Looking at the big picture, the latest GOES 17 satellite image shows a rather compact but dynamic low moving through southern California (LA/San Diego) today. This system dropped over 1 inch of rain over much of that area today. This system will rapidly move east into Arizona and weaken this evening ahead of a much larger storm system beginning to develop in the eastern Pacific at this time. The latest upper air analysis continues to show a rather amplified and split flow type pattern over the Pacific and much of western North America. The high over the west will break down and allow the next Pacific storm to continue to push toward the west coast.
As we look ahead to the forecast, what could be some of the colder and strongest storms of the season, there is high confidence that we will see significant rain in the valleys changing to snow and heavy snow in the Sierra this weekend into early next week. The latest operational GFS is in good agreement with most other models in developing a very deep 979mb surface low off the central California coast by early Saturday morning. This low will gradually move east-southeast through the weekend bringing our first round of heavy rains in the valleys and mountain snow. Snow levels will start out around 7,000 feet, then rapidly low to around 5,000 feet by Saturday morning. There will be little break in the precipitation Saturday afternoon into early Sunday, with snow being almost continuous in the Sierra, albeit light during this time.
Another decent surface low begins to develop just off the Oregon coast by Sunday into Monday to bring another round of heavy snow in the Sierra and possible moderate to heavy snow in the valleys of western Nevada. With a decent amount of cold air already in place, snow levels will start out near the valley floors ahead of the cold front. This is typically a classic setup for heavy snows into western Nevada. There is a good potential for decent accumulations especially in the valleys adjacent to the Carson Range by Tuesday morning.
Looking at the latest precipitation amounts currently being forecasted by the operational models, WPC (blend of models), GFS and ECWMF all showing similar amounts of liquid precipitation. To break it down, the Sierra especially along the crest will likely see amounts of 5-7 inches. The eastern Sierra and Carson Range will see amounts in the 2-4-inch range and the valleys of western Nevada will generally see amounts in the 1-2-inch range (highest near the foothills) over the next 5-6 days. These will translate to 3-6+ feet of snow in the Sierra, 2-4 feet in the eastern Sierra and Carson Range and several inches or more depending on elevation in the western Nevada valleys. After the snow ends by Tuesday, some of the coldest air so far this season will settle in for a few days with highs generally in the upper 20s to low 30s in the valleys and much colder in the upper elevations.
Looking further out, we will see a dry period in the 6-10-day outlook, possibly followed by another round of active weather by the 3rd and 4th week of February. Overall, it’s looking like an above average month for precipitation, which will be good news for the water supply situation. As always there is a lot of uncertainty this far out.