April 06, 2011

Convergence Zone Snow in April!

Bellevue, WA snow around 8 PM at 200 ft ASL. Picture courtesy of Julia Ziobro

Today we had a strong convergence zone and with cool air aloft the result was that in the heavier convergence showers the snow level was driven to the surface over some of the lowlands.

This morning dawned with a spectacular convergence zone over the north Sound.

Just beautiful. And there was also snow showers in the mountains, some quite heavy at the same time (over a foot today in the passes!)

The convergence zone moved south during the day, producing snow showers during the last afternoon in Brier and Lynnwood, and subsequently farther south on the eastside.

Here is a nice video of the snow action in Bellevue:

And with the convergence zone there was an extra bonus of lightning and thunder. My bike ride home tonight around 7 PM was enhanced by such atmospheric pyrotechnics in the vicinity.

Why does a convergence zone help produce snow?

When temperatures are close to being cold enough for snow, but not quite there, the precipitation from the convergence zone can start as rain and then turn to snow or mixed snow/rain. When precipitation starts there is first strong evaporation, that rapidly cools the air to the wet bulb temperatures. The air rapidly humidifies and after 15 minutes to a half hour the air is saturated and evaporation doesn't do much anymore. But the atmosphere has another cooling trick!---melting. The snow aloft melts as it falls into above-freezing air and it takes energy to do that...so that air cools further...sometimes enough to bring the snow level down to the surface. This happened today.

Want to see this in action? Here are the weather observations on top of the roof of my building. At roughly 5 PM (00z, time in red), you will see the precipitation started, the temperature fell, and the relative humidity rose (click to expand!). The initial evaporative cooling was very intense and short-lived. Then as the precipitation continued melting continued the cooling. Classic traces.


  1. got an inch of snow up here in the highlands above Issaquah! What a treat! I wish it'd stay!

  2. When will we have a Convergence Zone Heat Wave?

  3. Cliff, you say that "The snow aloft melts as it falls into subfreezing air." Could you explain that a bit further? I thought that "subfreezing" would mean cold air (below 0°C.) that would freeze moisture, not melt it.

  4. I don't mean to hijack this topic, but I wanted to check in with you. After contacting my representatives in Olympia about the adoption of the Common Core standards HB 1443 and the bill to delay adoption, HB 1891 (which has stalled) I received this via email from Sen. Andy Hill
    HB 1443, as amended in the Senate, will:

    ·Implement the recommendations of the Quality Education Committee regarding the Common Core State Standards; the definition of a high school credit; dropout prevention and graduation rates; parent involvement coordinators; a kindergarten readiness assessment; remediation programs; the Transitional Bilingual and Highly Capable student programs; teacher and principal evaluations; and additional tasks for the compensation working group.

    ·Require school districts to award credit for high school courses successfully completed by a student that meet the school district's graduation requirements and are provided by an approved multidistrict online provider.

    ·Provide that the increase in the minimum instructional program offered by school districts (from a district-wide average of 1000 hours for grades 1-12 with at least 450 hours in kindergarten to 1080 hours in grades 7-12"

    I don't know, but I suspect this isn't doing much to address the math concerns you outlined in your blog post 2/9/11?

  5. Yeah it was one wild ride across I-90. I think I counted 5 or 6 good lighting flashes during the hour I was out on the bicycle.

    The snow was still on the backyard grass this morning. (Lake Hills Bellevue) But with the rain it's gone now.

  6. I missed the lightning. Still, some of us would like to see spring get serious...

  7. I am having trouble figuring out this blog thing so please forgive any duplicate blogs... I just wanted to say: When are we going to break free of this La Nina thing and get some warmth?

  8. "producing snow showers during the last afternoon in Blier and Lynnwood" ...
    This is one of the few instances where I've ever seen my hometown of Brier mentioned, and it gets misspelled. :(

  9. Sea-Tac just finished the first week of April without making it to 50 degrees. The last time that happened was in 1920, 91 years ago.

  10. I apologize for duplicate messages, it appears my first one did not go through.

    We had graupel in the Newcastle area (near Issaquah) late Wednesday afternoon instead of just normal snow. What caused the graupel to fall instead of hail, sleet or snow?

  11. Frost here in Kingston this morning.

    Cold elsewhere. According to Wunderground its a little colder in San Juan Bautista CA where we have friends and in Valencia CA where my daughter is in school.

    I am so ready for summer!


  12. climo man, Sea-Tac also just completed its longest stretch of days with measurable precipitation on record (since records began there in 1948) between the middle of March to the middle of October, 15 days.

  13. We got a short but intense snow/sleet shower in Portland also, Wed. night, that dumped about 1/2 inch in 20 minutes in the vicinity of Going St. and Interstate Ave.


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