Officially, Hurricane season for the Atlantic N of the Equator begins June 1, and ends November 30. Below we will discuss seasonal forecasts, Hurricane Holes and risk minimization in the Caribbean, as well as timing and routing considerations for vessels departing the Caribbean.
Most credible seasonal forecasters predict an above average number of Tropical LO pressure systems this season in the Atlantic Basin. There are several interesting drivers of expected above normal activity in the Caribbean, Bahamas and GulfOfMexico.
ENSO: The El Nino Southern Oscillation (ENSO) influences Tropical activity in the Atlantic Basin. El Nino manifests in warmer sea surface temperatures in the E Pacific off S America. In the Tropical Atlantic and Caribbean, El Nino typically induces stronger wind shear which inhibits Tropical LO formation.
On the other hand, an E Pacific La Nina supports lighter Easterly Trade Winds (and hence less wind shear) in the Tropical Atlantic and Caribbean, enhancing Tropical LO formation and intensification.
The El Nino / La Nina cycle is presently neutral, but forecasts (see image, credit: @philklotzbach) suggest a weak La Nina may form during our summer (by August), and this could support more Tropical activity than normal in mid and late parts (August-December) of the Atlantic Hurricane season.
Wind Shear: Strong Easterly Trade winds cause wind shear (change in wind speed and/or direction with altitude), versus slow-moving upper-air in the Tropics. Shear inhibits Tropical LO formation through a combination of disrupting the vertical structure of a forming Tropical LO pressure system, and causing the Tropical WAVEs which spawn Tropical LO pressure systems to move W at too fast a pace for their convective engine to keep-up. If seasonal forecasts are correct, Wind Shear should decrease below normal (enhancing Tropical activity) beginning in August, and remain below normal rest of the year.
Sea Surface Temperatures: A troubling sign for the entire season is Sea Surface Temperatures (SSTs), which are running at least 1 degree C warmer than normal throughout the Caribbean, as well as Tropical Atlantic E of the Caribbean (which is commonly referred to as the “Main Development Region”, or MDR). Worse, SSTs in GulfOfMexico are 1 to 4 degrees C above normal, which is a tremendous anomaly. (see images)
Strong Easterly Trade winds often mix the column of water, resulting in slightly cooler sea surface temperatures, so the below normal strength of Trade winds expected from August onward may cause less mixing, and ensure abnormally warm SSTs persist all season.
Warmth enhances evaporation, and evaporated water (water vapor) is key to the Latent Heat cycle of Tropical LO pressure systems. Because we expect the positive SST anomaly to persist all season, oceanic heat content may be a key driver of enhanced Tropical activity through December.
One mitigating factor we have observed suppressing early season activity to a greater extent than normal in recent years, at least in the MDR Region (Africa to E Caribbean), is particulate matter in the atmosphere. Particulate matter, such as Saharan dust, filters sunshine and reduces evaporation. However, we have no skill predicting how much this may inhibit early season Tropical LO formation in the MDR this year.
Meteorologists also try to make “analog” comparisons – comparing present environmental factors to previous years with similar environmental factors. Dr. Phil Klotzbach of Colorado State University suggests 2020 may be analogous to seasons: 1960, 1966, 1980, 1996, and 2008.
“We look at these five analog seasons and four of the five seasons we selected had above-normal hurricane activity, while 1960 had near-normal hurricane activity.”
University College of London’s well respected TropicalStormRisk.com group predicts Atlantic activity “25% above the 1950-2019 long-term norm and 5-10% above the recent 2010-2019 10-year norm.”
Further, UCL predicts the ACE (Accumulated Cyclone Energy, which quantifies total accumulated wind from Tropical LO pressure systems throughout the season) has a 53% chance ACE of being above normal (in the highest 33% of seasons); 32% chance it’s near-normal (in the middle 33% of seasons), and only 15% chance below normal (in the lowest 33% of seasons).
Together, the above suggest a high probability of more Tropical LO pressure systems and more total wind energy associated with Tropical LO pressure systems.
So what is a “Normal season”?
Contrary to popular wisdom, Tropical LO pressure systems (Tropical Storms and Hurricanes) can form in the Atlantic N of the Equator anytime of the year. In the last 100 years the only month without a Tropical LO is March. However 112 years ago in March 1908 a Tropical LO developed along a stalled ColdFRONT several hundred miles NE of the Caribbean then moved SW through the Leewards as a Category 2 Hurricane! But that’s not “normal”.
Here’s what is “normal”:
Mid-Season: Over 80% of Tropical LO pressure systems in the Atlantic form during a 10 week period, from mid-August (August 10) through mid-October (October 20). Most of these form in the MDR (between Africa and the Caribbean). These features begin as storms over Africa, and emerge into the E Atlantic as Tropical WAVEs. As WAVEs propagate W-ward across the Atlantic, if environmental factors are just right, they can develop into Tropical LO pressure systems and some of these reach Hurricane strength.
These are the classic “Cape Verde Hurricanes” which are feared for good reason, and, as they move out of the Tropics, they can impact any area of the GulfOfMexico, US E Coast, CanadianMaritimes, and the entire Atlantic all the way to Europe.
Some of these classic Tropical systems form as early as June and as late as November, but the bulk form mid-August through mid-October.
Early and Late Season: Very few Tropical LO pressure systems form the other 42 weeks of the year (late October through July), and the triggering mechanism and geographic areas of formation are quite different from mid-season systems.
Early and late season systems most often develop when a ColdFRONT stalls over the ocean, and what we at Marine Weather Center call an IMPULSE of energy (cluster of persistent squalls) forms along the stalled FrontalTROF.
Normally the IMPULSE does not last long, and a hostile environment near the stalled FrontalTROF prevents development. However, if the IMPULSE persists for at least several days, and is quasi-stationary, and other environmental factors are supportive, then a LO with Tropical or sub-Tropical characteristics can develop.
Although this sort of Tropical LO formation can happen along a stalled FrontalTROF any time of the year, it happens most frequently in April-June, and again in October-December (when ColdFRONTs penetrate deep into warm waters of the Tropics or sub-Tropics before stalling). In January-March waters are typically too cool.
Early and late season systems typically develop in the SW and NW Caribbean, GulfOfMexico, Bahamas, and sub-Tropical Atlantic (N of the Tropics, but S of about 35N Latitude).
In the 50 years of the “Satellite era” of meteorology (when geostationary satellites allow constant monitoring of the oceans) we have seen the following number of Tropical LO pressure systems in each month:
April = 4
May = 19
November = 5
December = 8
Almost without exception these formed along stalled FrontalTROFs, as described above.
During the past 50 years the months of June and July saw roughly 30 (June) and 40 (July) systems each month, some of which formed along FrontalTROFs, but others formed along W-ward propagating TropicalWAVEs.
August through October saw roughly 100 (August and October) to nearly 200 (September) systems total in the last 50 years – the vast majority of which formed along W-ward propagating TropicalWAVEs.
Image shows typical areas of formation and track by month:
–The bulk of risk for Caribbean, GulfOfMexico, Bahamas, US E Coast, and entire Atlantic from W-ward propagating TropicalWAVEs is August-October, with less risk in June and July.
–In April-December (and to a much lesser extent January-March) folks should be very aware a Tropical LO could form along a FrontalTROF anywhere in the Atlantic Basin (including Caribbean and GulfOfMexico) – except typically not in the Tropical Atlantic E of the Caribbean.
Conclusions about early part of the 2020 Tropical season:
Although the risk of Tropical LO pressure systems is relatively low through most of July, unusually warm sea surface temperatures suggest greater risk for early season activity – particularly developing along a ColdFRONT stalling in the GulfOfMexico or the NW or SW Caribbean.
Above average SSTs suggest slightly higher than normal risk for an early season W-ward propagating WAVE to spawn a Tropical LO impacting the Caribbean (recall Tropical Storm Brett which hit Trinidad June 19, 2017).
“Hurricane Holes”: Unfortunately, restrictions on vessel movements may eliminate (or just delay) your access to some common Hurricane Holes, but we rate very highly:
–Up the RioDulce in Guatemala
–Anywhere in Panama and Colombia
The above locations are either outside areas at significant risk, or (in the case of RioDulce) in a highly convex coastline where an approaching system would weaken significantly as a result of interaction with landmass of Belize and/or Honduras before impacting up-river areas of Guatemala.
N Coasts of the DominicanRepublic and Cuba: Although within the high risk area, greatest impact is caused by either the eye or the right-front-quadrant of a Hurricane impacting your location. Significant impacts in these areas are most plausible if a Hurricane is moving W or SW, while normal direction of motion is N-of-W. However, there are rare exceptions – Irma devastated parts of the N Coast of Cuba when her eye stalled along the Coast in 2017.
Although some consider Grenada, Trinidad, and the ABCs (Aruba, Bonaire, Curacao) “safe”, there are far more instances of impacts from Tropical LO pressure systems in these areas than in Panama, Colombia or Guyana/Suriname. If your plans include Grenada, Trinidad, or ABCs, ensure you are prepared in case there is a Tropical impact (remember Hurricane Ivan in 2004).
If I were going to head for Guyana/Suriname: If I were on a vessel capable of really close reach sailing, April might be a good time to make a move, as the N Boundary of the ITCZ (Inter-Tropical Convergence Zone, with nasty squally weather) is generally near or S and E of FrenchGuyana, so most of trip is with moderate ENE Trade Winds, but adverse current.
In May-June the squally ITCZ can lift into this area, bringing more variable conditions but with intervals of inclement weather.
Later in June or July, when N Boundary of ITCZ often lifts N of 12N, vessels heading from SE Caribbean to Guyana/Suriname may find mostly mild conditions but adverse current. Meanwhile, I would stage in Trinidad/Tobago if possible, or in Barbados or Grenada (or as far S and E as possible).
Tropical Forecasting has become very good. We rarely have less than 2 weeks of warning in advance of a possible system. Of course the farther out we forecast into the future the lower our confidence, and the more risk there’s a false alarm (the potential for a Tropical LO, but one fails to form). We at Marine Weather Center continuously probe forecasts for any potential Tropical LO formation in areas where we have clients, so there are few instances when a Tropical LO develops that was not flagged as a possibility well in advance.
If you want to minimize false alarms but still have ample time to move or prepare, then focus on threats within the next 5-7 days.
Remaining in the Caribbean: This may be a viable plan for many folks – if you remain onboard and ready to cast lines and go sailing with a couple days notice. We typically have lots of warning before impact, and if you are prepared to sail as far to the SW as necessary to get well S of the path of an approaching Tropical LO, then you should be fine. We work with a number of vessels each year employing this strategy, but it is critical that you depart early enough and work with a professional weather router, because you need to be absolutely certain you will be well S of the path of the system – without getting trapped with no place to go.
For Irma, Maria, Dorian and other systems, we routed vessels SW from PuertoRico, the Virgins and Leewards…then, as these powerful systems passed well N of our clients, the vessels made gradual turns to the S then E then back N again, and after several days arrived back at their (often destroyed) departure point after a pleasant sail around the Caribbean. In the worst case, you might have to go all the way to the ABCs, Colombia or Panama, but eventually you’ll get out of the path.
Departing the Caribbean for US, Bermuda, Azores, Europe: Departure anytime from now through the end of June is reasonable.
A Sub-Tropical RIDGE (which in summer is typically referred to as Azores-Bermuda RIDGE or Bermuda HI) is fairly well established already in mid-April. This surface based HI is supported by an anomalously strong upper-RIDGE spread from the UK to areas S of Bermuda, through the Bahamas and Florida, all the way to Mexico and into the sub-Tropical E Pacific.
S of the RIDGE we find E-component winds, which in May should become ESE in direction, and may allow vessels departing E Caribbean to close reach in mild ESE winds most of the way to Azores, with nonstop travel possible from E Caribbean to Azores, Madeira Islands, Portugal, the Mediterranean, and CapeVerde Islands, and possibly even N Europe. Sometimes this does not work out: wind S of the RIDGE may fail to veer S-of-E, or may be too light, or squally weather may encroach toward S side of RIDGE, or RIDGE may shift around too much to allow vessels to remain in any “sweet spot” of moderate ESE winds and settled weather just S of RIDGE.
The alternative is to move generally N (or slightly NNE or even NNW is sometimes necessary) from the E Caribbean, and work your way across the wind-less RIDGE…then catch the prevailing SW winds for E-bound sailing N of RIDGE. This is the more conventional route, and the route more likely to be successful. Optimally you time departure from E Caribbean to reach N edge of RIDGE (where you intersect SW winds) at the time when these SW winds have penetrated farthest S.
Through most of May, the RIDGE is typically suppressed S-ward by LO pressure systems and ColdFRONTs, so vessels moving N from the Caribbean usually make their turn to the ENE or E near about 30N / 60W. In order to avoid GALE FORCE wind with ColdFRONTs through the end of May (and often into June) it’s common for vessels to pass about 32N / 40W before turning NE and riding the narrow zone of moderate SW winds toward Azores, on NW side of sub-Tropical RIDGE and to the SE of stalling ColdFRONTs.
As we move through June, RIDGE (and LO pressure systems / ColdFRONTs N of RIDGE) shift farther N…and vessels may need to get to 35N in June and at least 40N in July before turning generally E toward Azores and Europe.
Heading to the US E Coast: If you’re N of the RIDGE, then you need to be prepared for frequent intervals of GALE or near-GALE conditions associated with LO pressure systems, ColdFRONTs, and IMPULSEs of energy between systems. In April it’s common to see GALE or near-GALE conditions every 24-48hrs N of about 30N Latitude from the US E Coast into offshore waters of NW Atlantic.
Meanwhile S of the RIDGE (along N Caribbean and at least the S Bahamas and OldBahamaChannel to FloridaKeys and S Florida) we enjoy a mix of moderate ENE-SE wind for broad reach to downwind sailing.
Along the RIDGE (in mid-April the default position is about 27N/65W to the N Bahamas and S Florida) we see light wind. The width of the wind-less RIDGE is important if you’re crossing it – it can be quite broad. Especially in May it is not unheard of for the wind-less RIDGE to blanket areas from 20N to 35N. Vessels determined to cross the RIDGE in this situation need ample fuel, patience, or both.
As we move into May and June, the RIDGE (and the LOs/FRONTs N of area) shift N. We typically see reasonable weather from E Caribbean to Carolinas by the end of May, and to ChesapeakeBay in June.
Of course, these are just very broad generalizations, and it’s important to work with a professional weather forecaster and router, like Marine Weather Center, www.mwxc.com or 863-248-2702.