2005-12-17 WX Discussion (1950 UTC)
Observations at 17:45 UTC show fog in the valley and a band of light rain along a Sonora-Audubon-Chico line. Radar (WSR-88D) estimated hourly rainfall is less than 0.1". This band of precipitation is associated with a shortwave at 500 mb that is well resolved in the 16 UTC 13-km RUC analysis. Of importance for subsequent periods is a serious dent made in the HMT-death ridge. It is unlikely that the ridge will reform thanks to another shortwave trough that is extending westward a deep low over the northern tier of the United States. However, low-level wind in Chowcilla and Bagoda Bay profilers remains S-SEerly below 1.5 km, even after the passage of this shortwave. One of the key forecast questions heading into the 12-36 hour period is whether the low-level flow will turn SWerly prior to the arrival of the next shortwave. SSM/I water vapor imagery indicates a swath of mid-20s PW ahead of the next trough.
NWP model guidance (GFS, MM5, ETA, Short Range Ensemble, Canadian model) is in agreement in that precipitation totals are in the 1-10 mm range prior to Sunday 12 UTC. The models that produce ~10 mm prior to Sunday 12 UTC, which is a minority of the models, contain a weak short wave that is absent in the other models. The timing of much heavier precipitation after Sunday 12 UTC depends on the forecasted speed of a potent short wave trough. All model forecasts contain a potent short wave. The model solutions of the short-wave trajectory are consistent with a 24-hr extrapolation of the short wave as seen in water vapor imagery. Probability of precipitation in this period is very high. The relevant forecast questions are: when will heavy rain commence? and how much rain will fall?
A minority of models produces 10-25mm in the Sunday 06-12 UTC period.
About half of models produce 25-50mm in the Sunday 12-18 UTC period; the other half produces 10-25mm.
All but two models produce 25-50mm in the Sunday 18 UTC to 00 UTC period.
Total precipitation in the Sunday 12 UTC to Monday 00 UTC period exceeds 50mm in 21/27 models.
During the forecast discussion, weather forecasters indicated that widespread 1-2" totals between Sunday 12 UTC and Monday 00 UTC is a reasonable expectation. HPC analogue forecasts suggest amounts as high as 4" in the ARB headwaters. The snow level was expected to be about 3000-4000 ft at the onset of the wet period and ~6000 ft during the period of heaviest rain. Storm duration is expected to be midnight Sunday to early morning Monday.
Forecast models suggest a break between Monday midday and sometime on Wednesday. On Wednesday, the GFS ensemble has a river stretching into central CA with PW 30-32mm. Although the wave tracks north of CA, a trailing front pushes into central CA in the Wednesday 06 UTC to Thursday 00 UTC period, producing 30mm of precipitation. This shortwave creates a large ridge that deflects subsequent waves northward.
Chris Anderson