Videocast: Freezing Start, Sunny Finish To Christmas

Another round of frosty/freezing temperatures will start the day for our inland valleys but like we saw on Saturday, we’ll have sunshine mixed with some high clouds during the afternoon which will allow temps to rebound into the mid 50s near the East Bay valleys to upper 50s and low 60s around the Peninsula, inner East Bay, South Bay and North Bay locations. We will also see a Spare the Air alert again for particulate matter (smoke pollution) for Sunday meaning use of wood-burning appliances is prohibited. Cold air valley inversions w/out much wind are the cause of this sort of bad air pattern, hopefully with more onshore wind and other bigger changes ahead this week – air quality will soon improve.The week ahead is finally offering up some chances for rain and much needed Sierra snow. Look for high pressure to weaken start late Sunday as a weather system pushes some clouds our way late in the day and increases onshore winds -> which with the added moisture will hopefully bring up overnight temps some heading into Monday morning.By Tuesday a stronger system will drop into Northern California, likely spreading rain into the North Bay by the afternoon and evening. Some of this will spread south across the Bay Area on Wednesday into early Thursday, however with the jet stream anchored closer to the Oregon/Northern California border, its likely we’ll see mainly lighter amounts of rain with the highest totals clearly north around Sonoma/Mendocino counties northward.We call this type of pattern a ‘zonal’ flow where storms race mainly west to east along as fast moving jet stream, but in this case it appears the runway of this zonal flow will be just to our north from midweek and beyond so higher rain totals will likely stay away from the Bay Area. However, there will be enough wind, mixing of the atmosphere and more moisture to make our mornings less chilly and air quality much cleaner. Enjoy your Christmas Day and stay tuned for those rain chances developing as we move through the final week of 2011 — wow, can you believe the year is done?! – Rob Mayeda