A Peek at the Peak – Atlantic Basin Catches No Breaks in September

Valid September 14, 2017 at 9:00 AM CT

In the wake of two major hurricanes hitting the United States, Harvey to end August and Irma to begin September, much more attention has been put on the Atlantic hurricane basin in the media. But what is making many believe the Atlantic is more active than in past years? A few of the ingredients in the tropics simply say that activity is just getting back to normal for the peak of the season.

Sea Surface Temperatures


Figure 1 – Sea surface temperatures (top) and sea surface temperature anomalies (bottom) from a global perspective. Note the plume of anomalies across the center of the Atlantic basin, from the Bahamas to the Azores.

One of the key driving forces behind the strengthening and development of a tropical cyclone is the sea surface temperatures (SSTs). Warm SSTs above 26 degrees Celsius are paramount for a tropical wave to develop further into a full-fledged tropical storm or hurricane. The latest SSTs and anomalies show the warmth of the Atlantic basin, common for early to mid-September.

Temps remain elevated to bath water levels over the southern Gulf of Mexico into the Caribbean Sea, and even further east of the Lesser Antilles. The areas across the Caribbean and Antilles are slightly warmer than average, while the tight gradient of ocean temps across the north Atlantic is slightly shifted northward. This is especially true across the east and central Atlantic, with a plume of anomalously warm SSTs from the Bahamas to the Azores.

Total Precipitable Water


Figure 2 – Total precipitable water (TPW) across the Atlantic basin, with highest levels of TPW seen associated with Jose (left) and two tropical waves coming off the African coast (right).

Total precipitable water is an important factor to consider in the development of a tropical cyclone, especially a threat like Hurricane Jose, which continues to linger dangerously within range of the United States. As expected, the highest amounts of precipitable water are found in areas of convection and tropical cyclone activity, with Jose and two invest areas clearly visible.

It is worth noting that the second of the tropical waves exiting the Cape Verde Islands has a stronger signature of precipitable water than Jose, the latter which may be undergoing some weakening. Regardless, the atmosphere remains juiced across the Caribbean and over to coastal Mexico.

Saharan Air Layer


Figure 3 – The Saharan air layer analysis, showing a moderate amount of low to mid-level dust north of the two tropical waves under observation from the National Hurricane Center.

The Saharan Air Layer, which involves the influence of dust and sand from the Saharan desert in the development of tropical waves and tropical cyclones off the African coast, is noteworthy here. While there is some amount of low to mid-level dry air from the duty air mass off Mauritania, its influence on the two tropical waves currently under investigation is marginal. Both waves can be seen to the south of the majority of the Saharan Air Layer, but could move into areas of influence from this going forward.

Vertical Wind Shear


Figure 4 – Vertical wind shear across the Atlantic basin, with generally most favorable areas for development found over Jose’s circulation and east of the Lesser Antilles.

Vertical wind shear is one of those basic necessities for a tropical cyclone to develop, in that low shear will be conducive to development and strengthening. The current situation over the Atlantic basin would suggest that Jose may encounter some stronger shear once the meandering storm is directed more northwesterly. Combining this with cooling sea surface temperatures the further the storm moves north, and the future does not bode well for Hurricane Jose, at least in terms of intensity.

For the two tropical waves off of Africa, while there is a small area of less favorable shear they may have to negotiate, at least one of those systems has a chance to develop to the east of the Antilles, with generally favorable wind shear and anomalously warm SSTs.

The Projected Path


Figure 5 – The projected path of Hurricane Jose as determined by the National Hurricane Center (top), compared to the model run ensemble for Jose (bottom).

Considering the various factors that will play into the track of a tropical cyclone, there seems to be strong agreement with the ensemble models and the official hurricane track from the National Hurricane Center (NHC) for Jose. The cone suggests that Jose should pick up some steering in the coming days and potentially recurve out and away from the Cape Hatteras. A similar curve can be seen in the model ensembles, but member groups may tell a slightly different story.

While it may be difficult to quantify the differences between the ensemble models over the open ocean, it is important to see the slight shifts between the GFS, HWRF, and NHC. The GFS models, the AVNO and AVNI, are clearer in the recurvature of Jose off the coast of New Jersey. However, both the HWRF and NHC models do not curve the storm back out into the Atlantic as significantly, instead turning Jose northwest to north, moving nearly due north at the end of their runs.

Hurricane Jose will remain worth watching in the coming days, especially in terms of steering mechanisms and direction. The two tropical waves off the African coast will be pegged for development, but likely only one will survive considering their proximity to one another. Expect strengthening to occur when movement becomes closer to the Lesser Antilles.

Other Tropical Tidbits

Action in the Eastern Pacific basin has picked up significantly, with Hurricane Max, Tropical Storm Norma, and Tropical Depression Fifteen-E all lined up. In the Western Pacific, Typhoon Talim has Japan in its crosshairs, while Typhoon Doksuri heads for Vietnam. Finally, look for this tropical meteorology blog on Weather Unlimited every Thursday morning.

The tropical update blog is part of a weekly assignment for Dr. Kim Wood’s Tropical Meteorology class at Mississippi State University. Post created for academic purposes.

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