April 16, 2018 – Pacific storm continues today with showers, mountain snow and thunderstorms. Dry Tuesday, then a chance of showers again Wednesday and Thursday. Dry weather returns Friday.
Discussion
Cold front has moved well to the south and east and satellite imagery shows the cold trough beginning to move into NorCal early this morning. Upslope flow is continuing snow showers across the west slopes of the northern Sierra Nevada/southern Cascade Range while much of the remainder of the forecast area is currently dry. Current temperatures are considerably (5-15 degrees) colder compared to 24 hours ago and range from the 20s to lower 30s in the mountains to the mid 40s to around 50 across the Central Valley.
It promises to be an active spring day of showers and thunderstorms across interior NorCal, with snow continuing across the mountains, as the upper trough moves onshore. Broad cyclonic flow along with very cold 500 mb temperatures (-32C) will result in scattered showers and thunderstorms through the day across the region. Forecast soundings indicate relatively weak shear with thin, but deep, CAPE which often support thunderstorms producing copius amounts of small hail and the possibility of cold air funnels. Our local total shear chart does indicate potential today for a brief weak tornado or two with any stronger longer-lived storms (lower left on the box).
Light to moderate snow expected to continue in the northern Sierra into this evening with upslope flow and steep lapse rates up to 500 mbs. Another 4-8 inches can be expected with local amounts approaching a foot along the crest and areas south of Highway 50. Winter weather advisory continues in effect as mountain travel likely to be difficult with slick roads, reduced visibility, travel delays and chain controls.
Showers decrease tonight with drier and warmer weather returning Tuesday as upper ridging moves into NorCal. Ridging shifts east into the Great Basin late Tuesday as the next system moves in Wednesday afternoon and night. Appears this one will give us a glancing blow as it slides down the coast with the highest QPF along the western and southern edges of the forecast area.
Extended discussion (Friday through Monday)
Upper level ridge over the west coast will bring fair skies, light northerly winds and well above normal temperatures Friday. Daytime highs Friday are forecast to run about 6 to 10 degrees above season normals. Fair skies will continue through the weekend under upper level ridging but weak disturbances traversing over the ridge and through the Pacific Northwest may bring some high cloudiness at times. Despite this, daytime highs will remain well above normal through the weekend. Models diverge early next week. GFS brings troughing into the coast for at least a little cooling while ECMWF rebuilds a ridge over the coast. Have kept forecast dry with above normal highs for now and will wait for better forecast agreement before making any forecast changes.