Weak mountain storm expected this weekend

December 1, 2017 – Dry weather expected today. Chances for rain and snow for the higher terrain Saturday. Return to dry weather next week.

Discussion

Other than some high clouds, skies are mostly clear across interior NorCal early this morning. Current temperatures are chilly ranging from the 20s in the mountain valleys to the mid 30s to mid 40s elsewhere. Patches of fog have begun to develop across the Central Valley.

Dry weather will continue today under flat ridging. Highs will be similar to Thursday’s with mainly lower to mid 60s in the valley with 40s and 50s in the mountains.

The weekend will start out cooler and breezier as a stronger system will drops southeast from the Gulf of Alaska on Saturday bringing precipitation chances to mainly the mountains and northern Sacramento Valley as its forecast trajectory won’t be ideal for widespread precipitation at the lower elevations. The system will move thru by early Sunday with cool, dry and breezy weather expected to continue in its wake early next week under northerly flow.

QPF with the Saturday system is expected to range from around 1/4 to 1/2 inch over the mountains with around a tenth of an inch or less across the northern Sacramento Valley. Snowfall amounts of 1-4 inches at the higher elevations with around 6 inches possible around Mt Lassen.

Extended discussion (Tuesday through Friday)

WR Ensemble SA tool indicates 2+ positive NAEFS height anomalies over the Pac NW and Norcal Tue-Fri resulting in a warm and dry pattern for our CWA through the EFP. The Return Interval (RI) ranges from a 10 year event on Tue to a 30+ RI Wed into Thu before decreasing to 1 year interval by 00z Sat. Models are in general agreement especially later in the EFP, after a different beginning to next week. Now it the ECMWF that advertises a retrograding upper low resulting in a Rex Block along the coast, in contrast to the GFS which had a similar solution a couple of days ago and then backed off on that solution and followed the ECMWF. These differences early in the week become “ironed out” during this period with a similar pattern over Norcal, although the ECMWF may be a tad warmer late in the period. The ECMWF solution would certainly result in breezier Nly winds on Tue, and the position of the high pressure cell off the Pac NW coast would keep Nly flow over Norcal through Wed limiting fog potential. As the High nudges inland, the winds will subside Thu/Fri and increase the potential for fog in the Srn portion of our Valley.

The Nly winds would also result in some adiabatic warming effects and max temps a few to several degrees above normal, mainly in the low to mid 60s in the Valley. Record max temps at the main climo sites are in the upper 60s to mid 70s during the EFP, with perhaps RDD having the best chance of tying/establishing a record from the 5th-7th with their records 69 on the 5th and 6th and then 66 on the 7th. Meanwhile, the dry air, subsidence and strong temp inversions will 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.