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.

California Versus the Northwest

It is often true that California and the Northwest are often in contention for the wet stuff (and we usually win!). We have seen a number of periods this winter when a trough developing over the West Coast has sent the jet stream and all the moisture down into California. And it is going to happen again.

Last week I was staying in Monterey, California attending the Cyclone Workshop, a regular gathering of meteorologists interested in midlatitude and tropical cyclones. The locals were all talking about the heavy rain that had struck central California for over two weeks in March when the jet had moved south over them (see upper level map below for a sample).Monterey, CA, for example, had 4.95 inches in March, 3.29 inches above normal, and the mountains of coastal CA had much larger amounts. A major impact was a major slope failure along Route 1 between Monterey and Big Sur, closing the road. Broccoli and Cauliflower crops were damaged. And California became amazingly green.

Here is a video showing the failure.




The jet moved back up here during late March and they got warm and dry...and you know what happened here.

But those smug Californians are going to exchange places with us again in a few days! Take a look at the upper level chart for Thursday at 4 AM---a trough will drive the flow and rain into the palm-infested California climes.


Here is the 24h rain ending 4 AM on Friday. We dry out and coastal CA gets soaked. But they will probably take refuge in their hot tubs.

So enjoy our break on Friday....it won't last long. A weak system will hit Saturday and heavier rain on Sunday.