SUMMARY: Major pcpn event (at least relative to what we have had so far this season) on tap for the American River Basin, beginning Wednesday morning (7th) and continuing thru Saturday (10th).
SYNOPSIS and FORECAST: West Coast ridge has shifted inland and is losing amplitude as the first in what appears to be a series of upper air waves and associated surface lows has made it to the Pacific Northwest. The next in the series is near 40N/135W with a decent surface low and front and evidence for a cold-core upper trough based on presence of deep convection near 140W. The surface front has
a good push of NW surface flow behind it. This system has been given successively more emphasis by the numerical guidance over the last 72h and at this point looks likely to give > 1" of pcpn at Blue Canyon by 12Z Thursday. Pcpn (snow above initially 6000 ft, lowering to 5,000 ft or lower as pcpn tapers off early Th) will begin mid-late morning Wednesday and peak before midnight W evening. I expect a detectable frontal passage with this trough, most likely W evening.
Following this, details are more uncertain. It does still appear that there will be a break of at least 6-12h between the end of the postfrontal pcpn with the first system and the onset of pcpn from the next. This next wave to affect the west coast is apparent on water vapor imagery near 30N/165W at midday Tu. This is a well-marked feature with a strong rising/falling vertical motion couplet with it and a pronounced warm-advection bulge ahead of it N of Hawaii. The GFS high-resolution and the NAM runs from 12Z this mrng differed considerably, particularly beyond 36h, with the GFS more progressive with this central Pacific feature, and the NAM suggesting more tendency for this feature to cut off and slow in its eastward progress toward the coast. The result is that the open warm-frontal wave feature that yesterday looked as if it would give pcpn on warm advection on Th is maintained (but is a little slower) in the GFS, but in the NAM travels on a more NNE trajectory, is weaker, and affects mainly the coast ranges near and north of Cape Mendocino. GFS ensembles were a little less optimistic about this warm-frontal wave than the high-resolution run, but there was also quite a bit of spread.
In the end, I leaned toward the GFS, for the reason that the NAM beyond 48h with strong westerly flow is going to be dependent on 06Z GFS-provided lateral boundary conditions. Accordingly, I am sticking with a good warm-advection event beginning near noon on Thursday (8th). During this event, snow levels should rise to at least 7,500ft in the Sierra by early Friday morning. Another 1 to 1.5+" of pcpn should fall at Blue Canyon during this period. If the GFS is correct with its forecast of a surface low moving approaching the Northern California coast and then mvg northward overnight Thursday, there is likely to be a good barrier over the west slopes of the Sierra Thursday night. This is also suggested by this morning's ECMWF.
Looking farther ahead, all the models agree on amplification of a trough in the eastern Pacific by Friday and its progression inland over the weekend. (This trough will be some combination of the upper trough now near 170W, with another wave approaching the dateline.) As this happens, a frontal zone appears likely to remain over northern or central California, gradually settling southward as the trough approaches the coast. I anticipate that there will be frontal waves on this frontal zone that are not currently apparent in the global model forecasts, with the consequence that the continuous pcpn the GFS is forecasting Th thru Saturday over the ARB will actually come mainly as one or 2 more discrete and probably at times more intense 6-12h periods.
The main event looks like it will end sometime between late afternoon Saturday and during the wee hours of Sunday, with a break of at least 24-36h before any further system gives pcpn to ARB. At present the longer range progs suggest a resumption of the Ern Pacific-West Coast ridging pattern at upper levels by late next week. However, earlier in the week it appears likely that one or 2 light or moderate pcpn events should affect the ARB.