Puget Sound Convergence Zone

Late spring is the convergence zone time of the year and we have one tonight. Take a look at the latest radar image and you can see the band of precipitation north of Seattle (see graphic). This one is somewhat amorphous--partially because the atmosphere is not that unstable (this gives you a nice line of convection).


Now let me show you something that you won't see on TV tonight..the Doppler velocities for the same time. We don't call it a Doppler radar for nothing! But you never see that on TV.

Funny side story. KING-5 got their own Doppler radar years ago, and my colleague Jeff Renner, excited about it like a little kid on Christmas morn, showed both the reflectivity (precipitation intensity map) AND the Doppler velocities. This did not last long. These Doppler velocity maps are a bit difficult to read. They show you the component of the velocity of the target (here rain) towards or away from the radar--NOT the total wind speed. It takes some experience to really figure out what is going on...and the colors can be confusing. Anyway, here it is:

Makes perfect sense, right? Warm colors indicate flow moving away from the radar, Grey, no velocity towards or away, and cold colors, flow towards the radar. And remember the radar is on Camano Island. So looking a this radar one concludes that there are northerlies over the north sound, and a line of zero velocity--the convergence zone--stretching east-west over Seattle. In general, convergence zone precipitation is north of the low-level convergence line. Well, here are the winds at 9 AM (click to make big). Northwest winds on the coast and a nice convergence zone in the surface wind field. Classic.

Why do Puget Sound convergence zones prefer spring? Because the winds on the coast are more frequently from the west to northwest during this time of the year. In mid-winter, when the flow is more southwesterly, the convergence tend to be weaker and farther northwestward. And the air is generally more unstable during the spring as the surface warms more rapidly than the air aloft.

After the convergence zone passes south of you (with a switch from southerlies to northerlies at the surface), southerlies are usually still aloft. If you look carefully you can see it in the cloud motions. Here is a video for today...see if you can see the differing wind directions right before sunset:





Anyway, if you like Doppler velocities they are always available on the National Weather Service and UW websites, among others.

For those that are in the high tech industry and are located downtown, I will be giving a talk at a Blink open house on high-tech weather prediction on Thursday at 5:30 PM in Seattle. For more info and to RSVP, see: http://blinkux.com/insights/blog/blink-open-house-speaker-series/

And those of you interested in provide feedback to KUOW don't forget, this facebook site dedicated to listener comments:

KUOW Listeners Speak Out

I am now talking to a variety of local radio stations and will be finding a new home during the next few weeks.