Saturday, November 7, 2009
Heavy showers one minute and sun the next. Followed by more showers. The reaction one often hears is that NW weather is crazy and unpredictable! Such things don't happen over the east coast! But it turns out that showers and sunbreaks are an essential aspect of NW weather and if you are aware of what is going on (and can see a radar image on the web), you can take advantage of it.
Convective showers and sunbreaks typically follow the passage of cold or occluded fronts across our region. On satellite pictures, one sees a mottled looking fields of clouds, with the dark regions being the clear zones between the showers. These are instability clouds--cumulus, cumulus congestus, and cumulonimbus--- the occur when the atmosphere is unstable. (see my book for pictures of these)
The atmosphere becomes unstable when the temperature declines rapidly with height. The analog is your hot cereal being prepared in a saucepan over the stove. You put the burner on the bottom gets hot...a large temperature change occurs in your cereal and it starts to convect..with rising current of cereal interspersed with descending portions. In the atmosphere the upward motion produces clouds and precipitation and the downward moving portion the clear zones.
The Pacific Ocean stays relatively mild into the winter, but the air cools down aloft--with origins over Siberia and Alaska. Cold air above warm water gives you a big temperature change (known as lapse rate in the biz) and that leads to convection. So that is why showers and sunbreaks follow cold fronts--the cold air is coming in aloft behind the fronts. Why doesn't this happen over the eastern U.S.? Because the land is cold there...not warm like our Pacific Ocean.
The convective showers show clearly on weather radar images (see below) and such showers rarely last longer than a 1/2 hr. So if you view a radar loop on the web (such as available by going to http://radar.weather.gov/ or my department web site), you can see where the showers are and avoid them. I always bike to work on such shower days because I know I can avoid them by using the radar. Before the radar I would get soaked...no more! When we get the coastal radar we will be able to see the showers several hours out...which will really help us plan activities.
One final thing...we get most of our snow from such convective showers...the air is cool, plenty of moisture, and the snow level is low. Don't believe me? Take a look at the current cam at Stevens Pass below...lots of new snow!
Posted by Cliff Mass at 1:38 PM