The headlines are filled with dramatic talk of the "Bomb" cyclone hitting the West Coast. When you have a large segment of the population traveling to "grandma's house", this makes for great news copy but is it accurate and does it mean destruction? Let's take a look at where this storm formed and see if we should call it a "BOMB!"
September 2019 Sea Surface Temperature Anomalies. (Red indicates warm water.)
We have seen unusually warm ocean waters sitting off the coast of California this year (area of red/orange in graphic above). This resulted in an unusually large number of days of eddy winds for San Francisco. Eddies are counter clockwise wind patterns that do not extend up very high in the atmosphere and are cut off from the major flow. We can likely blame these eddies on the blob of very warm ocean temperatures. Frequently we watched as an area of Low pressure formed in this region and then drifted off the coast, forming large eddies that brought Southerly winds into San Francisco.
November 2019 Sea Surface Temperature Anomalies
Fast forward to November and that blob of super warm water is still in the Eastern Pacific, though it has shifted a little to the west. Well, these warm waters are now providing one of the ingredients for our "Bomb" cyclone.
850 mb Temperatures. (California is highlighted)
Bomb cyclone is a term used when a Low pressure deepens very quickly, about 1 millibar per hour. We see this type of quick intensification when there is a large temperature contrast between two airmasses that are wrapping around a Low. On Monday a wave of cold air (seen above as area of blue) slid across the Eastern Pacific,over that blob and met up with the warm Pacific air. As they met the Low deepened very quickly.
Pressure Reading for Buoy, 500 miles west of Eureka, California
The Low sat over the Eastern Pacific on Monday and rapidly intensified - hence the term "bomb". This buoy (#46006) showed a drop of 0.45" inches of mercury or about 15mb. That is fast but not quite enough to be considered a bomb.