March 03, 2016

Comma Clouds Hit the Pacific Northwest

They are the most important winter weather features that most Northwest residents don't know about: comma clouds.

Many of you are familiar with the large storms, the midlatitude cyclones, that dominate the satellite pictures and are frequently described on TV weathercasts.  For example, here is the infrared satellite image from 1 PM on Wednesday, and the huge cyclone/frontal system over the eastern Pacific is dominant.  A front extends NE from Hawaii to the Northwest and then heads due west in the Gulf of Alaska.  Look closely and you can see the swirl of clouds associated with the low center.

But now look at bit southward and you can smaller features, some in a comma shape...these are comma clouds.  And they can bring heavy rain and wind...and lots of snow in our mountains.

Here is the infrared satellite image around 7 AM today (Thursday).  The main frontal band has moved inland to Idaho, but two comma clouds are apparent: one crossing the Washington coast and the other offshore.

As I mentioned earlier, comma clouds can bring intense (but relatively short-lived) heavy rain.  Here is the weather radar at 7:30 AM-- this comma is packing intense showers  (yellow and red colors) in a narrow band.

What produces comma clouds?  Work here at the University of Washington revealed that they are associated with upper level disturbances (upper level troughs of low-pressure) that produce upward motion.  This upward motion works on a relatively unstable environment over the Pacific when cold air moves over the relatively warm water.   Here is a schematic showing the upper level flow and the clouds fields (shaded) for a typical comma cloud situation.  Comma clouds typically follow the main cyclone/front and form in the cold air behind. Notice the trough aloft just upstream (west) of the comma.

Comma clouds come in many sizes, although typically their dimensions are in the hundreds of miles.   Some grow in size to look similar to the big cyclones.


Public Talk: Weather Forecasting: From Superstition to Supercomputers

I will be giving a talk on March 16th at 7:30 PM in Kane Hall on the UW campus on the history, science, and technology of weather forecasting as a fundraiser for KPLU. I will give you an insider's view of the amazing story of of weather forecasting's evolution from folk wisdom to a quantitative science using supercomputers. General admission tickets are $25.00, with higher priced reserved seating and VIP tickets (including dinner) available. If you are interested in purchasing tickets, you can sign up here


  1. This post raises a basic question I've had about Northwest weather ever since I moved here from back east. There, the instability is in front of cold fronts where (in my limited understanding) the front forces the warm, moist air to rise into vertical development, often causing thunderstorms. Here the instability is behind the front where the cold air is on top of warmer water as in comma clouds. Does that cause the cold air to rise (seemingly against its normal tendency)?

  2. Wondering how many days hit above average temperatures in Seattle last month (February).


  3. "Big fleas have little fleas,
    Upon their backs to bite 'em,
    And little fleas have lesser fleas,
    and so, ad infinitum."

    While looking this up on Wikipedia, I saw an even more apt take:

    Lewis F. Richardson adapted the poem to meteorology, specifically discussing fractal wind patterns:

    Big whorls have little whorls
    That feed on their velocity;
    And little whorls have lesser whorls
    And so on to viscosity.

  4. Are the comma clouds related to the "popcorn" showers behind fronts, or are they something else entirely? Are they just a more organized form of the "popcorn" showers?


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

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