Kevin Myatt is The Roanoke Times' weather columnist.

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It got to 91 degrees on Friday in Roanoke.

That is usually not a landmark event in a typical summer. But it was Roanoke's hottest high temperature of summer 2004, the first time it had gotten above 90 degrees.

Our "heat wave" was short-lived -- more a warm ripple than a heat wave -- as a cold front swooped in, rained some on our Saturday and then cooled us off for Sunday and today.

But that little burst of heat could be a precursor of how our August will end.

A high pressure ridge is building aloft over the eastern United States. It's the kind of development that, were it mid-July rather than late August, might portend a substantial heat wave. This late in the season, it might gives us at least a few summery days before meteorological summer ends on Aug. 31. (By the astronomical calendar, it drags on to Sept. 21.)

Historically, you can't write off summerlike weather yet just because we haven't had much of it so far. Records show that Roanoke Regional Airport has been 90 or above as late as Oct. 6 -- 91 in 1951, back in its Woodrum Field days. It's been 100 or above as late as Sept. 5 --101 in horribly hot 1954, a year when it also got to 89 as late as Oct. 13.

But while we could still see some hot weather, a prolonged late-arriving heat wave does not appear likely. Those early 1950s Septembers followed hot, dry summers that left the ground parched and the vegetation withered.

Also, it's doubtful the weather pattern will stagnate now like it would need to for a late heat wave. In fact, this shift from our stable summer pattern may itself be a harbinger of fall.

A upper-level low pressure system is crashing ashore in the Pacific Northwest this week, bringing cool, wet weather to an area that has had lots of unseasonable heat and drought. It's breaking through the high pressure dome that has been in control of that region this summer, the same high that has in turned helped funnel unseasonably cool air into the southward dip in the jet stream over the central and eastern U.S.

Just like you can do by snapping a garden hose, this downward bend in the jet stream over the Pacific Northwest is springing an upward turn farther east, hence our East Coast high.

But just as a bulge in a hose travels down its length, so will the high translate eastward. It's expected to slip out to sea later this week. The result will be that it will start turning the winds out of the southeast aloft on its clockwise circulation, and this will start building moisture later in the week for showers and thunderstorms. While it will become plenty sticky, the clouds and daily showers will inhibit any kind of really intense warmup. And the Pacific Northwest storm may affect us by the weekend or early next week.

When ripples in the jet stream travel across the country with regularity, it is called a progressive jet stream pattern. Though uncommon in winter when Arctic air masses can get lodged in one part of the country or another, and even more rare in summer when heat-dome high pressure areas can bring the weather pattern to a standstill, a progressive jet stream pattern is a common feature of spring and fall.

Storm systems enter in the west, cross the country and exit east as a new one enters the west. Sometimes it even gets to be such a steady cadence where one can expect rain, say, every 3 days, followed each time by a cool, dry day and then a warm, more humid one.

We're not quite to fall or a fall-like weather pattern yet, even though many mornings have felt just a shade above frosty lately. But this is certain: the days are getting shorter, the sun angle is getting lower. The sun's heat is less potent with each passing day.

That's a progressive pattern that will continue whatever the jet stream is doing.