Get out your Hawaiian shirt and your umbrella, we're in for a Pineapple Express.

A gigantic river of warm, moisture-rich tropical air will bring torrents of rain this weekend.

The front is so named, University of Washington meteorologist Cliff Mass says, because it travels from Hawaii to the Puget Sound. Warm air holds moisture better than cool, Mass says, so when the balmy front hits Washington's chillier temperatures, that moisture is released — and we get very, very wet.

"There's going to be rain, and it's going to be a lot," said Weather Service meteorologist Johnny Burg, sizing up a system that has triggered flood watches for every county in Western Washington.

Intense rain is expected to hit late Saturday and continue into Monday, dumping close to 2 inches on Seattle, and more in outlying areas.

Expect spotty urban flooding. And at least several rivers likely will spill their banks.

The heaviest rain in Seattle is expected to occur south of the Lake Washington Ship Canal, according to a Friday news release from Seattle Public Utilities (SPU). While predictions say Paine Field in Everett will see only 0.6 inches of rain in the 72-hour period ending at 4 a.m. Tuesday, Boeing Field is expected to see 1.8 inches and Sea-Tac Airport 2.2 inches, Mass said.

A Weather Service alert describes the coming system as "a long plume of warm, moist air with subtropical connections ... aimed at Washington from Saturday night through at least Sunday night."

Heavy rain is expected to subside by Monday, but rain remains in the forecast each day through the middle of next week.

Melting, flooding

In the Cascades, the heavy snowfall of the past few days is expected to give way to rain by midday Sunday, with pass temperatures rising into the 40s and the snow level climbing to 7,000 feet.

Around Seattle and King County, preparations were under way Friday for the onslaught. Hundreds of pre-packed sandbags are at Meadowbrook and Delridge community centers; residents who live in flood-prone areas can take up to 25 of them to protect their homes. Extra Seattle Public Utilities crews will work Saturday and Sunday, SPU said.

Outside the city, "some rivers seem certain to flood," according to a Weather Service advisory, "including the Skokomish in Mason County, the Tolt in King County and the Puyallup River near Orting in Pierce County."

Rivers with at least a modest possibility of flooding include the Nooksack, Skagit, Stillaguamish and Snoqualmie flowing off the North Cascades and several rivers in Southwest Washington.

A "flood watch," according to the Weather Service, "means conditions are favorable for flooding, but flooding is not imminent or occurring."

If the threat becomes more substantial, a "flood warning" is issued. Just in case of major flooding, the Army Corps of Engineers has placed all its Western Washington flood teams on alert for the weekend.

Perfect timing

The city of Mount Vernon may not need the Army Corps' help — in a demonstration of eerily perfect timing, it finished construction Thursday on a flood wall designed to protect the entirety of downtown from river damage.

The Weather Service warns of urban flooding that could swamp roads and neighborhoods, especially where drainage is poor. Motorists are reminded not to drive through water covering roadways, a common cause of flood fatalities. Those who live on a floodplain are urged to prepare for disaster and consider buying flood insurance. SPU recommended that residents clear out any debris blocking drains on their neighborhood streets. County residents can subscribe to flood alerts at www.kingcounty.gov/flood.

Though flood preparations are in place all over the Sound area, let's be perfectly clear: This isn't rainmageddon.

"This is the thing we get once or twice in the typical season," Mass said.

Locals shouldn't expect the kind of epic damage that followed floods in November 2006 and December 2007, when "there were 10 to 20 inches of rain in the mountains," Mass said.

"Trails were taken out. Bridges were taken out. There are a lot of places that haven't recovered yet. We won't see that this weekend."

Jill Kimball: 206-464-2108 or jkimball@seattletimes.com

Jack Broom: 206-464-2222 or jbroom@seattletimes.com