A dramatic early-September weather pattern could bring near-freezing temperatures to parts of the northern Rockies while areas farther east push back toward 100 degrees, creating the kind of sharp temperature contrast that meteorologist Ryan Hall says could also rearrange the storm track across a large part of the country.
Hall, host of the Ryan Hall, Y’all YouTube channel, said the most important part of the setup is not simply the possibility of an early frost in higher elevations, but the large bend developing in the jet stream that could pull colder Canadian air into the West while forcing a stubborn late-summer heat ridge farther east.
That shift means some areas enjoying comfortable late-August weather may quickly return to dangerous heat, while parts of the Midwest and Great Lakes could again find themselves in a corridor favorable for strong thunderstorms.
First Frost Could Arrive In The Northern Rockies
Hall said the first major change should develop around September 3 through September 5, when the northern Great Basin, northern Rockies and northern High Plains are expected to turn much colder than normal.
Both American and European ensemble guidance, he said, are showing increasing chances for overnight lows below 40 degrees, with some higher elevations potentially falling to around 32.
That would be cold enough for the first frost or freeze of the season in some locations.
Hall was careful to point out that frost in mountainous parts of the West during early September is not especially unusual by itself, but the speed of the surrounding pattern change is what caught his attention.
A strong ridge of high pressure is expected to build over the northeastern Pacific, causing the jet stream to buckle southward downstream and carve out a trough over the western United States.
That trough provides a pathway for colder Canadian air to move south, and once the jet bends that sharply in the West, the effects begin spreading far beyond the areas actually experiencing the coldest temperatures.
Heat Could Surge Back Across The Midwest And Ohio Valley
As the western trough digs in, Hall said the heat ridge over the southern United States should be pushed farther east, which could produce the unusual situation of a cold shot in one part of the country helping drive another burst of intense heat somewhere else.
The Weather Prediction Center, according to Hall, expects dangerous late-summer heat to expand from the Southern Plains through the Mississippi Valley and Mid-South before reaching farther into the Midwest, Ohio Valley, South and Southeast during the coming week.
By around September 3, Hall said there is a moderate risk of extreme heat stretching from parts of the Central and Southern Plains through the Mississippi Valley and into western portions of the Tennessee and Ohio valleys.
High temperatures could reach 95 to 100 degrees in some locations, while areas farther south dealing with heavier humidity could see heat indices climb above 110.
Missouri, Illinois, Indiana, Kentucky, Tennessee and Ohio are among the places Hall specifically warned not to assume that the recent cooler spell means summer is finished.
Friday should still feel quite comfortable across many of those areas, with lower humidity and more manageable temperatures, but Hall expects the ridge to shift eastward as September begins.
The contrast is what makes the forecast notable: while some western areas flirt with freezing temperatures, parts of the central and eastern United States could still be experiencing conditions that look and feel like the middle of summer.
Storm Track May Reload Along The Heat Dome
Hall said the northern edge of the heat ridge may become just as important as the heat itself because small disturbances in the jet stream can ride over the top of the ridge and then dive southeast.
That kind of northwest-flow pattern can guide storms from the Northern Plains through the Upper Midwest and Great Lakes before pushing toward the Ohio Valley, Mid-Atlantic or Northeast.
The Storm Prediction Center has not yet outlined a specific severe-weather risk for the period, Hall said, but forecasters are already noting the broader setup in their extended outlooks.
In practical terms, the upper-level winds may become arranged in a way that allows disturbances to move over very warm and humid air, increasing the possibility of organized thunderstorm clusters.
Hall said that should sound familiar to people in Illinois because similar northwest-flow setups have repeatedly affected the state this summer.
He cited 209 confirmed tornadoes in Illinois so far this year, compared with a previous state record of 142 and an annual average of 54.
Hall said Illinois has become only the second state in history to reach 200 tornadoes in a single year, while Indiana has also broken its own record.
Some of those tornadoes came from major outbreaks, but Hall emphasized that many have also developed during repeated rounds of storms riding along the edge of the same general heat-ridge pattern.
Because of that history, another well-timed disturbance next week could quickly turn a hot and humid day into a much more serious severe-weather situation.
Tropical Moisture Could Surge Into The Southwest
The western side of the pattern may bring a completely different hazard, with Hall watching for tropical moisture to move north into Arizona and New Mexico around September 5 through September 7.
He said the Pacific has been extremely active, with numerous tropical systems between Mexico and Hawaii, and the larger trough-and-ridge pattern could help pull some of that moisture into the Desert Southwest.
That raises the possibility of heavier rainfall across southern Arizona and southwestern New Mexico.
At the same time, parts of Oregon, Nevada and Southern California have been dealing with critical fire-weather conditions involving dry air, strong winds, dry vegetation and active fires.
Hall said the colder air arriving roughly a week later could help ease some of those fire-weather concerns, making for another dramatic turnaround as parts of the region move from very dry, fire-prone weather toward temperatures cold enough for frost.
The rapid shift does not necessarily mean a widespread early start to fall, but it does show that the western pattern is finally becoming strong enough to challenge the heat dome that has dominated much of the summer.
Dry Ground Could Help The Heat Hold On
One reason the heat may prove difficult to dislodge is the increasingly dry ground across parts of the Central and Southern Plains and into the Mississippi Valley.
Hall said those areas have already been highlighted for a risk of rapid-onset drought, and dry soil tends to heat more efficiently than wet ground.
If the ridge remains in place long enough, that could reinforce the hotter pattern and make it harder for cooler air to spread eastward quickly.
This is where the major forecast uncertainty begins.
Hall said the American and European models generally agree on the first part of the evolution: a trough digs into the West, the Northwest cools significantly and the heat ridge is pushed farther east.
After that, however, the models begin to separate.
The European model pushes the trough farther east and gradually weakens the heat ridge, which would allow cooler air to spread into more of the central United States.
The American model keeps the trough farther west and allows the ridge to remain stronger, which would keep the South hotter and leave the Midwest closer to the active storm corridor.
September May Challenge The Pattern Without Ending Summer
Hall said official outlooks already reflect some of that uncertainty, with the extreme-heat signal easing slightly even though temperatures in the upper 90s and near 100 remain possible.
At the same time, the cold signal in the West is strong enough to appear on the Climate Prediction Center’s hazards outlook.
For several weeks, the heat ridge has been the dominant feature shaping weather across much of the country, but Hall said this incoming western trough is one of the first systems with enough strength to meaningfully disturb that pattern.
He cautioned against interpreting the first frost as a sign that fall is about to sweep across the entire country, because September still appears likely to contain plenty of significant heat.
The difference now is that the seasonal balance is beginning to shift.
Days are getting shorter, the sun angle is lowering, and each new trough has a better chance of making progress than it would have had in July.
Hall said this first cold push probably will not end summer for most of the country, but it may be the first serious test of a pattern that has remained remarkably persistent.
If the trough advances east, cooler air could finally begin cutting into the late-summer heat; if it stalls in the West, the ridge could reload, leaving the South hot and the Midwest exposed to more rounds of strong thunderstorms.
For the Great Lakes, Midwest and Ohio Valley in particular, Hall said the key will be the timing of those disturbances riding over the top of the ridge, because that is where a simple temperature swing could quickly become a severe-weather problem.
Mark grew up in the heart of Texas, where tornadoes and extreme weather were a part of life. His early experiences sparked a fascination with emergency preparedness and homesteading. A father of three, Mark is dedicated to teaching families how to be self-sufficient, with a focus on food storage, DIY projects, and energy independence. His writing empowers everyday people to take small steps toward greater self-reliance without feeling overwhelmed.
