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.

Pressure versus wind (in knots) scatterplot for Atlantic and East Pacific storms between 1989 and 2004. Isaac is shown in red. (Knaff and Zehr (2007))

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.

Tropical storms with very low pressure from tropical storm database . MaxWind (the second the last column on the right) indicates maximum sustained winds in knots. MSLP (the last column on the right) indicates minimum sea level pressure. (Phil Klotzbach)

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.