Introduction and Overview:
The last two winters were blowtorches. There’s really no better way to put it. The 2015-16 winter was anomalously warm across most of the United States, which was driven by a record-breaking El Nino episode. By that following spring, El Nino had begun to fade as the waters along the central and eastern equatorial Pacific began to cool, and as we approached the 2016-17 winter, sea surface temperature anomalies across that region in the Pacific favored weak La Nina conditions. But, that winter was also very warm from parts of the Southwest and Great Plains to the East Coast! We once again find ourselves in weak La Nina conditions; however, I’m going to explain why it appears that this winter will, at the least, be a bit more exciting across some regions compared to last winter.
This year’s winter forecast is broken down into 11 regions. I encourage everyone to read this forecast in its entirety all the way down to the region-by-region breakdown. Once reaching that section in this forecast, find the number on the map that represents your location, and skip to your (mostly brief) regional discussion. This year’s outlook is a bit more brief compared to previous ones. If you would like to receive more discussion-based, detailed forecasts throughout this winter, I encourage you to signup to receive our newsletter, which will be sent once a week beginning later this winter.
Current State of La Nina (the only semi-technical part of this forecast):
As mentioned above, La Nina conditions are back for a second winter in a row; however, this La Nina is different. But if you were to compare the sea surface temperature anomalies (SSTAs) to last year’s anomalies across the Nino 3.4 region over August, September, and October, you might convince yourself that there isn’t much of a difference. If anything, the SSTAs were slightly warmer over that time period this year. That’s why it’s extraordinarily important to determine how those anomalies are spatially distributed across the central and eastern equatorial Pacific. Notice in Figure 1a how the cooler anomalies are strongest in the eastern equatorial Pacific this year and extend along the Peruvian coast. That’s the complete opposite to what was occurring this time last year in that region (Figure 1b) when those anomalies were actually above average, and the cooler anomalies were more concentrated over the central Pacific. This distinction alone could result in the atmosphere responding completely differently than what occurred last winter.
While the above paragraph focuses entirely on the oceanic component of La Nina, it’s extremely important to not ignore the atmospheric component of this phenomenon, which could ultimately have an influence on our overall pattern in the United States this winter. In other words, below-average sea surface temperatures in the central and eastern equatorial Pacific are not as important of a consideration if the atmosphere isn’t responding to these anomalies. The recent Atlantic hurricane season can be used as sort of a proxy to measuring how well the atmosphere is responding. While there is a plethora of other factors to consider, vertical wind shear is generally quite low across the Atlantic when there has been an atmospheric response to developing La Nina conditions. Oftentimes, the amount of vertical wind shear across the Atlantic can be the difference between an active hurricane season and one that’s generally benign. Of course, we all know that this year’s hurricane season in the Atlantic was hyperactive, which likely signals that La Nina was playing a role.
Despite model guidance indicating a relatively weak La Nina event throughout the winter, there are other reasons to come to the conclusion that this event is becoming decently established. Last month, the Madden-Julian Oscillation (MJO) went through an active period. This is a phenomenon that can induce variability (changes) in precipitation, cloud cover, wind direction, etc. in the tropics and even outside of the tropics on a week to week basis. Research has shown that the MJO may play a role in triggering the onset of an El Nino event, and given that La Nina is the opposite of El Nino, the eastward propagation of the enhanced convective phase of the MJO through the central and eastern equatorial Pacific could have had a detrimental effect on allowing this La Nina event to become established; however, there’s no evidence to suggest that occurred. If anything, La Nina has strengthened since then, and there are other signals to indicate that this has taken place. Figure 2 shows that not only sea surface temperatures are cooler in the central and eastern equatorial Pacific, but also, those temperatures are cooler below the surface.
Figure 2: Sub-surface sea surface temperature anomalies across equatorial Pacific
Based on the reasoning above, previous weak La Nina winters, along with a few low-end moderate La Nina winters, were selected when producing this forecast. These years were then filtered based on other criteria that will be briefly discussed in the next section.
Brief Forecast Discussion:
This winter is probably going to be characterized by a lot of volatility in the pattern, particularly in regions 8, 9, and 11. We’ve already seen quite a bit of that this month (November). If you happen to take a look at temperature anomalies across the United States for the first 10 days of this month, the below-average temperatures have extended from the Pacific Northwest to the central and northern Plains over to the Upper-Midwest and Great Lakes region. The coldest of those anomalies were centered over Montana and northern Plains. To the contrary, anomalous warmth has occurred from the southeast quadrant of the United States to the East Coast. BUT (this is important), southern regions of the U.S. east of the Great Plains have had intrusions of colder air, but it has come and gone. I foresee that happening quite a bit this winter, and at times over the winter, colder air masses may become established pretty far to the south for several days. Regions 4 and 10 are the only regions that I expect consistent warmth. Temperatures are going to be back and forth across many regions, even if some of these regions end up with average to slightly above average temperatures. Some regions, like Regions 2 and 5, could have intrusion after intrusion of Arctic air into the area with only brief warmups, at times. The battle zone region characterizes, on average, where I believe the boundary between colder and warmer conditions will persist (the baroclinic zone), which could ultimately become a zone of enhanced storm activity (including winter storm activity). Due to the volatile nature of this pattern, I wouldn’t be surprised if at least two winter storm opportunities arise in region 9, and while that’s not completely unusual, this will be a stark contrast to the last two winters.
It’s worth noting that after I selected the initial analog years, which was an aid in producing this forecast, I noticed that there was quite a bit of variability amongst the different years. A sizable proportion of those years featured warmer conditions across the southeastern U.S. and along the East Coast, while those same years also featured very cold conditions from the Pacific Northwest to the northern Plains. The magnitude of those anomalies varied year to year. However, there were a smaller subset of years that featured below-average temperatures into south-central and southeastern regions of the United States. There is research that indicates that the easterly phase of the quasi-biennial oscillation (QBO) increases the likelihood of there being a weaker stratospheric polar vortex, which in effect, would result in deeper intrusions of colder air. We have moved into the easterly phase of this oscillation, and it is something that I took into account when putting together this forecast. The colder intrusions that are expected later in November are the result of a weaker tropospheric polar vortex and blocking over the Arctic within the troposphere. With that said, the stratospheric polar vortex is currently well-established and is quite concentric over the Arctic. Over the course of the winter, I’ll be watching to see if the stratospheric polar vortex weakens and becomes better coupled with what’s going on in the troposphere. Due to the uncertainty that exists and the fact that conditions associated with “similar” years varied, confidence wasn’t high enough to predict below average temperatures in regions 9 and 10. Also, I went with a “temperatures will fluctuate a lot” for region 11. With all of that said, I decided to go with slightly above average temperatures in regions 9 and 10 and didn’t expand that region as far northwestward as I otherwise would have, given the current state of La Nina. As has already been mentioned in this forecast, there’s probably going to be a lot of large swings in temperatures anyway, which will open the door for more opportunities for snow/ice relative to the previous two years, even if temperatures end up around average or slight above that.
This section is meant to provide a synopsis of what is expected to occur this winter in a given region. Some of this information was already included in the previous section, but each regional discussion will be much more brief.
Region 1: This region will remain active throughout most of the winter. Rainfall/snowfall totals should be above average. However, there will be times that ridging may tend to build into the region, which may shift the overall storm track a bit farther north, but this won’t be a permanent occurrence. Temperatures should run below average across the northern half of this region, and average to below average across the southern half.
Region 2: There will be a tendency throughout the winter for Arctic cold air to spill into this region, which is why I believe temperatures will generally be below-average. Some colder intrusions could be centered to the east of the region towards the northern Plains, but this will be only during the times when ridging has temporarily become established across the western third of the U.S. Snowfall should run average to above average, making this a good ski season for this region.
Region 3: For this region, temperatures should run about average, and precipitation should generally be average to above average (sorry about the typo on the map). It’ll be difficult for any anomalously colder air mass to dig this far south into this region, but I don’t have any reason to believe that temperatures will consistently be above average either.
Region 4: This region should remain consistently warm and dry throughout most of the winter. Of course, there will be variability from time to time, but overall, this should be one of the few regions this winter that won’t have to deal with a lot of back and forth in temperatures/precipitation. Confidence is higher that temperatures will mostly be above average across the southern half of this region, while the northern half could have average to above average temperatures. It’s possible that northern parts of this region could get closer to their average snowfall, but snowfall totals should run below average to the south.
Region 5: This region will be very cold throughout the winter. Temperatures will generally be below average and in places, could be well-below average. Numerous intrusions of Arctic air will spill into the region, and if the expansive snow cover over Canada persists and continues to build, those air masses will be that much colder. Snowfall should generally be average to above average, with this winter featuring numerous opportunities for snowfall. Heavy lake-effect snowfall just prior to and early in the winter will likely occur.
Region 6: This was actually a difficult region to pin down, especially since it runs parallel to the wintry battle zone. Temperatures should generally run average to below average across this region. Numerous Arctic intrusions should spill into this region, but I went conservative on the temperature forecast since it appears that there will be noteworthy warmups in between all of this. Precipitation should be about average overall, but I wouldn’t be surprised if precipitation and snowfall totals go above average across the right half of this region. Expect heavy lake-effect snowfall just prior to and early in the winter.
Region 7: I don’t expect this region to be excessively warm this winter, but on the other hand, the majority of the brutal cold should remain outside of this region. I certainly can’t rule out some decent snows falling at times across the northern third of this region, but overall precipitation should be about average.
Region 8: This is the region that I classified as the “battle zone.” On average, this is where I expect the dividing line to be located between the very cold air to the north and slightly warmer conditions to the south. Notice that I said, “on average.” The challenging aspect of this winter is that I expect a lot of volatility throughout the winter. So, the coldest air could, at times, remain to the north, but other times, colder air masses could dig to the south of the region. Nonetheless, I expect an active storm track across this region and do believe a lot of this region will have a “real winter” this year.
Region 9: While temperatures will probably be slightly above average across this region, this is going to be quite a different winter compared to the last two. I don’t expect consistent warmth across this region like I do farther to the south, and this winter will be characterized by a lot of back and forth in temperatures. Some of the more noteworthy Arctic air masses will make it into this region. At least a couple of winter storms will likely be on the table, and there could be an additional ice storm threat or two just to the east and south of the Appalachians, due to cold air damming events. It could be in the 60s/70s one day with cold and snowy/icy conditions the next.
Region 10: While some colder intrusions could make it into this region throughout the winter, this area should stay consistently warm. Overall, temperatures should be slightly above average. Conditions should be mostly dry; however, the southern regions of the Delta could actually end up with precipitation close to average. It’s worth noting that a small subset of my analog years did depict consistently colder conditions into this region, but confidence simply wasn’t high enough to put that into the forecast.
Region 11: Yes, temperatures are going to fluctuate quite a bit across this region. That may not seem like an actual forecast, but given the volatility expected with the pattern, it’s the most fitting forecast for this region. Temperatures will have a greater likelihood of being closer to average near the coast but may actually end up below average farther to the west across this region. Snowfall totals will likely be average to above average across inland regions, but I can’t rule out the typical nor’easter or two this winter that could bring wintry conditions closer to the coast. However, many of the systems that impact this region will bring snow to inland regions, but rain near and along the coast.
This year, we’ll be including sub-seasonal forecasts throughout the winter. Even though it’s mid-November, there is a higher level of uncertainty this year, and I expect adjustments will have to be made to this forecast. Also, temperature/precipitation anomalies can vary spatially from month-to-month, and sometimes (depending on the year), a seasonal outlook that averages everything out over a 3-month period doesn’t give the full picture. What if you have dry conditions one month and wet the next month, which averages out to “normal?” Seasonal forecasts are great at giving you a general overview of what to expect, but they don’t provide the nitty-gritty details that a sub-seasonal forecast does. Since I expect many regions this winter to have larger swings in temperatures, those sub-seasonal forecasts should be very beneficial to you.