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HMT Forecast Made 1930Z M 31 Dec 07

Models remain consistent in indicating a major pcpn event for the American River Basin Th through Sunday 3-6 Jan. Details, however, are, in my opinion still up for grabs, due partly to a complicated upper level situation near and northwest of HI that the models may or may not be capturing appropriately.

Here are the main aspects of the event as affecting the ARB, as I see it from here.
1. Deep and slowly progressive upper trough in the westerlies will be over the eastern Pacific by W - Th 2 and 3 Jan and shift inland by M 7 Jan.

2. The pcpn event still seems likely to occur in 3 parts. Models have been quite consistent about this aspect for the past few days. First part will be Th as a deepening low moves enewd toward the Northwest out of the eastern Pacific. The associated cold front will approach the northern CA coast by late Th but may not pass the ARB as an identifiable feature, as another surface low pressure advances eastward hard on the heels of the first. This one may be more intense than the first one, though I'm inclined to think that the GFSs with a 960 mb low center is too deep. This second part will bring pcpn on Friday, with probably no break between parts 1 and 2. It seems likely that F 4 Jan will be the wettest day. The final part will be on Sunday, as a short-wave trough traverses the Central Pacific ridge and into the larger-scale cyclonic circulation over the eastern Pacific. The GFS has this feature coming in fartehr south, over the Bay Area. As previous forecasts, this looks like a colder system for the ARB, with less orographic forcing.

3. It now appears likely that significant pcpn over the ARB will not commence in earnest till after daybreak on Th 3 Jan.

4. I'm still concerned about the potential for strong wind gusts due to combined effects of strong synoptic-scale pressure gradient and barrier jet. Friday 4 Jan looks like the highest risk for this, but Th could also see strong winds.

5. Both the ECMWF and GFS are forecasting 5" plus melted pcpn vicinity Blue Canyon for the Th - Sun period. I think that's still a good bet. Models now both have heaviest pcpn over the Sierra near Yosemite.

6. I'm sticking with ARB snow levels between 4,000 and 5,500 ft for Th and F, 3 and 4 Jan, with snow levels down to 3,000ft on Sun.

Looking farther ahead, M and Tu look to be drier days without significant (> 1" melted) pcpn at Blue Canyon. The ECMWF is indicating another significant storm on W 9 Jan, but the GFS takes this farther north, bringing the worthwhile pcpn into the Northwest.

John B.