May 23, 2018 – Afternoon and evening thunderstorms possible mainly over higher terrain this week. These are expected to extend through the Valley late in the week, along with some mountain snow showers Friday and Saturday. Slightly cooler than normal temperatures continue across most of the region this week warming to above normal early next week.
Upper level low pressure shifts to the north and east today. Showers and thunderstorms ended by late evening, with dry weather conditions overnight. Some morning showers are possible over higher terrain, maybe even a few sprinkles in the Valley. Unstable conditions in the afternoon will develop, with thunderstorms possible. These should be more limited to over the mountains than what we saw yesterday.
The Delta breeze has remained moderately strong overnight, keeping temperatures relatively cool in Delta breeze influenced areas. Marine stratus clouds have spread into the Delta and Sacramento metro as well, and will linger until late in the morning. Temperatures today will remain on the cool side with highs in the mid-70s around Sacramento, upper 60s in the Delta, and low 80s elsewhere in the Valley. The Delta breeze will continue to be factor through the next several days.
Wednesday will still have a decent Delta breeze in place and temperatures are expected to be slightly cooler across the region. Unstable air over the mountains will continue with possible afternoon and evening thunderstorm activity.
An upper trough currently off the coast will approach Thursday and move inland late Thursday night into Friday. This will bring the potential for widespread showers, including across the Valley and Delta. Some afternoon and evening thunderstorms are also expected,with unstable conditions from a cool core low. Snow levels will drop enough to allow some snow at pass levels, with up to 1 to 3 inches of snow possible over higher mountains from Friday through Saturday. High temperatures trend well below normal Thursday and Friday. Friday highs across the Valley are expected to be only in the low 70s, about 10 degrees below normal.
Showers linger across much of the area Saturday morning, then gradually shift eastward as the upper low exits to the east. Mountain showers and thunderstorms are once again an issue in the afternoon and evening. Anyone planning outdoor activities for the holiday weekend should be prepared for these storms, and have shelter available, especially from lightning. High elevation hikers and campers should also be prepared for unseasonably cool temperatures and snow showers.
Extended discussion (Sunday through Wednesday)
Upper low centered east of the forecast could keep a threat of afternoon showers and thunderstorms over the northern Sierra through early next week. Activity is expected to remain south of HWY 50. Northerly winds will develop Sunday into Monday in the wake of the departing low. Ridge of high pressure begins to build off the CA coast early next week. As a result, temperatures will begin to warm up across the region with most Valley locations potentially reaching the 90s. Another upper trough approaches from the west by midweek, and might bring precipitation activity over higher terrain as well as slightly cooler temperatures.