'Hurricane' Schwartz: December weather outlook

It’s the second warmest November on record in Philadelphia. The month has been five to six degrees above “normal,” which was only beaten in 1931. Highlights of the month:

  • No days with temps below freezing.
  • 19 days reached at least 60 degrees.
  • Not even a trace of snow, officially.

And it’s not just around here. Look at the most recent map of temperatures compared to normal across the U.S. (the bottom map):

 

As extreme as it’s been in the Northeast, with four-to-six degrees above normal, it’s been even more extreme in parts of Minnesota and Wisconsin, with temperatures more than eight degrees above normal. And the warmth covered the entire eastern half of the country. Only seven western states were cooler than normal.

And now for December 

The overall pattern shows no sign of changing for the next couple of weeks, at least.  Let’s go out a week to December 7, and predicted temperature anomalies (compared to normal):

The relative warmth across the northern U.S. continues, and the anomalies are highest in southern Canada. That area has to cool down a lot before any serious cold can invade our area. And that means air coming down from central or northern Canada. How can that happen? It would take a “Blocking Pattern” such as a negative AO (Arctic Oscillation) or NAO (North Atlantic Oscillation), as discussed last month.

For that, we look way above the surface. The map below is for 18,000 feet up for Dec. 7. There is no evidence of any kind of blocking pattern developing. That high up, winds blow parallel to the lines, so we can trace back from Phialdelphia to see where our air is coming from. Just as we saw earlier in November, it’s coming from the central Pacific. No Arctic (or even Canadian) air comes in from that direction!

The blue colors represent below average pressures, and the reddish colors are the opposite. To get a blocking pattern, we’re looking for some red colors on top of the blue, and there’s nothing close to that. It means a strong westerly jet stream racing across the Pacific, flooding most of the U.S. with unseasonably mild air. That’s for at least the first half of December overall. We’ll keep checking to see if any of those blocking patterns show any sign of developing.

 

Glenn “Hurricane” Schwartz

Chief Meteorologist, NBC10 Philadelphia