Showers will continue today with a chance of isolated afternoon thunderstorms. Another wet and windy system will move into the region tonight and Tuesday bringing moderate to heavy rain, high elevation snow and renewed localized flooding concerns.
Discussion
Moisture continues to stream into NorCal on westerly flow beneath the large trough off the PacNW. Numerous showers have been developing as warm-advection persists across the region. Snow levels continue to be hovering around 6,000-6,500 feet across the northern Sierra. With all the clouds and showers, temperatures will only warm up about 10-15 degrees than this mornings low temperatures.
Surface gradient continues to tighten, especially across far northern California where gusts of 20-30 mph remain across the northern Sacramento Valley and in the northern mountains. Precipitation will remain showery with intermittent breaks.
All the clouds and showers will likely inhibit potential for deeper convection, however, if sufficient clouds breaks materialize, there will be a chance for thunderstorms in the afternoon into the early evening. Forecast soundings still show plenty of shear with stretched hodographs and sufficient 0-1 km shear for longer-lived storms and a non-zero chance for funnel clouds. Some small hail will also be possible, but deeper instability doesn’t look quite as likely as over the weekend just to our south. Any thunderstorm development will be highly dependent on cloud cover.
As of 1pm this afternoon, more breaks in the cloud cover have developed across the far southern Sacramento Valley and the northern San Joaquin Valley. At this time confidence is increasing from low to moderate on enough instability developing for more organized convection in the southern Sacramento Valley. The better shear is definitely further north, in the northern Sacramento Valley, where cloud cover has been persistent through the morning and early afternoon.
Conditions will deteriorate tonight as light to moderate precipitation spreads in and winds pick up in conjunction with the arrival of the AR. Based on latest guidance, the heaviest precipitation will arrive Tuesday with moderate to heavy rain and some embedded thunder as the front approaches and moves through.
Rain and snow will then taper off from the north Tuesday night and early Wednesday, with only light, lingering snow showers through Wednesday morning in the Sierra.
This storm appears similar in strength and duration as the one late last week, but winds may end up a notch stronger as a surface low moves up into NorCal Tuesday afternoon.
Latest guidance indicates the bulk of the AR will once again come onshore to our south with stronger IVT forecast into central California.
Nevertheless, it will be another impactful system for the region with rises expected on rivers, streams, creeks expected resulting from the rain and some component of snow melt. Some urban and small stream flooding will be possible, particularly later tonight and Tuesday as heavier rain spreads in.
Little changes in forecast QPF values through Tuesday, with around an inch or two in the valley with 3 to 7 inches in the foothills and mountains.
Snow levels will be around 6-7k ft today, rising to nearly 8.5k ft Tuesday before lowering to around 3-5k ft by early Wednesday.
Accumulations of several feet of snow will be likely across the higher passes and along the Sierra crest leading to major travel impacts.
In addition to the precipitation, strong southerly wind gusts up to 55-60 mph will be possible in the valley with gusts over 70 mph possible in the mountains as the front moves overnight/early Tuesday.
Peak winds look to be between 7am Tuesday – 8pm Tuesday. Given saturated soils, gusty winds could lead to additional downed trees. Gusty winds will remain in the Sierra through Tuesday night.
Patchy fog is possible Tuesday night/Wednesday morning, but given the longer daylight hours, widespread dense fog is unlikely to develop. Mostly dry conditions are expected Wednesday, besides the early morning lingering snow showers in the Sierra.
Dry conditions will continue through at least Thursday. Cooler temperatures are expected Wednesday and Thursday morning, with the coolest morning being Thursday morning, with Valley temperatures generally in the upper 30s to mid 40s.
Extended Discussion (Friday through Monday)
Clusters and ensembles advertise a weak, relatively dry shortwave arriving to NorCal around late Friday-Saturday. Not all individual ensemble members are on board with precipitation; roughly 5-10% remain completely dry during this timeframe. Remaining members generally point to light precipitation.
Another potential system arrives for the second half of the weekend and into early next week. At this point, ensembles suggest a weak or moderate-strength system with plenty of details yet to resolve with respect to timing and placement.