December 19, 2019 – Light precipitation today, mainly north of I-80. Mostly dry conditions Friday. A wetter system is expected for the weekend into early next week, bringing hazardous mountain travel. Unsettled conditions may continue into the Christmas holiday.
Discussion
Warm air advection precipitation will continue to move into the region today, mainly over the northern mountains, and areas north of I-80. The highest QPF amounts will be in Shasta County, where up to 0.50-1 inches are possible. Rising snow levels are expected by mid morning, generally 5,500 to 6,500 feet with light snow accumulations possible. Precipitation on Friday looks to be limited to portions of Shasta County, whereas the rest of the area remains dry.
Widespread precipitation is anticipated this weekend as an upper level trough approaches the CA coast. Deterministic models still disagree on timing, with the latest 00z ECMWF coming in slower compared to the GFS/NAM. The NBM has also trended slower. Therefore, have followed the NBM with precipitation gradually spreading from northwest to southeast throughout the day Saturday, and reaching the I-80 corridor by Saturday evening.
The heaviest precipitation will be focused north of I-80, especially over the Coastal Range, western Shasta County, and the northern Sacramento valley. Overall, 0.50 to 1.50 inches of rain are forecast across the Valley with 0.75 to 2.50 inches, locally higher, of liquid precipitation possible over the mountains. These amounts will likely change as there are still QPF differences.
Snow levels will be generally 4500 to 5500 feet Saturday, then falling to around 4000 feet Sunday. Snow amounts of 6 to 12 inches are forecast at pass levels; higher amounts are likely over the southern Cascades. The heaviest period of Sierra snow appears to be Sunday. Gusty winds could lead to periods of significant reductions in visibility. Therefore, this storm could lead to hazardous mountain travel, especially since it is the pre- holiday weekend.
Extended Discussion (Monday through Thursday)
Model differences which appeared over the weekend will lead to uncertainties to commence next week. A compact upper low is likely to swing southward in the vicinity of northern California on Monday morning. Its exact track will dictate the areal coverage of showers across the region. The 06Z/00Z GFS runs are decidedly stronger compared to the 00Z ECMWF, roughly 10 dm stronger aloft, which would support a more intense band of showers. Until the guidance better resolves this feature, will continue to keep modest precipitation chances in the forecast. With cold temperatures in the mid/upper levels of the atmosphere, snow levels will likely be in the 3,500 to 4,000 foot range, potentially lower in convective showers. Thereafter, expecting a likely break from the active weather for Tuesday as a shortwave ridge tracks through.
Ensemble solutions become quite variable for Christmas Day through the subsequent days. Comparison of the main global means (ECMWF, GEFS, and CMC) shows quite a range of solutions amongst these typically more stable ensemble systems. The 00Z ECMWF ensemble mean shifted toward a much sharper and pronounced trough for Christmas Day which would bring wet weather to the area. Other means are a bit more progressive but still afford increasing precipitation chances. Given the unusual instability of these ensembles on a run-to-run basis, forecast confidence is below average.