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November 26, 2019 – A significant winter storm arrives today bringing heavy snow, rain, and strong winds to the area with major travel impacts extending into the Thanksgiving holiday. Much cooler temperatures are in store for the week.

Discussion

Satellite imagery shows the blossoming baroclinic leaf off the coast of the PacNW as the system begins to undergo very rapid strengthening, or “bombogenesis”, as it drops southeastward (models forecast the cyclone to deepen by around 30 mbs in 12 hours!). It will be very interesting to watch its development on satellite through the day as it moves toward a landfall near the OR/CA border after 00Z.

Skies are presently mostly clear across interior NorCal except for some thin cirrus beginning to move in from the north (thicker mid and high clouds loom just to our northwest and will move in early this morning).

Winds are generally light across most of the region early this morning except for lingering northerly winds of around 15-20 mph along the western edge of the Central Valley and some local easterly breezes over the ridges and near canyons across the northern Sierra Nevada.

Little overall change made to the forecast through the remainder of the week with the well-advertised strong system ontrack to bring rain, heavy snow (measured in feet) to the foothills and mountains, and strong southerly winds later this afternoon into tonight. Impacts will be severe during the peak pre-Thanksgiving travel period later today through Wednesday.

Mainly light to moderate warm-advection precipitation expected to develop this morning across the northern portion of the forecast area and spread south into the remainder of the area this afternoon. A period of heavier precipitation is forecast to move south this evening as the cold front sweeps in.

Strongest southerly winds look to be this evening as the surface low moves inland along the OR/CA border and fills. KMFR-KSAC gradient forecast to reach around 18 mbs around 06Z, but strongest winds may occur in the early evening across the Sacramento and Stockton areas as the cold front passes.

Main change was to introduce accumulating snow into the valley floor at the north end of the Sacramento Valley. Present indications point toward this being an anomalous event for Redding as strong southerly winds ordinarily usher in milder, higher moisture-content air. However, given the unseasonably dry airmass ahead of the system (valley dew points currently in the lower to mid 20s) and the fact that it will be quite cold with 850 mb temps around -2 C, snow is looking increasingly likely later this afternoon and evening as the low-level sounding becomes isothermal with evaporative cooling bringing the snow level to the surface. Southerly winds may finally mix in milder air after midnight leading to a switchover back to rain. We’ll be monitoring closely.

Extended discussion (Saturday through Tuesday)

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Another strong, cold winter storm is forecast to impact NorCal this weekend through early next week. Another closed low pressure system will deepen in the eastern Pacific throughout the weekend. Ensembles indicate this will be another strong low pressure system, though core of the low is expected to remain west of the California/Oregon coast. However, as the axis sits just off the coast through the weekend, the lee side of the trough situates over Northern California, bringing heavy Valley rain and mountain snow. Shortwaves along the trough will bring multiple rounds of precipitation to the area through Monday, though the heaviest precipitation is expected mainly Saturday and Sunday.

At this point, the storm is initially expected to begin very cold with snow levels down to 500 to 1500 feet early Saturday.

Snow levels will slowly rise throughout Saturday, rising to above 4000 to 5000 feet Sunday morning as the low taps into warmer Pacific moisture. Early models suggest another 2 to 3 feet of snow is possible through Saturday and Sunday. This system will also bring gusty winds over the weekend.

Showers could linger Monday and Tuesday with additional snow accumulations over the Sierra passes. However, confidence is low in details at this time as run- to- run ensemble consistency is low, so be sure to check back for updates. Either way, if you have mountain travel plans this weekend, consider traveling Friday as if this forecast holds, mountain travel will be significantly impacted beginning early Saturday.