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HMT Forecast made 1930Z Sat 29 Dec 07

A weak system is passing over the area at present, with smartly rising barometers on bouys off the Northwest coast behind a wind shift from south to west near shore. Light pcpn is likely to continue, over the higher terrain particularly, through noon tomorrow (Sunday 30th), as strong onshore W to WNW flow at mountain-top level continues. Cold air aloft over the Northwest will favor convection that area beginning later today, but I think convection farther south will be shallower and not an important pcpn producer (< 0.4" melted at Blue Canyon) over the ARB. By afternoon Sunday conditions will begin to improve.

Early next week should see calm, dry conditions over the American River Basin, with weak offshore flow as high pressure builds inland over the Northern Rockies and Great Basin and the Pacific High weakens. This will promote a mid-level (~700mb) flow of dry, relatively cool air from the east and, later southeast, over the area, and will be favorable for fog formation in the Valley under a weak subsidence inversion.

Situation still on track for a good event beginning Wednesday night or early Th 2 - 3 Jan, continuing through Saturday night the 5th and perhaps longer. As moist ascending air coming in from the SW aloft during the initial phases of the event experiences cloud and pcpn formation, there will be a period of evaporating pcpn (i.e., virga) aloft prior to pcpn reaching the surface. It remains to be seen how much of the high moisture near HI (up to 2.3" IPW) will work into this system, but the GFS is keeping 500-1000 mb thickness values mostly below 550dm in the ARB area, leading me to think that high snow levels (say, above 6,500ft) will not occur. Given that there seem to be only 5-6" snow-water equivalent over the higher elevations at present, it seems unlikely that this event will produce a major snow-melt enhancement of runoff. The HPC notes that GFS moisture-flux-magnitude anomalies are +4-6 standard deviations above the mean. GFS is indicating up to 8" water equivalent for the 5 days ending M 7 Jan, and with much of this coming as snow at KBLU, appropriate preparations are called for. GFS operational calls for 2-3 pulses of pcpn durg this period and ensemble members are in agreement with the overall timing of the event, in particular the onset as being after sundown on W 2 Jan. At higher elevations, it is likely that pcpn will be continuous for 48-72h beginning probably early Th 3 Jan, but there will be periods when the pcpn will become quite light. Snow level is anticipated to time-vary between 4,000 and 5,500 feet, probably near 4,000ft at start of the event, and dropping below 4,000 ft by Saturday. Details on the GFS operational from 12z this mrng suggest that there may be one or 2 good warm advection events during this period, including the initial event beginning W night or Th, 2nd or 3rd Jan.

Following this, it aprs likely, based on the GFS ensembles, there will be a quieter period beginning Sunday -Monday the 6th, and 7th. However, this is far enough ahead that confidence is only medium.