Cut-off low for California to bring showers and thunderstorms; calmer weather to come

The system that moved through late Friday into Saturday across California did bring a great deal more precipitation than expected across some parts of Southern California. Convective elements within and ahead of the cold frontal precipitation band brought some very high rainfall rates to the recent burn areas near Los Angeles (especially La Canada and Flintridge),  and there were some damaging mudflows/mudslides as a result. Fortunately, there were no fatalities. It does appear that all of the emergency debris basins that were set up in the wake of the fires to divert these debris flows are now full (due to sedimentation from the storms in January), so subsequent storms this winter and spring may produce more significant effects in the Station Fire burn area. This will be something to keep an eye on this week, as some strong convection will again be possible in the Los Angeles area on Tuesday/Wednesday…

GFS depiction of thicknesses and precip on Tuesday.

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A small but rather cold cut-off low is currently developing off of British Columbia, and this will drop south along the west coast over the next 48 hours. By late Monday night, I do expect showers and at least isolated thunderstorms to start to develop over Northern California. As Tuesday wears on, some fairly vigorous convection may fire in the Central Valley as very steep lapse rates and some vertical shear combine with differential surface heating. The main threat, though, will probably be small hail, as temperatures aloft will be rather cold ( -2 to -3 C at 850 mb). The threat will shift southward into Southern California on Tuesday night. Heavier rainfall, on average, may occur in the south as onshore flow provides extra moisture and convergence over the mountains. I would not rule out a severe storm or two on Tuesday/Wednesday, though I expect nothing like the widespread severe weather experienced in January. People near the SoCal burn areas will have to be wary of locally intense rainfall if thunderstorms do develop, however.

After Wednesday, I expect the state to dry out substantially as a fairly strong high pressure ridge builds in from the southwest. Much of the state will see clear skies and normal to above normal temperatures, though the Central Valley may remain largely socked in with Tule fog (as is ever the case this time of year). The MJO signal continues to remain stationary in the West Pacific and be in synch with El Nino, so I don’t think winter’s over just yet. A substantial break in the weather, though, will be a good thing right now…

© 2010 WEATHER WEST


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