This morning’s satellite image doesn’t look like mid-March. Really reminds me of June or early summer.
A large area of low clouds (mostly stratus and fog) covers the northeastern Pacific Ocean, with some clouds pushing into western Oregon and Washington.
The view from the webcam at LaPush on the WA north coast this morning was quite bleak.
Viewing high-resolution local satellite imagery at 8:06 a.m. this morning shows low offshore clouds moving into the Chehalis Valley and heading east down the Columbia River. Look closer and you’ll see a bit of fog in the Snoqualmie drainage system and around Bellingham.
Look even closer and you’ll see thin, prominent streaks, many going north-south and some going east-west. There are several broad (east-west) super streaks along the coast, their shadows cast on the low clouds below.
So why is there this summer-like low cloud pattern?
Because summer-like high pressure has developed off the eastern Pacific Ocean (see surface pressure map as of 10 p.m. Monday).
Offshore high pressure areas create northerly winds along our coast (typical in summer) and a low-level stable layer or inversion (temperature increases with altitude), ideal for creating low clouds.
High pressure is associated with sinking air, especially to the west of the high pressure area. Submerged air warms by compression (just like in your bike pump, which is warm to the touch after you’ve filled the tire). Obviously, the sinking force is weaker near the surface (air cannot move through the ground). With greater sinking and warming aloft, inversions often form at low levels when high pressure is nearby.
Inversions are very stable structures, suppressing vertical movement, which is great for low clouds and fog. They can also promote smog and air pollution.
Want to see the effect of current high pressure on low-level temperatures?
Below is the bottom of the vertical sounding at Forks on the Washington coast, with data coming from radiosondes launched twice a day at that location. The red line is the temperature and the blue dashed line is the dew point.
Mom Mia! It was a very strong inversion, with temperatures rising from the surface (about 1010 hPa) to about 950 hPa (about 2000 ft ASL) by about 13C (about 23F!!).
Temperature and dew point are the same in a very shallow layer: This is where low clouds are found.
Enjoy the flavors of this summer. Temperatures are much cooler and precipitation will return soon.
