By: Bob Henson , 2:00 PM GMT on August 01, 2016 From Weather Underground

Figure 1. Enhanced infrared image of Invest 97L as of 1245Z (8:45 am EDT) Monday, August 1, 2016. Image credit: CIRA/RAMMB/CSU.

The tropical wave in the eastern Caribbean dubbed Invest 97L continued to organize on Sunday night, and it will likely become a tropical depression or tropical storm on Monday and move into the western Caribbean as an intensifying tropical storm over the next day or so. Infrared satellite imagery on Sunday night revealed that shower and thunderstorm activity (convection) had become more intense and far more symmetric around 97L’s core, which was located early Monday morning about 350 miles east-southeast of Kingston, Jamaica. As of early Monday morning, 97L had not yet consolidated a closed low-level center of circulation, as seen by the lack of westerly winds in the ASCAT scatterometer image in Figure 2 below. However, surface winds were close to tropical storm strength on the north side of the system. As of early Monday morning, a NOAA Hurricane Hunter aircraft was tentatively scheduled to carry out a reconnaissance mission into 97L on Tuesday afternoon.

97L continues to chug westward at 20 to 25 mph, a speed that typically limits the ability of a tropical wave to intensify. In this case, however, upper- and lower-level winds are close enough in speed and direction to reduce the amount of wind shear that would otherwise affect fast-moving 97L. Long-range models agree that wind shear will remain light (around 10 knots or less) for at least the next three to four days, perhaps longer, along 97L’s path. In its 8:00 am EDT Monday tropical weather discussion, the National Hurricane Center gave 97L an 80% chance of development over the next two days. (On occasion, the NHC will bypass tropical depression status and upgrade a strong wave directly to tropical storm status.) 97L will be sweeping just south of Jamaica on Tuesday, perhaps as a moderately strong tropical storm (which would be named Earl], before approaching the Yucatan Peninsula later in the week (see below).

Figure 2. Low-level winds (in knots) detected by the Advanced Scatterometer (ASCAT) aboard the MetOp-A satellite as of 1302Z (9:02 am EDT] Monday, August 1, 2016. Image credit: NOAA/NESDIS/STAR.

The outlook for 97L
It’s now clear that 97L will survive its trek through the eastern Caribbean, which has long been known as the “hurricane graveyard” thanks to the climatological minimum in tropical cyclone formation over the area. According to a 2010 study in the Bulletin of the American Meteorological Society led by Owen Shieh (University of Oklahoma), this phenomenon is largely because of a predominant low-level southerly jet in the central Caribbean and an accompanying tendency toward low-level divergence over the eastern Caribbean. This pattern is most prevalent early in the season, peaking in July.

With a sprawling area of upper-level high pressure to its north, 97L should continue on a fairly straightforward west to west-northwest path for at least the next couple of days. There is strong agreement among our best longer-range track models, the ECMWF and GFS, that 97L will move south of Jamaica on Tuesday and approach the Yucatan Peninsula around Thursday. There are no signs of any major changes to the upper-level pattern that would divert 97L from this longer-term path, which could eventually bring it across the Yucatan Peninsula and into the Bay of Campeche. We will have to watch for any signs of a northward bend in 97L’s path later this week, but no members of the most recent GFS and ECMWF ensemble runs are indicating a track that would bring 97L into the heart of the western Gulf. Residents of Nicaragua and Honduras, and especially Belize, Guatemala, and eastern Mexico, will need to keep a close watch on 97L. Toward the end of the week, 97L could pose a threat to the western coast of the Bay of Campeche if it crosses the Yucatan Peninsula and survives the trek.

Figure 3. Oceanic heat content over the Caribbean on July 30, 2016, in kilojoules per square centimeter. The value is produced by integrating the vertical temperature from the ocean surface to the depth of the 26°C contour. 97L will be traveling over a large area of heat content greater than 100 kJ/cm2 on Tuesday and Wednesday. Image credit: NOAA/AOML.

Extremely warm water lies ahead of 97L
As it sweeps toward the western Caribbean, 97L will encounter a more favorable atmospheric regime as well as ominously warm water both at the surface and below. Sea-surface temperatures (SST) along 97L’s expected path from the area around Jamaica westward are around 29-30°C (84-86°F), which is roughly 1°C above average for this time of year. These warm waters extend to great depths, with large amounts of oceanic heat in the uppermost 200 meters (660 feet) of the northwest Caribbean (see Figure 3). Across the Caribbean and adjacent waters, the breadth and depth of oceanic heat content has been at near-record levels in recent weeks, as we discussed in a post on July 18. High levels of oceanic heat content are a major boon to tropical storm and hurricane intensification, because the storms do not churn up as much cold water as they otherwise would as they intensify. Rapid intensification is often associated with regions of high oceanic heat content, assuming that other conditions are favorable as well.

Figure 4. Infrared satellite image of Tropical Storm Howard at 1300Z (9:00 am EDT) Monday, August 2, 2016. Image credit: NOAA/NESDIS.

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