Atmospheric rivers that have been lashing the Pacific Northwest of the United States have prompted winter storm warnings in 11 states, with more heavy rain, snow, and strong winds predicted in the days ahead.
The warnings are currently in effect for parts of Colorado, Idaho, Maine, Nevada, New Hampshire, New York, Oregon, Utah, Vermont, Washington, and Wyoming.
An atmospheric river is a narrow space in the atmosphere that transports significant amounts of water vapor. When they hit land, the vapor is released through rain, snow, or a combination of both. According to the National Oceanic and Atmospheric Administration, atmospheric rivers can vary significantly in size and strength.
On average, they carry roughly the equivalent of the water flow through the mouth of the Mississippi River. More powerful ones can transport up to 15 times more water vapor.
AccuWeather meteorologists predict that at least through Dec. 6, locations across Washington, Oregon, Northwest California, northern Idaho, and western Montana might face even more wild weather due to an atmospheric river overhead. An additional 1 to 2 inches of rain in most affected regions is being forecast. Coastal areas are expected to be hit with far more rain; a further 4 to 8 inches is being predicted.
Flood watches and advisories are in place for numerous counties across western Washington and Oregon through late on Dec. 6 due to the steady stream of rainfall that will continue to spread inland.
“Both cities are expected to receive an additional 2 to 4 inches of rain through [Dec. 6]; the historical average rainfall for the month of December is 5.72 inches in Seattle and 5.77 inches in Portland.” AccuWeather meteorologist Elizabeth Danco said.
“Looking ahead to later this week, a storm moving into the Northwest can bring colder air back into the region on [Dec. 7], allowing for temperatures to drop closer to the historical average,” she said. “However, the stormy pattern should shut off.
“Mainly dry conditions are expected to continue on [Dec. 8], but another round of rain and mountain snow can sneak back into the Pacific Northwest by late ... into [Dec. 9].”
AccuWeather says that while the expected precipitation might cause travel disruptions, road closures, and flooding, it could gradually help improve the drought conditions.
Increased Avalanche Danger
Snow levels in the northwest mountains are also expected to rise significantly before warmer conditions set in early this week. According to Ms. Taylor, this brings some risk of flooding as well.
“Rising temperatures through Wednesday can lead to snow melting, especially in the Cascades where over a foot of snow has fallen,” she said.
“The snowmelt, in addition to the heavy rain Monday and Tuesday, can lead to flooding concerns and dangerous travel, especially in the mountains and foothills.”
Heavy rain falling in the mountains as temperatures rise could cause the snowpack on steep mountain slopes to become more unstable, increasing the risk of avalanches. The Northwest Avalanche Center has issued avalanche warnings for the slopes of the Washington Cascades, Mount Hood, Stevens Pass, and Snoqualmie Pass.
California Drought-Free
Most of these areas are familiar with massive rainfalls over a short period, especially from November to January. California, in particular, has been grappling with atmospheric river floods all year. Starting in March, forecasters warned that most of California was facing another atmospheric river storm system that would likely produce heavy rain.
