*Major rain event in progress; significant flooding possible in Southern California*

California is currently in the midst of an impressive multi-day precipitation event. A precipitable water plume of over 1.65 inches, originating from the Central Pacific southwest of the Hawaiian Islands is being dragged over central and southern portions of the state. A strong jet steam aloft has coupled with a strong low-level jet to provide an very favorable environment for precipitation production, especially in orographically-favored areas.

Visible satellite loop of complex storm system (NRL)

In addition to the present moderate to heavy rain band moving into Southern California this afternoon, a very unstable convective environment has developed in the wake of the strong front further to the north. NorCal is presently located under a region of tremendous divergence aloft, and when combined with a moist column, steep lapse rates, and veering wind profiles, it’s not surprising that lines of strong showers and thunderstorms are starting to develop. There is some potential for a few severe storms this afternoon and evening (and possibly again tomorrow).

In the Sierras, very heavy precipitation has been ongoing since later Friday afternoon. Snowfall has been extremely impressive at pass level–so far measuring in the 3-6 feet range–but rain has mixed in with the snow as high as 6000 feet. Now that the cold front has passed through, snow levels will plummet to below 4000 feet by this evening. Most of the precipitation for the rest of the week will fall as snow above about 4500-5000 feet, and some very impressive additional accumulations may occur above 6500 feet. Fears of snowmelt-related flooding have eased for the time being as the main warm and moist plume has been directed at Southern California rather than further north, but streams and rivers have still shown major rises throughout NorCal this weekend. Additional heavy precipitation in the coming days may aggravate this situation significantly.

Deep moisture and strong vertical motion will combine to produce very heavy rainfall in SoCal

Over the next 4 days, a complex storm system will evolve off the CA coast as a deepening low acts on deep subtropical moisture to bring copious rainfall to Southern California. Given the complexity of the pattern, it is important to remember that forecasts will probably continue to change as the situation continues evolve. That said, current rainfall projections (especially in the context of antecedent rainfall and expected future precip) have some serious implications. Significant flooding may occur in parts of Southern California over the next 3-4 days, especially on Tuesday-Wednesday as the most intense period of widespread precipitation moves through. Widespread moderate to heavy rainfall will be occasionally and locally enhanced by convective elements, and there’s even the chance of a few severe thunderstorms in Southern California depending on the strength of the front itself. Given the potential for serious flooding in some areas, it would be wise for those in flood-prone or mountainous areas to pay attention to official NWS warnings this week.

After the deep trough finally moves to the east by late Thursday, California may see a day or two of dry weather before what now appears to be a very prolonged period of major storm activity resumes by Saturday. The GFS and ECMWF both indicate a new strong storm, once again with a deep subtropical tap, taking aim at the entire state of California next weekend. Though it is still a ways out, this storm may have the potential to bring widespread heavy precipitation and potentially strong winds once again. In the week 2 period, it does appear that a very active and unusually far south jet stream will keep the storm track aimed squarely at California for the foreseeable future. Hydrological issues may continue to be present going into the new year if all pans out as currently projected. Stay tuned.