California needs the rain. The southern two thirds of the state are in a drought and have been for quite some time. The good news is, some relief is on the way, and rain is already falling across parts of the state. It won’t be a drought buster, but eye-popping rainfall totals will certainly make a dent in some regions, especially the Sierra Nevada Mountains.

California drought map.

An “atmospheric river” has its sights set on California. This moisture plume originates from the warm waters of the Pacific Ocean and is being driven northward by an abnormally strong subtropical jet stream. The moisture caught in this flow is exceptionally rich, and latest forecasts show potential for 1-2 feet of precipitation over Californian high terrain (including snow if you would melt it) and a broad swath of 3-6″ for all but the southern third of the state (which desperately needs rain as well).

The heaviest rainfall will be tonight through Sunday as a strong low pressure system from the Polar Jet Stream detaches and moves south. As it drifts south, this feature will interact with a subtropical branch of the jet stream and further enhance the speed of the atmospheric river. This “fire hose” effect is will pound the California coast and high terrain with a deluge of rain and wind gusts of 50-70 mph.

The map below shows precipitation projected by the GFS global forecast model. Totals over the Sierra Nevada Mountains are pretty staggering and consistent with official forecasts.

That strip of enhanced precipitation is due to a phenomenon called “upslope”, which is another enhancement factor. Terrain is a naturally lifting mechanism, and lift is needed to cool and condense moist air to produce rain. This explains why the windward side of mountains have a wetter climate than the leeward side. It’s almost like a “squeezing” effect (see graphic below). In fact, many of the higher precipitation areas in the map above are tied to areas of high terrain, and in this case, they are perpendicular to the “fire hose” of moisture, maximizing the upward transfer of energy by upslope.

Beyond the storm this weekend, there are strong signs that an active jet stream pattern will continue for much of next week, and although each consecutive storm would probably be less than the combination of factors this weekend, these types of patterns are exactly what California needs to ultimately bring an end to its extended drought.