Mostly splendid early autumn weather. Except for the Pacific Northwest and Florida.

In many cases during the first half of fall, the introduction of stronger upper westerlies will lead to widespread fair and pleasantly warm weather in much of North America. This is because the flow aloft is both downslope and continental, and chances for storms are marginalized.

If you live east of the Rocky Mountains you will be hard pressed to find anything resembling cold air. Even calling it cool will be a stretch, except perhaps in Canada, where nocturnal drops in temperature are steeper due to the overall dryness of the atmosphere. The benefit is abundant sunshine, great for sportsman, late harvests, and football games.

The proverbial “outsiders” in this pattern are the Pacific Northwest (nearness to the Gulf of Alaska Low) and Florida (slow approach of a TUTT signature from western Cuba). In the first case, the stretch from British Columbia southward to coastal California will be visited by cold fronts, clouds, showers and damp/chilly conditions. As for the Sunshine State, that upper disturbance could dump a lot of rain (thunderstorms involved), and may still be around ten days from now.

When will the cooler air start to move across North America? With the Madden-Julian Oscillation percolating in Phases 7 and 8, and a broad Kelvin wave entering the disturbed area from Southeast Asia, amplification of the jet streams is likely in about two weeks. Before that time, we are stuck with the cool West/warm Central, East alignment with strong spikes of hot weather in the south central states. If a severe weather event develops in the Great Plains and Mississippi Valley in mid-October, it will be time for a cool change.

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