Major pattern shift to come…

The reign of anomalously strong high pressure over CA and the West Coast appears to be coming to an end. A weakish trough is currently visible on satellite approaching the CA coast this evening, bringing with it much cooler temperatures and some light to briefly moderate rain along the cold front. Light rain will fall throughout most of CA during the next 24 hours, though totals should stay below 0.5 inches in most places and 0.25 inches in SoCal. Some convective showers will be likely behind the front tomorrow and an isolated thunderstorm is possible given the strong March sun angle, but not strong activity is forseen. The southern half of the trough now looks to pinch off and form a cut off low that will drop south to the pacific west of Baja CA. This will allow 95% of the state to dry out while the southernmost counties (mainly around the San Diego area) see a few wrap-around showers and possibly a few thunderstorms on Wednesday. 

Much more interesting is the extended discussion, however. There has been much discussion in recent days over the potential evolution of the MJO in the Pacific and the possible impact of an enhanced area of tropical convection over the Pacific north of Australia and east of Indonesia on our weather in the coming weeks. In addition to all of this tropical forcing, a Kelvin wave appears to be propagating across the Pacific, which will increase sea surface heat content and allow for more convection to form farther east. All of this tropical forcing has caused the East Asian Jet to strengthen dramatically, with a very wide and elongated core of 150 kt plus wind speeds currently in the Western Pacific. This enhanced jet stream will eventually cause the ridge downstream (currently located over the eastern Central Pacific) to amplify such that the deep and strongly digging trough on the eastern side of the high plunges south 500 miles west of CA by day 7. The GFS has a nearly 200 kt jet core impinging on the CentCal coast on day 8, with lots of baroclinic instability and cold air advective energy coming in from the north. There is the potential for some genuinely strong storms in this pattern…particularly in NorCal but eventually for the entire state. This pattern would potentially be a very cold and moist one…potentially not unlike the record-low snow levels that fell in March of last year. The Sierra snowpack, recently having endured 60 degree afternoons, will be bolstered significantly. With such a cold system arriving so late in the season with the strong near-April sun, thunderstorm activity is a near certainty. This is still a ways out…so we should wait before getting too excited…but it will certainly be a marked departure from the 100 degree heat in the Southland as of late…