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May 24, 2018 – Afternoon and evening thunderstorms possible mainly over higher terrain this week. These are expected to extend across the Valley later this week. Slightly cooler than normal temperatures continue across most of the region through Saturday, warming to well above normal early next week.

Discussion

A cool upper trough currently off the coast will approach and move inland late tonight into Friday. Showers and thunderstorms are expected over the mountains this afternoon and evening. Widespread showers are expected by Friday morning, including across the Delta, Valley and foothills. Thunderstorms will once again develop in the mountains and foothills, potentially into portions of the Valley. Snow levels will drop enough to allow some snow at high Sierra pass levels south of Highway 50, such as Ebbetts and Sonora passes. Current projected snow accumulations continue to be about 3 to 6 inches above 8000 feet through Saturday morning. High temperatures trend well below normal Thursday and Friday. Friday highs across the Valley are expected to be in the upper 60s, about 10 to 15 degrees below normal. For downtown Sacramento the forecast record low maximum is 65 degrees,set in 1915. The current forecast high is a few degrees higher, at 68, so it could be close.

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 possible again in the afternoon and evening hours. 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. Temperatures over the Valley will warm quite a bit as ridging builds in and skies begin to clear,with highs 10 to 15 degrees higher than Friday, in the upper 70sto lower 80s.

Sunday looks mainly dry, except for some lingering showers and afternoon thunderstorms over the Sierra crest south of I80. Highs in the Valley and Delta will increase about another 10 degrees,reaching the mid to upper 80s, near 90 for the northern Sacramento Valley.

Extended discussion (Monday through Thursday)

The end of the Memorial day weekend Monday will start out very warm under upper level high pressure over the west coast. All areas should see dry conditions with daytime highs expected to hit as much as 10 to 15 degrees above normal. ECMWF brings a weak shortwave into the Pacific Northwest on Tuesday for a threat of showers over the Sierra Cascade range while GFS is much weaker on this feature. Split the difference leaving a slight threat of showers or thunderstorms over the northern Sierra Tuesday afternoon but affect on daytime temperatures should be minimal.Models agree slightly better on Wednesday in bringing a stronger upper trough into the west coast on Wednesday for a better threat of Sierra showers or thunderstorms and a cool off in daytime highs down to just a bit above normal. Upper troughing remains over the west coast going into the end of the week with a little more cooling Thursday back down to near normal for this time of year.Sierra shower/thunderstorm threat gets reduced Thursday but still can not rule it out all together with upper trough axis expected to be somewhere near the Sierra range.