On an annual foundation, Seattle will get extra precipitation than Portland (39.34 versus 36.91 inches).
However there are sometimes durations, notably in Spring, when Seattle is significantly drier.
The very fact Seattle is usually wetter is smart: the jet stream and wettest circumstances are usually discovered over the northern portion of the U.S. West Coast and in British Columbia. Seattle is within the primo meteorological “hose” extra typically.
However at instances, Seattle is drier. And it seems that the Olympic Mountains are a giant issue.
When the winds are from the west, Seattle is within the rainshadow of that enormous mountain barrier, whereas Portland is downstream of a lot decrease coastal mountains, which permit extra precipitation to go (see terrain map under).
And it occurred this week.
Contemplate 6 AM on Friday. Beneath is the radar picture at the moment. A number of showers coming in from the coast. The showers had been enhanced as moist, unstable air was pressured to rise by the western slopes of the Olympics. However because the air descended on the jap aspect of the Olympics, it warmed and dried, producing a profound rain shadow over Puget Sound nation.
Because the air is pressured to ascend by the western slopes of the Cascades, extra showers developed.
Portland was not so fortunate, with the far decrease coastal mountain permitting Pacific showers to achieve the Rose Metropolis.
And simply to show the wind route was westerly (from the west) that morning, right here is the vertical sounding from the balloon-borne radiosonde at Quilayute, on the north/central Washington coast. The wind barbs on the correct present the route and velocity. The heights are in stress: 850 is about 5000 ft. Westerly winds confirmed.
Throughout mid-winter, the coastal winds are usually southerly or southwesterly within the decrease environment. Because of this, the Olympic rainshadow rotates to the north and northeastern aspect of the Olympics, offering drier circumstances from Port Townsend to Sequim That’s the reason people retire there! A number of dry days to play golf or take a stroll.
However within the late spring, the coastal winds within the decrease environment flip extra westerly, inflicting the Puget Sound area to be positioned within the Olympic rainshadow.
Now the northern a part of this rainshadow typically will get some rain from a Puget Sound convergence zone, however that function not often will get to SeaTac Airport.
Thus, throughout such westerly springtime circulate, we’d anticipate SeaTac to be drier than Portland.
Is that this true? To test it out, I plotted the month-to-month precipitation from each areas (proven under).
Eureka! Portland is wetter in Might and June, when the winds are extra westerly.
A climate subtlety maybe, however an attention-grabbing one.
