You can see why the weather models had such a tough time with yesterday's forecast for Los Angeles. Much of the day the main rain band associated with the upper level low spinning off the coast was stalled and producing moderate to heavy rain in areas of Ventura, Santa Barbara and San Luis Obispo counties. New daily rainfall records were set at Santa Barbara, Santa Maria, and Paso Robles airports.

With just a slight change in the position of the low, that rain band might have spent the day over Los Angeles instead. As it was, the rain did finally move into the area during the afternoon, resulting in an inch or more of precipitation in many areas, and snow in the mountains -- closing I-5 over the Grapevine. Here are PDFs of some preliminary Los Angeles County rainfall totals, and a NWS Public Information Statement with some preliminary rainfall totals for the ongoing rain event.

As wet as it's been, it could get wetter. This morning's 12z NAM shows the upper low currently off the coast being replaced by another -- stronger -- upper level low. The new low is forecast to tap into an area of high precipitable water in the tropics and draw that moisture into Southern California Saturday night. The result could be copious rainfall. The wet antecedent conditions would likely increase runoff. In addition, thickness values and 850 mb temps are forecast to rise, which could result in a higher snow level, with rain on snow a possibility.

But keep in mind we're dealing with another upper level low. The models continue to have difficulties with the forecast. The 12z GFS is astronomically wet, generating 6 inches of rain at Los Angeles between early this morning and Sunday afternoon. A BUFKIT analysis of 12z GFS data for Van Nuys shows a peak rain rate of 1.5 inch/hour Saturday night! The GFS is also wetter in the short term, producing about 2 inches over the next 36 hours. In comparison the 12z NAM generates only about 2.75 inches at Los Angeles through Sunday, and less than an inch through Friday. We'll see!

Update 01/24/08 4:00 p.m. The 18z NAM and GFS are not as wet as the 12z runs. The 18z GFS generates about 4 inches through Sunday evening, and 1.5 inches through Friday evening. The 18z NAM produces about 1.75 inches through Sunday evening, and 0.4 inches through Friday evening.

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