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A brief, but major change to autumn weather in the southern United States will be accompanied by a dose of heavy rain and thunderstorms into Monday.
A high-speed river of air at the level where jets cruise at, known as the jet stream, will soon plunge southward from Canada.
Ahead of the plunge, people can expect temperatures and humidity to reach unseasonable levels this weekend although it will not be as steamy as early October.
Moisture from the Gulf of Mexico will be thrown high into the atmosphere by the advancing cool air. The result will be a zone of heavy rain and locally gusty thunderstorms that pushes eastward.
After igniting over the southern Plains late Saturday, the band of heavy rain and thunderstorms will progress across the lower Mississippi Valley on Sunday.
A few of the storms on Sunday could turn severe with damaging winds and localized flash flooding being the primary threat.
"While most areas in the Southern and Eastern states should have no more than one stormy day, rain may be heavy enough to cause street and highway flooding," according to AccuWeather Chief Meteorologist Elliot Abrams.
People who have become accustomed to the long stretch of dry, sunny weather will have to make adjustments to their commute time and daily activities.
"In some cases, the rain will be beneficial, following a rainfall deficit that developed during the latter part of the summer," according to AccuWeather Senior Meteorologist Brett Anderson.
On Monday, rain is expected to stretch from the Ohio and eastern Tennessee valleys to the northeastern Gulf coast.
Much of the rain may fall during a period of a few hours, so there is a concern for urban flooding.
"Motorists should be especially careful where it has not rained in recent days," Abrams said.
"During the first few minutes, the mix of oil and water may be just as slippery as snow and ice and contribute to accidents," Abrams said.
By Tuesday, the heavy rain and storms are scheduled to reach the Atlantic Seaboard.
Highway and airline delays are likely to mount from Atlanta to Charlotte, North Carolina; Orlando, Florida; and Washington, D.C.
As cool air moves eastward, thunderstorms may organize into a solid line that brings not only heavy rain, but also strong wind gusts.
The most dramatic change to cooler weather is likely to be in and west of the Appalachians.
For example, in Nashville, Tennessee, highs near 80 F this weekend will be replaced with highs in the middle 50s at midweek. Highs near 80 in Charlotte this weekend will be replaced with highs near 60 on Wednesday.
Even areas as far south as Orlando can expect highs in the 80s to be replaced with highs in the lower 70s with low humidity by midweek.
Depending on where the chilliest air aloft sets up, there is the potential for some snow over the higher elevations of the southern Appalachians.
"If the pocket of cold air aloft sets up right over the Appalachians, then it may snow down to 3,000 feet," Anderson said.
"If that core of cold air sets up along the Atlantic Seaboard, then dry air may sweep in over the Appalachians and it would likely not be cold enough farther to the east to allow it to snow."
As has been the case so far this autumn, the upcoming dose of cool air is unlikely to lock in. Another warming trend seems likely during early November.
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