April 28, 2018 – Cooler with isolated showers and thunderstorms possible through the weekend. Drier and warmer early next week except a continued slight chance of mountain showers and thunderstorms through the middle of next week.
Latest satellite imagery indicates a closed upper low off the OR coast this morning. This system will bring a threat of convective showers, as well as cooler temperatures through early next week. Radar imagery shows scattered showers generally over Shasta County this morning, with some locations reporting a few hundredths over the last six hours.
The upper low will continue to move inland and slowly lift to the northeast over the weekend. As a result, showers and thunderstorms will be possible given cyclonic flow and enough instability. Best chances will be the afternoons and evenings over the mountains, foothills, and in the Central Valley, mainly from around Marysville northward. Some snow showers could be possible over the northern Sierra generally above 6000 ft, 1-2″ possible at pass level. The rest of the forecast area should remain dry with increasing cloud coverage.
Model guidance indicates a disturbance dropping down the back side of the low on Monday for a continued threat of mountain showers and afternoon thunderstorms. With the large upper trough remaining over the West, afternoon showers and thunderstorms can be expected through the end of the forecast period.
Synoptic cooling associated with the trough will keep daytime highs below normal through the weekend. Then, temperatures will warm back up to near normal by Monday.
Extended discussion (Wednesday through Saturday)
Early week trough forecast to slowly move east from the Four Corners region as ridge builds along the West Coast. A few late- day showers or thunderstorms may linger over the Sierra Nevada south of Lake Tahoe into Thursday, otherwise dry weather is expected with temperatures warming to above average.