A winter storm will bring moderate to heavy rain and snow and gusty winds through Friday. Thunderstorms are possible this afternoon and evening and some may be strong. There is a brief break in precipitation on Saturday and then a weak system brings light precipitation chances over higher terrain Sunday into Monday.
Discussion
A rather strong winter storm has moved into the area, bringing along moderate to heavy rain and snow, and gusty winds. Snow levels have lowered over Shasta County with a pocket of cold air over the area, with snow briefly reported as low as Redding Regional Airport at 500 feet. We have received reports and video of light snow in western Redding. We have also seen a mix of falling rain and snow on Caltrans webcams, such as on Eureka Way and Lake Blvd. As of 330 am there were chain controls on I-5 24 miles north of Redding. For current road conditions go to Quickmap.dot.ca.gov. The risk for snow in Redding should be end by late morning as snow levels rise.
Snow amounts of 2 to 6 inches of snow are possible for elevations between 500 and 2000 feet for Shasta County, with a Winter Storm Warning remaining in effect through 10 am this morning. A Winter Storm Warning will then remain in effect above 2000 feet through 4 am in the northern Coastal/Shasta County mountains. In the Sierra, warm air advecting in with a warm front has caused snow levels to rise some, with snow earlier falling at Blue Canyon (5280 feet) around midnight and then turning to rain by 1 am.
Snow levels currently are around 5500 feet early this morning over I-80, but are expected to drop to around 4500 feet by Friday as cooler air moves in. There are currently chain controls in effect on many Sierra highways. A Winter Storm Warning remains in effect for the northern Sierra, southern Cascades/Lassen Park until 10 pm Friday. There is the potential for 2 to 5 feet of snow above 6000 feet. Snow rates of 1 to 2 inches an hour are expected at times, and when combined with winds gusting to 60 mph could bring whiteout conditions.
Radar and surface observations show widespread rain across the area, with the CHP already starting to report local road flooding. Today the heaviest rain is expected. A Flood Watch remains in effect through 10 PM Friday for much of the area for elevations below 4500 feet. Expect slick roads, minor flooding, mud and rock slides and rises on area rivers and streams.
In addition to heavy rain showers, there is the potential (20 to 25 percent probability) for scattered thunderstorms this afternoon and evening. Highest potential is for the northern San Joaquin Valley, the Delta, the southern and central Sacramento Valley and the adjacent foothills. Models are indicating the potential for strong shear, which with sufficient instability (350-450 J/Kg CAPE could help to produce some severe storms. The Storm Prediction Center has the Central Valley in a Marginal Risk area. Can’t rule out funnel clouds or possibly even a tornado with this kind of pattern. Periods of heavy rain, strong gusty outflow winds and hail are also possible with any thunderstorms that form.
Gusty southerly winds have developed overnight and will persist through Friday morning before gradually decreasing through the day. Strongest gusts so far reported include 53 mph at Sacramento International Airport, 48 mph at Sacramento Executive Airport, 45 mph at Travis Air Force Base and at Mather Field, 40 mph at Stockton Airport. This winds are expected to peak today across the Valley and Delta.
A Wind Advisory remains in effect for these areas through 10 Am Friday. Potential impacts include outdoor objects blowing around, downed trees and branches, local power outages and difficult driving conditions for and around high profile vehicles.
Drier weather is in the forecast for Friday night through Saturday as the storm moves out of the area and we see a break in precipitation. Weak northerly flow returns to the area with colder morning low temperatures on Saturday (although not quite as cold as the past few days), generally in the mid to upper 30s in the Valley and Delta, though the Redding area could approach freezing. Ensemble guidance and cluster analysis continue to show a weak system arriving on Sunday bringing the return of some precipitation to the mountains and northern Sacramento Valley. Snow is expected to be light with limited impacts to mountain travel.
Extended Discussion (Monday through Thursday)
Ensemble guidance and the National Blend of Models (NBM) shows precipitation over the mountains and northern Sacramento Valley lingering through Monday, but rain and amounts look to be fairly light. Drier and slightly warmer weather returns Tuesday. There is some uncertainty beyond Tuesday but some guidance shows a weak shortwave passing by to the north that could bring some additional light showers mainly in the mountains later in the week.