While storms ravage Nebraska and Iowa, upsloping of winds along the eastern Rockies will result in isolated supercells developing over eastern Colorado and Wyoming. A belt of more scattered storms will develop in eastern Colorado, where instability will be greater. Low dew points in the upper 50s will prevent these storms from becoming wide spread. Damaging wind gusts and isolated large hail will be the main threats with these storms.
As a dome of high pressure expands northeastward, the Northern Country of New York and northern New England will be caught along the outer periphery of the high’s circulation. Very dry air aloft flowing all the way from the Rocky Mountains with the moisture of dew points in the low 70s will result in high instability over this area. An unseasonably vigorous jet streak will pass over this unstable layer, producing a narrow area of strong upward motion resulting in scattered severe thunderstorms.
The strongest storms will develop early in the evening Saturday in New York before spreading into northern New England by sunset. Strong wind shear associated with the jet streak will be sufficient to support large hail and a couple of weak tornadoes in some of the supercells. The tornado risk will be highest in the North Country of New York and in the valleys of northern Vermont including cities like Burlington, although the threat will not be as high as in the Midwest. Strong wind gusts will be the dominant threat elsewhere.
Heat will spread into northern New England behind these storms, unlike the refreshing airmass that awaits the Midwest. The cold front responsible for the Midwest’s storms will weaken as it approaches the heat dome, but it will still bring severe weather to Wisconsin, Michigan and Illinois on Sunday.