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HMT Forecast Made 1930Z W 2 Jan 08

Big event still on tap for Th - Sunday 3-6 Jan. Progs remain consistent in showing a 3-part event, beginning Th and ending Sunday. GFS from 12z this mrng and ECMWF from last night both indicate 5" or more melted pcpn KBLU vicinity by 00z M 7 Jan. I see no reason to think that this will verify too high, although the trend in the progs has been to shift the heaviest pcpn a little south. This mrng's GFS actually has the heaviest total on the San Gabriels behind LA, and the ECMWF from last night has a whopping 10+" near Yosemite south to Kings Canyon. The NAM is indicating somewhat more than the GFS and may be excessive due to problems with nonconservation of their water-substance variable.

As is typical, the situation is evolving to be a bit more complex than earlier progs. The very strong jet noted in the central Pacific yesterday is now diving down the back side of the amplifying eastern Pacific trough. Subtropical moisture has been entrained into the trof's circulation and on IR satellite a feed of moisture is evident in the wsw flow aloft from near 28N/128W to near 20N/145W and then west to HI. The surface front offshore from CA is showing signs of wave development near 37N/152W. The vorticity max that constitutes part 1 of the event appears near 40N/148W. All this seems likely to coalesce at the surface into a low center that will track NEwd toward a point offshore from the CA - OR border sometime Th night. As it does so, the moist tongue noted in the IR above will likely track more NE to NNEwd toward the Bay Area.

All this has bearing on the time of onset of serious American-River-Basin (ARB) pcpn. It is noteworthy that there is more divergence in the GFS ensembles on start of pcpn over the ARB this mrng than a couple days ago. I think it is still likely that serious ARB pcpn will hold off till after sunset Th, though there may be periods of light pcpn at higher elevations, or at least low-hanging virga that will give radar returns during daylight hours as mid-level moisture comes in from the SW over the dry air at the surface. This first event seems likely to have a more S to SSW fetch at low levels; this and trajectories of the low level air feeding into the first system will favor the Coast Ranges over the Sierra for Part 1, especially early in the event. I expect that with the dry air entrained into this system, the melting layer at KBLU will start out at the surface, and that KBLU will remain in the melting layer overnight Th.

Part 2 of the event looks similar to what has been forecast for the past 3 days: deep low approaching the OR coast from the west on Friday, setting up a very strong fetch from the SW and bringing abundant pcpn to the northern 2/3 of CA. It was pointed out during the discussion that the moisture flux anomalies (don't recall what level) are 6 standard deviations above the mean (although the distribution of these things is highly non-normal). Progs continue to indicate a frontal passage late Friday over the ARB, with the heaviest pcpn occurring with this. Winds are stil aniticpated to be strong all areas Friday, with combined barrier jet and strong presssure gradient. Dave Reynolds is expecting in excess of 100mph at Squaw Peak. Pcpn rates are likely to be ~ 1"/6h the last half of Friday in the upper reaches of the basin, heavier in the hour or 2 right around frontal passage. Melting layer should rise on Friday, and I think KBLU may well be just below the melting layer much of the day F into F evening. However, melting layer will descend back thru KBLU Friday night and with high confidence I think it will be below KBLU from Saturday daybreak onward.

There is at present no clear indication of a break coming between parts 2 and 3 of the event. Likewise, the timing and intensity of part 3 is uncertain. The GFS high resolution run continues to give more emphasis to this than does the ECMWF, and has it coming in Saturday night, rather than the later Sunday timing of the ECMWF. I think what we can say with some confidence is that there will continue to be strong orographic forcing behind the frontal passage Friday night, continuing at least through midday Sunday. This will result in continued snow above 3,000ft or so, with pcpn rates of 1/4 to 1/2"/6h Saturday and at least part of Sunday.

John B.