Precipitation starting later today, heaviest impact of atmospheric river on Tuesday

February 25, 2019 – An active weather pattern will continue to set up over northern California this week. Heavy snow is forecast for the higher elevations of the mountains while increasing flooding potential remains the threat for the valley and foothills.

Discussion

Radar continues to indicate light precipitation over the mountains to the north and east of Redding while scattered light rain presently extends about as far south as Chico in the valley. Profiler data indicating that snow levels are still holding around 4-5k ft across the northern mountains.

Southerly surface pressure gradients are tightening with SAC- MFR approaching 13 mbs, and many locales are now reporting wind gusts of 25-35 mph across the far northern portion of the forecast area. Temperatures are considerably milder across much of the region compared to 24 hours ago due to the clouds and wind and range from the 30s in the mountains to the mid 40s to lower 50s across the valley.

Not a whole lot of change to the forecast for the next several days with wet and windy weather expected into mid-week. Precipitation forecast to spread southward today as frontal system pushes into northwest California (reaching the Sacramento area in the afternoon and northern San Joaquin Valley by this evening). The front is forecast to become nearly stationary over NorCal tonight and Tuesday and focus several waves of moderate to heavy precipitation along with gusty southerly winds. Precipitation forecast to become more showery early Wednesday as the cold front finally sweeps through.

Flood potential for small rivers, creeks and low-lying areas likely to increase later tonight, and especially Tuesday, as moderate to heavy rain becomes more persistent. Strong low-level southeasterly flow will enhance rain along the western edge of the valley including the Fairfield and Vacaville area, and strong southwesterly flow above the surface will soak the foothills and west slopes of the northern Sierra where 5-8 inches will be likely (local amounts above 10 inches possible).

Another major snow producer is in store for the northern Sierra where amounts will be measured once again in feet! Amounts in excess of 5 feet will be likely above 5000 ft tonight through Wednesday. Snow will begin to pick up this afternoon and become heavy tonight, so travel over the Sierra should be completed today if at all possible.

Extended Discussion (Friday through Monday)

Dry conditions are expected on Friday as interior NorCal will be under short-wave ridging ahead of the next Pacific system approaching from the west. Precipitation will gradually spread inland late Friday evening into Sunday as the upper trough moves through. Models show differing solutions in terms of precipitation amounts, with the ECMWF being the least aggressive. Snow levels will initially range from 2500 ft to 4500 ft Friday night, but will raise to 5500 feet to 6500 feet by late Saturday. The GFS and ECMWF disagree with how quickly to eject this system, with the latest GFS showing a slower progression. Have kept precipitation chances until model differences are resolved. Dry conditions will return on Monday as the system exits the forecast area.