This has certainly not been a normal bingo card for a Neutral La Niña year.. but it also could be..
Why.. we had discussed back in Fall the importance of the position of HP in the Bering Sea. We experienced the HP in an undercutting position in October and early November only to have it move to a blocking position. Then, two weeks ago it moved again and allowed the storm door to open.
The impactful series of AR’s that hit the PNW were set up to be more impactful by HP over the SW which kept the jet stream aimed at the PNW and did not allow the fire hose to slide south. The HP ridge over the SW did eventually move and the jet stream drove a series of very powerful storms into CA. In a classic “Family of AR’s” pattern, a series of storms impacted the state and the plume of moisture hosed the state making multiple passes. The first two storms were zonal.. meaning east=west oriented with the core of the LP remaining well offshore. The Integrated Vapor Transport(IVT) set some records.. in basic terms.. IVT is the relative measure of the total amount of water vapor being transported in the jet steam both in length and depth. This is important.. had the core of the low been closer to the coast we would have seen some more consequential impacts from the first storm series. The result of that type of pattern is that there is no forcing mechanism to wring out all that rain. These type of set ups favor the higher elevations for heavy precip. That was very evident with the high water that was seen in the Shasta( McCloud0 rare high water event), Feather, Yuba and American water sheds as well as the NW coastal rivers. Many reached really high levels, while the CV only saw modest amounts of rain.
That changed with the last series of the storms around Christmas. A strong area of LP stalked the coast and came close enough to set up powerful pressure gradients and a change in the wind direction. Those winds aligned more SW rather than W-E. That meant more rain for the valley’s and less shadowing. This also brought in some cooler air which helped create more efficient precipitation rates and lowering snow levels. This was also the set up which enhanced the convergence zone east of Redding and produced significant flooding. A lot to be studied with the “Shasta Convergence” zone. My feeling.. a confluence of cool air moving down slope from the east with warmer overriding air coming up valley and from the west( Arbuckle Basin). The collision of the two air masses with the interaction of the terrain to the east and perhaps some cooler and denser air allowed the convergence zone to become established and persist for several hours with heavy and prolonged precipitation. This is not rare for this area to experience this phenomena, it just had some real consequences with this set up.
A new set of systems will make their presence felt this week and while not as powerful they will make an impact. Saturated soils and moderate dynamics will make for a stormy period.
What is causing this? Not unusual to see more AR’s in a La Niña.. it is rare for the precip to be so heavy south the Bay Area.
Are the warmer SST’s to blame, lack of arctic air making a southern advance( changing this week)? Only time will tell.
One thing to remember with weather.. we can only look backwards at weather to make sense of what is occurring now. We take past events and re-analyze the data and apply our evolving understanding of the metrics through the use of powerful supercomputers and researchers. By constantly challenging ourselves to examine the climate and its effect on our weather we continue to improve our understanding of the complex variables which drive our weather. We also have to accept that there is no absolutes in weather, as the climate changes the baseline changes.. Imaging playing a football game where the refs only told you the down and distance after the play was over and right before the ball was snapped, and throughout the game were constantly adjusting the metrics that they used to determine down and distance. That changing baseline would not allow you to form a long term game plan( we call that strategy).
The study of climate helps determine the baseline and allows us to apply very smart people and technology to analyze what the synoptic set up was and then what the outcomes were for a particular weather event. When we shudder or reduce those entities and people who do that kind of work we are effectively applying a blindfold to ourselves. This type of research relies on data and a long term review of that data. When gaps appear in the data.. it basically destroys decades of work.
The result.. you can only react to what the down and distance is as it occurs rather than building a playbook which helps to guide a more comprehensive game plan.