Atlantic Tropical Cyclone Steering Index and Activity Index: Description

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Based on , the Steering Index (SI) is a metric that describes large-scale MSLP patterns in the tropical Northern Atlantic, and thus can be used as a proxy for the steering imposed on tropical cyclones (TCs). Positive SI (SI>0) is generally associated with TCs that move westward across the tropical North Atlantic and make landfall in the Americas. Negative SI (SI<0) tends to occur when TCs recurve northward and miss North America. The Activity Index (AI) is a similar metric calculated via MSLP patterns that is a proxy for overall TC activity in the North Atlantic. Positive AI (AI>0) is generally associated with above average TC activity, where negative AI (AI<0) is associated with below normal TC activity.

From Figure 3 of . Positive AI (left) and Positive SI (right), where shading is relative TC occurrence.

SI and AI are calculated regardless of whether or not TCs are present. Also, while the current SI and AI calculation methods smooth MSLP, it is possible that any TCs in the domain may still impact SI because they are associated with a broad area of low surface pressure.

To determine the near-term steering regime that could impact the presence and direction of North Atlantic TCs, AI and SI outlooks using several numerical weather prediction ensembles systems are presented on the with daily updates.

DISCLAIMER: The current versions of SI and AI shown herein are experimental and should be treated as such. Please seek information from the National Hurricane Center to help with any decision making.

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