• Lowland gloom and smog likely through mid-December

    By: KIRO 7 Chief Meteorologist Morgan Palmer


    After a wet start to the fall, December is turning much drier.  The weather pattern looks set up to keep dry weather in place through at least mid-December.

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    The large ridge of upper-level high pressure across the West Coast all the way to southeast Alaska won't likely budge too much.  While this is bringing days of dry weather, the low sun angle this time of year combined with light wind will cause fog to persist each day and for low-level pollution to get worse.

    In the colder locations in the lowlands, freezing fog will be an issue each morning as well as the moisture in the fog droplets could freeze as ice on road surfaces (particularly bridges and overpasses) in locations where the air temperature is below freezing.   River valleys and rural locations are more prone to freezing fog and early-morning ice.

    Gloom and smog in the lowlands, sun in the mountains

    While most lowland locations will start off cloudy or foggy through the week, most of these spots will see the gloom break up in the afternoon hours, though there will be some spots that stay in the gray all day.

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    An "inversion" has set up in the atmosphere where a layer of warmer air aloft will "trap" the colder lowland air, not allowing it to mix with the air above and keeping clouds and fog in place.

    This lack of mixing of the layers of air in the atmosphere will also allow pollutants to build up through the week in the lowlands, likely prompting burn bans as one of the two primary sources of wintertime pollution is combustion in stoves and fireplaces (the other is vehicle exhaust). 

    I expect air quality to be unhealthful at times late this week into next, especially for those with respiratory ailments (asthma, COPD, etc.)

    Air quality will generally be at its worst at night and in the morning hours.

    The mountains will be above that inversion layer of warm air, so a short drive into the high foothills or mountains brings one above the low level clouds, gloom and pollution.  Skiers and snowboarders will find fine conditions with the sunglasses a must!

    When do we get back to normal?  What about La Niña?

    Presently, it appears this pattern will certainly last through all of this week and most or all of next week.  It could be close to Christmas before the weather pattern gets back to the more typical cool and showery.

    La Niña is still in place in the Pacific and this should help to get the weather pattern back to a cool and wet one toward the new year.  It is important to remember that the La Niña/El Niño phenomenon does not guarantee a certain result for the Northwest winter season.  Instead, it just makes a result -- in the case of La Niña, cooler and wetter than normal -- more probable than a typical year.

    Also, remember that La Niña primarly has its greatest impact on the winter season and winter doesn't begin until the solstice on Dec. 22.  

    There is no reason for snow lovers to give up hope as La Niña does produce better odds of lowland snow in a given winter.  While there is no guarantee, this is still very early!  Statistically, the best chance for lowland snow in Western Washington comes after the first of the year.


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