Law of averages brings a wet storm this week

The blocking ridge of high pressure over the Pacific Ocean off North America couldn't last forever. It just seemed that way. The approaching storm you have been hearing about is actually two systems and it's the latter one, expected on Friday, that promises to bring the deepest soaking to drought-parched California.

Like your weather observations wonky? "For the first time since December 2012, a deep, well-developed trough will arrive just west of the California coast and direct a deepening low pressure center over the Golden State," says the blogger at California Weather Blog. "This trough and associated surface low pressure system will coincide with the appearance of a temporarily enhanced East Pacific jet stream, which together will set the stage for a substantial statewide precipitation event."

It gets better.

California will be located underneath a region of strong upper-level divergence in the favorable left exit region of a strong jet streak rounding the base of this deep trough, which will allow the surface low to deepen as it approaches the coast on Friday. Given the presence of a sub-1000mb surface low near the NorCal coast, a strong Pacific jet overhead, strong upper-level divergence, and the presence of more than adequate lower tropospheric moisture, most of the necessary ingredients will be in place for a pretty impressive storm event across much of California, especially by the middling standards of the past year or so.

Heavy precipitation can be expected for at least brief period in many locations, especially near the time of cold frontal passage. Strong upward vertical motion and modest near-surface instability suggest the potential for convective activity as well, with possible high rainfall rates and thunderstorms (especially across Southern California). Strong and gusty winds may also be an issue, especially given the proximity of the surface low to the California coast and the potential for convection to mix strong winds aloft down towards the surface. I actually expect precipitation amounts to be surprisingly uniform from north to south, with generally 1-3 inches both along the coast and inland all the way from Eureka to Los Angeles, with areas to the south potentially seeing somewhat less than that but orographically favored regions (especially the Transverse Ranges in SoCal) locally seeing 3-5 inches. While these totals are well under the highly localized extreme 10+ inch values observed in NorCal during the recent atmospheric river event, I expect that this storm will bring more water to California overall given the widespread nature of the precipitation.

It also will be a colder storm than the ones earlier this month up north, so there should be significant snow laid down on the upper levels of the Sierra Nevada. Guess what: On top of this being the driest rain/snow year on record in California, the month of February itself is below normal in much if the state.

It's not too often you see the local rain forecast given as 100 percent, especially a few days out. Here's the latest National Weather Service statement:

A change to wetter weather is likely for Southern California later this week as two storm systems move across the region. The first storm is expected to move through late Wednesday through early Thursday morning, bringing light to moderate rainfall to the area. From late Thursday night through late Saturday night, a second and much stronger system will move across the area and that has the potential to be the most significant storm in quite some time. One to two inches of rainfall is likely in coastal and valley areas and two to four inches is likely in the foothills and mountains. Due to strong southerly flow ahead of the storm, locally higher amounts are possible in favored upslope areas in the local mountains. Snow levels will remain high on Friday at above 7000 feet, but will drop to between 5000 and 5500 feet Friday night and Saturday. Winter storm conditions will be possible in the mountains due to snowfall and gusty winds. Additionally, this storm will be rather dynamic and thunderstorms may develop which could produce intense short-duration rainfall. Communities in and around recent wildfires, especially the Colby and Springs burn areas, will need to be alert for heavy and intense rainfall which could produce mud and debris flows. Drainage areas should be cleared of debris to help reduce the chance of urban flooding.

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