Heading into the peak of the Atlantic hurricane season over the next 30 days, an uptick in activity is transpiring with Tropical Storm Dorian in the Caribbean and Tropical Depression #6 off the Eastern Seaboard of the United States. Along with Chantal from last week, this recent activity takes place after seasonal hurricane outlooks for the Atlantic Basin were increased in early August. Current storms and the increased seasonal outlook are discussed below.
Tropical Storm Dorian
According to advisories of the National Hurricane Center (NHC), Dorian is crossing the central Lesser Antilles as a strong tropical storm Tuesday morning and should pass Hispaniola and Puerto Rico as a strong tropical storm or possibly a category-1 hurricane on the Saffir-Simpson Scale.
Status – 5AM EDT (09 UTC) Today (NHC)
- Location: about 30 miles south-east of St. Lucia
- Maximum sustained winds: 50 mph
- Motion: west-northwest at 13 mph
- Minimum central pressure: 1005 mb
- Extent of tropical storm force winds from center: 45 miles
- Status: Strong tropical storm (strengthening)
Dorian should cross near or over Hispaniola/Puerto Rico on Thursday, most likely as a strong tropical storm or perhaps a category-1 hurricane per the NHC. Thereafter, large amounts of uncertainty exist with the future of Dorian, discussed further below. If landfall were to ultimately occur across the mainland United States, the earliest timing would be Saturday for Florida or Sunday/Monday for the Southeast United States given current model guidance. However, any ramifications for the lower 48 are highly speculative at this early juncture, as confidence is exceedingly low on longer term impacts until Dorian clears the Caribbean.
Tropical Storm Dorian as of 0535Z, Tuesday August 27, traversing the Lesser Antilles. Source: tropicaltidbits.com
While Dorian is expected by the NHC to almost reach category-1 strength on approach to Hispaniola and Puerto Rico Thursday, several factors are prohibiting more rapid intensification. Dorian is small, with tropical storm winds extending only 45 miles from center. Smaller sized storms such as Dorian are more difficult to forecast and subject to greater influence from changes in the atmosphere surrounding the storm. One of these factors is significantly dry air at mid-levels, which is impeding rapid, strong and consistent thunderstorm growth. Sea-surface temperatures are also running slightly below average across the Caribbean along the forecast track of Dorian and into the Bahamas region. Finally, if Dorian crosses directly over the mountainous regions of Hispaniola and/or Puerto Rico, it is possible the storm will decay due to the terrain and struggle to regain status. Yesterday, model guidance beyond the Caribbean with Dorian covers the entire mainland United States coastline. Overnight, model guidance is starting to converge towards Florida or the Southeastern United States. However, any potential mainland United States landfall is 5-6 days out, thus it’s too early to determine the long-term track or impacts with any confidence.
Sea-surface temperatures for the region of interest (August 26), running normal to slightly below normal across much of the Caribbean and adjoining waters of the Bahamas. Source: tropicaltidbits.com
NHC Position and Best Forecast. Source: NOAA/NHC.
Tropical Depression #6
In the last 24 hours, TD #6 formed off the coast of the Southeastern United States. The NHC forecast calls for this system to be named Tropical Storm Erin either Tuesday or Wednesday, thereafter tracking north towards the provinces of Nova Scotia and Newfoundland Canada . There are no weather models forecasting winds of hurricane strength at this time across Atlantic Canada. In contrast to Dorian, Erin is forecast to track across well above average sea surface temperatures, which should help maintain tropical storm strength through any interaction with Canada.
Status – 5AM EDT (09 UTC) (NHC)
- Location: About 365 miles southeast of Cape Hatteras, NC
- Maximum sustained winds: 35 miles per hour
- Motion: southwest at 2 miles per hour
- Minimum central pressure: 1010 mb
- Status: Tropical Depression (Potential Strengthening in next 48 hours)
Canadian weather model forecast for Erin on approach to Nova Scotia. 2 PM EDT Thursday, August 29. Source: tropicaltidbits.com
NHC Position and Best Forecast. Source: NOAA/NHC.
Seasonal forecast update
The recent increase in activity was consistent with the latest round of seasonal hurricane outlooks issued in early August, which slightly increased seasonal forecast expectations. Notably, Colorado State University seasonal forecaster Dr. Phil Klotzbach is now projecting a normal to slightly above average Atlantic season with 7 hurricanes and 2 major hurricanes. Meanwhile, NOAA has increased their seasonal outlook expectations to 5-9 hurricanes and 2-4 major hurricanes. The El Niño event of early 2019 has faded, along with the resulting winds that tend to suppress hurricane activity in the Atlantic Basin. Expectations from the NOAA Climate Prediction Center (CPC) are for ENSO-neutral conditions to persist through the coming winter. Tropical ocean temperatures in the equatorial Pacific have even been cooling towards weak La Niña levels in recent days and will be closely monitored for further implications to the second half of the hurricane season. La Niña periods are typically associated with increased hurricane activity in the Atlantic Basin. As discussed in earlier posts, hurricane landfalls closely depend on weather patterns and steering currents during the hurricane lifecycle, and are not reliably predictable over long timescales.
90-day trend of El Niño / La Niña conditions in the equatorial Pacific. Source: tropicaltidbits.com
Seasonal Tropical Outlooks from NOAA and CSU – Early August. Source: NOAA/CPC, CSU Tropical Meteorology Project
The next Sentinel update on the Atlantic Basin is scheduled for Thursday, August 29, unless major changes or escalating impacts become apparent with the forecast. For immediate updates from government agencies, please use the sources below.
National Weather Service, San Juan, Puerto Rico
Official watches and warnings, and statements from emergency management agencies supersede this update, and should be closely followed concerning matters of personal safety.
