My personal “line in the sand” past which lowland snow events become more and more unlikely is Valentine’s Day. After Valentine’s Day, longer days and higher sun angles make cold temperatures at the surface much harder to come by, even if temperatures in the upper atmosphere are just as cold as they were in January. But on rare occasions, if the air is cold enough and there is some moisture around, we can still see lowland snow after the 14th of February. Though significant snow is not in the forecast, there is a chance of light snow tonight on some of the hills above 300 feet as a chilly band of showers moves onshore.
Temperatures are still relatively warm (around 40 for most folks in Western Washington), but the radar shows some heavier showers moving inland, and these showers could give a quick dusting to areas above 2-300 feet. The Seattle NWS forecast office has a snow advisory for the potential for 1-3 inches of snow above 300 feet, with nothing to a trace below that.
In order to show you the root cause behind our chilly weather, I’m going to show a graphic I’ve never shown on this blog before! The graphic below shows the height in decameters (tens of meters) and absolute vorticity of the 500 hPa level of the atmosphere, and I made it myself with GrADS, which is a scripting language geared towards visualizing weather data. You can see a large ridge of high pressure in the Pacific and a trough off our coast… this trough is what is giving us these chilly showers and a chance of a few snow flurries.
At some point, I hope to have these graphics online and automatically updating with the latest model runs. In the meantime, if you would like to play with this code and make your own graphics, I’ve uploaded the code on WeatherTogether’s GitHub profile, and if you want to read about how to use Grads, I’ve posted some good resources on the forum to get you started. If you are interested in helping, PLEASE PLEASE PLEASE contribute! Even if you don’t know how to code, please let us know what type of maps you want us to make, and we’ll make them!
GitHub: https://github.com/WeatherTogether/model-charts
Forum: http://weathertogether.net/forums/topic/visualizing-weather-data/#post-6527
And here’s one more graphic, just for fun. Yes… that is yet ANOTHER very moist, very wet atmospheric river headed towards the Northern Sierra. This is a 180-hour forecast, so details are certain to change. And for Oroville Dam’s sake, let’s hope they do!
Thanks for reading!
Charlie