A tropical wave over the Northeast Caribbean (Invest 97L) is slowly growing more organized as it moves west-northwest at 10 mph towards Puerto Rico and the Dominican Republic. Winds gusts of 36 mph have been recorded this morning at buoy 41052 south of St. John in the Virgin Islands, and heavy thunderstorms from 97L have dumped over an inch of rain in many locations in the Virgin Islands and Northern Puerto Rico as of 11 am AST Wednesday. Puerto Rico doesn't need the rain, as they have had one of their wettest years on record; San Juan is nearly two feet (23.49") above average in rainfall for the year. Satellite loops show that 97L has a moderate amount of spin and heavy thunderstorms, with a respectable upper-level outflow channel to the north. There is no sign of a well-organized surface circulation. Long-range radar out of San Juan, Puerto Rico shows some modest spin to 97L's echoes, but no well-organized low-level spiral bands. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots. There is some dry air from the Saharan Air Layer (SAL) that is interfering with development, but this is becoming less of an issue as heavy thunderstorms from 97L continue to moisten the storm's environment.
Figure 1. True-color MODIS satellite image of 97L taken at 1:30 pm EDT on September 4, 2013. Image credit: NASA.
Forecast for 97L
Wind shear is expected to be low through Friday night, and ocean temperatures will be warm, 29°C. One wild card is how 97L will interact with a strong tropical wave about 500 miles to its east, just east of the Lesser Antilles Islands. This new tropical wave may compete for moisture, slowing development of 97L, and could also modify the track of 97L. The models take 97L to the west-northwest, bringing the center over the eastern tip of the Dominican Republic on Thursday morning. This track will allow the high mountains of Puerto Rico and Hispaniola to disrupt the circulation of 97L, forcing the storm to regroup on Friday over the Southeast Bahamas and Turks and Caicos Islands. The best chance for development of 97L would appear to be on Saturday, after the storm has had time to recover from its encounter with Hispaniola. The UKMET model predicts that 97L will become a tropical depression just north of the Southeast Bahamas and Turks and Caicos Islands on Saturday, but the GFS and European models show little development over the next three days. There will be a strong trough of low pressure off the U.S. East Coast this weekend, and the models predict that this trough will be strong enough to turn 97L to the north and northeast by Sunday, keeping 97L well offshore from the U.S. East Coast, but with a possible threat to Bermuda next week. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC boosted the 5-day odds of formation of 97L to 60%, and 2-day odds to 40%. The Hurricane Hunters are on call to investigate 97L Wednesday afternoon. The Hurricane and Severe Storm Sentinel Mission, a 5-year field project utilizing two remotely crewed Global Hawk aircraft to overfly tropical storms and hurricanes, will have one of their aircraft investigate 97L today. You can follow the progress of the aircraft using NASA's live-plane tracker map (Global Hawk AV6 is tail number NASA872, and Global Hawk AV1 is tail number NASA871.)
Regardless of whether or not 97L becomes a tropical depression today, the major danger from this slow-moving storm will be heavy rains. Three to six inches of rain are predicted to fall over Puerto Rico and the Virgin Islands by Thursday morning, with 3-day rainfall totals of 5 - 10 inches expected along the south and southeast shores of Puerto Rico. These rains are capable of causing dangerous flash flooding and mudslides, and a Flash Flood Watch has been posted. Similar rainfall amounts will fall in the eastern Dominican Republic, and heavy rains of 3 - 6 " are also likely to affect the Southeast Bahamas and Turks and Caicos Islands later in the week. Since 97L is relatively small, Haiti may see lower rainfall amounts of 2 - 4 inches.
Elsewhere in the Atlantic
A tropical wave over Mexico's Yucatan Peninsula and adjacent waters of the Gulf of Mexico is causing scattered disorganized heavy thunderstorms. This activity will move over the extreme Southern Gulf of Mexico's Bay of Campeche on Wednesday and Thursday, then moving ashore on the Mexican coast between Veracruz and Tampico on Friday. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC put the 5-day and 2-day odds of development at 20%.
A strong tropical wave is predicted to emerge from the coast of Africa on Saturday, and the GFS model develops this wave into a tropical depression near the Cape Verdes Islands by Monday. The storm is expected to track to the northwest into a region of ocean where very few tropical cyclones ever make the long crossing of the Atlantic Ocean to threaten North America.