Thunderstorms likely in some areas today; active early fall week ahead.

Well, it turns out that the GFS charts from a few days ago were correct after all. A fairly weak circulation off the coast (essentially a lone vorticity mximum) has succeeded in deepening the mid and upper-level moisture sufficiently over CA that elevated convection has been developing for the past six hours. Scattered thunderstorms have developed over all parts of SoCal, including coastal valleys and even offshore waters. Scattered showers have also developed over the entire Central Valley and there is some additional convective activity over far northern CA. I expect this activity to increase through the day today with the combination of daytime surface heating and the increasingly diffluent flow on the east side of the vort max approaching the coast from the southwest. Isolated to scattered thundestorms are possible over essentially the entire state today, and more numerous storms may occur in the mountains. Precipitation will generally be very light to light, but some very isolated heavy downpours will also occur. There is apparently a good amount of lightning associated with existing cells, a trend that will likely continue as the day progresses.

 Tomorrow will probably be less active, but some thunderstorms will still be possible over the Sierras and other mountain areas. Temperatures will be notably cooler, however, over inland areas, which have endured 95 plus degree readings over the past week. A huge change in the prevailing pattern–which has been in place more or less since February–will take place by Thursday as a very strong storm moves NE off the CA coast. This storm is already evident of satellite imagery and is apparently very convectively energetic–both the NAM and the GFS are having convective feedback troubles with this system, which is rare for these parts. Unfortunately, this system will most likely weaken and lift northeast before impacting CA, and all we will see from this one is some light rain in the north. A second and even more powerful system, however, will be close on its heels. By Friday afternoon, a rather powerful cold front may move across NorCal, according to the 12z GFS, bringing locally heavy rainfall and potentially gusty winds. This would be a memorably strong system by early October standards, and there would probably be convective activity associated with a system that is so strong and coincides with a still-steep sun angle. This is still a few days out, but a significant rainfall event is looking to be increasingly likely in NorCal this week and possible as far south as the Central Coast. SoCal could see a few showers out of this system, but that is much less certain that the NorCal rainfall. The models all lift the rain line back well to the north into Washington by late in the weekend, but keep a powerful 150 kt jet blasting into the Pacific Northwest. This leads me to believe that there may well be some more early-season rainfall within the following week. One other thing to keep an eye one is the tropical system that the models have been pretty consistent in developing by Friday or Saturday. Remnant moisture could become a relevant issue if we have deep southerly flow ahead of another cold front next week. Just a thought…

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