Cool and dry weather continues today, then precipitation chances return for the end of the week and the weekend. Dry weather expected through much of next week. Low-impact system tonight into Friday with light rain showers and light mountain snow. Weekend system has sped up in timing, with impacts now expected Saturday evening. Wetter system Saturday evening and Sunday bringing widespread rain and mountain snow with travel impacts. Gusty southwest winds in the mountains Tonight – Friday and again Saturday evening – Sunday.
Discussion
Cloud cover has increased across NorCal this afternoon ahead of the approaching trough that is currently spinning near 40N,135W. As this trough continues to move northeast, marginal moisture associated with a weak and decaying AR will move inland across the PacNW, brushing NorCal in the process.
This first system will bring light precipitation to portions of NorCal, leading to minor impacts. Latest guidance indicates a narrow band of light precipitation moving into interior northern CA between 3am-5am Friday.
As this band progresses eastward, it dissipates with lack of dynamic support, resulting in the ‘heaviest’ precipitation expected to be along and west of I-5, northwest of Sacramento. Precipitation totals expected with this band for the area northwest of Sacramento range from 0.25″ to 0.50″, with locally higher amounts north of Redding. Areas east and southeast of the I-5 corridor can expect precipitation totals of trace to 0.25″ Friday.
Snow amounts of 3 to 8 inches are expected to be limited to the higher elevations of the Coastal Range and southern Cascades, producing limited impacts.
Winds with this system will also be minor, with southerly winds gusting to 20-30 mph.
A brief period of mostly dry conditions is expected late afternoon Friday through Saturday morning, before another weather system moves in. The next weather system is expected Saturday afternoon – Sunday, bringing more widespread precipitation, including heavy mountain snow to the Sierra, stronger winds, and even some scattered thunderstorms.
Still some uncertainty in the timing of the second system as the the latest ECMWF is a tad faster than the GEFS with bringing in the second trough.
Regardless it does look like the heaviest precipitation is expected to arrive Saturday late afternoon/evening through Sunday morning. Precipitation totals range from 0.25″ to 0.75″ across the Valley (with highest amounts across the eastern Valley), 0.75″ – 1.75″ in the foothills, and 1.00″-2.50″ in the mountains.
Snow levels will initially be around 5000′-6000′ feet Saturday, dropping to 4000′ Sunday. With the heaviest precipitation expected Saturday-Sunday morning, the heaviest snow is expected above 6000′, with totals of 15 to 30 inches. As snow levels decrease through the morning Sunday, snow amounts of 6 to 15 inches will be possible above 4000′. With heavy snow expected across the northern Sierra/Lassen Park, decided to issue a Winter Storm Watch, highlighting the likely winter travel impacts over the weekend.
Added dynamics with this cold trough will allow for decent instability to develop Sunday, bringing the potential for some scattered thunderstorms. Timing and cloud cover will be key factors in determining where thunderstorms may develop Sunday. Even so, latest guidance is depicting CAPE values of 200-350 J/kg between 10am – 7pm Sunday.
Slightly stronger southerly winds are expected with the second weather system Saturday-Sunday, with gusts to 25-35 mph in Valley and foothills, and gusts to 45-55 mph in the mountains.
Extended Discussion (Monday through Thursday)
Ensembles and cluster analysis depict a ridge of high pressure building across interior NorCal for the extended period. This will result in dry conditions and near normal daytime highs. Temperatures will range from the upper 50s to low 60s in the Valley, with 40s and mid 50s over the mountains. Overnight lows are expected to range from the mid 30s to upper 30s in the Valley.