The West will see warmer-than-average temperatures all week with nearly 30 record highs possible. In the East, the arrival of autumnal air will mean a temperature tumble for many big cities.

The jet stream, a highway of fast-moving air that acts as a barrier between hot and cold temperatures, will take a more extreme shape this week that will allow for the heat to remain out West and intrusions of cooler air in the East.

From the Midwest to the Northeast and down to the Gulf Coast, a dip in the jet stream is expected to send temperatures 10 to 15 degrees below average. In Chicago, Monday's high of 78 will be a stark contrast to the end of the week, when highs are forecast to top out at only around 55 degrees.

Even in Florida, the temperatures and humidity levels will drop. The Tampa metro area is expected to drop from around 90 at the start of the week down to the upper 70s for Wednesday and Thursday.

The cool down in the East will follow possible rain, showers and a few thunderstorms. Locally heavy rain could lead to flooding in the waterlogged mid-Atlantic states Tuesday into Wednesday.

Meanwhile out West, the heat will build. Nearly 30 record highs could be tied or broken this week, according to Weathermodels.com. The persistent heat with high temperatures 10 to 15 degrees above average will last all week.

Medford, Oregon, will potentially break its record highs Monday and Tuesday as it flirts with the century mark. The high temperatures will be coupled with dry air, no rain, and gusty offshore winds, which will create critical fire conditions.

This pattern of warmth in the West and cooler weather in the East is forecast to last through the first weekend of October and into the following week.

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