Very unusual early fall storm to affect CA

This week’s weather across California is about to become very interesting. Models continue to indicate the development of a retrogressive cutoff low moving south over CA to a position off the SoCal coast by the end of the week. This low will be extremely deep for this time of year–500 mb heights will fall to what very well may be historical lows across portions of the state. Very cold air aloft (for this time of year, anyway) will accompany the cutoff, though initially moisture will be lacking. Still, scattered showers and thunderstorms are expected to develop as the low moves towards the Bay Area from the N/NE on Wednesday continuing into Thursday. Additionally, strong onshore winds will likely precede the low, gusting in excess of 40 mph. This could result in fire weather concerns. As the low moves south and west, it will pick up some Pacific moisture. Current runs of the GFS are not as emphatic with the entrainment of copious tropical moisture as they were yesterday, but given the low’s proximity to CA and the existence of some developing tropical systems in the E. Pac, this remains a (small) possibility. Even assuming the low does not develop a significant tropical moisture tap, instability will be sufficient to produce at least scattered showers and thunderstorms over most of the state, with the possibility of more organized band of precipitation developing at times. The best chance for significant precipitation will occur when the low finally lifts NE by next weekend. There is still much uncertainty regarding much of this pattern, especially about the track and speed of the ejecting low. Regardless, this week will be a very interesting one. It is also worth noting that several strong typhoons in the W. Pac. may contribute to an increasingly amplified flow pattern by the end of the week, which could affect the track of the low. The ECMWF, which has taken the low on a more “inside slider”-type track until this morning, may be keying in on this tropical forcing in its increasingly westward placement of the low. So…the wetter solutions of the GFS may yet verify. Stay tuned…