Tropical Storm Don Approaching the Windward Islands

July 18, 2017, 12:34 PM EDT

Tropical storm warnings were in effect for Grenada and for St. Vincent and the Grenadines at midday Tuesday as Tropical Storm Don chugged westward toward the Windward Islands. A Hurricane Hunter aircraft found a closed circulation and tropical-storm-strength winds in a western Atlantic wave on Monday afternoon, which led to Don’s immediate christening.

As of 11 am EDT Tuesday, Don was located about 155 miles southeast of Barbados and about 255 miles east of Grenada, moving nearly due west at 20 mph. Don’s top sustained winds were at minimal tropical-storm strength of 40 mph, down from 50 mph overnight. Relatively dry air (mid-level humidities of 55-60%) is not helping Don, and wind shear will be increasing to around 15 knots by Tuesday night as Don moves west.

Satellite imagery from Don at midday Tuesday showed an unimpressive cluster of showers and thunderstorms (see image above), and Hurricane Hunter observations on Tuesday morning found a weakening system that may have already lost its closed surface circulation. If the next flight confirms this, Don may be downgraded later on Tuesday to a tropical depression or a remnant low, perhaps just before Don reaches Grenada.

Radar image of TS Don, 10:45 am EDT 7/18/2017
Figure 1. Radar image of Tropical Storm Don taken at 10:45 am EDT July 18, 2017, from the Trinidad and Tobago radar.

Tropical-storm-force winds extend just 35 miles from Don’s center (mainly on the north side), and that may be generous. The impacts from Don on the Windwards will likely be little more than some heavy showers and thunderstorms and gale-force squalls. It's worth noting that the HWRF and new HMON models have been consistent in calling for Don to strengthen somewhat as it approaches the islands, so residents in the warned areas should not let their guard down just yet.

We’ve now had four named storms this year in the Atlantic basin: Arlene, Bret, Cindy and Don. Don arrived more than a month ahead of the average formation date of the fourth named Atlantic storm (August 23, based on data from 1966-2009). The first hurricane of the year usually arrives by August 10; so far, none of this year’s storms have become hurricanes.

Another tropical wave in the central Atlantic

A large but poorly organized tropical wave was rolling through the central tropical Atlantic at around 37°W latitude on Tuesday. This wave could develop into a tropical depression or perhaps a weak tropical storm over the next couple of days, but the European and GFS model ensembles agree that any development would be snuffed out by dry air and wind shear after 3 or 4 days, while the system remains far out in the Atlantic. In its outlook Tuesday morning, the NOAA/NWS National Hurricane Center gave 30% odds of this system becoming at least a tropical depression by Friday and 40% odds by Sunday.

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

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 on GOES-16 satellite, 2:30 pm EDT 6/19/17
Figure 2. Goes-16 image of Tropical Storm Bret affecting the Lesser Antilles Islands and north coast of South America at 2:30 pm EDT, June 17, 2017. Image credit: University of Wisconsin/CIMSS.

According to Colorado State University’s Dr. Phil Klotzbach, there were 11 other years between 1851 and 2016 that saw two or more tropical cyclones form in the MDR prior to August 1, as we’ve seen this year. Only one of these seasons (2013) was quiet, when measured by Accumulated Cyclone Energy (ACE). It should be a wake-up call that four of the eleven years with two early-season MDR storms were among the ten most active seasons on record: 1933 (#1), 2005 (#2), 1926 (#4) and 1887 (#10).

Below are the number of named storms, hurricanes, intense hurricanes, and ACE index for the seasons with two or more tropical cyclones to form in the MDR before August 1. The overall long-term averages for the past 65 years are 11 named storms, 6 hurricanes, 3 intense hurricanes and an ACE of 101. However, the years with early-season MDR activity have been considerably more busy than that long-term average, as shown below:

1887     19     11     2      181
1901     13      6      0        99
1926     11      8      6      230
1933     20    11      6      259
1944     14      8      3      104
1966     11      7      3      145
1979       9      5      2        93
1995     19    11      5      228
1996     13      9      6      166
2005     28    15      7      250
2013     14      2      0        36
Avg    15.6   8.4     3.6    163

The Madden-Julian Oscillation suggests a busy early August

The Madden-Julian Oscillation (MJO) suggests that the odds of a busy period in the Atlantic will be elevated in the first half of August. Each MJO phase includes a large area of rising motion, typically accompanied by showers and thunderstorms (convection), that moves east around the global tropics over the course of 40 – 60 days. In a Twitter thread on Tuesday morning, Michael Ventrice (The Weather Company) staked his claim as to when the Atlantic may get more active, based on the latest MJO projections:

Standard dynamical and statistical models do not extend several weeks out—and even if they did, they would not be able to predict individual hurricanes with any skill, given the inevitable chaos that emerges in day-to-day weather. However, the idea of flagging a busy period based on MJO status is not as far-fetched, since we know that a favorable MJO pattern can substantially boost the likelihood of Atlantic tropical cyclones. Climatology also favors a bump-up in activity during August, as the season is normally ramping toward its September peak by then.

The key, of course, is whether the MJO will behave as predicted. In this case, there is a fairly strong signal in the ECMWF extended-range forecasts of MJO development in the Indian Ocean over the next few days, with the MJO impulse predicted to enter the Western Pacific around the end of July. Should this occur, the typical eastward motion of MJO phases tells us that its hurricane-friendly forward flank would enter the Eastern Pacific around the first week of August, and the Main Development Region of the Atlantic toward the second week of the month. Odds of activity over the MDR should begin to increase in association to the transition towards the convectively active phase of the MJO, according to Ventrice.

Dedicated Atlantic hurricane watchers shouldn’t turn off their attention for the next couple of weeks, though. Even if the MJO makes hurricanes more likely at particular times, it doesn’t rule them out at other times.

Water-vapor satellite image of Hurricane Fernanda, TS 8E, and TS Greg, 16Z 7/18/2017
Figure 3. This water-vapor GOES-16 satellite image shows three tropical cyclones traversing the East Pacific at 1600Z (noon EDT) Tuesday: Hurricane Fernanda, Tropical Depression 8E, and Tropical Storm Greg. Data from GOES-16 are preliminary and non-operational. Image credit: NASA/MSFC Earth Science Branch.

Active in the Eastern Pacific:  Fernanda, Greg, and a new tropical depression

Three systems were clustered in an east-west band near latitude 15°N in the Eastern Pacific on Tuesday morning. Hurricane Fernanda, which peaked at Category 4 intensity, remained a Category 2 hurricane at 11:00 am EDT Tuesday, with top sustained winds of 100 mph. Fernanda is gradually weakening as it rolls toward the northwest at 9 mph. Located about halfway between Baja California and Hawaii, Fernanda is expected to continue decaying as it arcs to the west, ingesting dry air and encountering increasing wind shear later this week. In addition, Fernanda will be traversing sea-surface temperatures just below the 26°C (79°F) threshold for tropical development. It now appears Fernanda will be nothing more than a tropical depression, if that, as it approaches Hawaii early next week.

Meanwhile, a tropical depression about 400 miles southwest of the Mexican coast became Tropical Storm Greg on Tuesday morning, with minimal sustained winds of 40 mph as of 11:00 am.  Although models intensify Greg only gradually over the next couple of days, Fernanda developed much more quickly and dramatically than models had expected. I would not be shocked to see Greg become a hurricane by Thursday, given the very light wind shear, moist atmosphere, and adequately warm sea surface temperatures that will be in place over the next couple of days. Greg will be no threat to land as it marches westward, and it should be weakening by this weekend as it encounters drier air and increasing wind shear.

The newest member of the East Pacific conga line is Tropical Depression 8E, which formed on Tuesday morning in between Greg and Fernanda. TD 8E will be fighting strong wind shear from a nearby upper low as it pushes slowly to the west, so any development should be very gradual. There may be Fujiwhara interaction between TD 8E and Tropical Storm Greg later this week, which would tend to push TD 8E toward the south (assuming it survives). The next name on the East Pacific list is Hilary.

Dr. Jeff Masters co-authored this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Bob Henson

WU meteorologist Bob Henson, co-editor of Category 6, is the author of "Meteorology Today" and "The Thinking Person's Guide to Climate Change." Before joining WU, he was a longtime writer and editor at the University Corporation for Atmospheric Research in Boulder, CO.


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