An Arctic ridge along the eastern foothills of the Rockies will dominate the weather through Friday, maintaining cold and dry conditions over B.C. Strong northeast outflow gradients along the coast will persist through Thursday, then begin easing Friday.
The upper level pattern is a little unusual for an Arctic outbreak with an upper ridge flopped over into Alberta and an upper trough from Haida Gwaii to the south coast.
The upper trough will slowly pivot over the next couple of days to lie east – west along about the BC-WA border by Friday. The GLB and the ECMWF show an upper low forming in the trough just south of the Lower Mainland late Thursday and persisting Friday. Most of the associated moisture is expected to remain south of the trough so the South Coast and Southwest Interior should remain dry in this scenario. There is some uncertainty, however, as the GFS depicts the upper low and trough further north than the GLB or ECMWF.
The model details and the timing for the weekend are still uncertain. The GLB and ECMWF keep the qpf from a weak leading Pacific impulse on Saturday over WA and OR state while the GFS just brushes southwest BC with light qpf. There is near-agreement that another weak-ish Pacific system will give light qpf to southern BC on Sunday. At this point it appears that the full transition back to warmer maritime air will occur when a much stronger Pacific system arrives Monday.