May 2, 2020 – A weak weather system will produce scattered showers today, mainly over areas of Shasta County and into the higher terrain, mainly north of I-80. Isolated thunderstorms will be possible over Shasta County and the Southern Cascade range, extending into the Butte County foothills this afternoon into the early evening. Gusty winds and small hail appear to be the biggest impacts with these storms. Overall rainfall amounts are likely to stay between 0.10-0.25”, locally up to 0.50”, particularly where thunderstorms occur. Much of the shower activity should subside by around sundown. Otherwise, dry conditions will prevail into next week. Temperatures will be close to average this weekend while a gradual warm up ensues early next week. By mid to late next week, well above average temperatures are likely to return as a ridge of high pressure moves through.
Discussion
Pacific frontal system approaching the West Coast will move through interior NorCal later today. Ahead of it, moderate onshore flow is occurring however marine layer is slow to deepen at this time. NAM12/GFS20 are similar with frontal speed and push baroclinic zone to the NW California coast around 18z. Front then moves across the forecast area through the afternoon into evening. Southern extent of frontal precip forecast to remain north of I-80. Highest QPF depicted over the foothills and mountains. Afternoon CAPE to around 300-400 J/kg progged over portions of Shasta/Tehama counties and over the NE foothills and lower elevations of Western Plumas. Models indicate these areas best potential for isolated afternoon thunderstorm development. Bufkit soundings suggest potential for gusty outflows with thunderstorms and possible small hail. Southerly surface winds increase ahead of baroclinic zone in Central Valley today with highest speeds expected in the afternoon. Attm wind speeds looks to remain below advisory criteria. System will usher in cooler air with high temperatures today upwards of 10 degrees lower than Friday.
Front and associated upper troughing push east of interior NorCal tonight with zonal flow continuing Sunday. Weak embedded vorticity maxima move through Sunday for some cloudiness, but latest guidance is keeping forecast dry. AMS warms a few degrees Sunday with high temperatures near normal.
Model heights/thicknesses trend up early next as downstream ridging amplifies. However, GFS20 showing deeper troughing lifting into the PacNW and across northern portions of CA to suggest highs remaining near normal over northern third of the CWA. Forecast leans towards ECMWF/NBM which continue an upward trend in max T’s, with highs approaching 10 degrees above normal by Tuesday.
Extended Discussion (Wednesday through Saturday)
Hot and dry weather expected during the second half of next week as ridging strengthens along the West Coast. High temperatures will be in the low to mid 90s across the Central Valley by the end of the week.
