Unseasonably active pattern to continue for CA

The trend of unseasonably active weather over California will continue this week. Several storm systems will move into the state in the coming days, bringing precipitation and unseasonably cool conditions to much of the state.

Water vapor imagery of storm slated for Monday/Tuesday (NRL)


The first system, due in on Monday, does appear to have something of a subtropical moisture tap. In addition, despite the absence of very cold temperatures aloft, the system does appear to have some convection associated with it on satellite imagery. Since the front will be negatively tilted when it moves ashore and substantial warm air advection will occur ahead of the primary precip band, I would not be surprised to see a few thunderstorms develop despite the lack of an obvious source of strong surface-based instability. Lightning is sometimes observed with late-spring systems of this strength in NorCal, as it’s really the one time of year that relatively weak frontal dynamics can be sufficient to trigger sufficient upward motion for taller cumulus clouds without the additional presence of unusually steep lapse rates. Although the strongest dynamics and most moisture will remain in NorCal, SoCal may still see some light showers as a relatively slow-moving moist airmass drags itself across the state.

Later in the week, the models are indicating a rather strong trough moving into the Pacific northwest and possibly extending into California. A trough of this magnitude and strength–with 850 mb temperatures below 0C in NorCal–is quite anomalous. It’s not clear at this point whether there will be any moisture for convective activity except over the mountains, though the 12z GFS did indicate more widespread precip. The main story, though, will be very strong winds associated with the very sharp thermal and pressure gradients over CA and the interior West later this week. Stay tuned…


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