June 30, 2019

Freezing Temperatures and Puffy Cumulus Clouds

Before I discuss the impact of cold air aloft, let me show you a shot of the beautiful sunrise of Saturday morning for the Seattle PanoCam.   Love the reflection off Lake Union.

For the past several days, unusually cold air, associated with an upper level trough has been positioned over our region, resulting in cold nighttime temperatures east of the Cascades and puffy cumulus clouds over much of the region.  Below is a nice example from North Seattle Saturday morning.

An upper level chart (500 hPa, about 18,000 ft above sea level) for Friday morning shows the upper level trough (solid lines) and the cold temperatures (blue colors) over us.

With cold air aloft and relatively clear nighttime skies, some locations in western Oregon/Washington dropped into the mid-40s, while some locations in eastern Oregon plummeted to freezing Friday and Saturday morning (Saturday mornings lows shown below).

With cold air aloft and strong heating from the powerful near-solstice sun, there has been a large change in temperature with height (a large lapse rate).  This leads to instability--up and down motions in the lower atmosphere--as the atmosphere start to convect.    Such convection leads to the puffy white cumulus clouds, which form in the upward motion.   

The convection is enhanced over high terrain (which acts an elevated heat source).   A visible satellite image shows the cumulus clouds from space--you can see how high terrain is favored.

The troughing/low pressure aloft is weakening today, but should re-strengthen by Tuesday (see upper level map at 2 AM Tuesday)

As a result, precipitation is not over for us--with showers quite possible on Tuesday.  The accumulated precipitation through 5 AM July 4th, show substantial amounts over NE Washington and BC, with some extending down to Portland.

July 4th?   Looks dry over western Washington, with highs in the mid-70s.   Perfect for outdoor activity. 


  1. Cliff,

    Looking forward to the holiday week? Also, your holiday week forecasting. My plan is to participate in some summer activities if possible.

  2. Off topic, but do you know why the Western Washington Zone Forecasts web page (https://a.atmos.washington.edu/data/zone_report.KSEW.html) isn't updating? The last report was from Saturday afternoon. Likewise, the weather discussion is a bit out of date as well (there is nothing for today). Unfortunately, this isn't the first time this has happened, it seems to be a recent problem (last month or two).

  3. Cliff
    Could you comment on the reports of more than three feet of accumulated "snow" from a hail storm on June 30th in Guadalajara, Mexico? https://www.bbc.com/news/world-latin-america-48821306 Are there precedents for events like that in a semi-tropical place? or could this be evidence of climate change? I realize it is a mile high. Thanks
    Bill O'Neill
    a former Met 101 auditor


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