Based on National Hurricane Center climatological data from 1966-2009, there are two named storms during the average hurricane season through August 1. Tropical Storm Chris was the third named storm this year, putting the 2018 Atlantic hurricane season slightly in above-average territory. However, this small lead on the average could soon diminish. No tropical threats loom on the horizon as summer heads into August, less than four weeks away from the season’s most active period. Statistically, there is a sharp increase in tropical activity between late August and mid-September. Until then, prospects for a short term uptick in activity in the Atlantic basin look meager, as conditions remain hostile for tropical cyclone development.
The first and foremost ingredient necessary for tropical development is a wide swath of sea-surface temperatures of at least 26°C, or 78.8°F. These temperatures are the bare minimum required to support enough condensation for tropical cyclone development. As the most recent Reynolds Sea Surface Temperature (SST) plot depicts below, the region of acceptable SSTs is somewhat narrow in the Tropical Atlantic. The Reynolds analysis incorporates a blend of satellite radiometer measurements with buoy and ship measurements interpolated onto the measurement grid to correct for biases in SST measurements. These marginal temperatures could certainly fuel a tropical cyclone, but thankfully not high enough to support a major hurricane.
If these temperatures seem low for the tropics, that’s because they are. SST anomalies for the week from July 22 – July 28 indicate that the Tropical Atlantic is up to 1.5°C below normal between Africa and South America. These cooler temperatures limit the fuel available for hurricanes and reduce the surface area over which they can form. Since the vast majority of tropical cyclones in the Atlantic originate from thunderstorm complexes known as “tropical waves” drifting off the north African coast, these cool waters pose the first hurdle to intensification into a tropical cyclone. Thunderstorms and tropical systems are both fueled by convection and condensation, so neither can be supported by the lower potential energy in the cooler seas.
There are still more hurdles to cross besides cool sea surface temperatures when thunderstorms do move into the open Atlantic. Satellite water-vapor imagery depicts a considerable region of westward-moving dry air known as the Saharan Air Layer (SAL). This dry air is comprised of tiny dust particles blown into the Atlantic from the Saharan Desert. They stay elevated because the hot Saharan air is much less dense than the cool Atlantic air.
The warm colors in the imagery represent drier air because all of the measurable water vapor is contained in the lowest levels in the atmosphere where it is warmer. Tropical systems cannot tolerate dry air. Evaporation cools the warm core of a tropical cyclone and curtails condensation, the fuel of tropical cyclones. Additionally, the tiny dust particles attract whatever water vapor does exist in the SAL to condense on their surfaces. These droplets are too small to grow into rain or even cloud droplets and cannot grow larger because most of the water available condenses on the abundance of dust particles.
A third hurdle is also contributing to a hostile environment for tropical cyclones in the eastern Atlantic. The cool greens in the water vapor loop represent high cloud tops of thunderstorms. A few storms are noticeable leaving North Africa, but from close observation of the satellite loop, water vapor moving in different directions at different layers of the atmosphere implies a vast amount of wind shear. Model analyses suggest the wind shear ranges from 20-40kts over the eastern Atlantic. Wind shear rips apart a cyclone’s center of circulation. Even if wind shear were minimal however, westward propagating clusters of storms would still have to confront the SAL. Model guidance suggests wind shear will subside by this weekend. That waves of low pressure will develop over the Eastern Atlantic next week still looks slim, as numerical guidance also indicates that moisture will subside in tandem.
Despite these hurdles, numerical guidance suggests the formation of weak low pressure in the Tropical Atlantic this weekend. It does not appear at this time that it has a fighting chance to intensify into a named storm. Stranded between the flows of two impenetrable ridges of high pressure, the low will drift northwestward into cooler water and eventually dissipate.
Any hope for tropical development in early August will have to be rooted in systems emerging from Central America or the Caribbean. Wind shear is expected to be minimal over this region, which also overlaps the portion of the western Atlantic and the Gulf of Mexico, where sea surface temperatures range from 28 °C- 30 °C, sufficient to support a major hurricane. Roadblocks of a persistent easterly jet and intermittent intrusion of the SAL will continue to hamper chances of cyclone development. Intrusions of the SAL must subside and the Bermuda High needs to weaken or shift northward to allow tropical systems to bask in the warm waters of the Caribbean, Southeastern US, and the Gulf of Mexico before tropical development is likely.
While conditions are unfavorable for tropical development now, it is unlikely these conditions will persist indefinitely. As summer wears on, the Atlantic will continue to warm. Neither the SAL nor wind shear are permanent features of the tropics. It will only take a brief window of ideal conditions to prompt the development of one or more tropical systems as the peak of hurricane season draws near.