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December 18, 2019 – Precipitation chances continue through this evening, with moderate mountain snow. Light precipitation spreads late tonight into Thursday, mainly north of I-80. Mostly dry conditions Friday. A wetter system is expected for the weekend.

Discussion

Latest radar imagery indicates showers spreading northward across the region as the storm system pushes inland. Observed precipitation amounts over the last hour have been generally less than 0.10 inches. Overall, less than 0.30 inches of rain are forecast across the Valley with 0.50 to 1 inch of liquid precipitation possible over higher terrain. Highest amounts are anticipated to be north of the I-80 corridor. A brief break is possible for the southern Sacramento and northern San Joaquin valleys in the afternoon, before another round of showers moves in for the evening commute. Given cold air in place, a mix of rain and snow is possible down to the northern Valley floor including the city of Redding this morning. SREF plumes showing 0.23 of snow for Redding, although model spread is significant.

Shower chances diminish this evening, but the dry period of weather will be short-lived. Troughing over the PacNW will bring warm air advection over the northern part of the area north of I-80 late Wednesday night and continue into early Friday before retreating further to the northwest during the day on Friday. Snow levels Thursday morning start out around 4500 feet, but rise to 5500 to 6000 feet by Thursday afternoon.

Snow levels from 3000-3500 feet this morning will be rising above 4000 feet this afternoon, with a few inches possible down to 3000-3500 feet early this morning. Overall, this system could bring 5 to 10 inches of snow, locally higher, over the northern Sierra and southern Cascades through this evening. HREF 24-hour snowfall ensemble probabilities indicating highest snow amounts for these areas as well. Therefore, a Winter Weather Advisory is in effect for portions of the Coastal Range for elevations above 4000 feet through 4 PM today, and for the northern Sierra and southern Cascades mainly north of HWY 50 through 10 PM this evening. If traveling over the mountains, carry chains and expect travel delays.

A stronger system is forecast to bring widespread precipitation this weekend into early next week. There are still model differences regarding timing, with the latest model runs showing a slower precipitation onset. This system could lead to hazardous mountain travel, especially since it is the pre-holiday weekend. For specific storm details, see extended discussion below.

Extended Discussion (Sunday through Wednesday)

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Global ensembles continue to support a fairly wet system impacting the region this weekend. Tools used to diagnose the strength of an approaching Atmospheric River suggest this will be a low-end moderate event. Conditions should be higher up the scale up into the Pacific Northwest although impacts are still likely over interior northern California. Models have generally been trending slower with this system as precipitation does not arrive until early Saturday. Cold frontal precipitation reaches the Coastal Range by Saturday morning before spreading south and east throughout the day. The heaviest amounts will likely be across the Coastal Range into Shasta and Tehama counties, as well as northern Sierra foothill locations. Total precipitation amounts in these locations should range between 1.50 and 3.00 inches through Monday. Heavy snow would be possible across the higher elevations of these locations with lighter accumulations down toward the I-80/Highway 50 corridor. Would still anticipate some travel impacts and chain control requirements with snow levels possibly into the 4,500 to 5,000 foot range.

There is uncertainty in how quickly this upper trough exits the region with the 00Z ECMWF coming in a bit slower. Sunday should prove to be a showery day given the dome of colder air aloft combining with any appreciable surface heating. With 700-mb temperatures progged to reach the -9 to -11C range, snow levels may drop to around 4,000 feet or slightly less if this verifies. Some showers could linger over the Sierra on Monday given lower heights are expected to stick around. In terms of temperatures, readings should be quite seasonal during the day although extensive cloud cover will keep overnight lows on the milder side.

Looking further out, ensemble means show a brief lull in the pattern as a shortwave ridge sweeps through on Tuesday before another storm looms for Christmas Day. In such a progressive regime, timing will be difficult to pinpoint in a meaningful way. Given its a busy travel period for the holidays, it will continue to bear watching.