Dry and mild temperatures continue today as upper level ridging continues to dominate. Some patchy morning fog is already evident, and will continue through this morning in some portions of the Valley from around Marysville southward. Stockton could see another record high today, with a forecast high of 66 and a record of 64, set in 1973. Highs elsewhere are not expected to be record setting, but with widespread low to mid 60s in the Valley, Delta, and foothills, temperatures will feel unseasonably mild for late December.
The ridge will flatten on Saturday as a shortwave trough passes through the Pacific Northwest. A weakening cold front now appears like it will stall out and fall apart over far Northern California. Have removed a slight chance of precipitation for northern Shasta County, with just an increase in cloud cover expected and slightly cooler temperatures. The ridge rebuilds late Saturday into Sunday, with moisture from an offshore trough spreading high and mid clouds into the area. Sunday (New Year’s Eve) should be pleasant for outdoor activities such as fireworks shows.
Monday (New Year’s Day) will see little change as the ridge builds further. Highs are expected to be even a little warmer, for mild and dry start to 2018. A rather deep upper low is off the coast, though, with improved confidence in a significant pattern change by mid week as it moves inland. EK
Extended discussion (Tuesday through Friday)
Ridge of high pressure will remain anchored overhead early next week, resulting in dry conditions and above normal temperatures across interior NorCal. There is growing model consensus that the upper ridge will weaken as a trough of low pressure pushes onshore by the middle of next week. However, timing and precipitation amounts remain uncertain given different model solutions. The wetter GFS indicates a faster progression of the system, with precipitation spreading inland as early as Tuesday afternoon. Meanwhile, the ECMWF holds off precipitation chances until early Wednesday. Regardless of which solution is correct, there is increasing confidence in widespread precipitation across the forecast area by midweek with favored chances over higher terrain. Have expanded precipitation chances across Valley locations Wednesday into Thursday. Early precipitation estimates range from around a quarter to half an inch in the Valley, a half inch to two inches in the mountains.