2006-01-06 WX Discussion and Forecast
Broad trough in the mid Pacific continues with a weakening shortwave at 42N, 139W. This will be the next system to impact the ARB area as this wave moves east but weakens significantly. It should arrive on the coast by early Saturday (7/08GMT). This wave begins the breakdown of the ridge over the western US. A series of waves lined up across the Pacific will be on a path to impact first, the northwest (WA, OR) coast late Saturday (08/00GMT) and Monday (09/18GMT), but will slide further south as the ridge gives way. Two waves will move into central California one on Wednesday (11/18GMT) and another on Saturday (14/12GMT).
The GFS and ECMWF look to be in good agreement for the event on Saturday indicating cold-advection dominated showery precipitation in the range of 0.3-0.5 inches. Freezing levels drop from 9000ft to 6000ft during the course of the event. An IPW plume is very weak with values of 16-18mm. The GFS has it dissipating during the event. Bottom line: too weak to be of interest.
As discussed yesterday a better event is shaping up for Wednesday. Here, however the GFS and ECMWF forecasts are quite different. The GFS has an event predicted for late Wednesday/Thursday(12/00-12GMT) while the ECMWF rushes a minor shortwave onshore early and consolidates most of the energy in a trough further west that will impact the coast on Saturday (14/12GMT). The diversity in the forecasts are also captured in the GFS ensemble as solutions range from trough to ridge scenarios on the coast during this time. However, the Canadian CMC and Navy NOGAPS support the GFS in timing and intensity. Still, we should be a bit cautious at this time in specifying timing for this event. The GFS primary run shows that the event could be a good one if it verifies with 1-2 inches of precipitation beginning with a well defined IPW plume with values 40mm offshore and 30-35mm working into the San Joaquin Valley. It is curious that even though the GFS forecasts this plume to move in to the ARB with good upslope flow in the warm advection period, precipitation is late in starting, not getting rolling until Wednesday night (12/06GMT) and mostly post cold front in cold advection. Most of the precipitation during the warm advection period is focussed on the Coast Range. Freezing level is forecast to fall from 8000ft to 4500ft during the course of the event. The bottom line: it looks like a potentially good ARB event but there is much uncertainty about this storm and close monitoring will be necessary over the weekend to see if a deployment is warranted
The further outlook from GFS and ECMWF indicates another event for Saturday 14 January. The ECMWF has more intensity in this wave than the GFS but both agree pretty much on timing, starting the event Saturday morning (14/12GMT).
John McGinley