John Homenuk

The stratosphere, blocking, and a hemispheric pattern change

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What exactly could a ridge in the Kara Sea, the remnants of Hurricane Nicole, and a circulation in the stratosphere all have in common? They’ll all have significant impacts on a changing weather pattern in the Northern Hemisphere over the next two weeks. Meteorology is a fickle, uncertain, and highly detailed process in almost every regard. But every once and a while, the fluid process that runs through our planets entire atmosphere can give us a show — where multiple processes come together, and they can be easily visualized and understood.

What’s occurring over the next 7 days is, by and large, a hemispheric pattern change. The progressive pattern across the United States, which has been dominated by a Pacific Jet and relatively uneventful disturbances, will undoubtedly shift. Changing wavelengths are likely to make the pattern changes effects even more dramatic. But the most interesting aspect is where, when and why these changes are occurring.

The changes may very well have begun as Hurricane Nicole shifted from the Atlantic, near Bermuda, to the North Atlantic Ocean weeks ago — without us even knowing. At the time, a very large and strengthening stratospheric vortex over the higher latitudes was undergoing a bit of a transitional process. See, the stratospheric vortex — way in the upper levels of our atmosphere — does have an effect on our weather, as it can often dictate the pattern in the higher latitudes and near the poles. A stronger stratospheric vortex leads to less ridging (or “Blocking”) in those latitudes. Essentially, the cold air stays bottled up as the stratospheric polar vortex tightens up near its usual home.

But Hurricane Nicole disrupted this transition process. In a relatively unusual process, she was absorbed into the stratospheric vortex at multiple levels, disrupting the vortexes strengthening process altogether. This left a weakened vortex vulnerable for repositioning, stretching, and even splitting. Nicole’s break and phase into the stratospheric vortex also helped to strengthen a ridge near the Kara Sea, which became the most anomalous feature on the globe over the past several days.

Both the ridge near the Kara Sea, and the remnants of Hurricane Nicole, have left the stratospheric vortex weakened and vulnerable, and the ridge near the Kara Sea will continue to force a feedback process over the next several days. The stratospheric vortex, essentially, is under fire. Eventually, with repeated pressure from both ends, the stratospheric vortex is forecast to completely split by Days 5-10, by both the GFS and Euro ensemble suites.

Why is this so important? In very basic terms, a stratospheric vortex split can lead to dramatically increased opportunities for high latitude blocking. Cold air in the higher latitudes is easily displaced and ridging (yes, the same aforementioned “blocking) can easily develop near the poles and higher latitudes. This allows cold air, which typically resides in those areas, to be displaced farther south, and has major implications on the circulations throughout the hemisphere.

GEFS showing the progression of blocking and ridging from the Kara Sea to Greenland over the next 10 days.

GEFS showing the progression of blocking and ridging from the Kara Sea to Greenland over the next 10 days.

Forecast models have begun to pick up on the increasing likelihood of high latitude blocking through the beginning of November. Both the GFS and Euro ensemble suites show much above normal heights developing from Greenland through the Davis Straight during the Day 10 to 14 time frame, with colder than normal air being displaced into Southeast Canada. This, obviously, will have major impacts on the pattern over the United States.

Exactly how the stratospheric vortex splits, where the blocking develops in the troposphere (where our weather is), and how the hemispheric circulations respond? That’s yet to be determined — and will have the resounding impact on the changes in weather that the United States experiences.

However, one thing’s for sure: The hemispheric pattern is changing as we speak, and you’ll be noticing the effects sooner than later.