By Jim Purpura. Posted September 7, 2011, 8:11 PM.

You may have seen the Temecula forecast for Sunday night and Labor Day...

Tonight: Mostly clear and breezy in the evening, becoming partly cloudy, low around 65 degrees F. Labor Day: Partly cloudy, very warm, afternoon breezes, high of about 93 degrees F. Monday night: Partly cloudy, evening breezes, low near 65 degrees F.

...and noticed it just didn't work out. What happened?

What we saw happen meteorologists call "elevated convection". This is a case when showers and thunderstorms do not develop from the surface up (what we call the "boundary layer")... but instead have their bases above a stable (cooler) layer of air.

Thus elevated convection can produce showers and thunderstorms in places where we don't normally expect to see storms, such as over the ocean, or at night during the summer.

On Sunday night elevated convection developed over the LA area, and moved east across much of the Inland Empire on Monday. Widespread, light rainfall was seen, with Temecula topping the list at 0.09 inches. Even more significantly, high temperature forecasts for Labor Day were off by almost 10 degrees, with many places having highs in the low to mid 80s, vs forecasts highs in the mid 90s.

Elevated convection is one of the hardest situations to forecast in Southern California. Computer forecast models do not usually do a good job of forecasting these events well in advance. The base of this convection can be 15,000 to 20,000 feet or more above the ground.

Even though elevated convection may produce little rain on the ground, it can be dangerous. Winds of 50 MPH or more may descend from the storms. In 2008 a large area of elevated thunderstorms produced thousands of fire starts in northern California where lightning struck the dry ground. The resulting fires burned for weeks and caused serious air quality issues across Northern California and the central valley.

There is hope, however. Computer forecast models are becoming more accurate at predicting these events. Busted forecasts happen less frequently, although Labor Day was a case of this happening.

Finally, always be aware when you see mid level clouds (altocumulus or cirrocumulus), especially in the morning. This may be a hint that that layer of air well above the ground is unstable. This may mean there is a a chance of showers or thunderstorms, especially in the mountains, in the afternoon.

Jim Purpura

jpurpura@weathercurrents.com