Unseasonably deep low to bring active NorCal weather this week

Filed in Uncategorized by on August 4, 2009 37 Comments

A remarkably deep cutoff low for the month of August has been churning off the CA coast for several days now. The low is beginning to elongate a bit along a SW/NE axis as a strong jet streak rounds the base of the trough. There has been periodic thunderstorm activity associated with the low over the ocean and over mountain areas (some of these storms in the Redding and Lassen area sparked nearly 200 new wildfires, several of which are growing rapidly). As low continues to slowly approach the coast, dynamics will become rather impressive over the northern half of the state. Instability will be significant from about the Bay Area northward to the Oregon border as the low moves over the northern Bay Area and into the Central Valley on Thursday evening. The principal limiting factor in this setup is the profound lack of moisture at any level, though I think the powerful dynamics will probably overshadow this, at least locally. Still, I expect convective activity to increase significantly as early as late Wednesday evening and become more widespread to potentially include the entire Central Valley (except maybe the southern San Joaquin Valley) and the Bay Area by Thursday afternoon. Due to the lack of moisture, little precipitation is expected in lower elevations, though intense cells will probably have a small heavy rain/hail core just about anywhere and the mountains will benefit from orographic lifting. Record precipitation totals are likely if any rain falls across parts of the Central Valley and Bay Area, as there are some days in August on which measurable precipitation has never occurred in these regions! That should give some indication of how anomalous and unseasonable this low really is.¬† A further indication of the dramatic change in the weather will be the winds and much cooler temperatures, which probably won’t crack 80 in parts of the Central Valley¬† (which would qualify as record-low maximum temperatures). Fire danger will be high with this event, as the potential for widespread lightning and little rainfall exists in addition to areas of high wind ahead of the approaching low. A number of large and fast-growing fires are already ongoing across more northern parts of the state, so this will have to be watched closely.

One more item to note: a band of weak showers and maybe an isolated lightning strike developed across SoCal this morning in a plume of elevated subtropical moisture being drawn up on the east side of the low circulation. All of the activity has since dissipated, but a fairly moist plume continues to stream northward and it is conceivably possible that the strong upper jet dynamics could pop another convective burst across Central or Southern CA over the next 24 hours. Stay tuned…