For people who love winter this December has been a disaster. If there’s any silver lining for someone like me it’s that we’re going to absolutely obliterate the record for warmest December since 1905. Smashing records is fun – though I’d rather be skiing.
So what the hell caused this torch?
We can start with El Nino. A near record Nino has baked the waters of the Equatorial Pacific. As is typical, warm waters fueled powerful thunderstorms near the Equator south of Hawaii (see red shading below).
The latent heat release and outflow from these storms, in turn, helped a powerful jet stream to develop across the Pacific Ocean. You can see that here on the zonal wind anomalies for December.
This active jet stream helped bring milder/maritime air across the lower 48 but the nail in December’s coffin was the giant -PNA trough over the western half of the country. Notice the negative 500 mb height anomalies over the west coast (-PNA) as well as the gradient of above normal heights north of Hawaii and below normal heights over the Aleutians/Bering Sea forcing the abnormally strong Pacific jet.
An active storm track to our west and a remarkably persistent -PNA pattern flooded the eastern half of the country with the epic torch. Breaking a monthly record by a degree or two is impressive but by 5 degrees? Basically unheard of!
So where do we stand going forward?
A disturbance in the force (sorry for the lame Star Wars reference) will at least temporarily shake up the pattern. See yesterday’s blog post here. I’m worried, however, any change in the pattern is temporary as the stratospheric polar vortex will remain in place and the background El Nino forcing will stay the same.
Either way at least for now the torch is gone and we have a really sweet looking pattern for snow developing around 1/10. Let’s hope we cash in before the Heat Miser comes knocking on our door.