The visible satellite image this morning around 10 AM is impressive, with low clouds and fog enveloping the lowlands of Puget Sound, the Willamette Valley, and the entire Columbia Basin.
The clouds are associated with cool, dense, moist air trapped within the lower elevations of regional basins.
The cold air is quite shallow. I can demonstrate this with the temperatures at SeaTac around noon today (below). Temperatures were in the 30s near the surface, while lower 50s were observed above roughly 2200 ft.
A strong inversion, with temperature increasing with elevation, was evident between them.
Or consider the radiosonde (ballon-based) temperatures at Salem, Oregon, within the foggy Willamette Valley (see below, temperatures in °C, 0 is freezing, 10C is 50F), 900 hPa on Y-axis is around 3200 ft). This is from 4 AM this morning.
The right line is temperature and the left line is dewpoint. A cool, saturated sub-freezing layer near the surface is below a very strong inversion where temperature increases by about 15C (27F) over about 1000 ft.
These are wonderful days for hiking in the local foothills. Consider Tiger Mountain near Issaquah. Frosty and cloudy when you start but in the 50s with bright sun at the top (around 2500 ft).
Inversions are very stable features, meaning that they suppress vertical motions. Because of this, they can act as a lid for low-level pollutants from fireplaces, wood stoves, and other sources of particles, resulting in declining low-level air quality.
If you would like proof, take a look at the air quality situation near the surface around noon. Orange and red colors are the worst, green is good.
Notice the excellent air quality at high elevations above the cloudy murk.
If you want to see further proof of how temperatures increased with height this morning, consider the minimum temperatures today around Bellingham and Mount Baker (see below).
Below freezing and icy around Bellingham, but in the low 40s at higher elevations.
This kind of dry, frosty lowland pattern during the winter is associated with a ridge of high pressure aloft (see upper-level, 500 hPa map at 4 PM below). Red indicated much higher heights than normal.
With high pressure aloft, near-surface winds are light and there is strong sinking aloft, causing temperatures aloft to warm by compression. A lack of clouds with ridges allows the surface to cool by emitting infrared radiation from the surface.
This is our fog producer.
Yes, must be a super inversion. I heard freezing level will top 10,000 feet in some places. Not my favorite situation in winter.
ReplyDeleteYesterday, wanting to get some of that higher-elevation sunshine, I drove US-2 up to Stevens Pass. I emerged from the fog around Sultan, and it was beautiful until just as I got to the pass - it was heavily fogged in there and as far to the east as I could see, and the temperature had dropped to 21°F!
ReplyDeleteWe moved to Snohomish and have pretty much been in the fog continuously for the last few days. It is external fog not internal fog.... :-) However, if you drive to Clearview at about 400 feet, it is nice and sunny. Seems the fog layer goes to about 300 feet along Highway 9.
ReplyDeleteThat's an impressive temperature gradient. What is the largest temperature difference due to inversion you have ever observed in the region?
ReplyDeleteRemarkably low dew points and relative humidity along with very warm temperatures being reported from higher elevation weather stations this week.
ReplyDeleteOn 12/3 at 0200hr, Panorama Dome, the high point of the Mount Baker Ski Area at 5020', reported a temperature of 42F and dew point of -27F; good for a relative humidity of 4%. By around 2000-2100hr, the temperature had risen to nearly 60F!
On 12/3 at 0900, a station at 6240' above Chinook Pass reported a temperature of 46F and dew point of -28F, yielding a relative humidity of just 3%!
The graph showing the extreme divergence of temperature and dew point above the inversion layer is quite illustrative of these conditions. Its very good that this is occurring during the winter because I suspect such an occurrence during summer would lead to extreme wildfire danger!
Flying out of Sea Tac yesterday the cascades looked like an archipelago amidst an ocean of fog. Quite impressive.
ReplyDeleteWhy do they keep pushing out the rain forecast? First it was going to rain today, then it was Thursday, now it is pushed out to Friday night or Saturday.
ReplyDelete