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December 20, 2016 – Cool and dry weather for the most part this week. A weak weather system will bring some showers to mainly the mountains today. A winter storm with significant snow accumulations is possible Friday night and Saturday.
Temperatures are milder this morning as high clouds stream into the region. Current readings are in the lower to mid 30s thru much of the valley with 40s to around 50 in the milder thermal belts of the foothills and Sierra west slopes. The colder mountain valleys are in the 20s. Some patches of morning valley fog will be possible along with areas of frost.
Weak system rides over the ridge clipping the northeast corner of the state later today and tonight. Some light QPF will be possible over the northern mountains and northern Sierra, but most of interior NorCal will remain dry.
On Wednesday, north winds will develop behind the system in the valley and northeast winds in the mountains. Temperatures will continue to modify.
Dry conditions continue Thursday, then a potentially much stronger system lurks on the horizon for late this week. Timing and strength remain uncertain given some pretty significant differences in the different operational runs lately, but it’s looking likely we’ll at least see a more impactful system later Friday into Saturday.
Extended discussion (Saturday through Tuesday)
Showery weather is likely to continue into Saturday behind Friday’s cold frontal passage. The best chance of showers will likely be over the Sierra, where snow will likely continue to accumulate under the post-frontal cold airmass.
Confidence in the forecast is low beyond Sunday. The ECMWF maintains a dry forecast under northwesterly flow, whereas the GFS brings several potent shortwaves through the region. We have no inkling toward either individual model run, and opted to use a blend of the two to highlight the high degree of uncertainty.