|Above: The Saharan Air Layer (SAL) analysis from 8 am EDT Sunday, July 2, 2017, showed a large area of dry Saharan air in the tropical Atlantic north of about 13°N. A tropical wave worth watching was just to the south of the SAL. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.|
A tropical wave located at 12 pm EDT Sunday near 11°N 34°W, about 650 miles west-southwest of the Cabo Verde Islands, was headed west at about 10 mph. This system has the potential to develop into a tropical depression by the time it arrives in the Lesser Antilles Islands next weekend.
Satellite images on Sunday morning showed that the wave was in the early stages of organization, with a slowly increasing amount of heavy thunderstorm activity that had only a little spin. Development was being retarded by dry air, thanks to a presence of the Saharan Air Layer (SAL) just to the north. Wind shear was low to moderate, 5 – 15 knots, and sea surface temperatures (SSTs) were just warm enough for development, near 27°C (81°F)--about 1°F above the seasonal norm.
The main impediment for development is the presence of the dry air to the system’s north. Otherwise, conditions are favorable for development: wind shear should mostly be moderate, and SSTs will warm to 28.5°C (83°F) by Friday. The system does not have a high forward speed, giving it time this week to leverage Earth's spin to help itself get spinning. The Madden-Julian Oscillation (MJO), a periodic pulse of thunderstorm activity that circles equatorial regions of the globe every few weeks, is in a phase that will help development of Atlantic tropical cyclones this week. In their 8 am EDT Sunday Tropical Weather Outlook, the National Hurricane Center (NHC) gave the wave 2-day and 5-day odds of tropical cyclone development of 10% and 40%, respectively. The Florida State University Tropical Cyclone Genesis Portal--the top aid used by NHC to make genesis forecasts--gave 5-day odds of development of 44%, based on a consensus of the 0Z Sunday forecasts from the GFS, UKMET and Canadian models.
Our three top models for predicting tropical cyclone genesis--the European, GFS and UKMET--all showed some slow development of the system over the next five days, with the UKMET model clearly predicting development into a tropical depression. Over 90% of the 20 members of the 0Z Sunday GFS ensemble forecast predicted development over the next 7 days, with several of the forecasts predicting a hurricane by next week. The 0Z Sunday European model ensemble forecast was less aggressive, with about 40% of its members predicting development over the next 7 days.
The models had a strong ridge of high pressure steering the system west to west-northwest at 10 - 15 mph for the next five days, which would bring the storm to the Lesser Antilles Islands on Saturday. By next Sunday, a trough of low pressure expected to move off the U.S. East Coast may create a weakness in the ridge, allowing the storm to move on a more northwesterly path, potentially putting Bermuda, the Bahamas, and the U.S. East Coast at risk late next week. That is a long way in the future, though, and we’ll just have to wait and see if the wave will fight off the dry air and develop before we worry about long-range landfall threats. The next Atlantic “Invest” will be labeled 94L, and the next name of the list of storms is Don.