How strong could Irma get?
Several computer models have been confronting meteorologists with some eye-opening intensity forecasts for Hurricane Irma, especially for the period around next weekend, when Irma is currently predicted to be arcing northwest from the Eastern Bahamas. We cannot rule out the chance that Irma will reach Category 5 strength, but we can safely discount some of the most extreme model-generated intensities. The GFS global model and the new HMON regional hurricane model have consistently been deepening Irma to pressures below 900 millibars (mb). However, neither of these models fully incorporates the interaction between ocean and atmosphere that serves as a check on a hurricane's peak strength. A better guide to how strong Irma might get is the HWRF regional hurricane model, which extends out to 126 hours (just over five days). The HWRF has proven to be our most reliable model-based intensity guidance in recent years. The 0Z and 6Z Monday operational runs of HWRF deepened Irma to the 920 - 930 mb range, suggesting Irma will be a very formidable hurricane, but not a mind-blowing all-time record-setter.
For historical context, the lowest hurricane-related pressure ever measured at the surface north of the Caribbean and east of Florida is 921 mb in the Bahamas Hurricane of 1932. Hurricane Hunter dropsondes found a surface pressure of 919 mb within Hurricane Gloria (1985). Such pressures can support a Category 4 or 5 hurricane, but the peak winds depend on the size of the hurricane. As hurricanes move poleward, they typically get bigger. In a larger hurricane, the pressure force from a given central pressure will extend over a larger area, meaning that the top sustained winds will probably be lower but that a larger area could experience high winds and storm surge. This was the case with Hurricane/Superstorm Sandy, which brought record-low surface pressures and record-high surge across a large area despite winds of marginal hurricane force at best when Sandy made landfall.
