2005-12-31 1930Z Wx Discussion and Forecast
Present event is ending from NW to SE over the lower Sacramento basin area as upper short/wave trof comes onshore next hour or 2 and synoptic-scale dynamics begins favoring subsidence over ARB. Low-level onshore flow and lowering static stability due to cooling aloft nevertheless favors persistent but much lighter orographic pcpn over the Sierra above 2000 to 3000 ft through the day today, ending after sunset. According to reports, snow level has dropped to near 5500 ft following frontal
Next event is hard on the heels of the present one, as has been indicated by the progs for
the last few days. There will be 3 interacting aspects to this next system: a deep occluded low and associated s/w trof aloft that will track toward OR and then turn N paralleling the coast, bringing a frest batch of pcpn to the north coast of CA mainly N of Pt Arena by Sunday morning 1 Jan, the baroclinity left behind by the present exiting system, and a trailing lower latitude short-wave trough aloft presently near 35N/160W. The baroclinity, focussed in the 850-500mb layer but based at the surface, is passing into the lower San Joaquin Valley at present. It will not progress much farther south than the Transverse Ranges separating central and southern CA before the circulation with the aforementioned occluded surface low and later the trailing short-wave trof perturbs it north. Nevertheless it appears likely to remain south of the ARB, based on inspection of GFS control runs and ensembles. GFS runs have been suggesting a significant wave will initiate along this baroclinic zone (aka front) on Sunday as the trailing s/w trof approaches the coast. This
baroclinic zone will be the focus of the heaviest dynamically-induced pcpn, so I believe the heaviest pcpn from this next system, mainly midday Sunday thru late Monday, will be over the Santa Lucias along the central coast eastward into the central and southern Sierra, with Blue Canyon, being south of the strongest action, picking up a mere additional 1.5 - 2" between Sunday afternoon and midday Monday. Snow levels will remain about 5000 ft.
Looking farther ahead, the next wave and surface low in the strong zonal flow across the Pacific is forecast by guidance to take a more northerly track, affecting mainly the Northwest on Wednesday, with little or no pcpn over the ARB. As this takes place, GFS and ECMWF are both indicating massive baroclinic development in the central Pacific near 150W. Eventually this will shift eastward to
the coast, but the intensity of this development presages some ridge building along the
coast beforehand, so it still looks like midweek (Tuesday thru Thursday) will be mostly dry in the ARB.