A strong Pacific storm will start to affect the West Coast beginning Tuesday night and lasting through Thursday. Heavy rain, very strong winds, rough surf, and coastal flooding are all likely from British Columbia down through California. Along the Oregon coast, especially, winds could gust above 70 MPH at times.
The low pressure system in question is hauling tail and strengthening as it heads towards the coast of British Columbia at this hour, with the GFS model expecting its minimum central pressure to bottom out below 970 millibars by this time tomorrow afternoon. The approaching low will run up higher pressure over the inland regions of the west, resulting in an intense pressure gradient along the coast.
The steep gradient between high and low pressure will cause very strong winds, exceeding 70 MPH along the coast and at higher elevations from northern California through Washington, with even stronger winds possible in British Columbia. These winds can cause damage in coastal communities, so just as easterners will have to prepare for the potential for power outages with the nor'easter occurring at the same time, so should folks who live along and near the Pacific.
Winds aside, the storm will bring yet another round of heavy rain to almost the entire coast, with more than three inches of rain possible from Vancouver down to Monterey, California, with up to an inch (or more) possible as far south as Los Angeles. Locations at higher elevations could see more than six inches of water-equivalent precipitation, much of which will fall as snow above the freezing level. The precipitation is great news, since most of Oregon and California are still mired in a drought that seems like it'll never end.
The drought is improving, but not by much—just a few percentage points over last week. As of December 2, 55.08% of California was in "exceptional drought" conditions, and 99.72% of the state was in any level of drought.
The region has seen beaucoup rainfall over the past couple of weeks, with 14-day rainfall totals exceeding five inches in many locations. The back-to-back storms are also helping to replenish the Sierra's crucial snow pack, which provides drinking water for many populated areas in California. The latest snowfall analysis shows that some of the higher peaks are covered with 30 to 50 inches of snow, and another one to two (or more) feet is possible by the weekend.
All of this rain is great, but too much at once does more harm than good. Heavy rain falling on drought-stricken soil too quickly can cause the water to run off and create flooding instead of absorbing into the ground and helping soothe and moisturize the soils. Flash flood watches are in effect from the Bay Area into north-central California in anticipation of storm sewers and soil not being able to handle three to five inches of heavy rain all at once. Much of western Washington (including Seattle) is also under a flood watch.
In addition to being deeply in drought (which takes into account precipitation deficits from previous years, not only 2014), many parts of California from Fresno south have seen less than 50 percent of their normal year-to-date rainfall. Los Angeles Airport has only seen 49.95% of its normal year-to-date as of this afternoon (observed: 5.59" | normal: 11.19"), while downtown Los Angeles has seen 56.8% of its normal year-to-date rainfall (observed: 7.41" | normal: 13.05").
If you live in the area and need more localized information, the region's excellent National Weather Service offices are responsible for issuing all forecasts and advisories for the oncoming system. Dr. Cliff Mass' blog is also a must-read for Pacific Northwest weather events.
[Weather model image via WeatherBELL, all other maps by the author]