Valley fog Friday morning, unsettled weather continues through next week

October 28, 2021 – Dry and mild weather the next couple of days. Low pressure moving over the far northern portion of California will bring a chance of showers late Friday and Saturday, mainly to the northern mountains. Rain returns to the the majority of Northern California early next week.

Discussion

A little north wind down the Valley overnight provided sufficient mixing to limit fog in the Valley, while the night fog product showed moisture trapped in the mountain valleys/basins, i.e. Burney Basin and around Big Q. Increasing subsidence this morning, from the upper ridge axis building over the region, may lead to some fog in the Valley, as moisture from recent rain will be trapped below the radiative inversion. The HREF suggests some patchy chances of better than 50/50 of VSBYS less than 3SM developing before or after sunrise from the Nrn San Joaquin Valley northward into the central Sac Vly, mainly in the 0600-1000 time frame. At 0300, MCC and SAC began to report BR. Otherwise, a very pleasant day with max temps near to a little above normal in the Valley, and 3 to 9 degrees above normal in the mountains due to the warmer air aloft.

The upper ridge over NorCal shifts Ewd on Fri in advance of a closing mid-level low pressure center. Differences in the track of the low center provide a great deal of uncertainty in the areal coverage of PoPs/QPFs mainly Fri night and Sat. The GFS continues to be farther south and faster than the ECMWF. The 5H cluster analysis shows a trof over NorCal with only Cluster 3 heavily weighted to the ECMWF showing a closed low just off the N CA coast by 12z Sat. Looks as if the models, especially the GFS have slowed the timing of spreading precip into our CWA and have adjusted the PoP/QPF timing in the morning package to Fri night and Sat. This also includes limiting the chance of thunder to Sat with the consolidated low near the CA/OR border. Forecast water vapor transport shows NorCal in between moisture plumes so just light QPFs are expected over the Nrn mountain zones. The SREF QPF plumes show a mean of about 0.20″ at MHS, and less than a tenth of RDD, however the majority of members are higher and consolidated just under a tenth of an inch.

A narrow highly amplified ridge builds along the W Coast on Sun while the cut-off low fills/exits our area, but suppresses heights and warming in our CWA. Thus a cooling trend is expected over the weekend. The narrow, amplified ridge will quickly be shunted Ewd by the next progressive Pacific trof and front which is forecast to move inland on Mon bringing another round of precip to interior NorCal.

Extended Discussion (Monday through Thursday)

Models in good agreement with progression of next Pacific frontal system into NorCal Monday. Front progged to weaken as it moves into the CWA with overall light QPF. Drier weather expected Tuesday into early Wednesday under upper level ridge. Models differ with progression and overall QPF of next system, but point to more widespread light to moderate precipitation around the Wed/Thu timeframe.