Matt Makens

As we look at the remainder of December, it may remain just as "chaotic" as
November – at least that's how the modeling would like to project the month.

I say chaotic because seasonal modeling has been far from settling in on a
solution, both within a single model and across several models. This can be due
to the usual chaotic behavior in our weather patterns during the November and
the early part of December – it is a transitional period from a summer flow to a
winter one. This is commonly a time when we see the atmosphere and ocean pattern
work less in tandem than other months of the year. Thankfully, a properly chosen
analog-year package can offer steadier insights where as models may continue
fluctuating.

We know we have had a strong ridge (warmth) over Alaska which has worked to make
the weather pattern across North America amplified (a lot of north-south flow
versus a progressive west-east flow). The ridging in this region will fluctuate
in strength but likely remains in place due to the sea surface and ice
conditions supporting the warmth. As we go through December, we need to begin
considering a strong influence elsewhere, between Greenland and Siberia.

Greenland will also see ridging increase, coming in from Scandinavia/Siberia.
This area of warmth will act to reinforce the amplified pattern and can be
strong enough to act like a pattern block that favors more extreme winter
weather across the Eastern U.S.  (this is hyped on media as the -NAO) blocking
pattern which gets East Coast snow lovers very excited to hear. However, as
discussed in the outlook video, -NAO patterns are still subject to what is
happening in/near Alaska as to if we can send storm systems through to the East
or if we remain with some form of a blocking ridge there as we had in November.
However, this is where modeling for the month has been tricky so far.

This video walks us through the analog versus model support for December.
[https://youtu.be/zyVB_nJ1ry8]

Since that video, Euro now wants to maintain a bit more ridging closer to
Scandanavia which, if correct, implies the ability for colder air to sink
farther south into the Southern U.S., as you'll see below in some images.

Here's what is pretty clear as of this post, we will have western troughing
early in the month before potentially seeing that trough try to move east a bit
to increase the cold potential for the eastern U.S. a bit later in the month.

This is pretty clear in modeling and analog years where we have -PNA conditions
early in the month, that's our western trough, with cold support in the -AO and
-EPO indices. As the following index forecasts show, we transition more into
neutral territory as the month progresses.

Holding onto a -NAO through quite a bit of the month does increase the odds of
extending the colder air across to the eastern U.S., albeit a real shock of cold
air likely requires the AO to remain - longer than forecast. That is possible,
and I'm not saying to take the model outputs as truth, it is simply guidance
after all.

In analyzing how models compare to analog years, there's a relative consistency
and just for examples I'm including the output of the Euro Seasonal forecast for
the month, here. GFS or Canada's CANSIPS offer similar variations, too. 

I don't think we can take any single model or analog-backed forecast as the
absolute truth, but all together they do have similarities to the following.

From early in the month, this is Euro's thought on December.

We see the coldest areas of the month are the Western U.S., Central Canada and
the Northern U.S. Plains, with a lobe of cold extending toward the East Coast.

In the most recent weeklies update by Euro, with a different blocking pattern,
we can see the potential exists for a lot colder central and southern U.S.

I'm not saying one or the other is correct. Both have merit, and given the
chaotic nature of the atmosphere/modeling system the past week, both are
plausible and which one wins remains to be seen. An analog forecast, as shown in
the video, would be more supportive of the first Euro image with warmth across
the south versus the second image. This assumes the analog selection was correct
and is just another possibility. A gut hunch here, we are seeing La Niña's
transition to neutral and the atmosphere does not necessarily need to be coupled
with the ocean at any point this winter and this would support the colder
solution in the projections. The atmosphere hasn't shown much interest in
re-coupling with the oceans, but if that did occur the analog and warmer Euro
solution would likely come away the winner.

I don't want to muddy the water more with model solutions but if we did some
some kind of change to the precipitation outlook shown below, it is likely to
extend wetter conditions across the southern U.S.

In terms of precipitation, we are seeing a bit more of a tropical flow of
moisture scooting in from the West which can offer surplus snow and rain in
California and beneficial snows to the southern and central Rockies including
Colorado. But with a storm flow like this, we typically see rapidly drier
conditions east of the Rockies and over the Plains before storm systems can
engage with more humidity for the central-East and Northeastern U.S., also an
area that could see storm's slow as they interact with the blocking ridge over
Greenland.

In all, the remainder of December should be a valuable water month for
California and parts of the West, and may produce some heavy winter-precip
events for some folks in the East. Cold, will be most prominent in parts of
Canada, the West, and Northern Plains but may extend to cover the U.S. East for
a time, too.

Medium range weather outlook through end of 2022