November 07, 2009

Crazy Weather?

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 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 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!


  1. Well it happens in a few local spots in the eastern united states!?!

    And I know our ocean stays relatively warm during the winter but does it cool down a lot other places? I only say that because is a warmer ocean now a partial reason why mid-november is rainest time of the year and why mountain snows really don't start accumulating until after thanksgiving? Just been noticing that 925mb 0 C' moderate quite quickly over the ocean while I watch that line for good snow at snoqulamie.

  2. Cliff or other blog readers... any idea how much rain has fallen in Seattle with this storm? Know of a website that shows the measurements? We're getting water in the basement (that we just finished - ugh!)... wondering what our tipping point seems to be.

  3. The Portland area is getting slammed with rain right now!

  4. i was crazy! I saw a cold air funnel over chehalis.

  5. "Why doesn't this happen over the eastern U.S.? Because the land is cold there...not warm like our Pacific Ocean."

    I'm confused. I thought warm Pacific Ocean underneath the cold arctic air caused a pressure gradient to send the moist air above and this process is chaotic. Small gradient nothing happens clear sky. Slightly larger gradient and moist air sweeps up and causes clouds and precipitation.

    Is it the cold land over the East or the colder waters of the Atlantic (which carry the icebergs which sunk the Titanic) that prevents the instability?

  6. @Madrona Tree

    I frequent Weather Underground for most of my local weather observations.

    Once you're there and have entered your seattle are zip code. You could do one of two things.

    1) a less accurate but quick way to see total precip is to click on the radar. And once on the nexrad local radar page click "Total Precipitation" on the top of the radar readout. Then look to the left side of the radar panel to see when the "Begin Date" is.

    This is less accurate because it is just adding up all of the radar readings over time and assuming that what the radar saw is what fell to the ground in terms of measurable precip.

    2) the better method would be enter your zip code and get to the seattle area page. Scroll to the bottom, and find the "Weather Stations" list. Look for one that is close to your area. CLick on that weather station and it will list total precip. You can also change the date and look at historical data.

    Hope that helps. You can also view all local weather stations on a GoogleMaps mashup that wunderground puts together. It is pretty handy for realtime data in your area.

  7. JP,

    I can see why it's confusing, but think of it more vertically than a gradiant.

    Take a parcel of air over a relatively warm ocean. The ocean will warm that air parcel (like a bubble) and if the air above the warm air parcel is cold, it will rise from its surrounding envirment and condense into a cloud, etc.

    Take a large land mass and the exact same cold air mass above. Minus the moisture of the ocean too, that air parcel at the surface has no way to warm up except a weak winters sun, so it stays with its surrounding enviroment.

    Winter - oceans warm, land cold. Summer - oceans cool, land hot.

    See by that illistration if you can figure out what happens durng the summer when the upper air is relatively warmer?

  8. It's great weather, not crazy at all, really. One advantage of this weather is you get to see rainbows, like the really bright one outside in the east earlier this afternoon just before the sun set. The fall and spring are great for rainbows in the Northwest.

  9. JP: East coast weather is usually coming from the west (over land). When storms arrive over the warm Atlantic, they behave so differently, they are given their own name (Nor'easters).

  10. Crazy indeed, I was reading on Komo's website that the Lincoln City area experienced a tornado on Friday night!! Has that ever happened on the Oregon coast before?

  11. cliff, doesn't the east see those kind of showers sometimes? maybe not as frequent, but in the fall or spring when the ground is still warm or getting warm, and a cool air mass moves into the are from the north?

  12. @Paul and Smokejumper,

    Ah-Ha! I see now. There are two major differences between cold weather in the NW and cold weather in the East. Ours comes in over (1) warm (2) oceans, over there it comes in over (1) cold (2) land. Now here's a question, does the instability phenomenon exist in other parts of the world where cold weather comes over warm oceans? I'm guessing the candidates would be England, Australia's East coast, Argentina's coast and the South-East coast of Africa?

  13. @Ted - Thank you very much. That helps a lot. Radar shows 4-5" since Nov. 5... that seems like it may be right - basement stayed dry all last year, even thru all that melting snow... of course now that we put drywall in there, we're getting some leakage. >:( I'll do more looking after we get home from errands today.

  14. Meant to post this more current and accidentally placed under last blog.

    And now, going completely off subject:
    I've been following Hurricane Ida and see that it may transition to an extratropical cyclone before it arrives at the coast. It appears to be a very complicated forecasting problem. As I understand it, an extratropical cyclone is no longer dependent on the heat of the ocean for energy and draws energy from colder air high in the atmosphere. My question: Is it possible for such a storm to regain tropical storm force or is it still too warm? Fascinating phenomenon, extratropical transiti

  15. Wind is blowing around pretty good today in the South Sound, S@22 Gusts to 32.


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

Cool air and snow showers on Friday for the western lowlands. More crazy stuff in California.

 No, there won't be accumulating snow near sea level, but don't be surprised if you see a few flakes west of the Cascade crest on Fr...