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Winter hiatus expected to close out December

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After a two week period featuring a polar vortex intrusion, colder than normal temperatures, and frequent winter weather opportunities (especially in the interior) the pattern this week will be one of transition. The hemispheric pattern is undergoing some fairly significant changes, which are working to take then cold air back to its source region by late this week, over the arctic and north pole.

Yes, the arc is swinging back the other way, as the same cold air which was disrupted and pushed southward into the Northern 1/3 of the United States is retreating fairly dramatically. In the stratosphere, the polar vortex is tightening and strengthening over the North Pole, a dramatic difference from the vortex we observed earlier in the season which was stretched, nearly split, and elongated.

The hemispheric pattern by the end of this week on the GFS model -- showing most cold air bottled up near the Arctic regions.

The hemispheric pattern by the end of this week on the GFS model — showing most cold air bottled up near the Arctic regions.

While the stratospheric polar vortex doesn’t directly control our weather, it does have a large impact on the weather as its behavior gradually works down into the troposphere (where we experience our sensible weather). A split, elongated, or stretched stratospheric polar vortex can lead to a disrupted polar vortex in the troposphere, with large areas of very cold, arctic air moving south from the pole — like we saw last week.

As the polar vortex strengthens and returns “home”, the hemispheric pattern will be further impacted by other forces. The Pacific pattern is also undergoing changes as a result. Gone is the large ridge which built from the Aleutians to Alaska, now replaced by troughiness which seeps into the Western United States. This wave pattern has effects further east, with the deep trough in the Western USA allowing a Southeast Ridge to strengthen and expand.

Forecast models are in fairly good agreement on this occurrence, with the development of a strengthening Southeast ridge supported by both the European and GFS model suites and ensembles through the medium-range. This ridge looks likely to peak around Christmas week, as a deep trough builds into the Western US. The signal remains through the first week of January.

The height pattern through the end of December supports storm systems tracking well west of the Northeast US, limiting winter weather chances.

The height pattern through the end of December supports storm systems tracking well west of the Northeast US, limiting winter weather chances.

 

While this doesn’t necessarily mean winter weather will be completely “turned off” in the Northeast, it sure means it will be mitigated in terms of overall coverage and potential. The next large storm system begins to appear on forecast models around Christmas weekend, but the Southeast Ridge’s impact is fairly obvious. As energy ejects out of the Western USA, the height gradient in the mid levels of the atmosphere forces the storm well to the north and west through the Central US — with a storm system developing into the Western Great Lakes.

So, is there a light at the end of the tunnel for snow lovers? Not through the first week or so of January. It appears the slightly warmer than normal pattern will remain in place through then: owing to weak tropical forcing, a strong polar vortex near the Arctic, and a poor Pacific pattern. But by the fist and second week of January, forecast model ensembles are hinting at some changes as tropical forcing begins to wake up and a wave-breaking pattern in the Pacific starts to kick off some changes.

Until then, winter looks to take a hiatus through the holidays and into the beginning of 2017, with warmer than normal temperatures on average in the Eastern US.

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