• Thunderstorms and mountain snow: Active weather returns

    By: Chief Meteorologist Morgan Palmer


    After more than a month of mainly dry and warm weather, a cold weather system moving through this weekend will bring the chance for lowland thunderstorms and some snow in the mountains.

    We really do need rain!  After tying the record for the driest May on record in Seattle (at 0.12", tied with May 1992), we've only had 0.01" of rain this June up until the rain started Friday.

    Normally for the months of May and June combined, Seattle would expect about 3.50" of rain.  We're way behind!

    Showers, sunbreaks and... thunderstorms?

    In meteorology, we call this system moving by this weekend a trough of low pressure.  Basically, it's a cold pool of air aloft and is often associated with clouds and rain.

    But as is often the case, on Saturday and Sunday we'll have some periods of sunshine.  The weekend will NOT be a washout! 

    The sunshine will still bring a little warming each afternoon (about 60 degrees) but temperatures aloft will be colder than normal.  The greater the difference between the temperature of the air at the ground and the temperature of the air at several miles aloft, the better the chance for more vigorous downpours and even thunderstorms with small hail!

    We can examine storm chances by (among other things) looking at a map of CAPE (below right).  That stands for Convective Available Potential Energy.  It's a fancy term for the amount of energy available for storms to develop.   We'll have enough energy for a few isolated thunderstorms or hail showers each afternoon Saturday and Sunday. 

    The best chance for afternoon and evening lightning will be in the Puget Sound Convergence Zone from Seattle to Everett, nearer the mountains and in parts of the South Sound and southern Washington as well.  

    If you have outdoor plans (graduations, etc) in the afternoon or evening hours this weekend, be wary and prepared to go indoors if you see lightning or hear thunder!

    Snow down to Stevens Pass

    With these cold pools of air associated with low pressure, the snow levels fall. (The snow level is the elevation above which precipitation is mainly snow.)

    This weekend, we'll have snow levels down to 5,000 feet Saturday with a few inches of snow for Mount Baker and Paradise and on Sunday, snow could fall down to 4,000 feet at Stevens Pass!  We can expect just a couple inches of snow there too -- probably not enough to cause problems on US 2, unless the Convergence Zone gets involved.

    If you're hiking or camping or know someone who is going to the mountains this weekend, preparation for cold and wet or snowy weather is essential.

    Is this the start of "June Gloom"?

    No!  The weather pattern doesn't look to significantly change to a prolonged cooler and wetter weather pattern, spoiling hopes for a great summer.  In fact, the outlooks for next weekend are for warmer and drier weather and this general overall warm and dry pattern looks to persist through August. 

    This weekend's is just a passing weather system we should expect from time to time.  

    Also, the waters of the Pacific Ocean just offshore are running a bit warmer than normal which would tend to prevent prolonged "June gloom" -- the low stratus cloud deck that can sometimes form or move onshore bringing gray days. 

    We typically don't get much rain through September anyway, but the lack of May rainfall after a wet April has left grasses and other wildfire fuels a little taller than they otherwise would be, and drier.  I fear a tough fire season.  

    Keep up to date this weekend with the KIRO 7 Pinpoint Weather App, available at your app store or through kiro7apps.com 

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