Dense Fog & Degraded Air Quality Through Thursday Due to a Persistent Ridge Offshore

FastCast—Sunday, Jan. 23 to Thursday, Jan. 27:

A persistent and massive high pressure ridge will keep the same weather pattern going over the next 5 days. Expect fog, with visibility potentially under 1/4 mile, each morning through Thursday. Fog will be dense in places, and potentially become freezing fog if temperatures dip below freezing. This can create roadway icing, especially on bridges/overpasses. In the afternoons, fog will clear in some places, bringing partly to mostly sunny conditions and highs in the mid 40s to low 50s. Where fog doesn’t clear, high temperatures are significantly lower, with highs only reaching the mid 30s to low 40s. Lows region-wide will be in the low to mid 30s. Little wind and no “mixing” of air in the lower atmosphere will cause stagnant air and degraded air quality by Monday for most of Western Washington. Expect degraded AQI through at least Wednesday. Additionally, due to downslope flow caused by the persistent high pressure ridge, temperatures on the WA/OR Coasts will be much warmer than the interior (as warm as 70+ on the south Oregon coast!). Keep reading below for more information!

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A persistent area of high pressure over Vancouver Island and the Pacific Ocean will bring a similar weather pattern to the region through Thursday. The main weather impact in the lowlands will be fog. 

Dense fog is possible at times (seen below in Federal Way on Saturday morning), with visibility under 1/4 mile in some spots. 

With nightly low temperatures near freezing in parts of the region, freezing fog may form. This can lead to quick roadway icing, especially on bridges and overpasses.

The cause of this calm yet interesting weather is a large high pressure ridge centered offshore and near Vancouver Island, seen in the European model forecast below for upper-atmosphere pressure anomalies on Saturday evening.

The high pressure is also visible on surface-level maps. The UW forecast for 11 AM Sunday shows the ridge offshore of Vancouver Island.

Persistent high pressure ridges like this one bring stable atmospheric conditions, with little “mixing” or movement of air in the lower atmosphere. This increases the likelihood of stagnant air. Due to this, NWS Seattle has issued an Air Stagnation Advisory from Sunday evening to midday Wednesday. The alert is posted below.

It is important to not burn outside and limit wood burning as much as possible. Find air quality forecasts at airnow.gov and check your local AQI agency:

Puget Sound Clean Air

Olympic Region Clean Air Agency

Northwest Region Clean Air

A very interesting impact of this pressure ridge setup is the effect it has on coastal high temperatures, mainly from Long Beach, WA southward. Check out the UW forecast for temperatures at 12 PM Sunday below.

Wow! Red colors are 56°+, expected from Long Beach southward. Take a look at the southern Oregon coast…this area around Brookings is called the “banana belt of the Oregon coast” and frequently has very warm temperatures due to downslope winds. Brookings will reach the upper 60s to mid 70s on Sunday…summer temperatures in the middle of winter!

However…the reverse of this effect is also seen, as cold air is trapped in Eastern Washington and Idaho’s Snake River Plain, where temperatures will be below freezing. Pink and purple colors in Idaho are temperatures of 5-20 degrees.

Even calm weather can produce interesting effects in the Pacific Northwest!