Unsettled conditions after and unseasonably early hot and dry spell

What a strange winter it has been here in California! Intense December rains in the northern part of the state tapered off by the end of the calendar year, and our usually well-defined rainy season simply hasn’t returned since. NorCal has set numerous new records for minimum precipitation over various intervals since January–including, assuming provisional numbers hold up–the driest January-April period on record. SoCal has been quite dry too, on average, though occasional very dry years are less exceptional down south. Now that we’ve entered May, our typical rainy season should be well on its way out. Fire weather conditions over the past week have been disconcertingly extreme for so early in the year, with large, wind-driven wildfires both in far NorCal and SoCal. I don’t think this bodes well for the coming summer and fall.

Over the coming week or so, however, it does appear that a very unusual large-scale pattern will allow for some more unsettled conditions than we’ve seen in quite some time here in CA. By tomorrow, a cutoff low will retrograde over NorCal from Nevada under a rather strong Rex block. Cutoff lows are not uncommon near CA–in fact, our region is well-known for its propensity to produce these slow-moving features–but it is far from typical for a cutoff to form over the Great Basin and make it all the way off the coast of San Francisco, as is expected to happen over the next couple of days.

As it does so, this initially weak low will deepen and entrain some moisture over the ocean, slowly increasing lift and instability over a broad region of the state. Scattered showers and probably some thunderstorms should develop over the higher terrain by Sunday afternoon, slowly becoming more widespread Monday and Tuesday. Just about every part of the state stands the chance of an isolated thunderstorm at some point this week, though most places will see very little precip and lightning is most likely in the mountains. It is possible, as sometimes happens with spring/fall cutoff lows in CA, that some mid-level instability develops and produces more widespread thunderstorm activity than currently expected. If that happens, I anticipate the possibility of significant fire weather concerns, as such storms tend to be rather dry and we’ve seen how dry conditions already are throughout the week by the fires that have occurred. Could be an interesting week ahead!

© 2013 WEATHER WEST