May 13, 2020 – Precipitation chances and cooler temperatures continue through Thursday. Warmer and drier briefly returns Friday, followed by wet and cooler weather for the weekended through the middle of next week.
Upper level low remains centered off the Pacific Northwest today. Most precipitation has ended as shortwave energy exits eastward, except for a few lingering mountain showers. Another shortwave over the southern portion of the low will rotate into NorCal today, bringing a return of widespread shower activity. The best chance for showers will be north of I-80, in the afternoon and evening. The potential for thunderstorms is lower today, with more cloud cover limiting afternoon heating. Model soundings show shallow and thin CAPE. While thunderstorm activity should remain in more unstable air to the north, some heavier showers are possible in the late afternoon over Shasta County. Any snow will be limited to above pass levels, over higher peaks. After a bit of a lull late Wednesday night, another shortwave arrives Thursday to continue the showery pattern through the day.
The much cooler than normal temperatures will continue for our area today with highs 7 to 15 deg below normal in the Valley and as much as 20 to 25 deg below normal over the mountains. Thursday will see similar temperatures, maybe just a degree or two higher.
Upper level ridging pushes the low to the north and east late Thursday into Friday returning dry and milder conditions to Norcal. Valley highs in the 70s are expected Thursday afternoon, reaching the low 80s on Friday, which is normal for this time of year. This will be a brief break, though, as a large, cold closed low from the Gulf of Alaska approaches on Saturday. Wet weather is expected to move in from the west, over the Coastal and northern mountains in the afternoon, widespread over the area Saturday night. This system looks more like an early spring system, bringing widespread precipitation, mountain snow and potential convection as it moves inland.
Extended Discussion (Sunday through Wednesday)
Pacific frontal system forecast to move through interior NorCal Sunday bringing widespread rain and mountain snow. Main upper low remains off the NW CA coast Monday, shifting inland Monday night, and into the Great Basin Tuesday. This will continue unsettled weather over the forecast area early next week with below normal temperatures. Models indicate upper ridging Wednesday for drier weather with highs returning to near normal.