Chilly curtains of fog could loiter in the Rogue Valley until early next week, the National Weather Service in Medford says.
With that comes visibility of a quarter-mile or less, stagnant air and some light ice slicks on the roads. Exactly when the ground clouds will start to push on is anyone's best guess.
"That's the question of the month," meteorologist Brian Nieuwenhuis said.
The weather service extended its freezing fog advisory for the Medford area through 10 a.m. today. Thinning will continue during the late morning and afternoon hours before nighttime fog floats back into the valley floors.
A low-pressure system is expected to come through the area Wednesday, which could result in some thinning through Thursday, but not enough to push out the stagnant air completely.
"Most likely (it) will not be strong enough to clear us out. That will not happen," Nieuwenhuis said, adding the diluted fog likely will thicken back up after the system passes. "It's very uncertain as to when and where (the system) will go, but if it tracks over us, we might break out for a day or two."
The lingering inversion will limit mixing of the air, trapping pollutants. By Monday, Medford had endured a six-day streak of "moderate" pollution levels, according to Oregon Department of Environmental Quality data. Weather officials encouraged anyone with sensitive respiratory conditions such as asthma to limit outdoor activity.
The majority of the stale air and come-and-go fog has been seen in the Medford area and parts of the Illinois Valley. Weather officials said towns higher in elevation, such as Ashland and Grants Pass, have seen relatively quick periods of clearing during the morning hours.
But the gray won't last forever. The air could see a permanent shift back to normal starting next week.
"The long-term models, and we're talking like the end of the month, do show a pattern shift," Nieuwenhuis said, adding that shift could bring some much-needed precipitation to the area. "They change a lot, so that remains to be seen, but there is hope."
Reach Mail Tribune reporter Ryan Pfeil at 541-776-4468 or by email at firstname.lastname@example.org.