Warm(ish) daytime and widespread freezing nighttime temps this weekend

November 30, 2017 – Dry weather expected through the remainder of the week. Weather system may bring some precip to far northern portions of California over the weekend.

Discussion

Northerly surface pressure gradients have relaxed considerably compared to 24 hours ago with only local foothill locales still seeing gusts of 20-30 mph. Despite increasing high clouds, current temperatures are running considerably cooler across much of the region and range from the mid 20s in the colder mountain valleys to the upper 30s to mid 40s across the Central Valley and foothill/mountain thermal belts. Patches of fog have developed in the Central Valley from around the Sacramento area southward and in the mountain valleys.

Broad flat upper ridging will continue through Friday. Weak embedded wave moves through tonight with precipitation remaining confined to areas near the Oregon border. Some patchy valley fog possible again Friday morning. High temperatures through Friday will be near to slightly above normal.

Deeper system progged to move through over the weekend. Models continue to show some minor differences, but best chances for mainly light precipitation look to be over the mountains and northern third of forecast area Saturday afternoon into Sunday. Some light snow may fall over the higher terrain but little to no accumulations expected. Temperatures will be cooler.

Extended discussion (Monday through Thursday)

WR Ensemble SA tool indicates 2+ positive height anomalies over the Pac NW and NorCal Tue-Fri resulting in a warm and dry pattern for our CWA through the EFP. Generally, the models are in better agreement than yesterday wrt to the synoptic pattern affecting NorCal during the EFP. In the wake of the progressive Pac NW trof high pressure will build over the Wrn States next week resulting in a stretch of warm days, chilly nites, breezy Nly winds and dry wx.

The GFS is a little slower than the ECMWF building the ridge inland which would mean breezier conditions Mon/Tue before decreasing Nly winds Wed/Thu. The Nly winds will preclude much fog development in the Central Valley. The Nly winds would also result in some adiabatic warming effects and max temps a few to several degrees above normal, while the dry air, subsidence and strong temp inversions result in strong radiational cooling overnite.

Cold nites are expected in the Mtn valleys/basins Mon and Tue given the NEly, cold, dry flow from the NWrn NV. Min temps should also drop into the 30s in the Central Valley during the EFP but the dry air and Nly flow would keep frost minimal until winds weaken later in the period. Also, as winds decrease some mtn valley fog is possible later in the EFP.