OREGON STATE UNIVERSITY

It’s a heat wave – but 90-degree days in September aren’t that uncommon

09/07/2011

CORVALLIS, Ore. – Temperatures in the 90s – and threatening to hit the century mark – are forecast for the next few days, prompting many Oregonians to grumble about August weather during what should be  post-Labor Day nirvana.

But in reality, it isn’t that unusual for summer to hang on, even when the months start ending in “r.”

Kathie Dello, deputy director of the Oregon Climate Service at Oregon State University, says the high temperatures forecast for most of Oregon this week are the result of a high-pressure ridge sitting over the region. And yes, temperatures will be higher-than-normal, she says, but that does happen.

“Portland averages about 11 to 12 days a year of temperatures that hit 90 degrees or better,” she said. “Most are in July and August, but sometimes we get 90-degree days in June or September, too. Typically it is in the high-70s this time of year, but we do get the streaks of hot weather.”

In fact, the latest it has ever been 90 degrees in Portland was in 1980, when it reached that mark on Oct. 5. Corvallis has an even later date for hitting 90 – Oct. 11, which happened in 1936.

What makes this somewhat unusual is that much of the spring and early summer was cooler and wetter than normal, Dello said. Then starting in mid-July, summer arrived and hasn’t let up much.

“Medford has had no measurable precipitation since July 18 and hadn’t hit 100 degrees yet this year, though it should get there this week,” Dello said. “It usually hits the century mark about nine times a year in Medford, and gets to 90 degrees an average of 53 times a year.”

Normal highs for Medford this time of year are in the mid-80s. Forecasts call for the mercury to climb above 100 this week.

Much of western Oregon, in fact, has been dry this year, points out Dello, whose Oregon Climate Service office is affiliated with OSU’s College of Oceanic and Atmospheric Sciences.  Portland has had a mere .17 inches of rain in August, while Corvallis has had just .03 inches since July 19 – all recorded on Aug. 25.

“The tough part about the late-summer dry spells is the increase in fire danger,” Dello said. “The National Weather Service has a red flag fire warning for the Cascade Mountains and western Cascade foothills this week.”

The temperature records for this week are in the high 90s throughout much of western Oregon (and over 100 in Medford), and those records may be in jeopardy. And dry weather is forecast for the next several days, increasing fire danger.

“The only clouds we’re getting these days tend to be low, marine-influenced stratus clouds that keep it cool in the morning, but dissipate by the afternoon, when it begins to heat up,” Dello said. “Other than one or two thunderstorms, we haven't had anything convective that would produce rain.

“But summer will eventually transition into fall,” she added. “And we may be longing for the warm, dry weather again.”