September 16, 2019 – Pattern change will lead to significantly cooler temperatures and a chance of showers and thunderstorms late this evening into Monday. Another system could bring additional showers by midweek. Dry weather and warm temperatures return late this week.
Discussion
An upper level low pressure system is approaching the West Coast this afternoon, bringing cooler temperatures to interior NorCal. High temperatures today will peak around 5 to 10 degrees cooler than yesterday with Valley temperatures in the mid to upper 80s and mountain temperatures in the 60s and 70s. Winds have begun to increase this afternoon with gusts up to 20 to 25 mph, locally higher over the Delta region, observed across the Valley and Sierra.
Southwesterly winds will increase in advance of the trough. Gusts up to 30-40 mph will be possible near the Sierra crest this afternoon and Monday. This could result in a brief period of enhanced fire weather conditions across the Sierra today, though the expected precipitation should help mitigate fire weather concerns.
Deterministic and ensemble guidance indicate rain showers spreading into Shasta County and the Coastal Range late this evening, then reaching the I-80 corridor around mid morning Monday into the early afternoon hours. Forecast soundings indicate the potential for isolated thunderstorms on Monday, generally from Sacramento northward including the Valley and higher terrain. At this point, the most likely timeframe for convection appears to be late morning into early evening. Forecast rainfall totals are around one tenth to half an inch, and up to one inch over the mountains. Locally higher amounts are possible if storms develop. Snow levels should remain relatively high as bulk of precipitation moves in, but could briefly lower to around 6500 ft in convection. Coolest temperature readings of the week are expected to occur on Monday, with daytime highs as much as 10 to 25 below average.
The upper-level trough is forecast to shift east of the state by early Tuesday, ending precipitation threat across interior NorCal. However, this dry period should be short-lived as another system takes aim at the West Coast by midweek. At this point, it appears that precipitation will approach the northwestern portions of the forecast area by late Tuesday night/early Wednesday, with best chances over the foothills/mountains. In addition, guidance suggests the potential for thunderstorm development at least over higher terrain Wednesday afternoon. Below normal temperatures will continue on Wednesday, with Valley highs in the mid 70s to low 80s.
Extended Discussion (Thursday through Sunday)
On Thursday morning, an upper trough will be in the process of exiting the region while it reaches the Desert Southwest by the evening hours. Some lingering mountain showers are possible although these should become more scattered in nature by later in the day. Deep-layered northerly flow will follow the trough passage with such conditions continuing throughout Friday. A brief period of height rises are likely for Saturday although some ensemble members show a trough nearing the West Coast late next weekend. Ensemble means remain at odds so will maintain a warming trend in the forecast. From Thursday to Sunday, highs will likely rise by around 10 degrees which takes Valley temperatures to near 90 degrees.