Posts

John Homenuk

High latitude blocking a key in the late February forecast

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Over the past several days you have probably heard about a Sudden Stratospheric Warming event (SSW) — over, and over, and over again. With good reason, too – the event is important. We spoke about it several days back and will touch on it here again. Forecast models continue to suggest the development of a  very impressive stratospheric warming event over the next few days, beginning in just 24-48 hours and continuing through next week. The stratospheric vortex itself will split in two, with the dominant sister vortex centering over Canada.

Warmer than normal temperatures and higher than normal heights will surge into the stratosphere near the North Pole and Arctic regions. Most notably, the stratosphere and troposphere are well coupled heading into the event, which means the warming in the stratosphere will have “maximum impacts” on the atmosphere as a whole – with the troposphere reacting to the warming event happening above rather quickly.

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John Homenuk

Plummeting AO could offer some winter hints

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As we’ve spoken about several times already this Autumn, our winter forecast doesn’t typically come out until the first week of November. Although some will bark at us for this — we’ve been hearing it already this Autumn — there are reasons for it. Mainly, our forecasters have recognized the importance of analyzing the pattern during October and the ideas moving into November. The recognition of the pattern during the next few weeks will be immensely important to understanding how things will shake down this winter. And our first “hint” could be offered during the next several days, as the AO — or Arctic Oscillation — is forecast to take a nose-dive.

The Arctic Oscillation is a large scale monitor of climate variability, also referred to as the Northern Hemisphere Annual mode. More simply, it’s a climate pattern characterized by winds circulating counterclockwise around the Arctic — typically at around 55°N latitude. In even more simple terms — the negative phase of the AO typically displaces cold air farther south in North America, while the positive phase keeps the colder air farther north, nearer to the poles and arctic regions. Forecast models are in good agreement that the AO will nosedive over the next several days — and not surprisingly, a major shot of below-normal temperatures is expected late this weekend into early next week. 

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John Homenuk

AO Falls to lowest value ever for Mid March

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The Arctic Oscillation readings over the last several months show the dramatic and unrivaled drop in readings during mid March 2013.

The Arctic Oscillation readings over the last several months show the dramatic and unrivaled drop in readings during mid March 2013.

The Arctic Oscillation (AO) has fallen off the cliff in the last week, as incredible high latitude blocking ridges have developed both north of Alaska and over Greenland. The AO fell below -5.0 to -5.632 on March 20th, 2013 — making it by far the lowest reading after March 15th in the history of the AO’s recorded values. It also remains, historically, the lowest AO after March 6th.

GFS Model observed (black) and forecast (red) AO values

GFS Model observed (black) and forecast (red) AO values

The Arctic Oscillation, or AO, refers to the generally opposing pattern between pressure in the northern-middle latitudes and the Arctic. It is a climate pattern, that is characterized by counterclockwise winds which circulate around the Arctic near 55°N. The phase and degree of amplitude of the Arctic Oscillation can be a very useful forecasting tool, especially in the winter season. During a positive phase AO, a belt of strong winds circulating around the pole often acts to keep cold air confined to our north. But a negative AO opens the proverbial flood gates for the arctic air to penetrate much farther south.

It should come as no surprise, then, that this strongly negative AO phase has been accompanied by the presence of unseasonably cold air. The forecast, holds the potential for cold air through the next 7 days with chances for snow. The GFS ensemble and most medium range global models forecast the AO to rise over the next 7 days. Although it is typical for the AO to rise and fall rather consistently, the dramatically low values are being accompanied by rather high return values to normal, and in some cases a positive phase of the AO. Regardless, It remains to be seen if the wintry weather threats will work out Still, one thing is for sure: we are in uncharted territory as far as the Arctic Oscillation goes, for this late in the season.

For more on the Arctic Oscilation check out this article, which details the different phases and amplitudes.