A flood watch was issued Monday for Flathead, Lake and Lincoln counties as potentially record rainfall drenches the Pacific Northwest this week.

The flood watch is in place through Thursday afternoon as rain and snowmelt threaten to raise creeks, with urban flooding likely in areas with poor drainage.

“The amount of moisture carried by this system is forecast to be near all-time maximums for early December, essentially directing a firehose of warm, subtropical air into western Montana and north central Idaho,” the National Weather Service in Missoula warned in its forecast. 

The prolonged atmospheric river is expected to deliver between 4 and 10 inches of precipitation in the mountains, with 1 to 4 inches in the valleys.

“The warm nature of this system will likely melt existing low- to mid-elevation snowpack, combining runoff with rainfall to rapidly swell creeks, streams and rivers. Residents in steep terrain and near waterways should prepare for significant rises and potential flooding, as the unusual duration and intensity of this event will stress river basins far beyond typical winter norms,” the Weather Service warned.

Rainfall rates of 2 inches in six hours were reported in the higher terrain of north central Idaho on Monday night, while the Flathead River at Columbia Falls rose by about 3 inches overnight.

A second surge or heavy precipitation Wednesday will deliver an additional 2 to 5 inches of liquid precipitation to western Montana. A mix of rain and snow is possible in the Flathead Valley before snow levels rise above pass level.

The Flathead Avalanche Center raised its avalanche rating to "high" on Tuesday and advised that backcountry travel will become tricky in the week ahead.

"If you want to get a feel for conditions in the mountains over the next few days with minimal effort, ride through an automated carwash in a convertible with the top down," Flathead Avalanche Center Director Blase Reardon wrote in the Monday advisory. "You'll get the waves of moisture and the warm wind forecast through the week at mid elevations." 

Rain and snow remain in the forecast into the weekend.

"Looking ahead, confidence is increasing that anomalous moisture will persist through the remainder of the week," the Weather Service forecast stated.

However, an Arctic front is also knocking on the door. The forecast gets complicated if it creeps over the Continental Divide.

"The interaction between this dense, shallow cold air and the overriding subtropical moisture will be a critical focal point for the extended forecast," the Weather Service added.