The source of June Gloom is the large pool of low clouds that develop over the eastern Pacific. Here is the image this morning....the eastern Pacific is FULL of low clouds. Strangely enough, such extensive clouds occur under high pressure conditions, which develop as the East Pacific high builds and extends northward.
Why does high pressure help low clouds? One reason is that high pressure is associated with sinking and warming conditions aloft, which forms a low-level inversion that traps moisture at low levels. The vertical sounding (plot of temperature and dew point with height from a balloon-launched radiosonde) at Qullayute, on the WA coast, shows exactly this structure (see below).
Dew point is shown by the blue dashed line and red is temperature. Height is in pressure (900 is about 3000 ft up, 700 about 10,000 ft). In the lowest few thousand feet the atmosphere is saturated (low clouds), with inversion (temp increasing with height) above. Primo conditions for the gloom.
Here is the pressure pattern for 11 AM this morning...you can see the high offshore (solid lines are isobars, lines of constant pressure)
In June, we generally have the high parked offshore, lower pressure over land, and an onshore pressure difference that pushes the low clouds to the Cascade crest. June gloom. As shown in the satellite image above, their is often a sharp end to the gloom near the crest, so sun is just short drive east on I90. Yesterday, I did just that with a friend, mountain biking to the UW Manastash Ridge observatory near Ellensburg. Sun and amazing wildflowers.
We can escape the gloom for a while when easterly flow develops, displacing marine air with dry, continental air. That happened earlier this week when it warmed into the 70s. But real relief often has to wait until July, when the high pressure builds northward and the winds aloft become more northerly.
And if it makes you feel any better, June Gloom is a phenomenon that hits the entire West Coast, including our brethren in southern California (see pic). They always have their hot tubs.
June Gloom at Seal Beach, CA