The past week has been a dramatic one for many parts of the West Coast. Seattle, Portland, and points between experienced some record cold temperatures and very unusual frozen precipitation, from significant snowfall to accumulating freezing rain. Seattle broke its all-time record for monthly precipitation…with snow! The interior West is still frigid, with the lowest temperatures in the past 7 days having dipped below -15 degrees. Windchills, in the colder areas, were as low as -40. California was affected to a lesser extent by the cold, especially in more southern areas (single-digits mountain areas, locally as low as the 20s and 30s along the coast. The big story in CA, however, has been the powerful offshore winds that resulted from this dramatic thermal gradient and the bone-dry airmass that these winds ushered in. Mountain areas are currently reporting wind gusts of 50-70 mph, with dewpoints as low as -30 degrees! Needless to say, fire weather is a major concern, with several large fires already burning. Conditions will slowly improve over the coming week, but Red Flag Warning criteria will be met for a few more days. After that, a rather significant shift in the longwave flow will begin to take place. An active phase of the MJO has developed, and the wave of increased equatorial convection has reached the dateline. This will serve to strengthen the subtropical jet stream and amplify the flow off the West Coast over the coming week. A split flow will briefly develop on Wed/Thurs before a strong system ploughs right through the middle of the divergent jet flow into Central CA. This system may actually be able to maintain some significant rain/wind as it moves ashore, even after much of its energy has been lost to obliterating the slpit flow. Thereafter, a more consolidated jet will aim itself at CA, though a bit of a southern branch could still be maintained. This will be a favorable setup for major cyclogenesis to occur 750-1250 miles off the coast of CA, possibly setting the stage for heavy rain/high wind events days 7 and beyond. I mentioned the likelihood of an active phase of the MJO leading to a stormy pattern in the first half of December back in October, and it seems that it may very well play out. Stay tuned…Â
