Dry and seasonable weather with some occasional breeziness expected through the weekend and into early next week. A pronounced pattern shift toward active weather with is then expected from Tuesday through the end of the week. Periods of moderate to heavy rain and mountain snow alongside gusty winds will bring significant travel impacts to interior NorCal, especially through the middle of the week.
Discussion
GOES-West imagery depicts mostly clear skies across the region today, with attendant temperatures early this afternoon already eclipsing the high temperature marks from yesterday. This trend toward warm, dry, and occasionally breezy weather will be brought upon by building ridging aloft into the weekend. With the ridge axis centered offshore today, high temperatures into the upper 50s to mid 60s are anticipated throughout the Delta, Valley, and foothills, with 40s to 50s at higher elevations. While a drastic warm up is not anticipated, as ridging becomes more prominent over the region by the weekend, more widespread 60s to low 70s are expected at lower elevations with 50s to 60s at higher elevations through Monday.
A few periods of breezier winds are expected today ahead of the building ridge, Sunday as the ridge translates eastward, and late Tuesday as a strong, deepening trough begins to impact the region. As a result, north winds gusting 15 to 25 mph are expected today, with similar wind gust magnitude from a southerly direction on Sunday. Strongest southerly gusts are expected late in the day Tuesday with gusts 20 to 30 mph from Interstate 80 southward and gusts 25 to 35 mph north of the Interstate 80 corridor. Some gusts up to 45 mph will be possible along the Sierra crest as well. While increasing southerly winds will keep high temperatures seasonable, lowering heights as the trough approaches will work in contest to cool high temperatures closer to the upper 50s to mid 60s at lower elevations and 40s to 50s at higher elevations on Tuesday afternoon.
Extended Discussion (Late Tuesday through Friday)
Some minor timing differences exist between ensemble guidance, but by late Tuesday, a trough is expected to rapidly deepen from the Gulf of Alaska toward the West Coast. Given the slight timing differences, exact precipitation onset remains a bit uncertain, but general consensus indicates that widespread precipitation will be ongoing by Wednesday morning at the latest. Despite the deep nature of the expected trough, the majority of ensemble guidance indicates it being rather progressive. As a result, heaviest precipitation is anticipated on Wednesday as the trough is more directly overhead, with a very gradual tapering in intensity through Thursday.
Aforementioned increasing winds late Tuesday, look to continue their upward trend into Wednesday. Given the orientation of the trough, a south-southeasterly wind direction is favored at this time. While wind gust magnitude remains a bit uncertain, probabilities of gusts up to 45 mph currently sit around 30 to 60 percent from Interstate 80 southward, with 60 to 80 percent probabilities north of the Interstate 80 corridor and along the Sierra/southern Cascades. Winds are expected to taper off by Thursday, but additional breezy to gusty winds remain possible Friday as the next potential shortwave arrives.
A moderate intensity atmospheric river moisture plume is on tap to accompany this midweek system and given expected trough deepening, periods of moderate to heavy rain and snow are expected with this system. Current probabilities of precipitation up to 1.5 inches throughout the Valley and foothills sit around 50 to 80 percent at this time, with 60 to 80 percent probabilities of precipitation greater than 3 inches along the Sierra/southern Cascades. Snow levels are expected to begin around 4000 to 5000 feet late Tuesday through midday Wednesday, falling to 3000 to 4000 feet by Wednesday evening, and falling further to 2000 to 3000 feet by Thursday morning. Resultant probabilities of snowfall greater than 4 feet sit around 50 to 80 percent above 5000 feet, with some 30 to 80 percent probabilities of up to 1 foot between 3000 and 5000 feet (highest above 4000 feet).
Additional light snowfall accumulations will be possible between 2000 and 3000 feet, but uncertainty in this occurrence is high at this time. Heaviest snowfall at this time is expected on Wednesday with some potential for periods of snowfall rates 2-3 inches per hour at times. With periods of moderate to heavy rain and snow and strong southerly winds, take time now to prepare for significant weather impacts, particularly regarding mountain travel next week.