Today will be the last day of generally clear, warm, and comfortable fall weather across the state for a while. Quite evident in infrared satellite imagery is a very impressive-looking mass of enhanced clouds stretching far to the west and south, with its tail well to the west of the Hawaiian Islands. This incoming series of storms has a notable subtropical moisture tap, and it does appear that the models are beginning to pick up on this (but may still be underestimating the magnitude of it). All of this activity does still appear to be related to the MJO-induced tropical forcing that took place last week over the tropical Pacific. A sequential synopsis follows:
Tomorrow, the first system will continue to churn towards CA, with strong southerly flow ahead to the main front. The models now indicate that a weak disturbance off the Baja coast will become entrained by this deep southerly flow aloft, along with some additional subtropical moisture, and that the diffluent and increasingly unstable environment over CA will contribute to the development of some elevated convection. The potential for convection will be greatest over interior NorCal (possibly including the Central Valley), but some showers or even isolated storms may pop by late afternoon over pretty much any part of the state. Pre-frontal convection in the fall does sometimes occur, and it is often fairly significant (some strong to locally severe storms have developed in this fashion over the past few years), particularly over the Central Valley. We shall see–keep an eye to the skies tomorrow.
Then…the real action begins. The first storm moves into NorCal on Thursday night and early Friday, bringing widespread rain and breezy conditions, This will not be an earth-shattering event, but it will bring the most significant precipitation of the year so far. Rain will slowly taper off during the day Friday, though some showers will still be possible on Halloween evening. SoCal probably won’t see much out of this first system–perhaps a few light showers a little farther south than Point Conception.
Saturday will feel the effects of an approaching strong storm system. A low pressure center will develop off the coast of CA and ride north as it deepens significiantly, eventually setting up shop off the WA coast. The surface pressure gradient will be steep, even with the low center far away, and some strong winds still appear to be possible. This will become clearer over the next 24 hours. This second system will have a lot of deep moisture to work with, and plenty of dynamic support (with CA in the exit region of a strong jet streak), so I do expect rather hefty precipitation totals from late Saturday into Sunday. Rainfall totals of 2-4 inches in favored parts of the Bay Area and the foothills (along with far NorCal) are likely, with some heavier totals possible on south-facing slopes. The Central Valley and Bay Area in general may see upwards of 1-2 inches, and the coast of CA south to about Point Conception will see up to 1.5 inches. Most of SoCal will see under 0.50 inches with this storm, but mountain areas and south-facing slopes stand the chance of an inch or two. NorCal will remain just to the north of the polar front (jet stream) for a good 48 hours or more Sunday into Monday, so some convective precipitation is likely then with thunder possible. More (though probably weaker) systems may follow into the early part of next week.
So…winter does appear to be trying to make a rather early appearance this year. The million-dollar-question: will it last?