The Other Convergence Zone

Much of western Washington was dry today, but there was one exception:  a band of persistent precipitation extending southeastward from near Victoria towards Snohomish County(see radar images this morning around 6-7 AM)

As a result, there has been heavy rain in some parts of Snohomish County and over the NE portion of the Olympic Peninsula.

This is a topographically produced rainband resulting from  the interaction of the atmospheric flow off of the Pacific and our local terrain.   More technically, a topographicaly produced convergence zone.

Many of you are familiar with another convergence zone:  the Puget Sound Convergence Zone, which is produced by air deflected around the Olympic Mountains and then converging somewhere over Puget Sound.  As the air converges at low levels, air is forced to rise, producing clouds and precipitation.

But today we observed something else.   Air moves around the mountains of Vancouver Island (see figure) and then converged southeast of Victoria.   As in the Puget Sound convergence zone, converging low-level air produced upward motion and thus clouds and precipitation.

You could see the air streams converging southeast of Victoria in the observations at 6AM

This Vancouver Island convergence zone is apparent several times of year, particularly in the spring after frontal passages.  Another fascinating local weather effect of our region.  And another reason to watch the radar before planning any recreation.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/03/the-other-convergence-zone.html