Atmospheric river could drop 3 to 5 inches of rain on southern Southeast in a day

Ketchikan, Wrangell, Petersburg and other communities south of Frederick Sound could get 3 to 5 inches of rain at sea level in 24 hours when an atmospheric river moves in Thursday, the National Weather Service in Juneau said Monday.

Areas near Ketchikan and Misty Fjords could see 4 to 6 inches, especially in the mountains. For now, forecasters said, the main threat is many rivers and streams rising to the tops of their banks, with water pooling on roads and in areas with poor drainage. They also had advice for boat owners: "Mariners, check those bilge pumps."

Flooding is possible, but forecasters are not predicting it yet. "Significant rises and flooding in rivers is possible but at this time there exists large variability in how rain develops and persists across the southern Panhandle," the hydrologic outlook said. It also covers Prince of Wales Island, Hyder, Annette Island and western Kupreanof and Kuiu Island.

Heavy rain is expected across much of the Panhandle into the weekend. A separate special weather statement covering Yakutat to Hyder said a series of low-pressure systems will bring widespread moderate to heavy rain, gale-force winds over the Gulf of Alaska and strong winds in coastal communities. The timing is still being refined, but the statement said confidence is increasing that this will be "a more impactful system for Southeast Alaska."

Atmospheric rivers are relatively narrow regions in the atmosphere that carry most of the water vapor moving out of the tropics, according to the weather service's guide. When they hit Southeast's steep terrain, the guide says, the moist air is forced upward and rainfall increases, and fronts that stall against the mountains can stretch heavy rain out to one to three days.