May 19, 2019 – Rain and mountain snow continue today with Sierra Cascade travel impacts expected through early Monday morning. Thunderstorms are possible this afternoon. Another system will impact northern California Monday night into Tuesday bringing more rain and mountain snow. Cool temperatures continue through most of the week.
An upper low resides just west of the northern California coast early this morning, with widespread precipitation seen ahead of this feature. Conditions are beginning to deteriorate in the Sierra with snow levels expected to drop near 5kft by early this morning. This low will continue to track southeast, moving over central California by late afternoon. Accumulations of 6 to 12 inches can still be expected through early Monday morning, with the heaviest snow this morning and rates decreasing as this system moves out of the area.
For the Valley, convective models indicate shower and thunderstorm initiation by late morning with ensemble probabilities signaling the best chances along and south of I-80 where the strongest instability resides. Shear will be weak as the low moves overhead and the main hazards with any scattered or clustered storms will be heavy localized rain, small hail, and lightning. There is some indication rate rates may be near or exceed a half an inch an hour, especially in the late morning and early afternoon with storms.
This system will move out of the area by early Monday, but only a brief dry period will be seen before another system tracks over NorCal Monday night through Tuesday night. Snow levels, once again, look to be near pass level with snow accumulations around 5-10 inches. Travel impacts should be anticipated, especially Tuesday morning into the evening hours when the heaviest precipitation is expected over the Sierra. Thunderstorms may also be possible again Tuesday in the Valley.
Extended Discussion (Thursday through Sunday)
Ensemble guidance continues to feature a broad upper level trough across the region later in the week, continuing the threat for mainly mountain showers and below average temperatures. Models still vary in the details, so have continued to lean on the NBM and ensembles for precipitation chances and amounts.