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John Homenuk

Cool and dry weekend, snowstorm threat looms

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Canadian model (CMC) showing a significant snowstorm impacting the area on Monday. The CMC remains one of the farther north models within the envelope of guidance.

Canadian model (CMC) showing a significant snowstorm impacting the area on Monday. The CMC remains one of the farther north models within the envelope of guidance.

Not only is March madness taking place in the college basketball world, but it’s also taking place in our world of meteorology. Cooler than normal air remains well settled into the region to begin the weekend, as historically strong blocking near Greenland and the North Pole as well as a historically negative Arctic Oscillation (AO) has led to a late-March pattern that feels more like mid-February. High temperatures in the 40’s and cold westerly winds have been a staple in the area weather the past few days. Much of the same is expected through the first half of the weekend, with cooler and dry conditions prevailing. Attention will then turn to a potential snowstorm (yes, snowstorm) on Monday. Forecast models are struggling with the track and intensity of the system as well as the resulting impacts on our area. We’ve detailed the entire threat below.

SREF ensemble mean showing a significant snowstorm on Monday.

SREF ensemble mean showing a significant snowstorm on Monday.

It’s Spring. What is causing the threat for the snowstorm? Historically strong blocking over the high latitudes (extending from Greenland to the Pole) is displacing unseasonably cold air into the Northern 1/3 of the US. At the same time, a strong disturbance is ejecting out of the Pacific Northwest into the Central United States. Such a storm track could occasionally cause a rain storm in our area, especially this time of year, with the surface low tracking to our west. However, the presence of the unusually strong blocking is forcing the system farther south, to redevelop off the East Coast. The presence of cold air to the north and throughout our area is increasing the likelihood of snowfall if precipitation makes it to our area.

What are the uncertainties with the system?  One reason why blocking patterns can be so unseasonably cold is the fact that they force powerful Upper Level Low (ULL) pressure systems to meander at latitudes just to our north and northeast, which are a great source of cold air. However, when an ULL is too powerful and too close to our area, it can compress the height field out ahead of our storm and force it drift eastward instead of turning the corner and heading up the coastline. We do know that eventually, the storm will run into a brick wall and be forced to drift eastward. However, will it do this after it gets to only Ocean City Maryland’s latitude, south Central Jersey’s, or somewhere in between? This is the difference between a graze, a moderate rain to snow event, and a major snowstorm.

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John Homenuk

High latitude blocking may set up a cold finish to winter

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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).

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