Yes, a thunderstorm passed over LA this morning — but what caused it?

If you heard thunder and lightning in the early morning hours today, you weren't dreaming — there really was a storm system that passed over Los Angeles County.

These storms are rare during the summer, but certainly possible during heatwaves. These often take the form of dry thunderstorms with rainfall that often doesn't reach the ground.

It's less common to see the amount of rain we saw today — which even caused isolated flash flooding — but certainly possible, as L.A.'s early birds (and people with sensitive pets) saw this morning.

This thunderstorm formed over the Pacific Coast before moving northward across L.A. County, according to Ariel Cohen, meteorologist in charge of the National Weather Service in Los Angeles and Oxnard.

Moisture from the tropics

You can't have a rainstorm without moisture in the atmosphere, and today's rainfall was brought here by winds that are currently circling the heat dome over much of western North America. These winds are bringing more moisture into the atmosphere of this part of the world, according to Cohen.

But that wouldn't be enough to form the thunderstorm we saw today. There also needs to be additional disturbances in the atmosphere, which were present this morning.

"We're bringing in some very minor impulses, or areas of disturbed weather, northward from the low latitudes of the world," Cohen said.

Those spots of moisture interact with the disturbed weather to form this morning's thunderstorms.

"Think about it as ingredients all coming together at that particular time and place and spot to generate a thunderstorm area," Cohen said.

Similar to Arizona's monsoons

If you've ever been to Arizona during the summer months, you'll likely know that monsoons are common there, which provide the area with as much as 50 percent of its annual rainfall and may come as a surprise to visitors expecting a dry summer climate similar to Los Angeles.

That happens because of extreme heating throughout the region, like the current heat dome in the Southwest — the exact same mechanism that caused today's weather in L.A. County.

It's rare for these conditions to reach all the way over to urban L.A. County, but as we saw today, it's not out of the question if the atmospheric disturbances are in the right place.

Will more thunderstorms follow?

It's unlikely that there'll be another significant rain event in most of L.A. County. This afternoon, there's a chance that more thunderstorms could form in the far northeastern part of the county — which didn't see any storms this morning.

There are also spots of moisture currently heading north from the Pacific, though Cohen said the National Weather Service is not currently projecting that those will become thunderstorms — the likelihood is currently less than 10 percent.

As for this morning's storm system, it's now exited Los Angeles County as it works its way northward.