More rain for California this week

Filed in Uncategorized by on December 26, 2010 126 Comments

The “atmospheric river/Pineapple Express” event of the past week brought some very impressive rainfall to southern portions of the state. Precipitation totals in excess of 25 inches were measured at a couple of gauges on south-facing slopes in the Transverse Ranges, and even the lowands of SoCal saw storm totals of 8 to 12 inches. Quite a few places, in fact, saw close to their annual mean precipitation fall over the course of only a week, and more rainfall is on the way.

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GFS depiction of Tuesday/Wednesday system

Though the storms expected in California over the next week or so won’t be nearly as wet as the ones last week, the anticipated systems will pack a bit of a punch. Instead of warm and extremely moist, these upcoming systems will be modestly moist and very cold. A very cold airmass originating over the Canadian interior and the Gulf of Alaska will dive south behind the strong cold front late Tuesday. The front itself will bring a substantial amount of rainfall, and the models are indicating some very impressive upward motion associated with this system, so rainfall near the time of frontal passage may be rather intense and possibly convective in nature. A wave may develop along the front as it approaches the Bay Area (as indicated by the NAM), so precip totals could be further enhanced there. SoCal will see some significant rainfall out of this cold storm, though only the mountains will see totals much in excess of an inch. Gusty winds will precede and accompany the cold front, but these will be nothing to write home about.

Very cold air will filter into the region in the wake of the sharp cold front. 850 mb temperatures will drop well below 0 C and possibly approach -4 or -5 in Central California by Thursday. Some convective showers with small hail and isolated lightning will be possible on Wednesday, but the airmass thereafter will stabilize and dry sufficiently to prevent significant additional precipitation in the cold advection regime. Whatever isolated to scattered showers remain by late Wednesday and Thursday, though, will fall as snow above 1000 feet or perhaps even lower in the north. Snow levels towards the end of the event, even as far south as San Diego, will fall to around 3000 feet. More problematic may be the sub-freezing temperatures that are likely in the inland valleys Wednesday, Thursday, and possibly Friday nights. Though it does not look exceptionally cold, there may be a pretty long period of sub 32-degree temps in some places.

The GFS and ECMWF are in disagreement regarding the pattern for New Year’s weekend, but in general it does appear that California will see some unsettled conditions and probably continued below average temperatures going into the new year. The long-range GFS indicates a rather active cold pattern, with a retrogressive Pacific jet, for the next couple of weeks. It does certainly appear that the first half of the wet season has been very good to the state’s water tables and mountain snowpack…

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