December Forecast – Winter Forecast 2014-15

Today is officially the beginning of the meteorological winter! Firsthand Weather’s 2014-15 winter forecast is about to be put to the test, and this is the month when everything really should start to come together. November has been unusually active and unusually cold, and we are about to finally adjust back to more of a normal pattern before winter comes in with a vengeance. Don’t let anyone make you believe that this brief moderation in temperatures over the next one to two weeks is anything to worry about. It’s perfectly normal to shift back into a more zonal pattern while we’re making the transition into winter. The true test will come in mid to late December when I will be able to finally see if we’re locking into the pattern that will bring my winter forecast to fruition. Since December is going to be a transition month, I decided to not draw up any maps since it would be difficult to fully represent the volatility that December will bring.

One thing that I want to point out is that what occurred in November was extremely unusual. Most people across the United States got to experience winter very early, so this transition back to warmer weather may seem abnormal when it’s not. Given the upcoming pattern that will be setting up early this month, most of our air is going to be coming from the Pacific, meaning the upper-level flow is going to be moving from west to east (a zonal flow). This will translate to warmer conditions at the surface, and our source of Arctic air is going to temporarily be cut off. Notice that I used the word ‘temporarily.’

Let’s Talk About California and the West Coast:

Something that we have not had set up in several years is an active sub-tropical jet stream across California; and therefore, California and places along the West Coast have been dealing with an exceptional drought. The Pacific Northwest has been able to get some decent moisture this fall, but California has missed out so far. This isn’t unexpected at all, but it’s finally time for the heavy rainfall and mountain snows to pick up substantially across California. The state has already had some beneficial rainfall, and my concern is that they are going to get too much of a good thing too fast.

Even though an El Nino hasn’t technically been declared, the effects are about to be felt across the region. The reason for this is that there is a region of warmer waters along the equatorial Pacific and a cool pool in the north-central Pacific. Because of this tighter temperature gradient, there will be a stronger pressure gradient, which will support a stronger and more active jet stream. This will allow many low pressure systems to move across California this winter, and because of that, this will put a huge dent in the drought. With all of that said, I am concerned that flooding and mudslides will be an issue across the state, and unfortunately, some of the worst floods in the past have occurred when there was an active sub-tropical jet. I will keep a close watch on all of that, but if you’re in the area, you definitely need to be aware of that possibility. In my final winter forecast, I shifted the the above-average precipitation to include the entire state of California, and I am still happy with that slight modification.

Notice the cool pool across the north-central Pacific and the warmth along the equatorial Pacific.

Mid-To-Late December Arctic Blast Likely On The Way:

Although model guidance may beg to differ, everything is pointing towards a brutally cold mid-to-late December for the central and eastern United States. I do believe that the focus of the coldest temperatures will be over parts of the eastern United States in January and February, but like I mentioned above, December will bring a bit more volatility, meaning things will fluctuate some before we enter into a winter pattern. Enjoy the warmer temperatures that will kick off the first week or two of this month because it will abruptly come to an end.

We still have the warm pool over the northeast Pacific, which will promote ridging up over the West Coast and Alaska. Due to the rapid expansion of snow cover over Siberia in October, that will induce warming over the North Pole, which will displac much colder air to the south. December is the month when you typically begin to see the true effects of this occurring, and that is in line with what I am predicting.

The stratosphere has begun to significantly warm over Asia, and this warmer air will eventually begin to spread over the Arctic in the coming days and likely disrupt the polar vortex later this month. The true effects of this warming are yet to be seen, but typically when a stratospheric warming event occurs, the polar vortex can either become elongated or actually split in two. Now please understand that the polar vortex is NOT a new meteorological phenomenon and is something that the media hyped up significantly last winter. Nonetheless, it can have a significant influence on our winter temperatures and has to be discussed.

For those of you that may be wondering, the polar vortex is a cold low pressure system that is located in the upper levels of the atmosphere and stratosphere at the North Pole that keeps the cold air locked over the Arctic. From time to time, the polar vortex can be disrupted, and the Arctic air can begin to spill southward into the mid-latitudes. Sudden stratospheric warming events are not necessarily uncommon, but when they do occur, they can bring brutally cold air to the United States because of the warming it induces over the Arcitc. Since the sea surface temperatures and other factors already favor a brutally cold winter in the central and eastern United States, this is where the cold air will go if the polar vortex becomes elongated or actually splits. Even though I wouldn’t categorize this as a *sudden* stratospheric event as of yet, the warming that has already occurred will likely have effects by middle December. In fact, the warming that has occurred is currently at record levels for this time of year.

The polar vortex can weaken and split into two vortices, bringing brutal cold to parts of the United States.

Get Ready For A Potentially Big Winter Storm Later This Month:

As I’m sure you can already tell, things are likely going to get very interesting later this month. With the pattern that is about to setup for a brief time, it’s difficult to get big winter storms to move up the East Coast, but that is going to change quickly later this month. One of the things that I do when putting together these kinds of forecasts is look at what’s going on around the globe. In some way or another, what happens on the other side of the globe WILL affect us in the United States eventually. Forecast model guidance isn’t reliable beyond a week, but what I can do is look at things and tell you what could occur in two to three weeks, and in some cases, longer than that.

The Thanksgiving East Coast storm that occurred was definitely a big event, but the overall pattern was not a cold pattern. Inland regions along the East Coast were able to pick up some decent pre-seasonal snow accumulations, but regions directly along the coast (for the most part) and Southeast weren’t able to get in on this winter event. The overall pattern is going to become much more favorable in mid December and beyond for 1) big East Coast winter storms and 2) winter events much further to the south.

We’re going to start seeing some of that this month with a potential winter storm around mid-month, but given that the sub-tropical jet is starting to crank up, low pressure systems will begin moving into California and across the South, likely phasing with northern branch systems further east. Given that the pattern favors cold in the central and eastern United States, things could get very active. I’m not going to take the time to explain the MJO in this article, but the MJO will also be entering a phase that strongly favors troughing (cold) in the central and eastern United States and ridging (warmth) in the West later in December.

Notice that temperatures are typically below average across the central and eastern U.S. during the 7th and 8th phases of the MJO. We will likely be entering phases 7 and 8 later this month.

Final Thoughts:

As all of you know, anything with this forecast could change. I am far from an expert in meteorology, but I continue to feel that I have a good grasp on this upcoming winter. I strongly believe that this winter COULD rival some of the coldest winters in history, particularly in the eastern U.S. I try to be careful how I word things, but everything strongly indicates that this winter is going to be brutal. As I have been recommending since July, get prepared.

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