Patchy fog

[gswearin]

Hi All
Is there a weather app or other way to tell if fog is ahead? I’ve had the fog lift, all looks clear and then run into it again.
Thanks.

 

[SJI Sailor]

Not that I know of (and I'm familiar with lots of weather apps). The problem here in the PNW is lots of microclimates with varying temperatures that can easily drop below the dew point in a small region. Those essentially cannot be forecast because forecast models work on a larger scale and need lots of local data measurement (which doesn't exist for all those little areas on the water).

One example I often observe is clear weather at sunrise looking east from San Juan Island. Then a bit later, cold air pours south off of Shaw Island and a thick (but very local) fog bank settles in for an hour or two at the south side of Shaw. No way there could be enough weather data to forecast it.

Best bets are to go around; wait it out; go slow and use radar + sound signals; and/or use the radio to give and receive weather advisories from other boaters. The webcam feature on Windy is helpful, too.

One time I left Friday Harbor in dense fog, headed for Cattle Pass and the Strait. I hoped that the Strait would be clear -- otherwise would be quite daunting to go 15 miles in open water in fog -- but couldn't tell! Luckily a northbound yacht saw me on radar or AIS, got on the radio, and gave an FYI that it was perfectly clear starting at the pass. Many other times we've simply waited an extra day.

 

[gswearin]

Thanks! I was all clear at Langley but Clinton ferry was socked in. Would have waited if I knew. Radar and AIS got me through (with AIS I could see the ferry leave the dock).

 

[Kathy H]

While not an app, the NWS has a website that shows observed temps and dew points.

https://www.wrh.noaa.gov/map/?obs=true&wfo=sew
Choose the observation tab, then select dew point. Zoom in on the map.

Sent from my Pixel 4 XL using Tapatalk

 

[knotflying]

And for information, the closer the temperature and due point are the more likely fog. Due point is the temperature at which the moisture in the air condenses and will cause a cloud. Fog is a cloud low to the ground. So it the temperature is 70 degrees at the ground or lets sat sea level and the due point is 68 degrees one could expect a cloud formation at 500 feet above sea level since the air temperature usually cools at 4 degrees Fahrenheit per 1,000 feet. There are many other aspects to consider in the equation, but this is the down and dirty method.

 

[SJI Sailor]

+1 to knotflying it is certainly a good idea to forecast / expect the general possibility of fog. As temp and dew point get closer, the potential goes up.

Some weather apps can visualize this and may include color coded visibility as an option. I often look at those from HRRR forecasts. However, they are at best "informative" rather than precise due to all the local variations. Lots of times that says clear when I'm looking out at a fog bank; or says foggy when the sky is clear (although that makes me anticipate the potential!)

BTW a common misperception is that it can't be foggy and windy at the same time. It can -- although wind may help to clear fog out, it also may bring fog in. (Ask folks sailing around San Francisco about that!)

 

[Diatom]

Windy.com has a tab for fog forecast (and others for various cloud layers) on the right "more layers" dropdown menu. Selecting fog drops you out of ECMWF algorithm, leaves you in ICON (switch back manually for best wave/wind forecast). I've found the fog forecast a bit understated: fog encountered in a much larger radius than forecast, but accurately centered.

/tmm

 

[FlyMeAway]

Ventusky also has a fog layer and I find it easier to use / at least slightly more accurate than Windy (the times over the past however many years I've checked both).

I suggest toggling back and forth between the apps to see which interface you like more and which you feel is more accurate for you more often. Fog forecast (and even current fog conditions) is a bit of art in addition to a bit of science.

https://www.ventusky.com/

Also, if you want to go DEEP down the rabbit hole (I did this while stuck in a marina in the San Juans once, when I had my sailboat) you can divine your own fog forecasts from a few different publicly available real-time infrared satellite maps. The technique is different for daytime and nighttime fog, and frankly was one of those things that took me like 6 hours to learn across a bunch of meteorology blogs and was arcane enough that I promptly forgot it (mostly because Ventusky and Windy are not directly observing fog, but doing the same algos on infrared data). There are Coasties who are experts in this sort of thing, so said the one in the bar who started me down the rabbit hole 😀

 

[SJI Sailor]

+1 In Windy it's nice to cross-check this with their webcams (very handy!) Right now I see forecast fog in Des Moines WA, but webcams are clear. And they forecast it to be clear around Port Townsend, yet some of the webcams show fog on the water. It also shows fog forecast where I am but I'm looking out to 10+ mile clear visibility. Those are only the first three I checked.

So the accuracy of the forecast is not great in my experience in the PNW microclimates -- but OTOH it can give a sense of the odds and when to check for more info like cameras or airport readings. (Almost every paved, public airport reports visibility from an automated system.)