Enhanced satellite image of water vapor from Wednesday morning. Cool colors indicate drier air in the mid-upper layers of the atmosphere. An easterly wave is coming off the coast of Africa. (NRL-Monterey)

The 2016 Atlantic hurricane season got off to a torrid start in May and June, but has since shut down. We shouldn’t let our guard down, though.

Historical chances of tropical storm activity ramp up substantially over the next six weeks as we move into the height of hurricane season. Warmer-than-normal ocean temperatures over much of the Atlantic and Gulf of Mexico may only add fuel to the fire.

The season thus far

In early June, three named tropical weather systems — Alex, Bonnie and Colin — had already formed in the Atlantic basin, the most on record so early in the season. But as Colorado State University hurricane researcher Phil Klotzbach cautioned, a fast start to the season doesn’t necessarily carry forward.

Tracks of all four Atlantic tropical cyclones so far in 2016.

Since Tropical Storm Danielle washed ashore in Veracruz, Mexico, on June 20, as the earliest fourth named storm on record, tropical cyclone activity ceased.

Large amounts of dry air in the tropical Atlantic has significantly contributed to the lack of storms. Even when otherwise favorable conditions for storms have been present, very dry air in the mid-levels of the atmosphere has choked off organized thunderstorm activity. Huge plumes of dusty dry air have streamed off Africa, creating what is called a Saharan Air Layer, or SAL.

The SAL, generally toxic to developing tropical cyclones, has sprawled across the entire tropical Atlantic basin at fairly regular intervals.

Five-day animation of aerosol optical thickness, a measure of how much dust is in the air. Warmer colors indicate higher concentrations of dust, and dust acts as a proxy for dry air in this case. (WeatherBell.com)

The pause in activity over the past five weeks has transformed a record early hurricane season to one that is fairly close to average.

During an average season (using 1966-2014 data), there are typically two named storms, zero hurricanes and zero major hurricanes (defined to be Category 3 or higher on the 0-5 Saffir-Simpson wind scale) by Aug. 1, with an average accumulated cyclone energy, or ACE, of 7.3 by that date. This year, the count stands at four named storms, one hurricane, zero major hurricanes and an ACE of 5.8 — nothing too out of the ordinary.

While no named storms formed during July, that is not particularly uncommon. Other recent “stormless” Julys include 2012, 2009, 2004, 2001, 2000 and 1999. In 19 of the past 50 years, no named storms formed during July.

Abundance of warm water for future storms to draw from

Tropical cyclones draw strength from warm waters, and there is plenty of it to go around.

A very useful measure of the amount of warm water is known as “ocean heat content.” It provides a sense of not just how warm the ocean exists at the surface but also down to more substantial depths.

Presently, ocean heat content is unusually high in the central and western Caribbean Sea. Should a storm move through this powder keg of very deep warm water under the right conditions, explosive intensification is possible.

Weather Underground meteorologists Jeff Masters and Bob Henson noted the heat content even exceeds what was available during the 2005 hurricane season, which produced Katrina and Wilma, whose pressure dropped to the lowest on record in the Atlantic Basin. “This year’s high levels of ocean heat content in the Atlantic increases the odds of dangerous rapidly-intensifying major hurricanes if the other conditions needed for intensification are present,” they wrote.

Maps of sea surface temperature (top) and ocean heat content (bottom) from July 27. (UMiami/RSMAS)

Warmer-than-normal sea surface temperatures expand across much of the tropical Atlantic. The warmth acts as one favorable ingredient, but we stress many other environmental conditions must also come together for storm development. To add, while the Gulf of Mexico, for example, is roughly 1 degree Celsius above normal, the difference between 29- and 30-degree water is not much for a hurricane; either is more than enough to fuel a major storm.

Sea surface temperature anomalies averaged over a one-week period. (NOAA)

Looking ahead

At the beginning of the season, models were indicating a transition to a moderate-to-strong La Niña by the peak of hurricane season. La Niña years often feature above-normal hurricane activity because wind shear — which is detrimental to hurricanes — is usually lower than normal.

But more recent model runs now indicate a slower onset of La Niña and, on balance, a weaker event. This would decrease the chance of above-normal Atlantic hurricane activity by reducing the chance wind shear eases by the season’s peak.

Seasonal cycle of average daily accumulated cyclone energy (ACE) values. Daily contributions from Bonnie, Colin and Danielle are shown by yellow vertical bars.

The start of peak of the season, in mid-August, is now just a few weeks away. With or without La Niña, the probability of storm formation will ramp up substantially. And, irrespective of how many storms ultimately develop, it just takes one landfall to have major — if not devastating — impacts in a particular area. Especially given the amount of warm ocean fuel available to storms this year, we need to be especially vigilant.

The next name on 2016’s list of storms is Earl (The name Earl was first used in 1980, and this year will be its seventh reincarnation!). A tropical wave is just exiting the African coast Wednesday, and some models are forecasting possible development three to six days from now, though such forecasts have a high false alarm rate.

(Jason Samenow and Phil Klotzbach contributed to this piece.)