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Winds of change blowing as cooler risks set to return

Happy Monday! Much of the past several weeks have featured a similar synoptic weather headline across North America: Trough west, ridge east. This pattern had led to large scale above-normal temperature anomalies from the Plains eastward to the Mississippi River, exacerbated near the East Coast. But there are growing signs that this pattern will be changing over next few weeks.

Until now, the pattern has been largely driven by the presence of tropical forcing, from the ENSO regions where La Niña conditions have essentially taken over. We can use multiple measures and analytical approaches to understand exactly how the tropical convection is impacting global circulations, one of them being the SOI. This essentially lets us know if the atmosphere is responding in line with an El Niño, La Niña or neither.

In our current case, global circulations have been in a La Niña type pattern for several weeks now, with a 30-Day SOI value now at +11.03, which falls well into La Niña territory. We have seen the results firsthand, with the mentioned development of enhanced Southeast Ridging in the US and troughs from the Northwest USA into the Rockies. A large, deep trough or vortex has become established from near the Bering Strait to the Gulf of Alaska in the North Pacific Ocean during this time as well.

The winds of change are blowing, however, despite the fact that the same general wave pattern will remain in place across the Continental US for the next 7 days. In the North Pacific Ocean, a retrograding wave signal has already begun to develop, as tropical forcing shifts from over the Western Pacific/Maritime Continent into the Western Hemisphere and Africa beginning during the next couple of weeks.. A recurving typhoon over Western Pacific may act to enhance the process – but even without it, the retrogression of major wave features in the North Pacific Ocean will be significant.

GEFS showing the pattern retrogression at 500mb during the next 10 days over the North Pacific

GEFS showing the pattern retrogression at 500mb during the next 10 days over the North Pacific

The shift in tropical forcing is due to a more amplified MJO wave propagating into phases 6/7/8. As this MJO progresses, trade or zonal winds over the Tropical Pacific will be weakening and possibly reversing to a more westerly direction. This will likely damper or bring and end to La Nina strengthening conditions and influence  the atmospheric pattern over the Pacific, at least into early November. There are also some indications that tropospheric wave forcing driven by MJO influences may lead to some displacement of stratospheric polar vortex, particularly at the 30mb level. This is appears to be associated with larger-scale features in the troposphere retrograding – and may eventually lead to more high-latitude blocking.

500mb mean anomaly composite for October with the MJO in phase during negative ENSO period

500mb mean anomaly composite for October with the MJO in phase 6 during negative ENSO period

Overall, with a trough backing westward towards the Aleutian Islands, ridging will be able to redevelop near the US West Coast. This is gradually expected to lead to a more volatile weather pattern across the Lower 48, with shots of cooler than normal air from troughs and more transient warm ridging. Stronger cold fronts may quickly become a major part of the forecast as we look towards late October and early November. However, as is often is the case for the East Coast, the duration of below normal temperatures and potential for storminess will depend moreso on the exact position of the ridge near the US West Coast and amount of high-latitude blocking over Greenland/Davis Strait.

After another warm surge east of the Mississippi River this weekend, it looks like cooler weather is on the way late this month. We will continue to monitor the La Nina, MJO and perhaps the stratosphere for any significant changes. Stay tuned for more updates on our company website and social media accounts on the atmosphere pattern over the extended term, during the next few weeks.

This article was written by John Homenuk and edited by Miguel Pierre.

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