Periods of widespread Valley rain, mountain snow, and gusty winds expected this week into next week. Holiday travel will be impacted, especially over mountain passes. Lower elevation snow possible Tuesday or Wednesday. Cold and drier weather expected late next week.

Discussion

We are going to remain in an active unsettled weather pattern as long wave troughing has set up over the West. A short wave trough is currently pushing through the area early this morning and that is brining widespread showers to portions of the Sacramento and San Joaquin Valley and the Sierra. Rain rates with this activity has been generally light with less than 0.10″ /hr. but there are some localized pockets up to 0.25″ /hr.

Valley showers will become more isolated by the mid to late morning and the showers will become less widespread over the mountains during the afternoon as we see a short period of zonal flow set up. This break in precip will be very short lived as our next short wave trough will be digging down the coast Friday afternoon into the evening.

Precip will once again will become more widespread from north to south during the evening and overnight Friday. Widespread shower activity will continue on Saturday as the trough axis approaches the region. We do see some weak CAPE develop Saturday afternoon into the evening mainly from about Sacramento north to around Chico. The thunderstorm threat overall looks low but a few lightning strikes can’t be ruled out along with small hail.

For the most part the Valley will dry out Saturday night but widespread showers will continue over the mountains due to up slope flow. Showers will once again increase in coverage Sunday night into Monday as another short wave trough digs down the coast. Showers will be diminishing Monday evening into the overnight as the short wave pushes to out south and east. Behind each one of these short waves we will see colder air build in with falling snow levels.

Currently snow levels are between 3500 and 4500 feet over Shasta County and 4500 to 5500 over the Sierra. Snow levels will continue with a downward trend reaching 2500 to 3500 feet by this evening and will remain there into Saturday. The Sunday/Monday short wave will have quite a bit of cold air with it and snow is expected well into the Foothills as snow levels will be 1500 to 2500 feet.

Snow levels will bottom out Monday evening into the overnight and some snow accumulation will be possible down to 1000 feet over the foothills and down to Redding over Shasta County.

Several feet of snow can be expected over the mountains now into early next week. We are also going to see breezy winds in this pattern. The strongest winds will be ahead of the short wave troughs both Friday afternoon into Friday night and again Sunday afternoon into Sunday night.

Extended Discussion (Tuesday through Friday)

The cold, wet weather pattern from early in the week remains cold, but the heavier precip will be shifting southward into SOCAL by the middle and end of the EFP. Ensemble clusters indicate a cold upper trof will persist over Wrn NOAM with a short wave dropping Swd along the W Coast continuing precip in NorCal on Tue, whereas a subsequent short wave slightly retrogrades off the CA coast Wed/Thu continuing the cold wx but pushing the bulk of the precip Swd.

Earlier chances of snow in the Srn Sac Valley appear to be lessening on Tue given the NBM PoP/QPF forecasts. However, some light QPF is forecast with the short wave dropping Swd along the CA on Wed giving the chance of rain/snow in the Srn Sac Vly then.

Probabilistic guidance for the 24 hr period ending 4 pm Tue shows an 80% probability of a dusting (<1″) of snow in RDD with zeroes for SAC albeit the upper end of the range also indicates a dusting. Similarly for the 24 hr period ending 4 pm Wed the 80% probability is similar, although the upper ranges (and much lower probabilities) show an inch at SAC and 4″ at RDD. Thus, there is much uncertainty for any significant snow, with the most likely outcome a dusting at RDD.

Meanwhile, the Sierra and foothill locations should continue to see some light/moderate snow continuing to accumulate into the midweek period. A cold north wind is likely behind the subsequent wave which should drop min temps below freezing in the Valley as well.