a word on low clouds
Hi everyone! First post here for us.
It seems like June Gloom came early this year, with April Showers never really happening, and May Flowers were in March, but... anyway, it seemed like an appropriate time for us to have an important discussion on low clouds.
As meteorologists, low clouds (which you often see from marine layer, and in mountain areas) are the absolute bane of our forecasting existence. They are by far the most uncertain thing in our model. Tahoe and the Sierra skunked tonight because of low clouds.
In fact, the problem isn't just ours. Weather models and meteorologists have been struggling for many years to try to forecast the exact position of the marine layer, and they've gotten much better, but still they struggle. Why?
Weather models take a LOT of computational power to run. They can't process data for every single place in the world. Instead, they break down the country into a computational grid (here's an example). This is called discretization. The best weather models in the U.S., which we are based off of, use a scale of multiple kilometers for the length and width of each grid box.
That means, potentially, that yes, all of SF could be in the same grid box. And the weather models might not know the difference between, say, Rodeo Beach, which may be foggy, and Sausalito, which may be clear. Worse yet, low clouds can be very small and localized, so if the model thinks there will be low clouds in a grid box, there might be one small cloud in the middle of the box, or the whole box could be filled with clouds.
Yikes! Now you see why we need real-time reports from you all, and why we have a meteorologist stalking the webcams and satellite images all the time. This is also why the models often flip-flop on whether or not there will be low clouds at any particular Escaype station -- if one thing changes by a tiny amount, that could be the difference between clear sky and low clouds above your head.
The good news is that this grid box problem is hardly an issue for mid and high clouds, which are the source of most spectacular sunsets. The reason for this is that higher clouds are visible from much further away; that is, they're much less sensitive to where you are. If there are high clouds overhead, you could drive 5 miles and you'd hardly notice a difference. Very different from low clouds!
That's why the Escaype model forecasts often have trouble with low clouds, as does any weather model. If you ever find yourself in a situation when Escaype forecasted a mid/high cloud blowup, and you're socked in by low clouds, make sure to check and ask for reports from people nearby to see what's happening elsewhere, because 5 miles away, it might indeed be blowing up. And, of course, if you find yourself in position for Escaype forecast for a low cloud blowup, and it's clear over your head but you see the clouds a couple miles away... just drive underneath them. (You can always send us pictures and check in with us -- we'll do our best to let you know what's up.)
Fortunately for us in the Bay Area, we get the majority of our thick marine low clouds in the summer. In the fall, winter, and spring, when we get many of our best skies, low clouds often aren't as big of a threat. Now is our time to learn to work together and pinpoint where the low clouds are, so when the epic skies come, we'll all score every time.