Three days after San Francisco went through January without measuring a drop of rain for the first time in 165 years, it looks like a storm system roaring in from the tropics is set to produce immense precipitation on the West Coast over the next seven days. Heavy rain is likely at lower elevations, while mountain peaks could see some hefty snows.

The latest water vapor loop (above) shows moisture in the mid-levels of the atmosphere around 10,000 feet above sea level. The animation depicts a robust low pressure system pulling northeast away from the Hawaiian Islands towards the western coast of the United States and Canada. The plume of moisture accompanying the low will juice-up the atmosphere and make it rain for most folks from central California north through British Columbia.

If you live in cities like San Francisco, Portland, or Seattle, you can expect a very rainy, dreary end to your week thanks to this system...not that there's anything wrong with that, of course. The rain will start to push into the region on Thursday, cranking up through Friday and Saturday before tapering off on Sunday in most places.

This moist flow from Hawaii to the West Coast is often called the Pineapple Express (yes, that's a thing), and it's a great example of an atmospheric river, or a deep ribbon of moisture in the atmosphere that allows for the production of very heavy rain. Those of us on the East Coast usually hear about atmospheric rivers during the summer months, as they're usually responsible for those slow-moving pop-up thunderstorms that can produce many inches of rain in just a few hours.

The latest precipitation forecast from the Weather Prediction Center (which doesn't differentiate between precipitation types, mind you) shows the heaviest rain falling along the coast and lower elevations west of the mountains. The forecast shows that some areas could see rainfall amounts reaching double-digits, mainly along higher elevations where the precipitation gets an assist from orographic lift.

The region's long, dry January took a major toll on the state's water resources. The beneficial effects of December's drought-easing deluge have worn off, with exceptional drought conditions creeping back into areas that saw some relief from the double-digit rainfall totals seen a couple of months ago. As of last Tuesday, nearly 40% of California is still in the worst level of drought ("exceptional"), with more than three-quarters of the state experiencing an extreme drought.

Liquid precipitation isn't the only thing that matters in the Golden State. Much of California's water reserves are replenished by the mountains' melting snow pack, which requires, well, snow.

The drought isn't just depleting water tables and emptying reservoirs, but it's also left just a Boston of snow in the mountains. Only the highest peaks along the Sierra Nevada are covered in snow, which is far below normal for this time of the year. This morning's run of the GFS model suggests that one to two feet of snow (locally more) could fall on the highest mountain peaks in central and northern California up through Oregon, with much more falling on the ridges in Washington and British Columbia.

The storm won't come close to returning mountain snow or water levels to normal, but it's a start, and (almost) any rainfall is welcome at this point. Rainfall this heavy can be too much of a good thing, though, so if you're in an area susceptible to flooding or mud/landslides, keep an eye on alerts from your local National Weather Service office.

[Images: NASA, WPC, Drought Monitor, NOHRSC]

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