A New Discovery? The Chehalis River Fog Jet

The high-resolution satellite imagery yesterday morning revealed a transient, but fascinating feature:  a narrow current of fog emanating out of the western portion of the Chehalis River valley.   This fog current then passed over Aberdeen and Hoquiam before pushing out into the Pacific Ocean.

Here is the NOAA/NWS GOES satellite image around 8 AM Sunday that shows this amazing feature.  You can see the fog collecting in the Chehalis River valley and then jetting out to the west.  Just amazing how this current was able to maintain integrity for tens of miles.  About roughly ten miles offshore it lost its structure, with the cool, moist air producing a blob of low clouds.


If you look closely, a weaker fog jet was produced at the exit of the Columbia River.    

Just as a reminder, here is the Google terrain map for the region.  The Chehalis River Valley is well defined with relatively steep side slopes.  Cool air can drain from side valleys into the main Chehalis channel, resulting in cool air in the valley.   And if the air is moist enough, the air is cooled to the dew point, producing fog.


Surface observations Sunday morning at the same time as the satellite image shows easterly (from the east) flow at Hoquiam and temperature of 36F...and even cooler down in the valley towards Montesano.   Cool, dense air escaping the valley was aided by a regional pressure gradient with higher pressure inland and lower pressure towards the coast.



The observations at Hoquiam Airport illustrate the weather conditions during the passage of the "fog jet".  The time is in UTC (same as Greenwich Mean Time) with 1405 being 7:05 PDT.  The fog was only over them for about an hour, with easterly winds of 7-8 knots and temperatures in the mid-30s.


High resolution weather prediction models got a piece of this feature.  Here is the predicted wind fields for 7AM from the UW 4/3 km runs (the grid points are 4/3 km apart).  You can see the easterly flow and a relatively narrow jet of strong winds.


And the forecast for 9 AM, showing surface winds and temperature (shading), indicates cold air in the Chehalis River Valley and the plume of cool air moving out into the Pacific.


The much coarser NOAA/NWS HRRR model (3-km grid) had a difference solution.  Here is the forecast visibility for the same time.   A very weak plume of low visibility is coming out of the Chehalis River valley, but there is a distinct plume and an offshore fog area associated with the Columbia River valley.  The NOAA model doesn't have the resolution to properly simulate the narrow Chehalis River valley.


Anyway, this feature is perhaps a one on the meteorological Richter scale, but fascinating nevertheless, and a good test of my profession's high resolution models.




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