Winter has gotten off to a huge start this year thanks to a weather pattern that favors big storms on the West Coast — a complete reversal from just one year ago.

This is what the snow picture looked like in late December, 2014:

The USDA measures snow pack in “snow water equivalent,” which is how much water there would be if all of the snow melted. In addition to the reservoirs, snow stores additional water that can help replenish the reservoirs during the spring and summer melting season.

On Dec. 22, 2014, the snow water equivalent across much of the West — California, Oregon and Washington, in particular — was abysmal. Just 45 percent of normal around Lake Tahoe in California, an area known for its fantastic skiing, and 20 to 30 percent of normal up and down the Cascades.

Some areas of the Great Basin were doing okay at around or above 100 percent, but areas to the south like southern Utah, southwest Colorado, and ranges in Arizona and New Mexico were running well below average.

This year has been an entirely different story. The Sierra Mountains, southern Oregon and Idaho are buried in snow. Some regions are even reported twice as much snow this year as they normally get.

Statewide, California snowpack is an incredible 111 percent of normal for this time of the year, while last year it was just 56 percent, the National Weather Service in Sacramento reported on Twitter.

The central Sierra Mountains have seen the most snow in the state so far this winter, and are now running 121 percent of normal. Squaw Valley, a ski resort near Lake Tahoe, reported three feet of snow in just 24 hours on Tuesday.

The West can thank a few different things for its boom season. The December weather pattern has completely reversed since last year — instead of a “ridiculously resilient ridge” parked over western North America, it has shifted to the East Coast, which has been running unusually warm this month.

That has left the door open for huge troughs of low pressure to swing over the North Pacific into the West. These storms have featured something very important for big rain and snowfall totals: a strong atmospheric connection to the deep moisture of the tropical Pacific — what meteorologists call an atmospheric river.

These atmospheric river events can happen with or without a strong El Nino in place, though this year’s El Nino is certainly adding plenty of warm, moist air to the equation.