Heavy Rain Coming This Weekend

Our dry period is about to end dramatically.

Several plumes of moisture... popularly known as atmospheric rivers--will make landfall on Saturday, Sunday, and Monday.

Reservoirs will get substantial inflow and several rivers will approach minor flood stage.

Consider the regional accumulated precipitation through Tuesday morning (below).

Wow.  Some parts of the Cascades and coastal mountains will get as much as ten inches of liquid water.

Zooming into Washington State, one can view the profound rain shadows downstream of the terrain.  One is located northwest of the Olympic Mountains and the other one...much larger in extent...is east of the Cascades.

Precipitation picks up again as it rises on the western slopes of the Rockies.

As noted earlier, this heavy precipitation is associated with a potent atmospheric river in which warm, moist air from the subtropics pushes northward into the region.  You can see this moisture plume on Sunday at 4 AM below, with the air coming from north of Hawaii.

Associated with this subtropical plume, the freezingly level will be as high as 9000 ft over SW Washington and 5000 ft over the north Cascades (see map below at 7AM Sunday).

With the influx of warm air, most of the precipitation below roughly 5000 ft will fall as rain and not snow during the weekend and Monday.  

The snowfall total through Tuesday morning (below) indicates that the higher elevations in the North Cascades do well with snow, but Snoqualmie Pass gets very little (mostly rain)

Finally, this pulse of precipitation, coupled with a high freezing level, will cause many regional rivers to greatly increase their flows, some to near flood level.  For example, the Snoqualmie River will rapidly surge to near flood level early next week (see below).

Anyway, this event will greatly aid in alleviating the dry conditions of the past month.