2007-01-05_hmt_forecast
The big picture is not much different from what was discussed yesterday, with a strong jet extending off of Asia into the Western and Central Pacific. The main storm system at the leading edge of this jet is approaching the dateline and will ultimately be the next system of potential interest as it tracks down the West Coast mid to late next week. The models are fairly similar to yesterday with a tendency to favor this trough digging down the coast and then inland, as opposed to dropping off the West Coast, for later next week. By this coming weekend (6-7 Jan) the trough moves into the Gulf of Alaska and phases with the current upper level low over Alaska, with upper level ridging over the HMT area keeping any moisture to the north over the Pacific Northwest. The ECMWF and GFS again both leave a southern piece of the system behind as a weak closed low near Hawaii. Otherwise, the phased trough begins to push down the British Columbia coast Monday and Tuesday (8-9 Jan) as strong ridging occurs behind the system northwards across Alaska. By Wednesday the base of the trough is in the vicinity of the Pacific Northwest, with precipitation approaching but remaining north of the HMT area through most of the day Wednesday in the GFS solution. In the latest 12z GFS solution (which is pretty much in agreement with the 00z run) the center of the upper level low passes over the HMT region on Thursday (11 Jan), with the main shot of precipitation Wednesday night into Thursday. There is not a lot of precipitation in the GFS during this period, under a half an inch, but it is a cold system, with 1000-500 mb thickness values dropping to 522 dm. The issue will likely be whether the system will produce enough precipitation to match its interest as a snow producer at lower elevations.
Beyond Thursday the trough progresses eastwards, probably giving us another snowstorm this time next week, but for the HMT region not a long-lived event forecast by the GFS. Most of the ensembles follow this scenario, though a few are a bit slower and drop the trough a bit more off the coast, perhaps leading to more precipitation than the deterministic run is forecasting, though still not indicating a large precipitation event. The ECMWF deterministic run (and apparently the ensembles, per Wes Junker on the telcon today) are pretty much following the trend of the GFS. There are some members of the Canadian ensembles that do drop the trough more off the coast, not to the extent that would create a huge event, but probably enough to bring more considerably more precipitation to the HMT region that could linger until Friday of next week. My experience though with the Canadian ensembles is that they often show more spread than the GFS, but it is not clear whether such spread is indicative of a real potential of a different solution. Bottom line for today then is that there appears to be less of a chance that the trough coming down the coast for next week will drop off the coast and create a significant (higher precipitation) event for the HMT area, but a cold system will occur. However, it could be that the precipitation amounts will be too light to warrant interest. Plenty of time to monitor this, of course, and we will see if tomorrow's runs look any different.
Beyond day 10 (next weekend) the GFS and Canadian runs keep the overall pattern the same with a high amplitude ridge into the Gulf of Alaska and systems coming down the coast but moving inland with time. The Canadian run, however, even for days 10-15, trends with a stronger system closer to the coast than the GFS. One thought for the very long range (15 Jan and beyond) is that the northern portion of this high amplitude ridge could get pinched off and allow for a zonal jet in the Pacific to break beneath the ridge and comence a potential wet pattern for California, perhaps along the lines of what Klaus and Ed had suggested earlier this week.
ed szoke