If you ask any Western Washington meteorologist about the most important local weather phenomenon, the answer is immediate: the Puget Sound Convergence Zone.
Today, we had a spectacular example....let me show you.
For those of you who are unfamiliar with this feature, such convergence zones occur when the low-level winds on the coast are westerly or northwesterly (from the west to northwest).
The air is deflected around the Olympics and then convergences somewhere over Puget Sound (see figure below). Converging air at low levels forces upward motion, resulting in clouds and prediction.
A convergence zone is born!
We typically get 10-20 convergence zones per year, with the greatest frequency from March through June.
A strong convergence zone occurred this afternoon, with associated heavy rain quite evident on the weather radar around 3 PM (see below). You see the east-west precipitation band (yellow and green colors)/ That is the convergence zone, which extends into the western foothills of the Cascades.
The location of the flash is shown below (again, about 3 PM). We call this a "one-flash wonder"
When I was a graduate student many years ago, the origin of the convergence zone was not known. Weather Service forecasters thought there was a strong band of rain offshore that sometimes moved in off the ocean and then took up housekeeping over the central Sound.
Today we understand the critical role of terrain and particularly the Olympics.
But there is something else that changed: the ability of high-resolution models to predict convergence zone precipitation well in advance. For example, below is the predicted 3-h precipitation ending at 5 PM on Monday.
When it is sufficiently cold the convergence zone can produce a very well-defined band of snow across Puget Sound, something that happened in 1990. Even yesterday, there was convergence zone snow above approximately 750 ft ASL. Here is a picture sent to me by Dr. Peter Benda at around 1200 ft in the hills above Bellevue (3 inches of snow!).
Under very rare conditions, the snow band can get very, very narrow 😈
Love the "accurate" photo of very local snow.
ReplyDeleteWhat's the highest amount of precipitation you have seen the Puget sound convergence zone put down in a 6 hour period during your career for comparison?
ReplyDeleteOne time, when I lived in Kenmore, in the early 1990's, it rained about 2 inches in about 20 minutes. Very rare for around here. I don't know the cause. Another time, perhaps a couple years later, in August, I got a couple inches of half-inch hail stones. I was driving and I pulled over under a tree. No lightning as I remember.
DeleteI got absolutely soaked on my bike during the afternoon commute, Bellevue to Kirkland. And there was actual sun to the south, with a glimpse of Rainier!
ReplyDeleteThis after a pleasant dry morning ride in.
Indeed. Lots of hail in Kirkland and around UW. I was basking in the sun earlier in the day!
DeleteWe received small hail (sand-sized) in the mid-afternoon and snow in the late afternoon. The hail lingered for a half-hour, but the snow never accumulated. Now, the ground is saturated and too wet for rock removal (glacial till).
ReplyDeletegraupel here too in Bremerton, in several episodes -- one intense enough to cover the ground at 160' ASL. It was followed by a brief shower of ice chunks -- very wintry mix, given air temps well into the 40s!
ReplyDeleteBlizzard-like conditions on Tiger Mtn #2 yesterday
ReplyDeleteBothell hardly got anything. I guess you know that...
ReplyDeleteCliff states that when he was a graduate student, the origin of the convergence zone was not known. I do not know when Cliff was a graduate student, but I can say that the existence and basic cause of the convergence zone was likely known prior to his arrival in the Pacific Northwest. I grew up near Green Lake in north Seattle and became interested in meteorology about 1950. One reason for this was the existence of the convergence zone. I had friends and relatives who lived north of where I lived, in north King County and south Snohomish County. They quite often told me of the heavy rain and sometimes snow (to my chagrin since I wanted snow days off from school), falling at their locations while I had nothing falling at my house. It was in my head at that time that this was due to the Olympic mountains splitting the westerly airflow and having it converge again on the east side of the mountains. I do not remember how or where this idea came to me. It possibly could have been mentioned in one of the daily weather radio broadcasts that were issued back then from the Seattle Weather Bureau meteorologists. I have always considered that the existence and cause of the convergence zone was common knowledge, at least to meteorologists, ever since then, even though perhaps no official scientific paper was written about it. I do give credit to Cliff and his staff at the UW for developing the small scale computer models that can predict this and other weather factors that are important to accurate weather forecasts in our area.
ReplyDeleteI have lived in the Bothell area since the 1970s and now live in Snohomish. The convergence zone has been something I have lived all the time. There are so many times I have left Bellevue in clear blue skies to drive home and have it pouring in the Bothell area or sometimes vice versa. I remember one time I was to go to a meeting in Bellevue. I called to see if the meeting was still on because I had about 4 or 5 inches of snow in my yard. The Bellevue folks said there was no snow and it was sunny.
ReplyDeleteThe convergence zone moves around north and south all the time. The UW weather radar is great for figuring out where it is and if it is going to move my direction. A great PNW weather experience.