Current track brings this into our area Tuesday morning as a Cat 1 Hurricane....

Potential Tropical Cyclone Nine is expected to track westward into the Windward Islands Friday. It will still bring heavy rainfall and strong gusty winds to the Windward Islands Friday and Friday night. Interests on those islands should be prepared for localized flooding and perhaps very localized mudslides. Winds might be strong enough to cause some utility disruptions. The system will track westward probably north of the ABC Islands and remain north of South America. The system will move over very warm water and if the shear remains low enough, this system could become a strong tropical cyclone. Computer forecasts show it tracking westward across the southern Caribbean Saturday through early next week. The system could become a strong tropical cyclone before moving very close to and then over Central America next week.

Figure 1. Visible-wavelength GOES-16 satellite image of Potential Tropical Cyclone 9 as of 1515Z (11:15 am EDT) Thursday, August 17, 2017. GOES-16 data are preliminary and non-operational. The RAMMB site also has a "floater" loop zoomed in on PTC 9, offering vivid detail. Image credit: RAMMB/CIRA @ CSU.

Potential Tropical Cyclone 9 heading toward Caribbean

The NOAA/NWS National Hurricane Center launched advisories on Potential Tropical Cyclone 9 (formerly Invest 91) on Thursday morning. As we discussed back in June, the new PTC designation allows for more detailed guidance on systems that are not yet at depression strength but that have a chance of intensifying and bringing tropical storm or hurricane conditions to land areas within 48 hours. Potential tropical cyclones are assigned numbers as part of the same chronological list that includes tropical depressions.

At 11 am EDT Thursday, PTC 9 was located about 365 miles east of Barbados, moving west at about 17 mph. Our top computer models for track forecasting are in very strong agreement that PTC 9 will continue westward, taking it through the Lesser Antilles late Friday, across the eastern and central Caribbean over the weekend, and into the western Caribbean by Monday. There is no reason to doubt the model consensus, especially given that any track interaction between PTC 9 and the system to its east, Invest 92L, would tend to keep PTC 9 moving westward. The official NHC forecast makes PTC 9 a tropical storm by Friday morning. The next name on the Atlantic list is Harvey.

The ASCAT scatterometer has not made a pass directly over PTC 9 for the last few hours, so it has been unable to provide us with recent wind data on the system. A Hurricane Hunter flight will explore the area on Thursday afternoon, at which point we will see if PTC 9 has the closed low-level circulation needed to classify it as a tropical depression (or tropical storm).

Ahead of PTC 9, a tropical storm warning was in effect Thursday morning for Martinique, St. Lucia, Barbados, and St. Vincent and the Grenadines, and a tropical storm watch was in effect for Dominica.

Long-term outlook for PTC 9

PTC 9 has maintained a solid core of moderate to strong showers and thunderstorms (convection) since Wednesday night and has a modest amount of spin. Easterly vertical wind shear of around 15 knots has kept most of the convection focused on PTC 9s west side. This shear is predicted to continue until around Sunday, when it may drop below 10 knots. The wind shear is injecting dry air into PTC 9, with mid-level relative humidities averaging around 50%, but as the shear lessens, the environment around PTC 9 will be moistening from Sunday onward. Sea surface temperatures are more than warm enough for development along PTC 9s path, running at 28-29C (84-86F), or about 0.5C above average for this time of year.

If PTC 9 can keep the ill effects of wind shear and dry air at bay, it should be able to at least hold its own while moving across the eastern Caribbean, a region often hostile to tropical development because of predominant sinking air and strong trade winds. However, ensemble forecasts from Wednesday night suggest that PTC 9 may not make it through the central Caribbean. More than 60% of the European model ensemble members from 0Z Thursday, and more than 90% of the GFS members, bring PTC 9 to tropical storm strength, but the majority of both ensembles call for PTC 9 to weaken or dissipate Monday in the central Caribbean. Of our top three track models, only the UKMET takes PTC 9 into the western Caribbean as a robust tropical cyclone.

Assuming that PTC 9 makes it to the moist, low-shear environment of the western Caribbean, there will be a higher chance of more rapid strengthening early next week. The system will encounter higher oceanic heat content as it continues west, adding further credence to the potential for strengthening once wind shear decreases. The official NHC outlook issued at 11 am EDT Thursday brings PTC 9 to minimal hurricane strength by Monday morning into Tuesday. It is certainly possible that PTC 9 will become stronger than that if it survives its weekend trek.

At this point, it is too soon to know which land areas might be affected by PTC 9 next week. Only a slight shift in the overall westward track would make the difference between PTC 9 striking Nicaragua or the Yucatan Peninsula.

Category 6 Weather

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