Dry weather with above normal temperatures through mid-week with areas of frost and fog in the mornings. A fast moving weather system will bring rain showers and mountain snowfall Wednesday PM through Thursday. Wetter weather system arrives late this week and into the weekend with gusty southerly winds and renewed rain and mountain snow chances. Additional precipitation impacts possible with another system early next week, but details remain uncertain.
Discussion
Early morning GOES Satellite Night Fog reveals areas of patchy fog in the northern San Joaquin Valley nosing into the Modesto area, although visibilities haven’t dropped below a mile yet at the time of this writing. With light south to west winds in the northern San Joaquin Valley, there remains a chance that patchy dense fog will advect into the area as we continue through the early morning hours. Morning low temperatures will be in the low to mid 30s, so areas of patchy frost will be possible on grassy areas.
Today and most of the day Wednesday will be the last dry and mostly clear days, as upper level ridging continues to be in control of our synoptic pattern. The ridge will begin to break down as we move through Wednesday as a weak and fast moving trough slides into NorCal, then followed by a stronger/deeper trough moving over the Sierra Nevada on Thursday and quickly dig into the Four Corners Region Thursday night. Rainfall totals have stayed relatively the same from yesterday’s forecast, with 0.25 to 1.00″ inches across the mountains, and a trace to 0.50″ inches over the Valley from Wednesday AM through Thursday afternoon. Light to moderate snowfall will also accompany this quick hitting system, with little change to yesterday’s total forecast. Currently looking at around 2 to 6 inches in the Sierra and southern Cascades, with a peak amount at Lassen NP at around a foot of snow. The Coastal Range will also see some minor accumulations of around of around a dusting to 4 inches in the forecast. Precipitation is forecast to be out of the area by Thursday evening, with only isolated snow/rain showers with little to no accumulations/impacts expected.
The break in precipitation will not last too long, as another round of precipitation will enter the area on Friday afternoon. Precipitation will begin the northern Sacramento Valley, Coastal Range, and southern Cascades before spreading across the rest of the area late Friday evening. This system will be the wetter of the two, with around 0.10 to 1.50″ inches of rainfall in the Valley through Friday night, with the higher amounts in the northern Sacramento Valley and mountains/foothills. Snowfall amounts are forecast to be around 2 to 6 inches, through Friday night, mostly for elevations higher than 5000 feet. This system and the precipitation is forecast to continue through the extended period, so the totals listed here are not a storm total, just what can be expected in the short term forecast period.
Although models are starting to come into better agreement, they have shown inconsistency regarding track, timing, precipitation totals for a few days, so continue to monitor the forecast for changes. High temperatures are forecast to be in the upper 40s to mid 50s for the Valley through Friday.
Extended Discussion (Saturday through Tuesday)
Pacific frontal system progged to move through interior NorCal Saturday into Sunday AM bringing widespread rain with snow in the mountains. Category 1 AR associated with the baroclinic zone. Liquid WPC QPF range from around a few tenths in the N San Joaquin Valley, increasing to around 2 inches in the N Sac Valley with 1 to 4.5 inches in the foothills and mountains. Snow levels range from 4500-5500 feet with local snow amounts over a foot of snow possible at the higher elevations.
Decreasing showers expected Sunday as system pushes east and short wave upper ridging moves through. Another Pacific frontal system forecast to move through Monday into early Tuesday but QPF amounts look less than then system that moves through over the weekend.