April 19, 2018 – Light showers over the northern Sierra today and this evening with light snow over the passes. Dry Friday through early next week. A few showers possible over the mountains middle of next week.
Discussion
Dry upper level trough currently working its way across northern California. Trough axis is currently shifting into the Sierra with main cloud cover now south and east of the CWA. Therefore, most of the forecast area is under mainly clear skies this morning. Clear skies allowing for temperatures this morning to run a few to several degrees below 24 hours ago. Shortwaves dropping down the back side of the low as it shifts into the Great Basin could bring a few showers over the Sierra Cascade range today through this evening. Fair skies and warming airmass will bring up daytime highs to a few degrees above normal.
Upper ridge axis forecast to slide over the north state on Friday bringing fair skies and more warming with highs Friday expected to reach 10 or more degrees above season normals. Still more warming in store for Saturday under upper level high pressure bringing highs in the valley up into the 80s with upper 80s expected in the northern Sacramento valley.
ECMWF and GFS both weaken the west coast ridge temporarily Sunday as a weak Pacific trough quickly moves through the Pacific Northwest and NorCal. Both models indicate light showers over the northern most CWA so included some spotty showers there Sunday afternoon and evening. Northern forecast area will see a slight cooling as well. Otherwise the shortwave will have little impact with daytime highs Sunday most areas remaining well above normal.
Extended discussion (Monday through Thursday)
Upper level shortwave ridging rebounds on Monday as a weak disturbance exits to the east. Temperatures will be well above normal, with Valley highs in the low to mid 80s. Models diverge significantly on timing of an upper low moving inland next week, though, so confidence in the forecast is low. For now, have temperatures slightly cooler with a slight chance of some light mountain showers. Overall, this looks like a typical mid-spring pattern, unsetlled but without any particularly strong storm systems/wet weather expected.