[November Note] I put together this winter forecast in late October for the Portland newspaper UpPortland. If you’re in the Portland area or are interested in the local happenings of a cozy Maine town, be sure to give them a read! Not a whole lot has changed from my original forecast. The discussion below is unchanged from late October with my updated thoughts in [brackets] from mid November.
So what will this winter look like? The short answer is bound by two extremes. We will not have a repeat of last year’s sleeping baby season nor will 2014-15’s roaring beast of a winter make a triumphant return. Minimal analysis is needed to see that, and no two years (or storms) are ever alike. To get any more detailed though, we’re going to need to dig a little deeper. Before we do, however, I would like to offer a word of caution. Seasonal forecasting is a science still in its formative stages. The ideas presented here are merely ideas. There are many folks out there who know way more about this than I do and if you need to make any important decisions based on long range forecasting, you should check with them. Some of them include Michael Clark of BAM Weather, Stephen DiMartino of NYNJPA weather, Ed Vallee of Accuweather, and of course the folks at NOAA’s Climate Prediction Center. There are many others who do great work in long range forecasting and it would be nearly impossible to name every one here. With that said, here’s what I think this winter will look like and why.
I think the main theme of this winter will be volatility. There are a few factors that argue strongly in favor of cold weather and a couple that favor warm weather. The result is most likely to be deep cold blasts punctuated by short bursts of mild weather. Think of last year except that frequencies of warm and cold periods will be switched. Instead of mostly mild weather with a few cold blasts last year, signals point to this year being mostly cold with a few mild blasts. This of course is also in contrast to 2 years ago when there were no mild blasts and supremely cold weather dominated. Storm tracks also appear to be volatile with a mix of clippers, nor’easters and inside runners. This of course contrasts both last year (only inside runners) and 2 years ago (only nor’easters/clippers). Now that I’ve gone over the general theme of the winter, let’s look at some of the ‘why behind the what’.
Snow Growth: Eurasia
First let’s look at the factors in favor of cold weather. Notice I’m not talking about snowy weather, just cold weather. Of course you can have cold without snow but you can’t have snow without cold. I’ll discuss snow potential in a few paragraphs. The first factor that argues in favor of cold weather is that there is a lot of snow in Siberia currently (see Figure 1). When there’s a lot of snow in Siberia in October, cold air in that region gets colder due to the snow and pushes south. That pushes warm air north over the Gulf of Alaska which then pushes cold air south over Eastern North America (us). The black line on Figure 1 represents the rough boundary between warm and cold air. The blue arrows are where expansive snow growth pushes cold air south (causing more snow growth) and the red arrows show where a lack of snow growth lets warm air run north (preventing further snow growth). Also note that this cycle is a feedback loop meaning that the patterns it shows/causes are likely to stick around. The description I gave of this process is of course a gross simplification and I point you to the research of Judah Cohen from MIT for analysis that does the process justice. The point is that, in general, strong Eurasian snow growth promotes cold weather here and there is currently strong Eurasian snow growth.
[November update: The snow cover anomaly map has changed fairly significantly over North America. Snow cover is well below normal over most of North America as a strong Pacific jet has kept the continent flooded with maritime air. The biggest consequence of this is that there is little cold air to our north west. North America’s cold air is in Eurasia where snow growth has been abundant. To get to us, that cold air has to cross a lot of warm water including an Arctic Ocean that is covered in record low amounts of sea ice. A pattern shift is underway and a blizzard this week (11/17) will likely erode some of the red on the map in South-Central Canada allowing for a cold pool to begin to build. Overall though, things are progressing nicely in Eurasia and the current Canadian anomalies will act to postpone slightly, but not cancel, winter’s arrival.]
SST’s: The PDO, La Nina, and The West Atlantic
Another factor is the current Pacific Decadal Oscillation (note: it was named the Pacific Decadal Oscillation because it was originally thought to move on a decadal (10 year) timescale, however observations indicate it moves in a 1-2 year timescale, much like ENSO (El Nino/La Nina) phase. The PDO is currently positive as shown by the so-called “blob” of warm water just off the Alaskan coast (figure 2). This “blob” helps push warm air north in the Gulf of Alaska (see Figure 6 showing the typical patterns in +/- PDO years). As we discussed in the previous paragraph, when warm air goes north over the Gulf of Alaska, cold air comes south over Eastern North America leading to cold weather here in Maine. One thing to note about the current positive PNA pattern is that it is showing signs of weakening.
Cold water is moving east from Kamchatka and undermining the blob of warm water. This indicates that the PDO pattern could be trending towards neutral or even negative by the time we reach the tail end of winter. This could act to bring Spring in on time compared to the lethargic approach Spring has taken in recent years. Despite what may happen in the springtime, we know that at least through the first half of the winter, the +PDO pattern will be going strong.
[November Update: The positive PDO has weakened in the past month, as forecast. A large area of cool SST’s is located south and west of Alaska. Despite this, warm water remains near the North American coast and seven day SST anomaly changes show warming of SST’s along the western edge of the cold pool. In addition, raw PDO values have rebounded slightly since September and October. The PDO forecast looks good to me.]
Now that we’ve established that we’ll have at least some cold air, let’s take a minute to look at the factors that promote some intrusions of warm air from time to time. The first important thing to note is the warm ocean temps just offshore (figure 2). With such warm water in the Gulf of Maine, all you need to bring mild air into the region is a quick flip of the wind to east, south east, or south. This indicates that there is potential for mild periods here and there, especially for the coastline.
The other factor that argues in favor of some warmer temps is the forecast presence of a weak La Nina. La Nina occurs when water off the west coast of South America is cooler than average. Notice that currently, that is the case (figure 2). The CPC and climate models (Figure 3) continue to forecast the development of weak La Nina conditions for this winter. What does that mean for us? Years with weak ENSO (background oscillation of Equatorial East Pacific, positive phase El Nino, negative phase La Nina) values are highly dependent on other factors to drive the pattern.
However, weak La Nina events tend to favor slightly warmer temps and average precipitation (Figure 5). The weak La Nina may help to get a few periods of warmth going but it’s not the same setup as last year’s El Nino that flooded the entire continent with warmth. Also note the deep cold that usually lurks to the west during weak La Nina events. Proximity to cold air is a dangerous proposition in Maine in a winter forecast and it wouldn’t take too much to send some of that our way despite the overall La Nina pattern arguing for warmth. Also remember that in weak ENSO years, ENSO is rarely the driver of the pattern. It is merely an influence, one of many vectors in a pile that must be correctly added up to determine the eventual outcome.
As with any ENSO (La Nina/El Nino) event, there are complications. Notice in Figure 2 that there is cooler water on the western edge of the La Nina “bubble”. This indicates a west-based La Nina. A west-based La Nina shuffles up the longitudes which thunderstorms form at near the equator. This impacts the MJO (Madden-Julian Oscillation, an eastward propagating disturbance that enhances tropical convection) phase which has been shown to impact our weather. A west-based La Nina predisposes the MJO phase to be favorable for cold weather in Eastern North America, however that’s not a rock solid connection. The MJO often has a mind of its own and could also be negated by other factors we’ve talked about. For more on that, check out Stephen DiMartino’s winter forecast from NYNJPA weather.
[November Update: La Nina continues to develop in the east pacific and this loop from the CPC shows that. Notice that this loop is the actual temperature of the water, not the departure from normal (anomaly). Looking at the updated anomaly map, the west-based idea remains solid. No major news regarding the development of La Nina.]
Will this year feature the same slow start we’ve seen the past couple years? The answer is a bit complicated as two important factors are giving conflicting signals. One important thing to look for is how warm (or not warm) the waters of the Gulf of Maine, the Great Lakes, and Hudson Bay are. If the waters are warm in the Great Lakes and Hudson Bay, cold airmasses moving in from the North-West will have a hard time staying cold as they move over warm water. We’ve already gone over the impacts of warm Gulf of Maine waters, if you missed that, look up a few paragraphs. The water this year in all three locations is quite warm (see Figure 4). Hudson Bay is only moderately above normal but the Great Lakes and the Gulf of Maine are downright balmy (compared to average). This means that as cold airmasses move south, they will modify (become less cold) as they pass over the waters. This would argue for another slow start, but as with anything in weather, it’s never that simple.
[November Update: This looks good and because SST anomalies in this area have changed little since I wrote this, I won’t repost what is pretty much the same map. Concerns over warm SST fueled airmasses remains for this winter but promises of nor’easter jet fuel also look good.]
Snow Growth: North America
The other important factor in the early season behavior is Canadian snow growth. If there’s snow just to our north in Canada, cold airmasses can become colder and stay colder for longer as they travel south. A check of Figure 1 reveals robust Canadian snow growth which would tend to support very cold airmasses spilling south. So what do I think will happen? Currently we’re seeing cold airmasses spilling south and being significantly modified by the time they get here (if they get here at all). By December, I think the airmasses will have become cold enough to bring the water temps down to a point where they will have negligible influence over the incoming cold. So I think we’ll start slow but not nearly as slow as last year.
[November Update: Snow cover over Canada has slowed down dramatically since October (look at Figure 1 vs Figure 1a). However, guidance is pointing to an advance in Canadian snow cover over the next 10 days and this is consistent with a shift in the pattern that is currently ongoing. With this new pattern, I think we’ll make a slow return to the idea I discussed in October. Again, this looks like a temporary postponement of winter’s arrival rather than a cancellation of it. However, this will be something to watch because if guidance is wrong and snow cover remains lackluster, we could be in for another slow start.]
How about storms/snow? There are a few clues to storm track/characteristics and snow but it is important to remember there’s just simply no way for us to tell exactly where a storm will go even a couple of days before it hits. The storm tracks I describe below are generalizations. For example, I think we see at least a few intense nor’easters. Will these track inside the benchmark and bring us rain? Possibly. Will they track just offshore and bring us heavy snow? Possibly. Will they track farther offshore and miss us altogether? Also possibly. The point is to outline a general pattern, not to throw darts at a board for specifics. With that said, let’s look at some of the clues.
The warm SST’s off the east coast go both ways for cold and snow here in Maine. First, they argue for intrusions of warm air, not good for snow. However, they also provide intense fuel for powerful storms. Energy in meteorology comes almost exclusively from gradients or differences. The greater the difference between characteristics at two places and the closer those two places are, the more energy there is for storms to develop. You see this in thunderstorms for example. The greater the difference in temperature between the surface and aloft (lapse rate) the more energy there is for storms to develop. In the case of winter storms, the greater the temperature between the ocean water and the cold airmass moving over it, the more energy there is for coastal storms to develop. Figure 2 shows us that there is PLENTY of hot water off the coast to fuel developing storms. This means there is increased potential for nor’easters and redeveloping clippers. That doesn’t mean we’re in for the powderfest known as 2014-15 though.
Notice the warm waters in the Gulf of Mexico and just off the SE US coast. These waters could act to help prolong a current phenomenon known as the South East Ridge which is an area of high pressure that stretches across the SE US and north east into the Atlantic. This ridge, combined with warm SST’s up and down the coast are likely to force at least a few storms west of the area following tracks we saw last year. I do not think these will be nearly as frequent or dominant as last year but I also think they will bring us some of the warm periods the winter of 2014-15 was missing.
[November Update: All these general themes continue to look good. The La Nina continues to develop, SST’s in the Great Lakes and off the East Coast remain well above average, and the PDO is still clinging to positive territory. The one thing that is dramatically different from October is that Canadian snow growth has slowed down dramatically. However, this looks to reverse in the coming days and weeks which means that I will stick with my original forecast. Everything looks good, except that perhaps the start of true winter weather will be delayed a little more than originally expected.]
To summarize, my forecast for the upcoming winter calls for
-A slow-ish start but not as slow as last year. No December snow blitz but no Christmas shorts either.
-Alternating periods of fairly intense warm/cold. Slightly more cold than warm.
-An on-time arrival of spring. No dragging out of winter like the past few seasons.
-An active but variable storm track. Mix of clippers/cutters/coastals, none dominate.
This is based on
-A weak, west based La Nina
-A +PDO moving towards neutral and eventually perhaps weakly negative
-Robust Siberian and Canadian snow growth
-Warm SST’s in the Gulf of Maine, the Great Lakes, and off the Eastern Seaboard
Some links to sites I used for the data presented in the forecast:
Between watching the data on these sites and reading the forecasts of those I linked to above, as well as others such as Crankywxguy and Judah Cohen, you can get a pretty good idea of what to expect this winter. Of course, Mother Nature is always fickle and it’s entirely possible all the pundit’s predictions will once again be wrong. That being said, lots of really smart people agree on a similar structure to the winter forecast so it’s not a total crapshoot. Regardless of how this forecast pans out, winter in Maine will undoubtedly bring a few notable events and I look forward to covering them this winter.
Thanks for reading and enjoy the holidays!