Low pressure systems to bring showers, thunderstorms and mountain snow over Memorial Day weekend and beyond

May 24, 2019 – Scattered rain showers and thunderstorms mainly mountains and foothills today and Saturday as one low pressure system moves east and another drops in to take its place. Locally heavy rain possible. More widespread showers or thunderstorms on Sunday and Sunday night with snow higher Sierra elevations. Afternoon mountain showers continuing through mid week.

Discussion

Upper trough has shifted mainly south and east of the forecast area this morning resulting in dry conditions and fair skies most locations. A low pressure system dropping southward into the Pacific Northwest this afternoon will bring diffluent flow aloft to northern California. Resultant lift, instability and residual moisture over the region will bring a threat of showers or thunderstorms this afternoon and evening. Most of this activity will be over the mountains but some shower activity may slip into the Sacramento valley this evening as the upper low drops farther southward. Overall precipitation amounts are expected to remain light but some embedded heavier showers/thunderstorms could bring locally heavier precipitation. Daytime highs today should come in similar to Thursday or near or a little below normal for this time of year.

On Saturday, the Pacific Northwest trough drops southward through NorCal mostly as an inside slider type of system. Therefore, the biggest chance of showers or thunderstorms will be over the eastern half of the forecast area. Again precip amounts overall should be light but heavier showers will be possible. The passage of the trough will bring cooler air into the north state for another drop in high temperatures bringing highs down to a few to several degrees below normal.

A stronger low pressure system is forecast to drop southward down the Pacific Northwest coast and into northwest California on Sunday. Showers and thunderstorms will again be likely but over the entire CWA this time. This system will usher in still cooler air with daytime highs down to 10-15 degrees below normal. Snow levels will likely drop to below pass levels with enough snowfall forecast over the Sierra passes to bring travel impacts right in the middle of the coming Memorial day weekend. The upper low shifts eastward into the Great Basin on Monday. This will shift the shower threat back eastward by Monday afternoon so main shower activity will be over the mountains. By the afternoon, snow levels should be above pass levels with daytime highs climbing a bit although remaining below normal.

Extended Discussion (Tuesday through Friday)

Unsettled pattern continues through the period as ensembles support troughing across the area. Chances for showers or thunderstorms expected over higher terrain throughout the week, mainly in the afternoon and early evening hours. Elsewhere, dry conditions are expected. Temperatures will be on the rise through the period with Valley highs in the upper 70s to low 80s Tuesday, reaching into the mid and upper 80s by late next week.