Low-energy Tropical Storm Don collapsed near the Windward Islands on Tuesday after barely 24 hours as a weak tropical storm. The NOAA/NWS National Hurricane Center reclassified Don as an open wave at 11 pm EDT Tuesday after Air Force Hurricane Hunters were unable to find westerly winds at the surface, implying that Don no longer had a closed center of circulation.

Don passed well to the southeast of Barbados and just south of Grenada during its final hours. A dropsonde launched from a Hurricane Hunter flight on Monday measured winds of 70 mph at the 902-millibar level (about 3000 feet above sea level). Here are the top winds reported at ground level on Tuesday:

Barbados: sustained winds of 33 mph at 2 pm AST (EDT)
Grenada: sustained winds of 23 mph at 12 pm AST
Tobago: sustained winds of 14 mph gusting to 32 mph at 9am AST
St. Lucia:  sustained winds of 29 mph at 4 pm AST; 40 mph gust at 6 pm
Martinique:  sustained winds of 25 mph, gusting to 37 mph at 3:30 pm AST

Heavy rains south of Don—attributed to local effects by the Trinidad & Tobago Meteorological Service—produced widespread reports of flash flooding and several landslides on Tuesday, according to the Trinidad Express.

First Bret, then Don:
Likely harbingers of an active Atlantic hurricane season

As we discussed on Tuesday, the most dangerous hurricanes are the ones that get their start from tropical waves traversing Main Development Region (MDR), which includes the waters from the coast of Africa to Central America between 10° – 20°N, including the Caribbean Sea. Tropical waves that traverse the MDR are responsible for 85% of all major hurricanes (Category 3 and stronger). When hurricanes and tropical storms form in the MDR during June and July, it usually portends an active hurricane season, since it shows that atmospheric and oceanic conditions are primed to assist development of tropical waves that will come off the coast of Africa during the peak mid-August through mid-October portion of hurricane season. We’ve now had two tropical storms form in the tropical Atlantic before August 1: Bret and Don. This early season low-latitude activity is likely a harbinger of a more-active-than-usual Atlantic hurricane season.

TS Bret in June and TS Don of July were at tropical storm strength for 1.25 days each, bringing the total number of MDR Atlantic named storm days for June – July 2017 thus far to 2.5. If we look at the history of MDR named storm days before August 1 in the Atlantic, we see that 2017 now ranks in the top 20 seasons for this statistic, according to a spreadsheet provided by Colorado State University’s Dr. Phil Klotzbach (where he defines the MDR as south of 23.5°N and east of 75°W). Only a few of these top-20 MDR named storm day seasons (1861, 1867, and 2013) were quiet, when measured by Accumulated Cyclone Energy (ACE). It should be a wake-up call that some of the most active seasons on record appear on this list:

Year	June-July NSD (South of 23.5N, East of 75W)	Post 1-August ACE
1933	9.75	209.9
1901	7.5	77.7
2005	6.75	182.0
2013	6.75	28.7
2008	6	107.4
1996	5.75	144.0
1887	5.25	152.4
1966	5.25	111.8
1926	4.75	207.2
1916	4.5	103.8
1969	4.5	161.7
1944	4	93.1
1960	3.75	62.3
1861	3.25	37.0
1995	3	212.9
1998	3	179.9
1867	2.75	48.2
2017	2.5 (through July 19)	TBD
1990	2.75	88.8
1917	2.25	59.1
1961	2.25	191.1

Atlantic tropical wave 96L little threat

A tropical wave (Invest 96L) located at 8 am EDT Wednesday in the middle tropical Atlantic, about 1000 miles west-southwest of the Cabo Verde Islands, was headed west-northwest to northwest at 10 – 15 mph. This wave has a marginal potential to develop into a tropical depression late this week.

Satellite images on Wednesday morning showed that the wave was disorganized, with little spin and only a modest amount of heavy thunderstorm activity. Development was being retarded by dry air, thanks to a presence of the Saharan Air Layer (SAL) just to the north.Wind shear was moderate, around 15 knots, and sea surface temperatures (SSTs) were warm enough for development, near 27°C (81°F).

There is limited model support for development of this wave; approximately 30% of the 70 members of the 0Z Wednesday European and GFS model forecasts predicted that the wave would develop into a tropical depression later this week. However, none of these forecasts showed the storm surviving long, as the system is expected to enter a region with high wind shear of 20 – 30 knots Friday through Sunday. In their 8 am EDT Wednesday Tropical Weather Outlook, the National Hurricane Center (NHC) gave the wave 2-day and 5-day odds of tropical cyclone development of 30%. The next name on the Atlantic list is Emily.

Figure 1. The East Pacific tropics were packed with activity in this GOES-16 blended satellite image from 1500Z (11:00 am EDT) Wednesday, July 19, 2017. GOES-16 data are preliminary and non-operational. Image credit:RAMMB/CIRA at Colorado State University.

East Pacific continues to bubble: Fernanda, Greg, and TD 8E

Hurricane Fernanda is taking its time weakening in the remote East Pacific. After having peaked as a Category 4 storm, Fernanda is now a Category 1, with top sustained winds of 85 mph as of 11 am EDT Wednesday. Located about 1300 miles east of Hilo, Hawaii, Fernanda is moving west through an environment that’s just unfavorable enough to keep the slow decay going: sea surface temperatures about 0.5°C below the 26°C (79°F) benchmark for development, plus moderate wind shear of 10-15 knots. By Tuesday, southwesterly wind shear will be considerably higher and more dry air will be infiltrating. NHC projects Fernanda to weaken below hurricane strength by Thursday and to become post-tropical by Saturday, passing well north of Hawaii as a remnant low. Part of a midlatitude trough will be cutting off northwest of Hawaii over the weekend, adding a bit of intrigue to Fernanda’s long-term future. NHC notes that the upper low could allow Fernanda to maintain its identity as a tropical cyclone longer than expected. Should this happen, Fernanda would likely stay well north of Hawaii, whereas a faster decay would bring Fernanda’s remnants a bit closer to the islands.

Tropical Storm Greg has a good shot at becoming the East Pacific’s next hurricane as it follows in Fernanda’s footsteps well offshore. Greg’s top sustained winds were only 45 mph at 11 am EDT, but conditions will be very supportive of strengthening over the next day or two. The 12Z Wednesday SHIPS model output shows low wind shear over Greg, with shear predicted to remain below 10 knots into Friday. Greg is in a very moist environment (mid-level relative humidities around 75%) and is traversing quite warm water (SSTs around 27-28°C). If Greg’s large area of showers and thunderstorms (convection) can become more organized and consolidate around its center of circulation, Greg could intensify fairly quickly, though models suggest its growth should be more gradual.

In between Greg and Fernanda, Tropical Depression 8E has been struggling, with top winds still at 35 mph at 11 am EDT Wednesday. The depression is so disorganized that satellite data could not confirm the location of TD 8E’s center amid a batch of disorganized convection. Southwesterly wind shear of 20-25 knots has plagued TD 8E, and the shear will be slow to abate over the next day or so. It’s possible 8E will intensify just enough to become a named storm tonight or Thursday, but after that point it will likely be overwhelmed by the influence of nearby Tropical Storm Greg.

What’s with Don and Hilary?

The next name on the Pacific list is Hilary—but thanks to Don’s demise, there will be no simultaneous tracking of Don in the Atlantic and Hilary in the East Pacific in 2017. That’s probably a good thing, since there were folks on social media wondering whether the names had been chosen with politics in mind.

As it happens, Hilary (spelled with one L) is one of the most venerable storm names in the East Pacific. The first Tropical Depression Hilary was in 1967, and since then the name has been applied to six hurricanes (1981, 1987, 1993, 1999, 2005, and 2011) and one tropical storm (1975). Don is a more recent arrival, approved by the World Meteorological Organization when the name Dennis was retired following Category 4 Hurricane Dennis in 2005. The only Tropical Storm Don prior to this year was in 2011. Unless the eventual Hilary of 2017 is high-impact enough to be retired (very unlikely in the East Pacific, where many hurricanes stay offshore), we will see both Hilary and Don reappearing in 2023, when the current lists will be reused.

As Marshall Shepherd put it on Tuesday in a Forbes explainer on hurricane naming, “Perhaps the tropics have a sense of humor or are trying to tell everyone to lighten up.”

Dr. Jeff Masters co-wrote this post.