Subtropical moisture threat and a possibly massive heat wave?

Well, I have to say: this is not turning out to be one of our more boring summers. Temperatures today–though hot–will not be incredible by summer standards. Inland, temperatures are expected to stay rather hot for the foreseeable future–well into August, that is.

There are several features of note on a satellite view of the Eastern Pacific. The first is Tropical Storm Dolores, which is looking rather healthy as she tracks northwestward from a position southwest of Baja California today. Hurricane Carlos, which at present is looking quite anemic, is churning in the general direction of Hawaii (though it is not expected to significantly impact the islands). A rather strong trough for late July is currently setting up off the Pacific Northwest, which is continuing the trend towards an unusually active pattern resulting from the fact that the mean jet path has been unusually far south this summer. A bit of a monsoonal surge associated with an easterly wave from the Gulf of California is also propagating westward at the present time. All of these features will contribute to fairly strong mid-level S/SWly flow over the state for the next few days, which will probably allow for some tropical/subtropical moisture to flow over much of the state. This will be a pattern similar to last week’s in the Bay Area, where moisture from an old tropical storm produced scattered showers and isolated thunderstorms. In this case, however, there is a much deeper and broader moisture plume, and there are also multiple sources of mid-level moisture contributing to overall enhanced moisture field. As such, I do expect that there is at least a somewhat better chance of mid-level convection over the state than there was last week. The models performed very poorly with last week’s convection and moisture entrainment event, so I do expect that they may again be seriously underestimating convective potential across the state this week. Certainly there will be  an uptick in mountain convection by tomorrow afternoon, but some of this may spread to lower elevations if the moisture field is deep enough and there is some form of trigger. Stay tuned on that front…

The GFS and ECMWF have been indicating for several days that California may be in for a truly impressive heat event beginning around the middle of next week. 850 mb temperatures exceed 30 C on both models for at least a week, peaking over 33 C on some runs. Keep in mind that the current moderate event has 850 mb temps in the mid to upper 20s, so widespread readings above 30 C would probably bring record temperatures to much of the state (and that is a serious threat, because this is climatologically the hottest part of the year). Also, as the high builds into CA from the SE, the flow of subtropical moisture will again increase, leading to elevated surface humidity and creating the potential for a dangerous heat event. Heat indices in such an event would easily peak above 115 in inland areas. Again, stay tuned…