2006-11-27 12 UTC Synopsis
Massive, cold upper-level low is positioned along the NW US and British Columbia coastline. Tropopause pressure in the core is about 375mb. Core of highest speed upper-level winds appear to be west of Quillayute, WA. One aircraft report exceeded 140 knots at 300 mb. There are multiple reports of 100-120 knots extending from northern CA into OR. Precipitable H2O values in the 12Z GFS analysis are in the 0.6-0.7" range in the San Francisco Bay area and westward over ocean points. SSM/I passes between 05 and 09 UTC similarly show values less than 1", and Bodega Bay IPW at 15:30 UTC is 0.5". The juiciest air appears to be south of the target region with 0.8-0.9" pcp h2o near Santa Maria in the GFS analysis.
The system may be characterized as one with vigorous upper-level dynamics with modest moisture supply. It is also a cold system with below 0C temperatures near the core at Quillayute, WA. Closer to the ARB region, Bodega Bay reports snow-rain line at 2000 meters.
A front passed through the ARB region Sunday afternoon around 19 UTC. Bodega Bay reported 0.6" of rain during the frontal passage and a wind shift from 110 to 300 degrees. Precipitation reports for the period Sunday 4a - Monday 4a PST are: Blue Canyon 1.03", Auburn 0.2", Georgetown, 0.2", Huysink 0.32", Farad 0.04". Much of this precipitation fell as snow in the Blue Canyon region. The surface met from the ETL Blue Canyon site reports and increase of the snow depth fom 0.5" to about 7" since 00 UTC, during that time abou 0.9" fo precipitation was recorded.
Current conditions in the ARB region
Snotel measurements at 7a PST are mostly 1-6" and some reports of 6-12". 17 UTC surface obs show upslope wind direction with wind speeds about 10 knots. Overcast skies reported in the valley and freezing fog and snow reported at Blue Canyon and Truckee, respectively. Temperatures are in the low to mid 40s in the valley and upper 20s in the Blue Canyon to Truckee region.
Bodega Bay reports 50F and 0.55" of precipitable h2o. Wind direction is southerly at the surface and veering to westerly by 2000 meters. Wind speeds in the 2000 to 4000 meter layer are 30-35 knots. Precipitaiton is currently not being reported, and the freezing level is near 2000 meters.
0-72 hour forecast
The core of the jet stream will continue to dive southward, pulling the upper-level low with it. The forecast questions are: what will the be southern extent of the trajectory of the core, how quickly will it move eastward, and how much of that 0.5" of pcp h2o can it squeeze out. Specific model information is given below. In summary, the forecasts are in very close agreement on the trajectory of the low, pushing it into the four corners region by Thursday midnight. Expect precipitation to be light but steady between now and 00 UTC Tuesday. Three hourly accumulation is not expected to exceed 0.2", and the storm total precipitation should be not much more than 1.0" range at higher elevation, all of which should fall as snow.
The NAM solution moves the core southeastward over the four corners by midnight Thursday. An upslope orientation to the 700 mb height field persists between now and late Tuesday afternoon. Three-hour precipitation rates peak at 0.2" early Tuesday morning, when, apparently, the combination of on-shore flow and upper-level dynamics provides the most vigorous flow.
The GFS has similar timing and trajectory of the upper-level low. PCP h2o remains steady near 0.5" through today and drops rapidly overnight as the 700 mb flow begins to have a northerly component. Precipitation amounts remain steady at 0.1" per 6-hours through Tuesday afternoon.
>>72 hour forecast
A shortwave is progged to move through the NW Wednesday into Thursday. As a trailing front pushes into central CA Thursday night, pcp h2o values in the 0.7-0.9" move into the area, producing a chance at a brief (less than 12 hours) preciptiation event. More details about this system will be provided in tomorrow's forecast discussion.
Generally, a ridge will be positioned in the Gulf of Alaska. This will create unfavorable conditions for heavy precipitation in ARB through next week.