August 17, 2016 – Warm and mainly dry weather continues. Monsoonal moisture will push into northern California today bringing a threat of mountain thunderstorms. A threat of mountain thunderstorms will continue for the next few days but this likelihood will become less each day.
Fair skies over the CWA this morning as weak upper disturbance that brought isolated thunderstorms to Plumas county Tuesday evening shifts ito the Great Basin. Daytime high temperatures ran a little higher on Tuesday and this trend has continued into the early morning hours today. Another upper disturbance is forecast to drop into Norcal this afternoon and evening with enough instability projected for another threat of mountain afternoon thunderstorms. Areas of forecast instability indicate a threat of isolated afternoon thunderstorms over the Sierra Cascade crest and parts of the coast range as well as Norcal moves under upper southeast flow. Projected PWAT values of better than .8 inches would suggest that any thunderstorm activity should contain at least a little rain although not a lot. Overall airmass and onshore flow should be similar to Tuesday so daytime highs today should come in within a degree or two of Tuesday.
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Upper disturbance drops southward on Thursday bringing northeast upper flow to the region. For this reason…believe thunderstorm threat to coast range on Thursday is minimal and confined the mountain thunderstorm threat to the Sierra Cascade crest on Thursday. Again, little change expected in daytime high temperatures Thursday although slightly cooler temperatures will bring slightly cooler highs. Low pressure to the south and High pressure over Norcal on Friday should bring mainly fair skies and continued above normal temperatures. Easterly flow aloft could bring a few Sierra crest thunderstorms south of about Tahoe but again believe thunderstorm threat to coast range is minimal. Little change in pattern expected for Saturday so little change in overall weather. Latest ECMWF does bring a little disturbance northward across the coast range around the Norcal high in the afternoon so have kept a slight afternoon thunderstorm threat in western Tehama county in case this solution pans out. Other models do not have this feature.
Extended discussion (Sunday through Wednesday)
Models continue to be in good agreement to the positioning of the upper level high pressure system in the eastern Pacific that will influence northern California in the extended forecast. The positioning of the upper high will cause an easterly wind component to develop and tap into areas of higher atmospheric moisture in the Great Basin and desert southwest. Moisture advection over the higher terrain will create slight chances for showers and thunderstorms to the mountains during the afternoon and evening hours on Sunday and early next week.
The valley is expected to remain warm and dry through the period with temperatures near to slightly above normal in the mid to upper 90s. Higher elevations will reach the 70s and 80s.