Above: Sea surface temperatures (SSTs) analyzed at 2 am EDT Tuesday, July 3, 2018, across the North Atlantic, as compared to the 1981-2010 average SST for this time of year, in degrees C (see scale at right). SSTs across most of the tropical Atlantic between the Antilles and Africa were running cooler than average, while the subtropical Atlantic SSTs were generally warmer than average. The cooler waters over the deep tropics are expected to play into a less-active-than-usual Atlantic hurricane season. Image credit:  tropicaltidbits.com.

Chiller-than-usual waters in the eastern North Atlantic have prodded the forecast group based at Colorado State University to reduce the amount of tropical cyclone activity they project for 2018 from the values predicted only one month ago. In their July update issued on Monday, CSU’s Dr. Phil Klotzbach and Dr. Michael Bell are now calling for a total of 11 named storms, 4 hurricanes, and 1 major hurricane for the season, including the named storm we’ve already had (Alberto).  This is a major downgrade from the outlook issued on May 31, which had been calling for 14 named storms, 6 hurricanes, and 2 major hurricanes. The amount of accumulated cyclone energy (ACE), which takes into account both intensity and duration of each cyclone, is now projected to be 60 for the Atlantic season— only about two-thirds of its typical value for the period 1981-2010.

“I don't really see any way that this season ends up active at this point,” said Klotzbach in a Twitter message. “The Atlantic remains colder than normal, and the odds of El Niño look to be increasing too. We're also already starting to see stronger than normal wind shear in the Caribbean.”

CSU’s early July outlook was introduced in 2016 based on a fairly simple but quite effective statistical technique that relies on just two factors: the May-June sea surface temperature (SST) across the eastern North Atlantic and the June sea level pressure over the eastern tropical Pacific (see Figure 1 below). Together, these two easily assessed ingredients explain about 60% of the season-to-season variation in Atlantic ACE from July onward.

Predictors for CSU's July Atlantic hurricane outlook

Figure 1. The two main factors that feed into CSU's July hurricane outlook are the the May-June sea surface temperature (SST) across the eastern North Atlantic (in a region bounded by latitudes 10°N and 50°N and longitudes 10°N and 30°W) and the June sea level pressure over the eastern tropical Pacific (in a region bounded by latitudes 15°S and 15°N and longitudes 150°W and 110°W). Image credit: Courtesy Phil Klotzbach, CSU.

The main driver for this year’s tepid July outlook was the cooler-than-average May-June SSTs over the eastern North Atlantic (see image at top). Cooling in this region often goes hand in hand with relatively cool waters in the Main Development Region (MDR) of the tropical Atlantic, and in turn with slower-than-average hurricane seasons. In fact, the waters of the MDR were the coolest for mid-June since at least the mid-1980s, as Klotzbach pointed out in a Capital Weather Gang post on June 18. The cooling happened quickly over the last several months, Klotzbach explained, as high pressure intensified over the Atlantic subtropics, strengthening the trade winds and stimulating upwelling of cooler water.

In contrast, there was little influence on the July outlook from sea level pressures in the eastern tropical Pacific. With neither El Niño nor La Niña in place, these were running near average. However, an El Niño Watch is now in effect, and the odds appear to be rising for at least weak or borderline El Niño conditions by August, which could put a further brake on tropical activity in the Atlantic.

Overall, the CSU outlook is for a hurricane season much more like those in the relatively calm period from the 1970s to the early 1990s as opposed to the more active period that’s prevailed from 1995 onward. “My thoughts are that this season will be quiet in the deep tropics. It's going to be more the subtropical-type formations that we need to watch for,” said Klotzbach. This is a good time to remember that 1992 was a largely tranquil year—apart from Andrew, which hurtled into the Miami area as a fast-developing Category 5 and caused massive destruction. Andrew didn’t strengthen in a big way until it moved out of the deep tropics and approached the Bahamas. Right now, Atlantic SSTs are running above average north of about 25°N, and more than 1°C (1.8°F) above average over the northern Gulf of Mexico and over large parts of the North Atlantic between about 30°N and 35°N.

In their outlook, Klotzbach and Bell reiterated their usual note of caution: “Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much activity is predicted.”

Forecast

Figure 2. Predicted tracks of a potential tropical depression or tropical storm from the 0Z Tuesday European model ensemble forecast (left) and 0Z Tuesday GFS ensemble forecast (right). The tracks from the operational version of the models is shown in red (presumably the best forecast). About 20% of the ensemble members of the European model and over 50% of the members of the GFS model predicted that a tropical depression would form late this week. The purple dots show where the predicted storm is at tropical depression strength, and the blue dots, tropical storm strength. None of the ensemble members predicted that a hurricane-strength storm (light blue dots) would form. Image credit: cfanclimate.com.

Category 6 Jeff Masters