Dense fog that prompted weather advisories could last for days

Heavy fog forces flight cancellations in Vancouver The fog that's blanketed parts of the South Coast all day has forced officials to cancel flights out of Vancouver harbour.

Dec. 8 update: The fog advisory was extended Friday, and the weather is expected to stick around until at least Saturday afternoon. Latest information here.

Metro Vancouver commuters are being warned about the possibility of near-zero visibility due to dense patches of fog blanketing the area.

Environment Canada issued a fog advisory Wednesday night as the fog descended on the area, and renewed the advisory Thursday morning. The advisory applies to the City of Vancouver, as well as Burnaby, New Westminster, Coquitlam, Maple Ridge, Surrey, Langley, Richmond, Delta, North and West Vancouver.

The foggy forecast was caused by moisture trapped by a temperature inversion, the weather agency said.

"That's due to this ridge of high pressure that's built in for numerous nights," Environment Canada meteorologist Lisa West said.

"The clear skies we get with a ridge of high pressure allow for a maximum nighttime cooling. That's why temperatures near the surface do get so cold overnight."

The cooling at ground level and warmer temperatures at higher elevations lead to thick, persistent fog.

The weather agency warned anyone travelling to be prepared for low visibility at least until the afternoon, when the fog is expected to "become more patchy," especially in areas further inland.

Environment Canada forecasts the fog will again blanket the region in the evening, then dissipate again around noon Friday. The high for Thursday and Friday is 8 C, but in areas where fog is most dense, the temperature could be as low as 4.

A weaker front is expected to move in over B.C.'s South Coast over the weekend which could provide a break from the fog, but the ridge of high pressure will rebuild Monday, West said, bringing fog back to the region.

"We could be in for multiple days here," she said.

The city's last major fog event was in October 2013, and lasted about nine days. West said weather systems like this typically hang around for three to four days, but the high pressure ridges can "dig in (their) heels" in the fall and winter.

While the foggy conditions last, those flying into or out of the affected areas are advised to check their flights ahead of time. Helijet flights were grounded Wednesday and Thursday, and passengers were advised to call 1-800-665-4354 for more information on cancellations.

Harbour Air had to cancel some of its flights Wednesday and Thursday, and smaller planes taking off from Vancouver International Airport were grounded.

A spokesperson for YVR told CTV News the fog had cleared enough that the airport was no longer in low visibility operations as of 10:20 a.m. Thursday, and while there were some delays and cancellations earlier in the day, she did not know whether they were due to fog or other issues.

"Pilots use an Instrument Landing System which is a navigation instrument that guides pilots safely to the runway during good and bad weather conditions, and is very useful in fog," Tess Messmer said in an email.

The Sunshine Coast and Southern Gulf Islands also fall under a fog advisory.

Similar conditions are expected on the eastern side of Vancouver Island between Campbell River and Nanaimo. In the Comox Valley area, the Ministry of Environment has also issued an air quality advisory due to high concentrations of fine particulates in the air.

Open burning restrictions are in effect for parts of the valley within 15 kilometres of Courtenay City Hall.

With files from CTV Vancouver's Nafeesa Karim and Sheila Scott