April 23, 2010

Why are our fronts different?

Friday Afternoon Visible Satellite Image: Front Approaching

Update: Here is Saturday Morning's Visible Photo--you can see all the showers offshore

It is now 7:30 PM on Friday and a modest Pacific front is moving through western Washington . The above (top one) high-resolution satellite image at 5 PM shows the story. At that time most of eastern Washington was clear and wave clouds induced by the mountains was found over and immediately to the lee of the Cascades (they are those periodic lines of clouds in the image). The front will be through by midnight and the continuous showers will end. If we were living over the eastern two-thirds of the U.S. it would then be all over. Fronts bring rain. After the fronts move past the skies clear and the precipitation is over. Here is an example of the lack of clouds and precipitation after a midwest front.

But that is NOT what is typical here. After the front passes in the Northwest the weather show is just beginning, because we typically see postfrontal convective showers in the 24-h after a front moves through during winter and spring. You know what I mean--the typical showers and sunbreaks. The media tends to call this wacky and strange...but those that make such remarks don't understand our weather.

Here is an interesting fact: in the winter much of our mountain snow does NOT come from frontal precipitation, but from the showers after the fronts in the cold, unstable air that follows. Tomorrow will be no exception. A field of postfrontal convective showers are out there waiting to come into our area, and the forecast is for showery precipitation on Saturday. Don't believe me? Take a look at the larger view satellite image at 5 PM today (Friday). You see the front? Then you see a narrow zone of clearing (this is due to sinking motion immediately behind the surface front) and then a thousand kilometers of more of convective showers, which look like a field of cotton balls surround by dark (clear) areas. Our number is on those clouds. In the business we call them "open cellular convection" because generally there are more clear areas than precipitating areas.

Why do we get these showers AFTER the front? Because there is plenty of moisture (not surprising) and the atmosphere is relatively unstable there. The ocean surface is "relatively" warm--roughly 50F year-round off our coast, and the air behind the front (cold or occluded front) is even colder, coming off of Alaska or Siberia. Cold air over warmer water is an unstable situation, with temperature decreasing rapidly with height. Cold air is dense and wants to sink, warm air is less dense and wants to rise. Put them cold air over warm air and they want to switch places. Or think about your hot cereal, which convects when you create a large change of temperature in the vertical when you turn on the burner. (My book has much more on this if you are interested in learning more).

So why doesn't this happen over the eastern U.S.? First, there is less moisture. But more importantly, the land surface is cool or cold in the winter, so you have cold air over cold land. No good.

But wait! We have more special weather here! The coastal winds often switch to the west here after frontal passage, particularly during the spring. Such a wind direction leads to the formation of Puget Sound convergence zone, a zone of clouds and precipitation over Puget Sound...generally from Everett to north Seattle, with clear zones to the north and south. And guess what, the computer models indicate a good chance of this happening tomorrow (see forecast of the 3-hr precip ending 8 PM on Saturday). And there will be plenty of showers in the mountains, including snow showers at higher elevations (see graphic). Typical spring in Seattle!


  1. I noticed those alien shaped clouds overhead this evening. They were pretty cool. Nice sunset. Also a sign of cool dry and windy tomorrow.

    Where I live, precip falls with the front. Not even, its like pre front. If its a 24hr storm, the window of opportunity of rain is about 3 or 4hrs. Main reason why it rains so little

  2. The convergence zone has set up perfectly tonight... it's raining off and on between Snohomish and Monroe, but when I dropped down into Mill Creek it was dry.

  3. A NWS meteorologist in Norfolk once told me forecasting weather on the east coast was so easy because the Atlantic acted as a giant stationary front.

  4. The convergence zone was in full effect on Saturday on the Puget Sound. I was on a sailboat, and we had steady 25mph winds from the South all the way from Seattle to the southern part of Whidbey Island, and then the wind died down to almost nothing. We would sit in the water going nowhere for minutes at a time, then we'd get a gust of 8 to 9 knots, then it would die again.

  5. 26 April 2010 @ 1:35pm
    Hi Cliff,
    Have you checked Arlington Airport today? It's 72F as I write this! We're having excellent SE winds. It's a perfect example of downslope warming through compression. I wonder if we could have some T-storms tonight?


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

The Weather Regimes of Summer

 Weather patterns tend to get "stuck" for extended periods and we have certainly seen such persistent conditions this summer.    W...