The 8 a.m. update on tropical storm Isaac from the National Hurricane Center headlines it as “nearly a hurricane.” In fact, it’s about as close to being a hurricane as it can get without actually being a hurricane. It probably will be one by 11 a.m.
Its minimum pressure is 976 mb which is unusually low for a tropical storm. Characteristically, a storm with a pressure this low has significantly stronger winds.
“This does seem like an amazing outlier in terms of the pressure-wind relationship,” our tropical weather expert Brian McNoldy said in an email.
Usually, a storm with Isaac’s pressure would have maximum sustained winds closer to 80 knots or 90 mph.
Phil Klotzbach, a tropical weather researcher at Colorado State University, noted just a handful of tropical storms have had lower pressures than Isaac. The lowest pressure he could find for an Atlantic tropical storm was the 952 mb reading associated with Hilda in 1952 (which had maximum sustained winds around 70 mph).
Below is a list of other tropical storms with very low pressure Klotzbach compiled.
Why are Isaac’s winds so low relative to its pressure? Simply put, it’s a large storm whose wind field is spread out and has not yet consolidated. There are indication it’s trying to consolidate, hence some intensification of maximum winds is likely before landfall tonight.