July 4, 2017 – Slight chance for t-storms in the Sierra south of Lake Tahoe Wednesday. Otherwise warm and dry. There will be slight chances for afternoon and evening isolated thunderstorms in the Sierra today and Wednesday south of US Highway 50.
Confidence is decreasing that mountain storms will develop. Any developing storms in the Sierra will bring small hail, gusty wind, and lightning. Lighting in thunderstorms will increase the threat of fire starts in the vicinity of any storms, but semi-moist fuels should keep rapid fire expansion to a minimum.
Discussion
Upper ridge is over NorCal providing clear skies to the interior. Current temperatures are pretty similar to 24 hours ago and generally range from the upper 40s and 50s in the mountain valleys to the 60s and lower 70s elsewhere.
Little overall change is expected for interior NorCal on Independence Day compared to Monday. Enough onshore flow will continue through the Delta to keep warmest readings in the Central Valley around the century mark. The airmass still appears too dry and stable for any late day thunderstorm activity in the northern Sierra.
Some forecast uncertainty Wednesday and Thursday as the ridge edges a bit further east allowing mid and upper level moisture to move up from the south. At this point, forecast soundings don’t indicate much instability across NorCal with strong mid and upper level capping present so thunderstorms likely to be limited to the Sierra south of Lake Tahoe.
Nevertheless, expect to see an increase in mid and high clouds from the south and we’ll continue to monitor for increased potential for elevated convection with embedded short-waves moving up in southwesterly flow aloft. Temperatures will hold steady.
By Friday, the Southwest high is forecast to rebuild westward bringing warming to the region as onshore flow is reduced.
Extended discussion (Saturday through Tuesday)
Expansive ridge of high pressure centered over the Four Corners region will remain to be the predominant weather feature through the extended period. Northern California will be on the western fringe of this ridge, and high temperatures are expected to be several degrees above normal as a result. Portions of the Central Valley are likely to reach triple-digit heat during this timeframe.
The desert monsoon is also likely to be active through the extended period. With S to SW flow over NorCal, thunderstorm activity will tend to favor areas east of the Sierra crest. Can’t rule out a few thunderstorms in our CWA (mainly south of Lake Tahoe) though.