Cool risks exacerbated by recurving typhoon in the Western Pacific

While much of the focus over the past few days has been on Hurricane Harvey, and rightfully so, another tropical cyclone will have an impact on the weather in the United States. We have spoken many times in the past regarding the importance of re-curving typhoons in the Pacific Ocean. These tropical systems can have major impacts on the mid level atmospheric patterns there, which eventually results in changes downwind over the United States.

Recurving typhoons are, by all measure, powerful systems which have major implications on their surroundings. Their movement into the general mid and upper level jet stream can completely disrupt the atmospheric wave pattern as a whole. They can also result in “wave breaking” events, or in a simpler term, the breaking of significant energy eastward across the Pacific — like a wave crashing onto shore.

Illustration of how recurving typhoons can lead to blocking and cooler air in the USA. (Eric Webb)

The next several days will be a great example of this, in all likelihood, as a typhoon strengthens in the Western Pacific and then recurves into the mid latitude jet stream. A tropical storm over Western Pacific named “Sanvu” is forecast by most ensemble guidance to become a typhoon and then recurve into the North Pacific over the next several days. As the system becomes absorbed into the mid latitude jet stream, a wave breaking event will occur across the Pacific Ocean, which will in turn enhance ridging near the Western USA and Canadian coasts. This ridging will drive northward and disrupt arctic air in Canada, which will then be forced southward into the USA — all exacerbated by a recurving typhoon from the Western Pacific Ocean.

Forecast model and ensemble guidance is in good agreement regarding the effects over the United States. As a large ridge builds in the Western United States, guidance has accordingly trended more amplified with troughing over the Central United States, particularly the Great Lakes and Agriculture regions, with cooler air also leaking eastwards towards the Northeast United States and the Eastern Seaboard as a whole.

With this intrusion of cooler air, there are a few things to note. First — the cooler air will almost certainly be enhanced by this aforementioned Western USA ridging. This suggests that models will continue to play “Catch-up” in regards to the amplitude of the cooler air and anomalies across the Central and Eastern United States. Second, the source of the cold air is genuine — which is very important in terms of sensible weather and actual effects.

Euro ensembles showing significant cool risks over the US, particularly Great Lakes and AG regions, September 5th through September 10th.

Essentially, straight south-southeast advection of cold air through Canada on the east side of a ridge will allow this air to travel quickly, unabated into the Northern 1/3 of the United States. This unmodified airmass will present the opportunity for impressively cool temperature anomalies. In addition, the potential exists for early frost risks across many growing regions of the Northern 1/3 of the United States, especially from September 5th onward.

Over the coming days we will begin to get a clearer picture in regards to exactly how the typhoon will behave, leading to added confidence in regards to the shots of old air. Our updated long-range outlook on Wednesday will include some further detail in regards to anticipated impacts during Week 2 (9/3-9/10).