Winter Storm Kori Bringing an Ice Storm to the Northwest, More Feet of Sierra Snow and California Rain

Linda Lam
Published: January 17,2017
Winter Storm Kori will pound parts of the West with mountain snow, ice and flooding rain this week, as another atmospheric river sets up over the region.
(MORE: Winter Storm Central | How Winter Storms Are Named)
An ice storm warning has been issued for the Portland, Oregon, metro area, as well as for parts of the Columbia River Gorge and the northern lee slopes of the Washington Cascades, including the cities of Moses Lake and Wenatchee, Washington, for potentially damaging accumulations of ice.
Freezing rain advisories for lighter accumulations of ice have been posted for portions of northeast Washington and the chimney of Idaho, including Spokane, Washington.

Winter Alerts: West

Portland, Oregon, reported 0.35 inches of ice Tuesday afternoon, and freezing rain was still ongoing early Tuesday evening. Troutdale, Oregon, about 15 miles east of Portland, reported 0.49 inches of ice.
Ice accretion of 0.10 to 0.25 inches has been reported across portions of Washington, including 0.25 inches in Vancouver.
Additionally, flood watches have been posted in parts of western Washington and western Oregon, including the Willamette Valley.
On the heels of a period of flooding and prolific snow last week, another deep plume of moisture known as an atmospheric river has pushed ashore along the West Coast.

Pacific Atmospheric River Watcher

Kori will then slide down the West Coast and push into the Rockies, raising the threat of flooding rain, more heavy mountain snow and, in some areas, more freezing rain and sleet.
Let's get into the particulars of this forecast, starting with timing, then with potential flooding, snow and ice impacts.

Tuesday Night

• Wind-driven precipitation continues in the Northwest, from northwest California to Oregon and Washington. 
• Snow levels will rise up to at least 7,000 feet in the Cascades, resulting in snowmelt. This snowmelt, combined with any heavy rainfall, will increase the chance for flooding.
• Freezing rain is expected in the Columbia River Gorge, upper Hood River Valley and the far northern Willamette Valley. Cold air trapped in other areas of eastern Washington and the chimney of Idaho may lead to sleet or freezing rain, there.
• Precipitation has changed over to rain in much of the Willamette Valley, but northern areas, including Portland, Oregon, likely won't warm above freezing until after midnight.

(FORECAST: Seattle | Portland, Oregon | Medford, Oregon | Spokane, Washington)

Tuesday's Forecast

Wednesday

• Moderate to heavy rain and snow will spread farther south and east into northern California.
• Some rain and mountain snow should reach southern California Wednesday night.
• Snow levels will start high in the Cascades, but will begin falling later.
• Snow levels should fall to 3,500 to 4,500 feet in the Sierra Wednesday night.
• Windy conditions are also expected with the cold front's arrival, with some tree damage and power outages possible.
• Snow will spread into parts of the inter-mountain West and and northern Rockies.
• Parts of the Great Basin, from eastern Washington to eastern Oregon, Idaho's "chimney" and the Snake River Plain may see freezing rain and/or sleet.

(FORECAST: San Francisco | South Lake Tahoe | Portland, Oregon | Boise, Idaho)

Wednesday's Forecast

Thursday

• Showers will continue in California and the Northwest, but most heavy rain should be over, temporarily.
• Snow levels will lower to between 2,000 and 4,000 feet in the Cascades and Sierra.
• Pockets of heavy snow are expected from the northern Rockies to the Four Corners higher elevations.
• Winds will be gusty once again, especially along the coast and into parts of southern California.

Thursday's Forecast

Friday

• Another strong Pacific front, a second phase of Winter Storm Kori, will surge into the West Coast.
• Unlike the first phase of Kori, this second front will have low snow levels and send most of its rain and mountain snow into central and southern California.
• Strong winds are likely to accompany this front, as well.
• Light to moderate snow will continue with the first phase of Kori Friday in the Rockies from Montana to New Mexico.

(FORECAST: Los Angeles | San Diego | Reno, Nevada

Friday's Forecast

Saturday

• Pockets of snow, possibly heavy in spots, will persist in the Four Corners, mountains of southern California, Sierra, Siskiyous and Cascades.
• Some light to moderate snow, associated with Kori's first phase, may blanket parts of the Dakotas.

(FORECAST: Flagstaff, Arizona | Salt Lake City | Albuquerque | Pierre, South Dakota)

Saturday's Forecast

If that wasn't enough, yet another powerful Pacific storm lurks behind Kori for Sunday with more rain and mountain snow, including in southern California.
(MAPS: 7-Day U.S. Rain/Snow Forecast)

How Much Ice?

The greatest chance for accumulating ice will be in the Columbia River Gorge, the northern Willamette Valley, including portions of the Portland metro area, and in western parts of the Snake River Valley. At least a glaze of ice is also possible in the rest of Idaho's Snake River Valley, including Boise, the leeward slopes of the Washington Cascades and in eastern Oregon.
Slippery roads, not just bridges and overpasses, are likely.
In the ice accumulation forecast below, the following definitions are used:

• Glaze: Hazardous travel, spotty power outages
• Damaging: Some tree damage, more numerous power outages

Potential Ice Accumulation

How Much Rain/Snow?

Contrasting with last week, heavy rain will also fall below snow level in western Washington and western Oregon, in addition to parts of northern California. Fortunately, snow levels in the Sierra won't nearly be as high with Winter Storm Kori, and the atmospheric river won't nearly be as potent to trigger the kind of flooding we saw last week.
Despite that, a swath from western Washington to northern California, including the Sierra foothills below snow level, will likely see at least 3 inches of rain through Saturday. Much higher amounts are likely in the coastal ranges and windward slopes of the Olympics.

Rain and Snow Forecast

Water levels in most of northern California have dropped, and given the recent storm track and cold weather, water levels in rivers and streams in western Washington and northwest Oregon are below normal for mid-January.
However, that steady, at times heavy, rain is likely to trigger at least some river flooding the next several days on the most flood-prone rivers.
There is also concern for rain falling on areas of existing heavy snowpack, including in the Portland, Oregon, metro area, where up to 15.5 inches of snow fell from Winter Storm Jupiter.
Fortunately, Kori won't dump another 10-plus feet of snow in the Sierra, but another 1-2 feet are likely through Saturday.
Other parts of the West, including the Cascades, mountains of Idaho and northeast Oregon, southern California mountains and the high country of the Four Corners may pick up a foot of snow through Saturday. This includes parts of Arizona's Mogollon Rim and the Grand Canyon.
If all that wasn't enough, high, possibly locally damaging, surf is expected along parts of the California coast into early next week.

A "Mixed Blessing" of Drought Relief

Last week's heavy rain and feet of Sierra snow effectively wiped out the California drought north of Interstate 80, which had been in place, there, since Dec. 20, 2011.
A big player in California's water supply is spring snowmelt of the Sierra's snowpack. Fortunately, thanks to the recent barrage of storms, the water content of the Sierra snowpack is now almost double the average for mid-January.
Furthermore, reservoir levels in most of the state's major reservoirs are near or above average for this time of year.
However, parts of southern California remain in an extreme to exceptional multi-year drought.

Current Drought Status

Drought relief in southern California is more tricky, often posing a Catch 22.
While some rain is good to moisten soil and vegetation to reduce any fire risk, a wet winter season can fuel the growth of brush that can serve as fuel for wildfires during the summer/early fall dry season.
Furthermore, heavy rain in southern California has two additional impacts:

• Flash flooding triggered from rapid runoff over impervious surfaces in the L.A. Basin's "concrete jungle."
• Debris flows over recently-burned wildfire scars. 

The rain rates needed to trigger debris flows in southern California, according to the U.S. Geological Survey, are as low as 0.2 inches per hour.
So, the reality of the situation is, unless the storm's water ends up in a reservoir, one could argue that despite the drought this upcoming barrage of storms may be more headache than helpful for southern California.

MORE: Flooding in California and Nevada (PHOTOS)