Periods of rain are forecast over the weekend, along with slightly below normal temperatures. A drying and warming trend will begin on Monday and continue through much of next week.
As of 03:13 AM PDT Saturday, Bay Area radars continue to pick up a few showers ahead of the next storm system. These leading edge showers are rather light. A few automated gauges tipped with Livermore Airport reporting a trace. Forecast for this next few hours remains unchanged - hit or miss showers as a storm system approaches from the west.
Current synoptic set up reveals an upper level low pressure system off the Oregon coast tracking southward. This is easily seen on the overnight water vapor imagery. 00Z model suite brings this low southward into far Northern California Coastal waters this afternoon. As the low approaches expect shower activity to increase across the Bay Area from roughly 6-9AM this morning. By this afternoon there could be some stronger embedded showers due to some daytime heating and weak surface instability. After sunset Saturday evening there may be a lull in activity with only a few scattered showers lingering overnight. Another burst of shower/rain activity is expected on Sunday as an upper level trough and weakening surface boundary pass through. The second round of shower/rain activity will once again be in the morning and into the afternoon on Sunday. Precip will diminish Sunday evening and be dry by Sunday night. This weekend will not be a total washout, but periods of rain/showers may hamper people venturing outside responsibly for some fresh air. Rainfall amount today through Sunday evening will generally be 0.25-0.5" most areas, up to 0.75" coastal mountains, less than 0.25" southern interior.
Unlike earlier storm systems this one will not be as cold. Snow levels are on track to be 4500-5000 feet and there is not much real estate that fits that elevation in the forecast area. Most likely area for a few wet snow flakes will be the highest peaks in the Santa Lucias and Gabilan. For what it's worth - Chews Ridge and Mt Hamilton are both hovering right around freezing.
Monday will kick off a drying trend as the storm system exits eastward. As mentioned on the previous discussion some medium range models continue to show a brush by of light showers for the North Bay on Monday. Confidence still remains low at this point, but a few showers have been added for far northern areas of the North Bay. There after a notable warming and drying trend will develop through next week with temps in the 70s by Wednesday.
As of 4:51 AM PDT Saturday, Radar is picking up precip just west of the Bay Area. Cigs dropping to near MVFR conditions with light rain by 15Z. Models forecast low-level moisture through early tonight so will see MVFR cigs continuing through the day. A brief break after 04Z before a secondary front arrives Sunday morning bringing rain and lower cigs back. Winds will switch to a south to southeast direction this morning with speeds under 10 kt. A switch to the west is expected in the afternoon.
Vicinity of KSFO, Light rain after 14-15Z with cigs becoming MVFR. Bases lowering to 2500 ft after 15Z then lowering further to 1500 ft after 20Z. Southeast winds under 10 kt becoming westerly to 10 kt in the afternoon.
SFO Bridge Approach, Similar to KSFO.
Monterey Bay Terminals, Becoming MVFR after 15Z with light showers after 18Z. Southerly winds 5-10 kt in the morning becoming west in the afternoon.
As of 04:32 AM PDT Saturday, High pressure will be pushed into the southern California waters as a low pressure system moves into the Pacific Northwest waters. This will result in light winds through early next week. A light southwest swell will ontinue to mix with a larger northwest swell through Sunday.