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May 15, 2020 – Warmer and drier weather briefly returns today and Saturday. Precipitation spreads back into NorCal Saturday night into Sunday and continues into the middle of next week as a late-season weather system affects the area.

Discussion

Ridging from the EPAC beginning to move back over NorCal resulting in less cloud cover across the region early this morning. Radar shows just a few lingering showers extending from western Plumas County northward into southeastern Shasta County. Current temps range from the mid 30s to lower 40s in the mountains, to the mid 50s to lower 60s across the Central Valley.

Dry and milder weather returns today and Saturday as the upper ridge moves overhead. Highs both days will top out in the upper 70s to mid 80s across the Central Valley, close to average for mid-May.

Wet weather spreads back into the region for the second half of the weekend and continues early next week. Initial frontal system gradually slides southeast through NorCal beginning late Saturday afternoon and continues into Sunday. NAEFS standardized anomaly of 4-6 standard deviations and IVT of 300-400 kg/m/s along the front indicates good potential for widespread impressive QPF for this late in the season. Amounts in the valley will vary from 1/4 to 3/4`s of an inch (highest in the north) while foothill and mountain areas see 1-2 inches of rain. Snow levels will be high (above 8k ft) with this initial wave of precipitation. Gusty winds will be widespread with the strongest winds in the mountains.

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Best post-frontal convective potential on Sunday still appears to be across the northern half of the Sacramento Valley. Forecast soundings for that area indicate plenty of shear, and backed surface winds, for storm organization should sufficient instability materialize. We`ll continue to monitor.

Upper trough settles over the region on Monday with numerous showers and thunderstorms expected. Snow levels will lower to around 6000-6500 feet, so there will be increasing potential for snowfall accumulations impacting the high northern Sierra passes.

Extended Discussion (Tuesday through Friday)

Ensemble guidance depicts the deep, upper low will be centered over Northern California on Tuesday bringing additional chances for precipitation. Scattered showers and possibly an isolated thunderstorm will be seen Tuesday as this low continues to move out of the area towards the Northern Plains. Some light accumulation of snow near pass levels is possible through Tuesday night impacting mountain travel. Lingering light mountain precipitation may be seen into Wednesday, although the majority of the area will see dry weather by this time. Below average temperatures Tuesday will be on the rise through the week reaching near normal values by Thursday.