Typical late-summer and early fall conditions, then a possible change

An  early-fall trough has contributed to cooler-than average tepmeratures and some localized light precipiation in NorCal over the past 48 hours, but the vast majority of areas did experience a return to sunny skies and seasonable temperatures today. This trend back towards climatological norms will continue this week, and inland highs may actually surpass normal values by Tuesday. Comfortable afternoon temperatures will be felt virtually everywhere in CA this week–including the coast–and though it will be a little on the hot side in the Central Valley,  overnight temps will cool sufficiently to avoid any discomfort.

As this ridge biulds into CA from the southeast. however, and increasingly amplified pattern will be developing in the far Eastern Pacific. A split flow pattern is expected to develop by next Friday, with a cutoff low developing off the CA coast by the weekend. The GFS is now fairly consistent in bringing substantial subtropical moisture into CA from the south as the result of deep-layered flow on the east side of the low. The model also forecasts the mid and upper-level moisture, combined with diffluent flow on the east side of the low, to be sufficient for convective activity. The 18z GFS actually brings quite a bit of convective precipitation to the Bay Area and Central Valley next weekend. Although this is almost certainly rthe result of convective feedback issues in the modelling scheme, there is the real potential for some interesting weather next weekend ahead of this cutoff low. By the end of the weekend and early next week, the pattern gets even more interesting. The GFS has been indicating for some time that a major trough of low pressure, associated with a longwave shift in the jetstream (which will be screaming at nearly 140 kts at 300 mb–way stronger than usual for this time of year), will swing as far south as central California in the 7-12 day period. Now, with all this subtropical moisture already in place, the potential is there for this system to bring widespread significant precipitation to NorCal and possibly places farther south. This is a long way out, but certainly bears watching. A further complication to the weather from this Friday onward is the possibility of tropical development in the Eastern Pacific. The GFS is now indicating that at least two tropical systems will be caught up in the deep southerly flow on the east side of various troughs near CA over the next two weeks. The NHC is indicating that the potential for tropical development is indeed increasing, so this will also have to be monitored.

 Stay tuned…the weather may be about to get much more interesting than it has been in over six months.