Where Will Tropical Storm Henri Land? It’s Tough To Tell.

Adam Sobel is a professor and director of the Initiative on Extreme Weather and Climate at Columbia University. He is an atmospheric scientist and author of “Storm Surge,” a book about Superstorm Sandy.

There is a good chance that Tropical Storm Henri will make landfall as a hurricane in Massachusetts, Rhode Island or Connecticut in the coming days, which would make it the first hurricane landfall in New England since Hurricane Bob in 1991.

Landfall is expected on Sunday or Monday, but the forecast has been highly uncertain.

Over the past day or so, Henri has been kept from intensifying by substantial vertical shear in the Atlantic Ocean — different winds at different altitudes are keeping it from standing upright. But the exact degree of this suppressing effect has varied between models, affecting the forecast.

Models that project Henri to strengthen more quickly have pushed it closer to the Atlantic Coast in the near term, predicting a stronger hurricane that makes landfall further west, possibly even in New Jersey or New York City. Models that expect Henri to remain weaker project it to make landfall in New England, or even remain offshore until reaching Canada.

A weaker, more tilted storm would be mostly steered by the large-scale winds in the lower atmosphere, which have been blowing from south to north. A stronger, more vertically stacked storm would feel the upper atmospheric winds to a greater degree, and those have been blowing from east to west.

Before Henri makes landfall, wherever that is, relaxing of the shear will probably allow the storm to strengthen into a hurricane over the warm subtropical Atlantic waters that have been made a bit warmer by climate change. The National Hurricane Center’s intensity forecast never projects the storm to grow beyond a Category 1 hurricane.

As recent aircraft reconnaissance flights have pinned down Henri’s intensity and structure, the models have begun to agree a bit more, making landfall in either New England or Long Island look most likely. But some uncertainty remains as Henri stalls and drifts in weak steering winds.

A hurricane watch was in effect for parts of Long Island, Connecticut and Massachusetts including, Nantucket and Martha’s Vineyard, the center said on Friday morning. A tropical storm watch was also in effect for areas west of Fire Island, N.Y., west of Port Jefferson Harbor, N.Y. and west of New Haven, Conn.

Henri is expected to dump up to five inches of rain over New England on Sunday and Monday, with isolated totals near eight inches. Heavy rainfall across the area could bring some flooding. Some coastal areas could see storm surge as high as five feet.

A left turn — vaguely Sandy-like, but less dramatic — is possible, with a landfall point that would probably still be on Long Island. Landfall in New Jersey or New York City now appears unlikely, but it is not entirely out of the question. Everyone near the coast from New Jersey to Maine should be keeping a close eye on the forecast.

Derrick Bryson Taylor contributed reporting.