Posts

Miguel Pierre

What the stratosphere and tropical Pacific can tell us about December

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For the better part of the last three weeks, our forecasters have been discussing and monitoring the potential for a pattern change throughout the hemisphere. Medium range model guidance has often been gung-ho with the pattern changing within 7 days. As is often the case, forecast models were too quick to change the pattern. Much of this has to do with poor forecasting of the stratosphere and tropical Pacific ocean. Not coincidentally, these two features can give us clues as to where the pattern is heading in the weeks ahead.

As it stands this afternoon, model guidance and ensembles are in good agreement that a -NAO (North Atlantic Oscillation) pattern will develop over the next two weeks. This pattern is often defined by large ridges or higher than normal heights in the atmosphere over Greenland and the North Atlantic Ocean. -NAO patterns can allow cold air to be displaced farther south into the Northern 1/3 of the United States, particularly the Central and Eastern United States.

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John Homenuk

Premium: Active pattern to begin December, but uncertainties remain

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The hemispheric pattern will continue to undergo changes as the month of December approaches, and this comes as no surprise to those who have been following our long range forecasts for the past few weeks. The main changes in the pattern are unfolding throughout the Pacific Ocean, where a large vortex in the Gulf of Alaska is being gradually replaced by a propensity for ridging. This causes a domino effect downstream towards the United States.

However, questions still remain in regards to the results of this change in terms of sensible weather. The source of cold air, speed of the pattern, and strength of individual storm systems are the main concerns; most stemming from the uncertainties in regards to the pattern in the higher latitudes from Canada towards the North Pole.

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John Homenuk

Pattern in transition as December quickly approaches

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Over the past few weeks, much has been made in regards to a changing hemispheric pattern. Global circulations are in flux, and a much warmer than normal pattern has in fact changed across the Continental United States. However, the changes have been slightly less stout and slightly delayed when compared with initial ideas, and the atmospheric pattern has been left in an overall flux as we head towards December.

The Thanksgiving holiday is often an important time-frame in meteorology as one looks ahead to the upcoming winter. Gone are the days of wild speculation and assumption when it comes to the winter ahead. December is just a week away — and the ideas formulated regarding the pattern as we move into winters first month are beginning to obtain more weight. Some ideas are clearly losing steam, while others garner more attention.

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Miguel Pierre

Premium: Late month pattern change likely to be gradual

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As discussed in the winter forecast, the stratospheric polar vortex has been weakened since late October. A wave-breaking event, mainly induced by a large ridge in the Kara Sea, has resulted in 100mb heat fluxes remaining above average. We still believe the polar vortex is likely to be weaker and more disturbed going into this winter, especially compared to the past several years.  However, some resistance from the polar vortex and tropical forcing influences will likely result in more gradual step down into pattern changes over next several weeks.

While westerly zonal winds have decreased to below average levels, a wind reversal or easterly winds aren’t being forecasted, particularly around 10mb and 60N. EPV at 10hpa also appears to shift from being poleward to more equatorward by the third week of November. Despite being severely disrupted; these winds allow for the polar vortex to remain quite large and profound over the high-latitudes. This will likely result in more neutral modality with the AO/NAO over next few weeks.

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