This December's Weather Will Be A Real Wild Card. Here's Why

NOAA on November 30 issued this long range
forecast covering December 8 to 14. It featured
enhanced chances of colder than normal weather
over most of the United States. 

 I've noticed some very conflicting longer range weather forecasts for December, at least for everything after the middle of the upcoming week. 

That's because there are at least two big forces battling it out. Whichever one wins will greatly affect whether its warm, or cold, or snowy or rainy or dry.

The big chatter in social media circles among weather geeks is the Greenland Block gradually setting up. I mentioned this one the other day in a previous post. 

A Greenland block is when massive high pressure sets up over and near Greenland.  When this happens, the jet stream over North America gets squashed further south than it usually is. 

When the jet stream is south like that, it tends to get colder in much of the United States. The jet stream, suppressed to the south, opens the door to frigid Canadian air that can flow southward. Sometimes, the cold air interacts with storm systems passing through, and you can - but not always - get big snowstorms in the eastern United States.

Here in Vermont, it often, but again not always gets cold when there's a Greenland block.  Big snowstorms usually go too far to our east to affect us much, but not always. 

However, with all this in mind, there's another factor in play here big time, at least for now. 

Fighting the Greenland block is a ridge of warm high pressure setting up over and near the southeastern United States. Depending upon how strong this ridge gets, it can make it on the warm side in Vermont. 

Nobody is quite sure if the Greenland block or the southeastern ridge will dominate. For either, it could be win, lose or draw.  Or one can dominate, then the other. 

You see the problem. Long range forecasts are always dicey. With this battle going on, any forecast beyond a few days is very suspect. Take those predictions with a grain of salt. 

To illustrate the forecast iffiness, the first image in this post is the long range 8 to 14 day forecast for the United States that was issued on November 30.  It covers the period from December 8 to 14.  It kind of shows the Greenland block winning, with cold air favored over most of the nation. You can see hints of the southeastern ridge with warm air forecast near the Gulf of Mexico and Florida.

This forecast also anticipated below normal precipitation over New England, as the Greenland block would smush storms too far to our south to affect us that much. 

NOAA yesterday issued this 6 to 10 day forecast that
covers pretty much the same period as the forecast
map above. Now it features mostly warmer than
average temperatures over the eastern half of the nation.
Forecasts this month beyond a few days will be
especially iffy due to questions about how two
large weather systems play with each other. 

Yesterday, the six to 10 outlook was issued, which will cover the period from December 9 to 13.  That's almost the same time period from the earlier, longer range forecast I just mentioned above.

Instead of cold weather dominating the nation, the chill is relegated to the western third of the nation. Warm air would bathe most of the eastern United States. The southeastern warm ridge for the win!  

This updated forecast issued yesterday also favors wetter than normal weather over New England. Because we'd be close to the battle line between cold and warm, so that's where storms would go.

The other big questions are how long will the Greenland block last and how strong will it remain while it's there?  Same issues for the warm southeastern ridge.  Does it move away? Weaken over time? Strengthen? Who knows?

Between the two forecasts I described above, I'd give the edge to the one that was issued yesterday.  Only because it's more recent and covers a time period not as far off in the future as the prediction issued on November 30.

Even so, don't count on either forecast being correct.  I always tell people don't count on forecasts for more than four or five days beyond the current date. That's really, especially true for the rest of this month.  

Whatever happens, we'll probably see an active weather pattern through most of December. That suggests fairly frequent storms, but we don't know whether they will be strong or weak.  We don't know if they will be snow, rain or a mix.  And we don't know how many of these storms will come close enough to Vermont to make a big difference in our weather. 

In the short term, the pattern favors warmth.  A couple of modest storms will come through midweek. They'll produce mostly rain, with maybe just a little ice in the coldest pockets. 

If you want to know what will happen after about Thursday, you're on your own.  I'll let you know later in the week.