Hurricane Earl was upgraded to a category 4 Monday night, but forecasters are still unsure what kind of impact the storm will have on New England.
According to the National Weather Service, Earl's maximum sustained winds surged to 135 m.p.h. on Monday as it was about 100 miles north-north east of San Juan, Puerto Rico. Kevin Cadima, a meteorologist at the National Weather Service in Taunton, said the storm is moving north-northwest at 14 m.p.h. and is expected to be somewhere west of the Bahamas by early Tuesday.
A hurricane is considered to be a category 4 when it has maximum sustained winds between 131 and 155 m.p.h.
If Earl stays on its current course, by Friday it is expected to be at about 150 miles southeast of Nantucket, Cadima said, but he cautioned that the projected track of the hurricane can vary widely and said much could change between now and then.
"This is still four days out," he said Monday night. "It could be further east or further west, [but] the worst case scenario, obviously, is if it goes further west of the current track."
Meteorologists should have a more definitive idea of the storm's impact at about 48 hours prior to its arrival in the region, Cadima said.
"Right now, itís looking like itíll give us a brush,Ē said Charlie Foley, another meteorologist with the National Weather Service.
Besides the looming threat of Earl, Massachusetts is currently in the midst of what could end up being a five-day heat wave, meteorologists said.
On Monday, the temperature crested at 92 at 2:23 p.m. at Logan International Airport; similar numbers were logged elsewhere in Eastern Massachusetts. And more 90-degree heat was expected for Tuesday, which would be the third day in a row of such high temperatures, according to the National Weather Service.
Three consecutive days of 90 or more is enough for an official heat wave. But the heat is expected to persist two more days, through Thursday, with increasing humidity making things even more uncomfortable beginning on Tuesday.
On Friday, clouds may arrive and temperatures may drop as Earl passes offshore, Foley said.
If the storm stays on its current track, the Cape and the Islands stand to see the most adverse weather in New England from Earl, with a lot of heavy rain and strong winds expected, Cadima said.
The rest of Eastern Massachusetts will experience heavy rainfall but probably not strong winds, Cadima said, if the hurricane stays on course.
"It's important to note the margarin of error can be 200 miles in either direction when we're four days out," Cadima said. "We'll see what happens over the next few days."
In the coming days, Cadima advises everyone, especially those on the Cape and Islands, to closely monitor all advisories that come out of the weather service's National Hurricane Center.
Cadima said the last landfalling hurricane to reach New England was Hurricane Bob in 1991.
Globe Correspondent L. Finch contributed to this report.
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