Dr. Jeff Masters ·

Above: MODIS view of Potential Tropical Cyclone 15 (later to become TD 15) on Saturday morning, September 16, 2017. Image credit: NASA.

Tropical Depression Fifteen formed on Saturday afternoon in the waters east of the Lesser Antilles Islands, and tropical storm watches were hoisted for the islands of St. Lucia, Martinique, Guadeloupe, Barbados, St. Vincent and the Grenadines, and Dominica. TD 15 has the potential to be a hurricane by the time it passes through the Lesser Antilles Islands on Tuesday morning.

TD 15 had very favorable conditions for development on Saturday afternoon. Wind shear was light, 5 - 10 knots, SSTs were a very warm 29 – 29.5°C (84 - 85°F), and relative humidity at mid-levels of the atmosphere (as analyzed by the 12Z Saturday run of the SHIPS model) was moderately moist--about 60%. Satellite loops showed that TD 15 had a good deal of spin, and heavy thunderstorm activity was steadily increasing and growing more organized, with several curved low-level spiral bands forming. The first hurricane hunter aircraft is scheduled to investigate TD 15 on Sunday afternoon.

Figure 1. The 20 track forecasts for TD 15 (formerly Invest 96L) from the 0Z Saturday, September 16, 2017 GFS model ensemble forecast. Image credit: CFAN.

Figure 2. The 50 track forecasts for TD 15 (formerly Invest 96L) from the 0Z Saturday, September 16, 2017 European model ensemble forecast. The operational European model for TD 15 is the red line, adjusted by CFAN using a proprietary technique that accounts for storm movement since 0Z Saturday. The track of the average of the 50 members of the European model ensemble is the heavy black line. Image credit: CFAN.

Track forecast for TD 15

The model forecasts on Saturday morning were in better agreement than on Friday, predicting that TD 15 would be steered west-northwest at speeds ranging from 10 – 20 mph through Thursday, bringing the storm through the Lesser Antilles Islands on Tuesday morning, to the vicinity of the Virgin Islands and Puerto Rico by Wednesday morning, and near the Dominican Republic by Thursday morning. Beyond Thursday, the path of TD 15 will depend, in part, upon what Jose is up to. If Jose is still wandering off the U.S. New England coast, this will create a weakness in the ridge of high pressure steering TD 15 to the west-northwest, and TD 15 will turn more toward the northwest or north-northwest. If Jose is out of the picture, the ridge could keep TD 15 rolling west-northwestward. In either case, it is possible that TD 15 will approach the U.S. East Coast more than a week from now. A strong autumn-like upper-level trough will be moving across the western and central U.S. late in the week. This will tend to pump up a ridge toward the northeast U.S. and northwest Atlantic, which could end up blocking TD 15 from moving out to sea.

Intensity forecast for TD 15

The 12Z Saturday run of the SHIPS model predicted that TD 15 would have very favorable conditions for development over the next five days. Wind shear will be low, TD 15 will have very warm SSTs near 29.5°C (85°F), and there will be moist conditions at mid-levels of the atmosphere. These conditions should allow for strengthening of TD 15. On Saturday morning, our top intensity models—the HWRF, COAMPS-TC, DSHIPS, LGEM, and HMON—all predicted steady intensification of TD 15 into a hurricane by Tuesday, when the storm is expected to be moving through the Lesser Antilles. Further strengthening is likely through Thursday, until TD 15 potentially gets disrupted by passage over Puerto Rico and/or Hispaniola. Note that the official NHC forecast of a Category 1 hurricane on Tuesday and Wednesday is conservative; the 12Z Saturday runs of the HWRF and HMON models show TD 15 attaining Category 3 hurricane status by Tuesday afternoon. That may be too aggressive, but I would not be surprised to see TD 15 become a major hurricane by Wednesday, given how Hurricane Irma exploded into a major hurricane under similar conditions and in a similar location. The next name on the list of Atlantic storms is Maria.

Figure 3. GOES-16 view of Hurricane Jose at 11 am EDT September 16, 2017. Image credit: NOAA/RAMMB. GOES-16 data is considered preliminary and non-operational.

Hurricane Jose headed towards New England

Hurricane Jose was headed northwest at 9 mph towards North Carolina on Saturday morning, but is expected to turn to the north by Saturday night and pass several hundred miles east of the North Carolina coast on Monday. Jose's tropical storm-force winds cover a large area of ocean, extending out 150 miles to the northeast of the center. This wind field is expected to expand to 160 miles to the northeast of Jose’s center on Monday, then 230 miles on Tuesday. These winds are generating large swells that are bringing rough surf and the risk of rip currents to Bermuda, The Bahamas, the northern coasts of Hispaniola and Puerto Rico, and the U.S. east coast. Jose may bring up to 1” of rain to North Carolina’s Outer Banks on Monday, and heavier rains of 2 – 4” to Eastern Massachusetts beginning on Tuesday. The U.S. coast from Long Island, New York to Maine is at the edge of Jose’s 5-day cone of uncertainty.

Satellite images on Saturday morning showed that Jose had plenty of intense heavy thunderstorm activity that was well-organized, with a hint of an eye. The storm was less developed on its northwest side, where strong upper-level winds out of the west-northwest were tearing into the hurricane. An Air Force hurricane hunter aircraft will investigate Jose on Saturday afternoon.

Track forecast for Jose

A high-pressure system to the northeast of Jose will steer the storm mostly to the north over the remainder of the weekend, putting Jose a few hundred miles east of North Carolina’s Outer Banks on Monday. In their 11 am Saturday Wind Probability Forecast, NHC gave Cape Hatteras, North Carolina a 28% chance of seeing tropical storm-force winds from Jose. Jose is then expected to turn to the east-northeast and pass close to the coast of southeast Massachusetts on Wednesday. NHC gave Nantucket, Massachusetts a 50% chance of seeing tropical storm-force winds from Jose by Thursday morning. The large waves from the storm will be capable of causing high surf and considerable beach erosion along the shores of the mid-Atlantic and New England coasts during this period. Jose’s fate beyond Wednesday is coming into better focus, with the Saturday morning runs of the GFS and European models predicting that a trough of low pressure passing to Jose’s north will continue to pull Jose to the east-northeast, out to sea. However, it is still possible that the trough of low pressure passing to Jose’s north will not be strong enough to do this, leaving Jose wandering in an area of weak steering currents a few hundred miles southeast of Massachusetts late in the week. This was the solution of the 0Z Saturday run of the UKMET model.

Figure 4. The 20 track forecasts for Jose from the 0Z Saturday, September 16, 2017 GFS model ensemble forecast. Jose is a potential landfall threat along the coast from Long Island, New York to Southeast Massachusetts, according to these forecasts. Image credit: CFAN.

Figure 5. The 0Z Saturday September 16, 2017, track forecast by the operational European model for Jose (red line, adjusted by CFAN using a proprietary technique that accounts for storm movement since 0Z Saturday), along with the track of the average of the 50 members of the European model ensemble (heavy black line), and the track forecasts from all 50 members of the ensemble. The European model ensembles show less of a threat to New England than the GFS model ensembles. Image credit: CFAN.

Intensity forecast for Jose

Jose was under high wind shear of 20 – 25 knots Saturday morning. But with sea surface temperatures (SSTs) near 29.5°C (85°F), Jose should be able to maintain hurricane status though Sunday morning. Higher wind shear of 30 – 35 knots is expected to affect Jose on Sunday and Monday, which should cause weakening, perhaps to a strong tropical storm with 70 mph winds, by Wednesday. Wind shear will likely fall to the moderate range again on Tuesday and Wednesday, but the waters beneath Jose will be steadily cooler as the storm heads north, discouraging any strengthening. SSTs are about 28°C (82°F) a few hundred miles off the coast of North Carolina, where Jose will be on Monday. Once Jose moves north of about 37°N (the latitude of Washington D.C.), the storm will be beyond the warm waters of the Gulf Stream, making it difficult for Jose to intensify. Water temperatures within about 400 miles of the coast, from New Jersey to Maine, are generally below 26°C (79°F). Most of the intensity models show Jose as a strong tropical storm with 60 – 70 mph winds throughout its closest approach to the New England coast, Tuesday and Wednesday.

Figure 6. Visible image of Lee at 10:45 am EDT Saturday, September 16, 2017. Image credit: NOAA/NESDIS.

TD 14 develops into Tropical Storm Lee, but not expected to be a threat

TD 14 developed into Tropical Storm Lee at 11 am EDT Saturday, becoming the 12th named storm of this ferociously active Atlantic hurricane season. We are just 5 days past the climatological mid-point of the Atlantic hurricane season, and have already had a full season’s worth of activity: 12 named storms, 6 hurricanes, and 3 intense hurricanes. An average season has 11 named storms, 6 hurricanes, and 2 intense hurricanes by the end of November.

Lee is expected to continue rolling westward at about 12 mph on Saturday, with a gradual turn toward the west-northwest by Monday. Our top track models agree that an upper-level trough should pull Lee northwestward long before it has a chance to reach the Lesser Antilles, and high wind shear from strong upper-level winds associated with this trough should be able to destroy Lee by Thursday. Lee is not expected to be a threat to any land areas.

Bob Henson contributed to this post.