Wednesday, September 26, 2007
Weather columnist Kevin Myatt: Remember when September meant fall?
Kevin Myatt is The Roanoke Times' weather columnist.
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The calendar flipped to fall Sunday, but summerlike heat is back.
Sunday's high reached 90 in Roanoke and Tuesday hit 93, breaking a 37-year-old record for the date. That makes 52 days this year and nine days this month that the temperature has climbed to at least 90. Only four other years and four other Septembers have had more 90-degree days since official temperature records began at Roanoke Regional Airport in 1948.
For those of you who prefer more fall-like temperatures this time of year, let's step back to a September that was actually cool, when leaves were turning colors because of some chilly days and not just because they were drying out and falling off.
Let's go way back to 2006.
That's right, last year.
In September 2006, there were no days that hit 90 or above in Roanoke. There were no days that hit 85 or above, compared with 11 so far this September.
The warmest low temperature in September 2006 was 63 degrees. With five days left in this month, 10 mornings have been at least that warm, and two have been 70 or above.
September 2006 was the sixth coolest such month on record in Roanoke, with an average temperature of 65.1 degrees. The month's average high temperature, 74.2, was the coolest of any September on record.
If September had ended Monday, it would have been the fifth warmest on record at 71.7 degrees. Forecasted temperatures the rest of the week are likely to keep this September among the top few warmest, but probably will not be enough to carry it past the record of 73.6 set in 1998.
Why the big difference in September in consecutive years?
It all goes to the prevailing jet stream pattern.
As August began turning to September last year, strong high pressure developed over the Western U.S., bringing extreme heat to that region. The jet stream was channeled up and over that high and then forced southward into the Eastern states, allowing cool air from Canada to settle in unusually early.
This year, the pattern is almost reversed.
Except for a few days of cool weather in the middle of the month, high pressure has been in charge over the central and Eastern U.S., keeping the jet stream in Canada. That has allowed hot air, prevalent to the extreme in August, to hang on most of this month.
The jet stream is expected to start dipping far to the south over the Western U.S., so expect to see some cold weather developing out there. The mountains in the West will probably start getting some substantial snow over the next couple of weeks.
Though we'll see the warmth edged off later this week by a cold front, October appears likely to start warm and dry in most areas of the country east of the Rockies.
It's going to take some work by the jet stream to root out the warm high pressure system that has kept returning.
But as the sun angle lessens, the days get shorter and snow starts covering the ground across Canada, summer heat will have to work harder to keep its grasp on our weather.
