Tropical Storm Erin to become major hurricane within days; pivot away from U.S. more certain

Chicago Tribune

2 days ago

Tropical Storm Erin is forecast to become a major Category 3 hurricane this weekend as it approaches the Caribbean, but long-range models are increasingly confident that the storm will turn north before the U.S. East Coast.
Still, since some outlier forecast models show the storm tracking farther south and west, everyone along the U.S. coast, from Florida to New England, should continue to monitor the storm's track, the National Hurricane Center said.
Erin's forecast path will take it close to some island in the Caribbean. 'Erin could move close enough to the northern Leeward Islands, Virgin Islands, and Puerto Rico over the weekend to produce some impacts on those islands. However, the magnitude of those impacts is still not known,' the NHC said.
The National Hurricane Center's cone of uncertainty is just north of the Dominican Republic and Puerto Rico as of 11 a.m. Tuesday. Tropical-storm-force winds could arrive at the island as early as 8 p.m. Saturday. High winds could spur power outages.
As of 11 a.m. Tuesday, Erin was still in the east-central Atlantic, moving west at 17 mph with sustained winds of 45 mph. Erin is encountering dry air and mild sea-surface temperatures, which will 'keep the small vortex in check,' the hurricane center said.
But warmer waters will eventually fuel strengthening, and Erin is forecast to become a hurricane by Friday and a major hurricane — Category 3 with sustained winds of at least 115 mph — by late Saturday, just north of Puerto Rico.
Forecasters also said there is 'even greater uncertainty' regarding impacts to the east coast of the United States, and Bermuda next week.
'Bermuda could be soaked with 2 to 4 inches of rain and wind gusts of 60 to 80 mph when Erin passes by the island next week,' said AccuWeather Lead Hurricane Expert Alex DaSilva.
Erin would be the first Atlantic hurricane of the season and is the fourth tropical storm. The average date for the first hurricane is Aug. 11.
There's little doubt that Erin will strengthen. The question is: Will the storm steer into portions of the U.S. coast, or will the path of the storm take it out to sea?
Three forces will interact to determine Erin's track.
A high-pressure system to the storm's north, called the Bermuda High, is currently keeping the storm on a westerly track, toward the U.S., said Bryan Norcross, hurricane specialist at Fox Weather.
High-pressure systems block and thus steer hurricanes as they cross the Atlantic. But a dip in the jet stream, which will bring in a low-pressure gap from the continental U.S., is expected to soften the Bermuda High's western edge, allowing Erin to turn north, away from the U.S. East Coast. If the pressure door opens, the storm moves into it.
A sharp north turn would keep the system away from the Southeast coast of the U.S.
It's unclear when that turn would occur, or how sharp it would be, said Norcross, but he said the dip in the jet stream will create that gap this coming weekend.
'Will the Bermuda High stick to the west far enough to push the storm farther west? Are the front and Jet Stream going to be weaker than forecast?' He said it's unclear.
A subset of scenarios show a threat to the east coast, said Norcross. But even without a direct strike, he thinks the coast will still feel marine effects.
Forecasters at AccuWeather agree. 'Erin will churn up rough surf and dangerous rip currents along the Atlantic coastline from Florida all the way north to Maine starting this weekend into next week,' said AccuWeather lead hurricane expert Alex DaSilva on Tuesday. 'Beach erosion is possible in some areas. A major hurricane in the open Atlantic can produce very rough surf that will impact beaches hundreds of miles away. Hazardous beach conditions are likely across parts of the Caribbean, Bahamas and Bermuda.'
Norcross also said a storm that strengthens quickly would more likely cut north, because it's taller. 'A weak storm doesn't feel the gap in the steering flow like a strong storm. All that steering (from the Bermuda High) happens aloft, say 15,000 to 20,000 feet in the atmosphere. But if it's a weak storm, it's more or less caught in trade winds, which come much farther west.'
If the storm stays weak, then strengthens near the Bahamas, it could affect the U.S. East Coast. He said South Florida impact is the least likely scenario.
As of Tuesday, most forecast tracks — known as the 'spaghetti models' — show the turn coming well before reaching the U.S. coast. But many of those tracks have moved that turn a bit closer to the U.S. than previous runs.
Forecasters warned that long-range forecast tracks commonly swing back and forth, and Erin's forecast path is likely to continue shifting.
The current forecasting models show the turn bringing Erin close to Bermuda.
From Erin's location early this week, storms take an average of 10 to 12 days to reach the U.S. coast or the Gulf.
'History tells us that activity picks up in the middle of August, so it would be unusual if that didn't happen,' Norcross said. 'The overall atmospheric pattern looks like it will be generally conducive to storm development for the next couple of weeks.'
A new system of disorganized showers and thunderstorms emerged Tuesday night in the western Gulf near Honduras and Nicaragua. It is expected to move west-northwest across the Yucatan Peninsula on Wednesday. It has a 20% chance of developing in the next seven days, the hurricane center said.
One other system in the northwestern Atlantic a few hundred miles off the coast of Nova Scotia has a near 0% chance of developing into a tropical system, as of 8 a.m. Tuesday, according to the hurricane center. Its move north into cooler water is ending chances for development.