Wet week ahead for Bay Area

After a fairly average weekend in the Bay Area, with periods of sun, clouds, drizzle, and onshore winds, we have a much more active weather pattern in store this week.  As I alluded to in earlier blogs, several signs were pointing to a wet period in mid-March, and it's about to start.  In fact, this may very well be the wettest week of the winter season.  Very moist air from the subtropics is moving northeastward across the eastern Pacific, currently headed for the Pacific Northwest associated with a strong upper-level trough developing off the coast.  This system brought thunderstorms, heavy rain, and large hail to Hawaii a few days ago.  This trough will gradually sink southeastward across California this week bringing periods of rain, sometimes heavy.  Rain will begin in the Bay Area on Monday night as a warm front moves through.  As warm air moves in from the southwest, it is forced up and over cooler air at the surface.  This results in clouds and precipitation.  Rain will generally be light to moderate into early Tuesday before a strong cold front moves in with a period of heavy rain and wind late Tuesday.  The first image is a model 12-hour precipitation forecast for the day Tuesday, indicating at least an inch of rainfall for much of the area.  The other computer models are in agreement as well, which gives us more confidence in the forecast.  Another cold front will drive through Thursday into Friday with more rain.

The parent upper-level trough will then settle over California this weekend as indicated by the second image.  This will bring in much colder air, especially aloft, and will result in very low snow levels - perhaps lower than 2000 feet at times.  With such cold air aloft, combined with the relatively strong March sun, the atmosphere will become quite unstable (by California standards).  This may result in a few thunderstorms in the Bay Area, but it's still too early to say for sure.  Regardless, we can expected perhaps 2 to 4 inches of rain in the Bay Area this week with several feet of snow in the Sierra.  This is very good news considering the lack of rain so far this winter!

Also on the weekend forecast map, notice the strong upper-level ridge over the eastern United States.  Many locations there, even the northern states, may have temperatures in the 70s.  That would be extraordinarily warm for this time of year, considering that often such locations (Great Lakes and the Northeast) are covered with snow in early to mid-March.  It is not uncommon though to have one side of the country cool and wet while the other is unseasonably warm.  In fact, this is often the case.  Strong troughing over the western U.S. forces strong, downstream ridging over the eastern U.S. in response, and vice versa.  This is why it may often seem that during periods of sun and warmth here, the Midwest and East are getting severe thunderstorms and/or snowstorms.