This is not a drill, Vancouver. A full-fledged fall low-pressure system will sweep across Metro Vancouver and the South Coast on Friday.
With such an abrupt transition, they'll be some issues and dangers with such a quick start to the fall fury.
As of Wednesday, a jet stream racing towards B.C. at over 300 km/h is anomalous for mid-September, and it'll trigger an atmospheric chain reaction. It'll build an amplified pattern and fuel a deep trough that resides on the fringes of climatology for September 17th, 2021.
Friday morning: The light rainfall builds, and we're comfortably in the 'it's not so bad' stage. It's also snowing on Whistler at this time – you hear that skiers?
Friday afternoon: The core of the atmospheric river pivots across the South Coast, likely triggering some pooling and ponding on roads – with impressive rainfall rates.
Check out the hardest-hit regions: they're the usual suspect spots, getting some added totals with the help of the mountains aiding in some forced ascent:
Friday Evening: Rainfall rates will slowly ease after sunset before tapering to unsettled showers throughout the day on Saturday.
The totals are impressive for mid-September; the forecast atmospheric river happens roughly 3-10 times per year, but rarely in September.
The cold trough left in its wake means isolated thunderstorms are now possible on Saturday.
A large portion of the summer remained rain-free, creating significant stress on the trees across the South Coast. During the wicked windstorm of August 2015, 710,000 customers lost power from winds up to 90 km/h. The damage was certainly high for relatively modest wind speeds, but the dry, antecedent conditions were the primary factors.
The forecast wind gusts will remain below 70 km/h will be along the immediate coastline.
The trees also still have their foliage, which can act as a sail with strong wind gusts.
As a cool trough maneuvers over the South Coast, temperatures cool significantly with height, naturally developing instability and the threat for isolated thunderstorms into Saturday.