Key Takeaways
- An atmospheric river is a long, narrow corridor of water vapour in the lower atmosphere that can dump torrential rain on B.C.'s coast.
- A Flood Warning means rivers have exceeded, or imminently will exceed, their banks; a Flood Watch means they may.
- An Evacuation Alert means be ready to leave; an Evacuation Order means leave immediately.
Rivers in the sky
An atmospheric river is a long, narrow corridor of concentrated water vapour moving through the atmosphere. These systems can transport large amounts of moisture from tropical regions, including areas near Hawaii. Atmospheric rivers originating near Hawaii are commonly nicknamed the "Pineapple Express." When moisture-laden air reaches B.C.'s coastal mountains, it is forced upward, cools and produces precipitation. The effects vary considerably depending on the storm's position, duration, intensity and local geography. Atmospheric rivers are an important part of B.C.'s water cycle, particularly during fall and winter.
Flood Watch vs. Flood Warning
The BC River Forecast Centre uses three main categories to communicate rising river risks. A High Streamflow Advisory means river levels are rising or expected to rise rapidly, but major flooding is not expected. A Flood Watch means river levels are rising and may approach or exceed their banks, potentially flooding adjacent areas. A Flood Warning means river levels have exceeded their banks or are expected to do so imminently, with flooding of adjacent areas expected. These classifications describe river conditions and flood potential. They are separate from evacuation notices issued by local authorities. The Pemberton Valley climbed that ladder in three days: a High Streamflow Advisory on November 9, a Flood Watch on November 10, and a Flood Warning on the morning of November 11. The Squamish-Lillooet Regional District then issued an Evacuation Alert for most properties in the Pemberton Valley, and a shelter-in-place advisory at midnight on November 12.
Evacuation Alert vs. Evacuation Order
An Evacuation Alert means residents should prepare to leave on short notice. An Evacuation Order means residents must leave the affected area immediately, following directions from local authorities. A Flood Warning does not automatically mean an Evacuation Order has been issued. Different agencies manage river warnings and evacuation decisions, and conditions can change rapidly. Residents in flood-prone communities should:
- Prepare an emergency bag containing medication, important documents, clothing, food, water and phone charger.
- Identify safe evacuation routes and possible accommodation.
- Monitor official emergency notifications and road conditions.
- Keep vehicles ready for evacuation where practical.
- Check on neighbours who may require assistance.
- Never walk or drive through floodwater. Anyone facing an immediate life-threatening emergency should call 911.
The Twikup Take
Atmospheric rivers are a normal feature of B.C.'s fall and winter, not a freak event — the risk is when a strong one stalls over steep mountains and rain plus snowmelt push rivers over their banks within hours. That is what happened in the Pemberton Valley this week, and it is why the province keeps repeating the same three instructions: know your alert level, have a bag ready, and never drive through floodwater. Image: Illustrative image.