Shrouds of smoke from wildfires in the Pacific Northwest again are congesting the skies over the Philadelphia region — and much of the nation — and they are forecast to persist for a good part of Wednesday.

While smoky haze from distant wildfires isn’t all that unusual, said Tom Kines, senior meteorologist with AccuWeather Inc., “to have it this thick, it doesn’t happen very often.”

The veil of smoke has dimmed the sun the last two days, and on Tuesday night painted the moon blood orange.

“It’s an extraordinary event,” said Sean Greene, air quality programs manager with the Delaware Valley Regional Planning Commission.

The region was under a “code orange” advisory for air quality Wednesday and had bumped into the “code red” more-dangerous level at times on Tuesday and could do so again Wednesday, he said. The region hadn’t had a code red event in three years, he added.

The primary pollutant was ultra-fine particulate matter, he said, the fallout of smoke from the western fires.

Health officials were advising those with heart or lung issues to eschew strenuous outdoor activity. However, the densest concentrations of the smoky veil would remain in the high atmosphere, meteorologists said.

A strong front, which might ignite severe storms Wednesday afternoon, was expected to launder the skies and return the blue by Thursday. The government’s Storm Prediction Center has posted a severe-thunderstorm watch for the region until 8 p.m. Wednesday.

Canadian fires on Wednesday were concentrated along the British Columbia-Washington border, and the massive “Bootleg Fire” was engulfing close to 400,000 acres of woodlands in south-central Oregon, forcing evacuations, the Northwest Interagency Coordination Center reported.

Prevailing upper-air winds were exporting the smoke all the way to the Atlantic seaboard.

The smoke was stacked in a roughly two-mile layer, five to seven miles above the surface, said Patrick O’Hara, meteorologist with the National Weather Service in Mount Holly.

If you think you’re smelling smoke, you’re not necessarily hallucinating: A subtle odor might be detectable on occasion, he said, as particles could mix down close to the surface.

Wildfire smoke made the 2,500-mile journey to the East Coast as recently as two weeks ago, but it was far less substantial than this refill.

» READ MORE: That persistent milky sky over Philly is smoky haze from the Pacific Northwest wildfires

And while the extent of the Bootleg wildfire and the smoke plumes has been impressive, it evidently hasn’t had the impacts of recent deadly fires in more-populated areas.

Oregon is one of the nation’s largest states — bigger than the combined areas of Pennsylvania, New Jersey, Delaware, Maryland, and West Virginia — and has vast unpopulated regions. New Jersey has 30 times more people per square mile than Oregon.

.

Locally, in addition to fouling the air, the smoke might be shaving a degree or two from the temperature, said AccuWeather’s Kines.

But the front due to broom through the region later Wednesday was expected to do some more-significant temperature shaving, along with routing the humidity and perhaps setting off fresh rounds of thunder and lightning.

Thursday the air mass should be so pleasant — low humidity, highs in the mid-80s — you might think you moved. And you might even see blue in the skies.