So obviously it’s been quite some time since I last wrote here, since last year actually – but let’s be honest – when it comes to winter weather threats this season, there just haven’t been any. However, that is about to change.
All the talk this Fall has been about the El Nino, some calling it a “godzilla” El Nino, some saying it would mean a warm and snowless winter for many in the east, some saying it would be the strongest ever. A whole bunch of that is false, or weather hype, or some combination thereof, in my opinion. One of the biggest comparisons I’ve heard with this years Nino is with the strong one that occurred in ’97-’98, which is comparable to this year’s event in some ways, and very much delivered a warm and nearly snowless winter in the eastern United States.
That being said, you may ask yourself, how is this El Nino going to produce anything different than what happened in ’97-’98? It’s easily in the top three strongest, so what gives? Notice I said comparable in SOME ways. Not all. And that’s important.
First of all, the location of the warm waters in the equatorial Pacific is paramount. You can see from the graphic below (courtesy of Ryan Maue, Weatherbell Analytics, LLC) that the warm waters extend from and just west of the dateline (180 degrees longitude) to the off the South American coastline, with the warmest anomalies in the western 2/3 of that area.
This is vastly different from what was seen in 1997-1998, where the warmest waters were concentrated primarily off the South American Coast, and the waters were significantly warmer at this point in time then versus now (1997 on the left, 2015 on the right).
And here’s a plot comparing the temperature differences between the two directly:
Additionally, this El Nino, while there will be some fluctuations from week to week, is starting to weaken, and will continue to do so over the course of the Winter and into Spring:
And finally, a graphic showing the sea surface temperature (SST) change in the last week alone (courtesy of Levi Cowan, tropicaltidbits.com):
So, from the above information, we can gather that (1) this years El Nino is NOT the same as 1997-1998, not as strong when compared to the same point in time, and in fact it’s a basin wide phenomenon, with the warmest SST’s in the western 2/3 of the basin, closer to the dateline, (2) this Nino is weakening, and will continue to do so, and (3) that cooling that is occurring is greatest in the eastern regions per the last graphic above. These facts lead to more thunderstorm convection along the date line (180 longitude), which in turn promote a jet stream configuration over North America that leads to ridging (relative warmth) over NW Canada and troughing (relative cold) over the southern and SE states of the U.S.
Now, if that’s the case, why has it been so damn warm in the eastern US? Because of course, there are other factors at play here. The biggest in my opinion has been the strength and positioning of the Polar Vortex, which so far this year has been centered over the pole and VERY strong, effectively locking in almost all of the arctic air over northern Canada and the arctic circle.
You can see the coldest values directly over the pole, with a ridge in the east, trough in the west, just as it’s been for weeks now. But that is about to change.
The graphic above is the 10 day forecast from the European ensemble guidance, but I could’ve picked any of the models quite honestly, because they are all showing the same thing. The cold pool over the pole becomes elongated, and splits its concentration into two locations – over eastern Canada, and to a lesser degree NE Asia/Bering Sea. This a result of the weakening of the Polar Vortex (PV). The reasons behind this are somewhat complicated, and beyond the scope of this blog, and involves the transport of warmer air from the mid latitudes of the troposphere (where we live) to the stratosphere (high in the Earth’s atmosphere), which in turn disrupts the PV circulation; but understand that this weakening was in fact favored given the well above normal snowpack over Siberia in October (courtesy of www.judahcohen.org – for those of you interested, I highly recommend reading his work work):
Although the PV will only be weakened in the near term, it will be enough over the next few weeks to aid in causing displacement of colder air from the north into eastern North America, as well as a deepening eastern US trough, as depicted in the second image from the European above at 240 hours. This will likely not result in a prolonged Arctic air mass, but rather temperatures closer to normal, at least to start the month of January. As the PV is disrupted further later in the month, the cold COULD be more severe.
So, the disruption of the Polar Vortex is one of the reasons why we’re seeing a change in our pattern in the face of a strong (yet weakening and more west centered than 1997 remember!). But there are others as well.
The Madden Julian Oscillation, or MJO, which again is complicated but is basically a coupling of circulation in the atmosphere with tropical/equatorial convection (thunderstorms). In an El Nino such as this, the MJO, to borrow an explanantion from Steve DiMartino (nynjpaweather.com) is more of a modifier than a pure driver when it comes to our weather in the United States. To make a long story short, the MJO is forecasted to move into a more favorable phase 8 in the first week of January:
Which corresponds to, again, more convection towards the date line, and a western ridge and eastern trough, resulting in temperatures across the US in the winter months:
And finally, we have the warm waters in the Pacific west of North America, also know as a positive PDO:
Which results in this temperature pattern across the US:
So, given all of the drivers and modifiers above, it’s no wonder we’re seeing the jet stream configuration go from this, the so called “firehose” Pacific jet stream, flooding warm Pacific air into most of the country as it attacks northern CA below:
To this graphic below, in about 10 days or so, where it splits, where one branch heads far to the north over Alaska and the Yukon and then south into the US, supplying the cold, and then other branch heads south into Baja California and then into Texas and the eastern US, supplying the moisture.
It all adds up to lots of fun on the near horizon with regards to winter weather. Expect a mild Christmas weekend, and then sharply colder this coming Monday, with the chance for ice and snow from northern PA and especially upstate NY and New England Monday night into Tuesday. I don’t want to go into specifics yet regarding that storm, but it needs to be watched over the next few days. This system isn’t even the result of the changes I’ve described above, but it is the beginning of a lot more to come.
Expect this website and my page to get busier over the coming weeks and months as we step down into winter from this record breaking warmth we’re currently seeing. Don’t worry, #WinterISComing, it’s just taking a little longer than usual – but it won’t disappoint.
In the meantime, I wish everyone a safe and merry Christmas!