What is best prediction of fog in San Juans?

Listen for fog horns from Ferries in the distance
 
I've never used this feature of the Windy App, but it does have a cloud base map that shows in purple when the clouds are at zero feet. Not sure if that would relate to probability of fog or not? I'm usually somewhat happy to find fog along the trip since it typically correlates to calmer sea conditions and the radar overlay on the chart plotter is a good safety tool in those situations. In my experience, often the fog bank doesn't go quite all the way to the surface leaving a reasonable distance where we can still visually pick out obstacles (crab traps, logs) ahead. Even if the fog does come right down, it seems I still have enough forward visibility that slow ahead is possible.
 
Look at a weather report. If the temp and dew point temps are close, there is a very good chance of fog.
 
Fog in the San Juans comes in two forms: everywhere and unavoidable (and obvious), and highly localized. For everywhere fog, you just wait it out and check the dew points as noted. Usually it clears by noon (or earlier) but occasionally lasts almost all day.

As for localized fog, it can be a surprise. It may be clear in Friday Harbor, for example, but have thick fog between Shaw and Lopez (that's a common pattern) or clear at Anacortes and foggy in Rosario Strait, or clear at Rosario Resort and foggy in Thatcher Pass. As noted clear skies and relatively high dew point are indicators. One surprise is that it often starts out clear in the SJs and then fogs up an hour or two after sunrise!

A good tool in those cases is the radio + AIS -- find other vessels on AIS and ask them what's going on where they are (using channels 68, 69, 72 after hailing on 16). I have navigated heavy fog from FHR to Cattle Pass on the basis of radio assurance that it would be clear in the Strait of Juan de Fuca (and it was, clearing up right at Cattle Pass. Would not have wanted to cross the Strait otherwise.)

Adding to the others here, I also check general aviation airport reports (an app like Avia Weather, using near-water airports like KFHR, KORS, KBFI, etc.) and also the Webcam feature on Windy (ferry terminal cameras can be very helpful).

Currently looking out my window in Seattle I see clear skies and the Olympic Mountains. But Boeing Field just a few miles away reports low instrument conditions with 400ft ceiling! SeaTac reports 100ft ceiling. So I know there must be some fog out there. If I were headed out, I would check webcams around my destination, and AIS tracks for more insight.
 
Windy also has a webcam layer where I discovered rockridgeweather.com. The RR website is useful for viewing Guemes Channel and Rosario Strait. Another app is Zoom Earth. Zoom Earth has a 20 minute delay of satellite photos. It shows fog, radar, temperatures, etc. I'm not sure I agree with Zoom Earth's wind measurements yet, but found the fog photos accurate in the San Juans.

Sent from my Pixel 6 Pro using Tapatalk
 
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