High latitude blocking may set up a cold finish to winter

GFS Ensembles showing 500mb height anomalies well below normal across the Northeast US by next week in response to blocking developing in Greenland and Northern Canada.

GFS Ensembles showing 500mb height anomalies well below normal across the Northeast US by next week in response to blocking developing in Greenland and Northern Canada.

It may be a bit premature to say that Spring will be off to a cold start. Medium and long range forecast guidance, however, would say that is a fair forecast. Forecast guidance has been pointing to the development of a very strong high latitude blocking episode within the next week, which could include a NAO and EPO ridge. The combination of the two could force a piece of the Polar Vortex (and very cold arctic air) into South-Central and South-East Canada, meaning our chances of below normal temperatures would increase … especially the farther north you go.

It seems, at least to the meteorologist, that there is a bit of an underlying passiveness towards March snowfall events in the New York City Area. I guess this comes somewhat naturally due to the past several years, which have offered very little hope for March snowfall (running well below normal in our area over the last 5-10 March’s). In fact, last year around this time much of the area saw temperatures soar into the 70’s for highs. Lets face it, March has been a nonexistent winter month in our area with the exception of a storm in 2009 which produced significant snowfall in parts of the area.

GFS Ensemble forecast Arctic Oscillation readings.Notice the rapid dropoff over the next week.

GFS Ensemble forecast Arctic Oscillation readings.Notice the rapid dropoff over the next week.

With all of that said, most of you are probably asking yourself: Why would it suddenly get cold and potentially snow this March? What makes it so much easier this time? The answer is that it won’t be that much easier. It still will be more difficult to get snow this time of year given the longer days, stronger sun, and warmer ground. But it is important to remember that forecast guidance has been strongly pointing towards the potential for a significant blocking episode — not just a run of the mill Greenland block or positive height anomaly. We can begin to see this response just glancing at the Arctic Oscillation forecasts, with remarkable ensemble agreement on the AO readings falling off the proverbial cliff. Almost every ensemble member brings the Arctic Oscillation below -4 and some of them to -5 — this is not your basic – AO development. It’s worth noting, too, how much lower these readings are compared to even the lowest readings of the past few months. A word of caution though, we should carefully watch the AO over the next several days as it has outrun the 7 day ensembles forecast in a positive direction recently (bottom chart).

The high latitude blocking teleconnection response is also extremely impressive. In this situation, there are blocking signals (robust, impressive ones at that) on both sides of Central Canada. One over the NAO region

GFS Ensemble mean showing a -5 anomaly blocking feature developing from Greenland into North-Central Canada.

GFS Ensemble mean showing a -5 anomaly blocking feature developing from Greenland into North-Central Canada.

which builds into Greenland, and another over the EPO region north of Alaska with a strong 552+dm ridge. This essentially helps dislodge the core of the Polar Vortex into Central Eastern Canada and it will elongate there. This is important because we are now dealing with polar cold air not just on our side of the globe, but hanging out just to our north over Canada. The GEFS and ECMWF ensemble means are showing -4 to -5 SD anomalies over the higher latitudes from Greenland into North-Central Canada. This would be the most impressive high latitude blocking signal we have seen in this area since the winter of 2010-2011 — and the most dominant blocking feature since the -4 to -6 SD block near Newfoundland during Hurricane Sandy which forced her recurve.

That being said, the timing of the individual disturbances remains somewhat up in the air and forecast models are going to struggle with them as we move forward. This is important, because it seems like there is some concern in the community about the forecast models not showing any major snowstorms or tremendous cold as we move forward. I say to give it some time. Forecasting individual disturbances at this range can be difficult even in basic patterns — and this is not one of them. Here, we have some very abnormal and unseasonable features which are developing and will undoubtedly wreak havoc on the forecast models. One of the best forecasting techniques at this range is to use ensemble forecasting to pinpoint a few sets of potential dates.

There are several which catch our eye — none of which are in the short to medium range at the moment. One of the more impressive seems to be around the 20th-24th of the month. We’re starting to see this threat appear on the medium range guidance (GFS, GEFS, CMC and ECMWF) over the past few model cycles. With the aforementioned blocking, favorable MJO state and mid and upper level height field across not only the CONUS but Canada and the high latitudes, this storm system has the potential to produce significant wintry weather across much of the Northern 1/3 of the US from the North-Central Plains, through the Ohio Valley and into the Northeast. It’s too early to speculate on further details. Concluding, it appears the potential is higher than normal for colder than average temperatures across the Northeast US through the end of March…with the potential for snow likely becoming a concern as well especially across the interior.

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