March 18, 2018 – Dry weather today into Monday with warming temperatures. Wet weather returns Tuesday into Friday, and possibly into next weekend.
Discussion
Mostly clear skies and dry weather have settled into Northern California as a shortwave ridge moves across the state. Clear skies and light winds allowed for strong radiational cooling to occur, with most Valley locations dropping down into the 30s this morning, and 10s and 20s over the mountains. Isolated showers may develop over the mountains this afternoon as a very weak trough passes through.
Most of the region will remain dry Monday and Monday night. The next weather system will then begin to move into the region on Tuesday. This upcoming system has the look of an atmospheric river, with Integrated Vapor Transport (IVT) values of 250-500 kg/ms being projected. The first surge of moisture will likely make landfall to our south, either toward central or southern CA on Tuesday. NorCal should still some light precipitation during this timeframe.
Heavier precipitation will begin on Wednesday across Northern CA as the upper trough pushes inland. Snow levels should initially be rather high: on the order of 6500-7500 feet. Periods of moderate to heavy precipitation will be possible, particularly over the mountains.
Our snowpack will likely be able to absorb much of this rainfall across the higher elevations: snowmelt will only occur in the lower elevations, along the periphery of the current snowpack. Gusty winds are also possible on Wednesday across the region.
Extended discussion (Thursday through Sunday)
Heaviest precipitation of the week begins early Thursday as a Gulf of Alaska trough merges with subtropical moisture bringing rain and mountain snow to the entire forecast area. Snow levels will drop during the day Thursday as colder air filters in to the region with major Sierra passes expecting significant snowfall by afternoon. Combination of heavy snow and wind will create significant mountain travel impacts. The lower elevations will see moderate rainfall as daytime temperatures drop to around 10 degrees below season normal.
This disturbance is forecast to shift east of the state by Thursday night but a parent upper low over the northeast Pacific and Pacific northwest will bring a continued threat of showers through Friday. Overall airmass continues to cool and snow levels drop to below 3000 feet most areas. Precipitation amounts will drop significantly under the more convective pattern Friday but mountain travel impacts are still likely.
Models indicate that at least one more impulse will rotate through the region Friday night or Saturday keeping the precipitation threat going especially over the mountains where orographics will play a role. Models becoming more consistent in pushing in ridge up along the west coast on Sunday pushing any precipitation threat east into the Sierra Cascade range and by Sunday night models indicate the start of another dry period going in to early next week.