May 28, 2018 – Well above normal temperatures continue through Tuesday, peaking Memorial Day. Afternoon and evening thunderstorms are possible over higher terrain Tuesday through Thursday. A significant cool down is expected for middle to late this week.
Upper level ridging peaks today, Memorial Day, bringing the hottest weather of the week and this year so far. Temperatures will reach the mid to upper 90s across the Central Valley with the warmest readings expected over the northern Sacramento Valley,where a few spots could reach triple digits. As a result, there is the potential for moderate heat risk for sensitive groups including the elderly and the very young. Ridging will keep stable and dry conditions conditions across the forecast area today.
The upper level ridge is anticipated to flatten on Tuesday as an upstream upper trough approaches from the west. Onshore flow and a Delta breeze will bring a slight cool down, mainly near the Delta influenced areas. However, there is still the potential for moderate heat risk, mainly over the northern Sacramento Valley,where highs in mid to upper 90s are expected. Elevated instability could be enough to bring a few afternoon and evening thunderstorms on Tuesday over the northern Sierra mainly south of Highway 50, and over eastern Shasta County and Lassen National Park.
The hot weather period will be brief, just Monday and Tuesday. Synoptic cooling associated with the upper trough, as well as onshore flow, will result in a significant cool down Wednesday, with highs 10 to 15 degrees lower than on Tuesday. A fairly strong Delta Breeze will bring the most cooling, through the Delta area and into the southern Sacramento and northern San Joaquin valleys. Daytime highs will be near or slightly below normal, with Valley highs in the 80s, and 50s to 70s over the mountains. Afternoon and evening showers and thunderstorms are possible over the Shasta County mountains and Lassen National Park area, and near the Sierra crest south of Highway 50 Wednesday and Thursday.
Extended discussion (Friday through Monday)
Temperatures will return to around average by the weekend as the mid-week trough shifts east. Another trough lurks for early next week, but models differ on the strength.