Maritime Polar Air Masses

During the winter, cP (Continental Polar) air masses move east-southeast from Asia over the Pacific Ocean around the Aleutian low.

The ocean water modifies this air mass by adding warmth and moisture to it.

It gradually transitions from a cP air mass into a mP air mass.

By the time it reaches the USA, it is cool, moist and unstable.

You often hear the television weather folks call this a warm storm, and it usually brings a lot of rain to the Pacific Northwest with some high mountain snow.

We can see mP air masses move into the New England states off the North Atlantic, but it isn't nearly as common due to the westerly winds that prevail up at the jet stream level.

Maritime Tropical Air Masses (mT) originate from the tropics and move across oceans toward land.

The best example of this is the "Pineapple Express" -- which is a flow of moisture from Hawaii toward the west coast of the US.

This flow of moisture can be really powerful -- with the best example coming from January 1997.

The "pineapple express" slammed central and northern California with tremendous flooding that sent thousands running for higher ground.

Yosemite National Park was closed for 2 months due to flood damage after the storm event.

In the eastern US, maritime tropical air masses that impact the weather mostly originate over the Gulf of Mexico or the Carribbean Sea.

Finally, we have Continental Tropical Air Masses and these originate in the deserts of the southwestern US and northern Mexico.

This is when we see an area of high pressure bring several days of 100+ degree heat to the center of the country, typically in late June to early August.