Hurricane Irma strengthened to Category 4 on the Saffir-Simpson Scale on Monday and a Hurricane Watch was issued for Puerto Rico and the U.S. Virgin Islands.  At 5:00 p.m. EDT on Monday the center of Hurricane Irma was located at latitude 16.7°N and longitude 54.4°W which put it about 490 miles (790 km) east of the Leeward Islands.  Irma was moving toward the west at 13 m.p.h. (20 km/h).  The maximum sustained wind speed was 130 m.p.h. (215 km/h) and there were wind gusts to 160 m.p.h. (260 km/h).  The minimum surface pressure was 944 mb.

Hurricane Warnings are in effect for Antigua, Barbuda, Anguilla, Montserrat, St. Kitts, Nevis, Saba, St. Eustatius, St. Maarten, St. Martin, and St. Barthelemy.  Hurricane Watches are in effect for Guadeloupe, the British Virgin Islands, the U.S. Virgin Islands, Puerto Rico, Vieques and Culebra.  A Tropical Storm Warning is in effect for Guadeloupe.  A Tropical Storm Watch is in effect for Dominica.

The circulation of Hurricane Irma has been going through a series of eyewall replacement cycles.  A plane flying through Irma reported concentric eyewalls earlier today.  The two eyewalls appear to have merged.  A small circular eye is at the center of circulation.  A ring of strong thunderstorms surrounds the eye and the strongest winds are occurring in that ring of storms.  Additional bands of showers and thunderstorms are revolving around the core of Hurricane Irma.  Storms around the center are generating strong upper level divergence which is pumping away mass in all directions from the hurricane.

Hurricane Irma has been increasing in size.  Winds to hurricane force extend out about 40 miles (65 km) from the center of circulation.  Winds to tropical storm force extend out about 140 miles (225 km) from the center.  The Hurricane Intensity Index (HII) for Hurricane Irma is 25.1.  The Hurricane Size Index (HSI) is 13.6.  The Hurricane Wind Intensity Size index (HWISI) is 38.7.

Hurricane Irma will move through an environment very favorable for tropical cyclones.  Irma will move over water where the Sea Surface Temperature is near 30°C.  The upper level winds are weak and there is little vertical wind shear.  Hurricane Irma could strengthen further and it has a chance to reach Category 5 on the Saffir-Simpson Scale.  Hurricane Irma is likely to go through additional eyewall replacement cycles, which will cause fluctuations in its intensity, but Irma should remain a strong hurricane as it nears the northern Leeward Islands.

The subtropical high pressure system over the Atlantic Ocean has been steering Hurricane Irma toward the west and that general motion is expected to continue for another 12 to 24 hours.  After that time Irma is expected to turn more toward the west-northwest.  On its anticipated track Hurricane Irma will approach the northern Leeward Islands late on Tuesday.  Hurricane Irma could approach Puerto Rico on Wednesday.