Remnants of Hilary move rapidly to the north today bringing rain to the northern Sierra. Much of the remainder of the region will see a chance of showers and thunderstorms today with locally breezy southerly winds. Drier Tuesday, then warmer temperatures by mid-week. Flash flood watch for the Mosquito burn scar in effect.
Discussion
A large area of light rain has spread northward across the northern Sierra since Sunday evening. This will probably mark the western extent of precipitation associated with the remnants of Hilary as they move north up through Nevada today. Meanwhile, the persistent closed low remains off the central California coast and is responsible for the showers and thunderstorms offshore, and some newly developing showers across the northern San Joaquin Valley early this morning. This system will likely end up being a more significant factor for NorCal’s weather today.
Deep moisture (TPW in excess of an inch) and instability have pooled across NorCal ahead of the low, and as it drifts onshore near the Bay Area later today, showers and thunderstorms will increase across the region. Two areas in particular are highlighted for increased potential for significant weather today. The first is the Coast Range to the west of the Sacramento Valley where persistent wrap-around showers and a afternoon thunderstorms are forecast to the north of the low.
Lift from the low may be enough to overcome extensive cloud cover in this region leading to a risk of heavy rain as storms will be slower moving. HREF probabilities of exceedance of 1″/hr rainfall rate is around 25-30 percent, so a non-zero risk of seeing some debris flow issues around the burn scars in that area.
The second area of heightened concern will be across the northern San Joaquin Valley up into the Motherlode in the afternoon into the early evening. This area is likely to see more sunshine today from late morning into early afternoon allowing deeper instability to develop, and CAMS have been pointing toward some stronger storms developing there beginning around mid-afternoon accompanied by downpours, gusty winds and perhaps some small hail.
Showers will be decreasing tonight and Tuesday as the low finally kicks out to the northeast ahead of a stronger system moving into the PacNW. Temperatures will be on the cool side today and Tuesday, but will warm back above average during the middle of the week under short-wave ridging. Additionally, much of the region will see breezy southerly winds today from a combination of Hilary lifting out to our east and the incoming low.
Extended Discussion (Friday through Monday)
Closed upper low lingers off the West Coast through the extended forecast period. Models differ with how it tracks, but both support southwesterly flow aloft over interior NorCal with increased onshore flow. This will result in dry weather through the extended forecast period with high temperatures near to below normal.