by Meteorologist Kerry Anderson
Each day when we create a forecast for winds we use many different tools, surface and upper observations, numerical forecasts models, soundings, wind profilers to name a few. For today’s forecast the most important tools have been the upper air soundings and satellite pictures because in order to determine how strong the winds will be at the beaches this afternoon we needed to know how much insolation the clouds would block. If the inland valleys don’t heat up there is little thermal gradient and then less support for the afternoon sea breezes.
At midday the visible satellite picture (Fig 1) showed clearly that the marine layer stratus was well developed and pushed right up against the mountains. Marine layer clouds develop when the ocean surface is cooler than the air above. That happens frequently in this area because of the cold ocean currents that flow into the area.
Figure 1 Visible Satellite picture for 21 April 2015 for Southern California. You can clearly see the marine stratus that have banked up against the mountains keeping the Los Angeles basin socked in with clouds. Also notice the high level clouds coming off the Santa Barbara coastline spinning around an upper level low and the thunderstorms building over the San Jacinto and San Bernadino Mountains
You may have noticed in the forecast today that we called these clouds marine stratus rather than marine layer clouds. They have that distinction because the base of these clouds is higher than the usual marine layer clouds. An upper level low in the area is giving the air some lift. When the air is lifted to the point that it condenses a cloud will form. We call that the Lifted Condensation Level (LCL). That level is calculated and noted on a sounding ( a plot of the temperature, moisture and wind profile for a location). You can see the plot from San Diego this morning in Fig 2. It clearly shows why these clouds developed. The LCL is quite high at about 4000 feet, so we don’t have fog and drizzle but a higher based stratus deck that has pushed up to the mountains.
