Deep upper-level low brings unseasonable weather this week

Filed in Uncategorized by on August 2, 2009 22 Comments

This will be another brief update today. A deep upper-level cutoff low is currently positioned off the Central California coast and has been producing episodes of very impressive elevated thunderstorm activity across interior NorCal and southern Oregon. Severe storms have occurred at times, and as many as 200 new wildfire starts have occurred in the past 18 hours alone. This low is expected to very slowly drift towards the far northern California coast through the coming week. Dynamics will become increasingly strong as the week wears on, with a large 130+ kt jet streak over most of NorCal for several days, deep southerly flow at all levels, and strong upper-level divergence on the east side of the low. The center of the low is already convectively active with occasional thunderstorm activity, but other than at the core and on the far north side of the circulation the low is extremely lacking in moisture. As such, the models do not project much significant convective activity outside of the mountains this week except for the far north coast. The caveat: the upper low will be over water for an extended period of time, and lows such as these do have a tendency to pick up some moisture for the ocean due to prolonged vertical motion. Therefore, it is conceivably possible that the models are underestimating the convective potential in lowland areas. Also, the NAM is beginning to key in on some subtropical moisture being tapped by the low, which could potentially bring some convective chances to SoCal later this week. One thing that is certain: temps will be rather cool through the period. On the horizon: an even more robust cold front could hold together long enough to bring precipitation to parts of NorCal by next weekend. More later…