Kevin Myatt is The Roanoke Times' weather columnist.

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It won't get to 100 in Roanoke this summer.

OK, so that's not really a huge limb to go out on, considering it is already Aug. 21 and it hasn't gone higher than 92 yet.

But there is an atmospheric shift in the works that will just about eliminate any chance of an extreme summer heat wave in Southwest Virginia for 2009. It might even give us an early taste of fall.

The pattern that is developing is actually a return to that which kept July unseasonably cool, bringing unprecedented midsummer chill to several states just to our northwest.

Throughout most of July, the nation's hottest air banked in the West. The jet stream was rerouted up and over that dome of hot air, diving southeast into the East. The jet stream was blocked from its typical summertime west-to-east glide across Canada by strong high pressure near Greenland.

That brought unseasonably cool air from Canada into the Upper Midwest, Great Lakes and throughout much of the East.

A cold front arriving today or early Saturday, will reintroduce that cooler regime to the same areas it controlled earlier this summer.

Today should be the last day of the recent sticky tropical pattern as a strong cold front presses toward the region. The cold front will squeeze out another round of showers and thunderstorms today, some of which could produce heavy rain or strong winds. A few showers may linger into Saturday.

The advancing cold front will also deflect Hurricane Bill away from the East Coast, though there's a chance it might scrape New England.

By early next week, highs will struggle to reach 80 and most Southwest Virginia locations will see lows in the 50s.

This general weather pattern, with a few ups and downs on the thermometer (and maybe even a brief hot spike near 90 late next week), appears likely to hang on for at least the next couple of weeks, taking us into September, the start of meteorological fall.

So by the time it might let go, we would be already very much on our downward slide into fall, as the days continue to shorten and the sun angle lessens each day.

Summerlike heat waves are not unheard of in September and even October, but autumn hot spells are generally more severe if they have been preceded by a hot, dry summer.

It has never been 100 or above in Roanoke after Sept. 5, though it was 99 as late as Oct. 6 in 1941. We had a run of record-setting low 90s heat on Oct. 7-9 just two years ago, but that followed a record-hot August and warmer-than-normal September.

So the coming cool pattern will be a reflection on a summer that has been far from hot and a reminder of an autumn that is close at hand.

Weather Journal appears on Monday, Wednesday and Friday.