March 13, 2019

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.


  1. Yes!
    I have always wondered about that convergence zone! Living on the Quimper Penninsula, some of our heaviest rains have occurred, when no apparent storm is and sight and the distant skies are bright, while while we get a deluge.

    Bet that Vancouver island convergence zone is partially responsible for the precipitation difference between Sequim and Port Townsend.

  2. last month's snowfall really exposed the variation in convergence zone,(s), because everyone was focused on the snow and roads. The same thing happens with rain but we just cuss instead of watching the local news folks driving around. Speaking of that, it is fascinating to me to watch guys riding shotgun talking about the roads; and how every few miles you have completely variant road and snow depth. Yes mostly due to elevation, but as today's blog shows, it can be from many factors. Air moving around the topography -valleys and hills and wind direction makes all the difference. Can you imagine living in south dakota, or anywhere flat, where you pretty much know what is coming well in advance? Cliff's blog would be a broken record!!

  3. This is off subject.
    NOAA now says that the weak El Nino will extend into the summer and possibly the fall. Should this occur, what type of summer can we expect?

  4. THIS is what happens when we mess with the chemical composition of the atmosphere and oceans and the climate system breaks down....

    severe, erratic weather....

    "Remarkable view of a deep mid latitude cyclone that depend rapidly across Colorado and Kansas earlier today. This produced blizzard conditions in places like Denver, and severe storms to the south and east of the surface low."

    20 second satellite video embedded at the link

  5. On the last image showing the SE of Vancouver Island convergence zone, what do the numbers next to the wind arrows represent? The "feathers" on the wind arrows show a wind of 10 (one long bar) while the numbers next to each arrow are around 40. In the future how should an image like this be read, numbers or arrow barbs?


  6. Hi Jon. The black number is the temperature, the red number is the dew point, at the time the readings are taken and recorded. And of course the wind barb points INTO the wind, ie, the direction the wind is FROM.

  7. Convergence zones are very interesting, Cliff. Thanks for making it simple to understand!

  8. Sahila
    THIS is what happened almost a hundred years ago, before we had significantly messed with the chemical composition of the atmosphere and oceans....

    “The path of the Tri-State Tornado was 235 miles with an average width of 3600 feet, however there were moments when the width reached over 1 mile wide.
    The Tri-State Tornado lasted for 3 1/2 hours, starting down at 1:01pm 3 miles NNW of Ellington, MO and dissipating at 4:30pm 3 miles SW of Petersburg, IN.
    Average forward speed of this tornado was 62 miles-per-hour with a record setting speed of 73 miles-per-hour at one point.
    The Tri-State Tornado was an F5 on the Fujita Scale with winds reaching near 300 miles-per-hour.
    695 deaths and 2,027 injuries are associated with the Tri-State Tornado.”

  9. Nice blog! We experience similar cases with the Channel Islands in southern California. This especially true with post frontal thunderstorms.

  10. I live directly in the path of that Vancouver island convergence zone near lake roesiger and I can tell you when it hits in the winter it is no joke as I commute to monroe. Very narrow as well, it could be a foot of snow and one mile to the west nothing. When I see this on the radar in the winter now, I definitely try to avoid it.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

Substantial Snow Returning to the Pacific Northwest

During even the strongest El Nino years there is often a period at the tail end of winter when the atmospheric circulation associated with E...