March 20, 2020 – Few afternoon showers possible today and Saturday over the eastern mountains, otherwise dry weather is expected into the weekend with warming temperatures. Widespread rain and mountain snow returns later Sunday and continues into the middle of next week.
Discussion
554 DM upper low centered over the Sierra Nevada this morning will slowly retrograde back to the coast today. Low will keep a threat of afternoon showers over the Motherlode and Sierra Nevada. Some model CAPE depicted in this area later today, so will need to continue to monitor potential for isolated afternoon thunderstorms. AMS continues to warm today with highs today returning to near seasonal values.
Upper low continues to linger over the area Saturday, keeping a slight chance of afternoon showers over the higher elevations of Western Plumas county and the Northern Sierra Nevada. Associated shower threat ends Saturday evening as low weakens and pushes east into NV. Dry weather forecast elsewhere over interior NorCal with warming trend continuing through the weekend.
Lined up behind exiting low is next system which models suggest will spread some light precip into the SW portions of the CWA Sunday afternoon. Precip threat expands over the forecast area Sunday night into Monday as upper low opens to trough and tracks inland across south-central CA. At same time, next cold low from GOA is digging down the West Coast and will maintain a threat of widespread precipitation over interior NorCal Monday through Wednesday. Another 1 to 3 feet of snow is possible over the higher elevations during this period, impacting mountain travel. Snow levels start out around 5500 to 6500 feet late Sunday, gradually lowering into the foothills by Wednesday morning. Liquid storm QPF Sunday into Wednesday attm looks to be upwards of an inch in portions of the Central Valley with 1 to 3 inches for the foothills and mountains.
Extended discussion (Tuesday through Friday)
Model guidance favors a continuation of wet weather into the middle of next week as a longwave trough descends from the Gulf of Alaska. As often is the case, these systems are cold in nature which keeps snow levels on the lower side. While not able to tap into abundant moisture, it will make up for this deficiency given potent dynamics coupled with prolonged upslope flow.
Although the Valley can expect light to moderate rainfall with this system, the bigger impacts will accompany the mountain/upper foothill snowfall event. Relative to the upper trough which just brought several feet of snow to the Sierra, this will not be able to provide such majestic numbers.
However, a widespread area of 1 to 2 feet of snow is likely around 4,000 feet and above. Generally speaking, snow levels will sit in the 3,000 to 4,000 foot range during a majority of the winter storm. By early Wednesday, 700-mb temperatures plummet to around negative 14C which allows snow levels to drop to around 2,000 feet.
Much of the moisture should be scoured out at that time so only minor accumulations are possible at those elevations. The system eventually exits late Wednesday night with a progressive shortwave ridge moving through in the wake. Height rises will bolster temperatures a bit as Valley highs reach the low to mid 60s late next week. Some additional precipitation chances loom for next weekend but not expecting much given the low amplitude of these waves.