Portland’s weather will be stuck on broil for much of the next week and Western Oregon may be about to experience the longest stretch of 100-plus degree temperatures on record -- a situation meteorologists say could have knock on effects for the entire summer and bodes ill for fire season.

The key messages from the National Weather Service’s latest forecast are rather dire: “Extremely dangerous...intense, widespread and prolonged duration…increased risk of heat related illness.”

Hydrate. Hydrate. Hydrate.

So is this a heat wave? A heat dome? Both? And what the heck is an omega blocking pattern anyway?

Meteorologists say we’re certainly in the midst of a heat wave, which can have multiple causes and simply refers to an extended period of higher-than-average temperatures.

With a high pressure system parked over most of the West Coast, Portland temperatures are expected to build from an expected high just shy of 100 degrees on Friday to 104 degrees Tuesday, according to the National Weather Service. The average daily high in Portland for July, meanwhile, is 82 degrees.

Nighttime won’t provide much relief, with temperatures expected to remain in the high 60s to 70 degrees, compared with lows averaging 57 degrees.

“The temperatures compound on each other,” said David Bishop, a meteorologist in the agency’s Portland office. “When it doesn’t cool down at night, it’s much easier to heat up to 105.”

Models show some cooling midweek, but not much.

“The temperature is not going to go down a lot,” said Larry O’Neill, state climatologist and an assistant professor at Oregon State University. “It will be in the mid to upper nineties continuing through next 10 days to two weeks.”

So is this a heat dome? Opinions vary.

Cliff Mass, a professor of atmospheric science at the University of Washington, says it’s a fairly new term popularized by the media that is not very descriptive of what’s going on. “Everyone is using it but it’s pretty meaningless.”

Kelley Bayern, a meteorologist for KOIN 6 News, said not every heat wave is a heat dome and she wouldn’t want to overuse the term.

But “this is potentially deadly heat and I want to get people’s attention,” she said.

O’Neill says it’s a legitimate term that he uses too, and is apt in the current pattern.

Regardless, all three agree on what that pattern is: A massive high pressure system in the upper atmosphere – at about 18,000 feet - with sinking air that compresses and warms as it goes down, supercharging the surface temperature readings.

They also agree that this pattern has set up as so a so-called “omega blocking pattern,” an unusually stable high-pressure ridge sandwiched between two lows to the east and west, which looks like the uppercase Greek letter omega (Ω) on a weather map. That forces the jet stream to the north, and impedes the normal marine flows that might otherwise push the high pressure system to the interior.

“This one is so stagnant it’s not moving anywhere for days,” Bayern said.

Tropical storm Beryl, which is expected to return to hurricane strength as it moves toward the Texas-Mexico border this weekend, may also factor in how quickly the high-pressure ridge moves on and the weather here cools, O’Neill said.

“It’s a wildcard in there that could block the movement of the high-pressure system,” he said.

Another worrisome aspect of a prolonged heat wave this early in the summer is that the evaporative effects could exacerbate wildfire risks all summer long as Oregon enters its dry season.

Just two weeks ago, the Oregon Department of Forestry issued a three-month forecast. It said Oregon had officially moved out of drought conditions, and called for relatively warm weather from July through September, “but without extreme/prolonged heat.”

“That’s all gone right now,” said O’Neill. “The heat and low humidities are causing huge evaporation of soil moisture. The fuel moistures are going to be really low. Everything is curing and drying out, so this will be a very interesting fire season.”

He added that the dry soils exacerbate the heat in general, compounding the effects of global warming.

“This is shaping up to be the hottest July we’ve had,” he said. “It’s something we’ll be looking at very closely.”

- Ted Sickinger is a reporter on the investigations team. Reach him at 503-221-8505, tsickinger@oregonian.com or @tedsickinger

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