2006-12-15 Daily Wx Discussion & Forecast
The latest IR satellite loop shows the cold-frontal comma-cloud tail laid out WSW-ENE across the ARB and extending well out into the Pacific, with a frontal wave forming offshore of CA. The SSM/I imagery captures a continued weakening of the narrow SSM/I PW plume intersecting the central CA coast, with core values now only marginally exceeding 2 cm. Cold fropa occurred at the Bodega Bay, CA profiler rather early… in the 06-08z time period, while more subtle evidence of cold advection aloft can be found at the Chico and Sloughhouse profilers after ~13Z. A hard wind shift to Nerly has just been observed at Chico starting at 1530Z. Only moderate amounts of precip have fallen in the ARB region since ~06Z, including ~0.9” rain at Blue Cyn and ~0.5” at Alta.
The 12Z family of model solutions show moderate postfrontal onshore flow at ~700mb today, slowly weakening in intensity. Hence, continued light to moderate showery precip can be expected in the ARB today, with snow levels lowering to <5kft by this evening. After 00Z this evening, the onshore component of the flow slackens off to minimal values, at about the time the primary zone of upper-level dynamics begins propagating across the ARB. Upper level dynamics should persist through tomorrow midday… deep within the lower-tropospheric cold air. Hence, spotty light precip will fall into tomorrow, with snow level lowering to <=4kft. The ARB will be in cold-core conditions aloft tomorrow into Sunday, as a 500-mb cyclonic circulation tries to cut off to the S and SE of the area. Spotty light precip may intermittently affect the area during this period.
By Sunday afternoon through Monday, the 500-mb trough should begin exiting the area, and generally Nerly flow should become established downstream of an amplifying ridge in the eastern Pacific. If the 500-mb trough cuts off, then the ARB can actually see significant downslope NEerly flow. Regardless, the atmosphere should dry significantly during this period.
For the long range, expect midtrop ridging over CA for at least the first half of next week, with generally dry conditions. Some remnant wave energy may break through the ridge but should not yield any precip.