El Nino! It has been all over the news this year and will continue to be as we progress into the upcoming winter. Why? Because El Nino (Part of the “ENSO” phase in the tropical Pacific) is often a major driver of our weather pattern. However it is not the only player at work when influencing weather patterns across North America. In addition, no two El Nino’s have ever been the same and this year’s is already into uncharted waters.
Strong influence from El Nino is hard for other major weather players to overcome and this year’s Nino is undoubtedly a strong one. Thus the overall winter forecast is weighted towards our working knowledge of what a strong El Nino *should* produce. It is worth noting however that we work from a very small sample size of “super” strong El Nino’s. With a variety of other factors thrown in, the forecast comes together. Now comes all the technical information, but if you get bored you can skip to the end for the final result. (If you’re not quite geeky enough about the method behind the madness)
*Click on the links if you want a visual reference aid*
ENSO (El Nino Southern Oscillation)
El Nino has been building throughout 2015 and we may be nearing its peak. The only other Nino on record for being this strong is 1997-98 and we have surpassed it in some regards. El Nino monitoring is broken into “regions” 1 + 2, 3, and 4. (3 and 4 have a combined section called 3.4) As of November 19th, the latest weekly region 3.4 anomaly has broken the all-time record (1997) to become the warmest anomaly ever recorded. However, region 1 + 2 is quite a bit cooler than 1997. One thing that is more impressive about this year is the areal extent of the warm water. The winter of 1997-98 featured an “east based” Nino meaning the warmest water was east towards South America. This year, one could justify the Nino as being “basin wide” meaning widespread warm anomalies across the entire tropical Pacific basin. It is almost certain now that this El Nino will go down as the strongest “basin wide” El Nino on record even if 1997-98 beats it in specific areas.
In the comparison plot of 2015 to 1997 (October through mid-November), note how much warmer the tropical Pacific is this year with the exception of the eastern area. (Region 1.2)
Second, note the abundance of warmth in the northern Pacific and near the west coast of the U.S. which is much more widespread than in 1997. Water temperatures from California down to Hawaii are also well above normal. These regions are not part of the equatorial Pacific where El Nino is measured. However, they can still have an impact on how the jet stream behaves.
El Nino is expected to peak coming up in December. It is expected to then weaken through 2016 and possibly even snap to La Nina conditions (cooler than normal) by next winter.
Three major “teleconnection” areas of the world near the U.S. each have their own numerical index because the jet stream pattern in these areas can have large impacts on the weather in other parts of the world. The North Atlantic Oscillation (NAO) refers to a jet stream pattern in which either a ridge (negative phase) or trough (positive phase) exists over Greenland. The Arctic Oscillation (AO) refers to ridging over the Arctic (negative phase) or more frequent troughs (positive phase). The Eastern Pacific Oscillation (EPO) refers to either a ridge (negative phase) or trough (positive phase) over the far eastern Pacific near Alaska and northwestern North America. Note: All three of these indexes favor colder weather in the Ohio Valley if in a negative phase and the opposite during a positive phase.
An extremely negative EPO (very strong ridge over Alaska) was one of the major culprits of the cold weather that we experienced during the past two winters. It is likely that extensive warm water anomalies in the northern Pacific and along the west coast of the U.S. (think back to the maps above) helped sustain a massive reoccurring ridge in the jet stream over Alaska and northwestern North America during the past two winters. This feature frequently caused harsh arctic air outbreaks across the central and eastern portions of the U.S. by creating a path for polar air to march southward. However, the opposite pattern tends to form during strong El Nino’s. There is typically a large trough of low pressure in the jet stream near the Aleutian Islands which floods the CONUS with mild air. If it is positioned slightly farther west, this low can force a ridge in the jet stream over northwest Canada (essentially a –EPO) which would lead to occasional cold air outbreaks in the eastern U.S.
Strong “east based” El Nino’s (think: 1997) seem to have a higher correlation with an Aleutian Islands low that bring about mild conditions for the Ohio Valley. This year’s “basin wide” variety tends to favor a slightly more westward position. Although likely not far enough west for a predominately – EPO.
Very warm waters in the northern Pacific remain in place which could favor a tendency for ridging in the jet stream near Alaska and/or northwest Canada from time to time. Overall, my forecast is for the EPO to AVERAGE positive this winter due to the overwhelming forcing from El Nino causing the trough in the jet stream to develop frequently. However, occasional dips to negative are likely due to the aforementioned factors. This will likely lead to periodic episodes of below normal temperatures particularly later in the winter but possibly early on as well.
The AO and NAO are much harder to predict with much certainty beyond a few weeks. There has been research done on certain key indications from around the globe that can correlate (but don’t always) to either negative or positive phases of each index. The Snow Advance Index (SAI) is a representation of how fast snow cover is advancing over the Northern Hemisphere relative to average. Faster building snow packs have been shown to favor negative AO/NAO modes later in the winter. This year, snow has advanced much faster than normal through the middle of November especially in Eurasia. Without getting too technical, this helps perturb the stratospheric polar vortex (which is over, you guessed it: The North Pole!). When the larger stratospheric polar vortex is strong (which it is presently), it tends to lock up the coldest air by the pole. However when it weakens, it is more likely for pieces of it to branch off and send harsh periods of winter weather into lower latitudes.
Given the above information and the fact that El Nino’s often favor a negative NAO (especially at the end of winter), I predict the AO and NAO to both average slightly negative this winter. Although it is important to note that these negative modes will be less likely during the first half of winter given the currently strong stratospheric Polar Vortex.
In addition another index called the “QBO” is positive this year which favors a strong Polar Vortex and positive AO especially at the start of winter. (That is more technical, but you can read more about the QBO here) The net result should still be a slightly negative AO/NAO when averaged across the entire winter.
Bringing It All Together
These are just a few of the factors that go into forecasting the winter weather pattern and frankly any seasonal weather pattern. Overall, a strong El Nino signal exists this winter and will likely dominate the pattern for the most part. Some conflicting indicators (such as negative EPO, AO, NAO) may temper the effects and/or cause slightly more frequent outbreaks of cold air than would otherwise be expected in such a strong El Nino. That being said, significant periods of time, if any, below 0 are highly unlikely this winter for the Tri-State. It will also be unlikely for us to remain below freezing numerous days in a row like we did the past two winters.
The sub-tropical jet stream (the southern branch of the jet) will be very juiced up this winter thanks to El Nino and the warm waters between Hawaii and California. This will lead to a very active, stormy pattern meaning large temperature swings. This means we could see temperatures literally range from 60 degrees to 10 degrees in January, and that will probably happen multiple times. The warm days in both December and January will likely pull the average temperature for both months above normal despite some cold outbreaks in between.
As the winter progresses into February, there will be a higher chance for the cold weather drivers to influence the pattern more significantly. Therefore I expect February to be the coldest month (relative to average) and snowiest. The cold in February will be very unlikely to balance out the above average months of Dec/Jan, thus slightly above average is the forecast for the winter as a whole (Dec/Jan/Feb). Even with the active storm track, the heaviest rain events will be favored across the southern U.S. and thus I am expecting slightly below normal precipitation as a whole.
Predicting seasonal snowfall is kind of like predicting who will win the super bowl at the start of the football season. You may be able to predict known trends within teams and who has the highest chance, but on any given day, any team can win. The overall trend this winter will favor below average snowfall locally. But on any given day, all the right cards could happen to line up for a big one that pulls us closer to average. (There was a 12” snowstorm in February 1998 but very little snow the rest of the winter) Even with the prospects of snowfall looking better in February, it will likely not make up for the lower than average amounts we receive earlier in the season.
So is this El Nino the ghost of El Nino’s from the past? Not exactly, other than the extremely warm water temperatures in the equatorial Pacific. When looking at the overall El Nino picture and even global sea surface temperature anomalies, we are in unchartered territory and have never really seen a setup like this since our better record keeping began in 1950. This makes the winter forecast extra challenging this year.
If I had to characterize the forecast in one sentence: Warmer and less snowy than the last two years, but not necessarily a walk in the park.
Forecast CVG temperature anomaly DJF: +1 to +3F
Forecast CVG snowfall: 14-18 inches (24 is average)
Forecast CVG Precipitation: 60-80% of average