July Tropical Update

By: - July 7th, 2017

With the formation of Tropical Depression Four (TD4) on July 5th, I wanted to remind the insurance industry of what to expect this hurricane season. TD4 is now located 690 miles to the east of the Lesser Antilles and is not currently a threat to the insurance industry due to expected weakening in the coming 24 hours. However, keep an eye on the location of it’s left over moisture plume later next week as it could be a sign of future storm tracks towards the heart of the 2017 hurricane season.
In my May 15th tropical update, I went into detail discussing how many early Atlantic Basin seasonal hurricane forecasts were putting a lot of weight into the formation of an El Niño event during the 2017 Atlantic hurricane season, but spring El Niño forecasts can be notoriously misleading. In fact, the seasonal climate models continue to back off the idea that an El Niño will develop, and currently there is only a 40% chance of an El Niño with neutral condition developing during the heart of the hurricane season.

I also mentioned in the May 15th update that many of the early season forecasts would likely be adjusted upward to a “more active or normal season,” and this is what has happened. In fact, on July 5th, Phil Klotzbach and the team of seasonal forecasters at Colorado State University revised their forecast and now call for an above average hurricane season with 15 named storms, including 8 hurricanes and 3 major hurricanes during the 2017 Atlantic hurricane season.

A great collection of 18 different seasonal hurricane forecasts can be found here:

As indicated in my May 15th update, although sea surface temperatures are conducive for tropical development in the Main Development Region (MDR), a large negative influence on named storm activity over the last few years has been the Saharan Air Layer (SAL). This is a factor this year and is likely the reason why TD4 has struggled as it tracks across the MDR.

Dusty dry air to its east of TD4

The SAL will likely continue to be an influence on named storm development in the MDR for the remainder of the season, but there will be windows of less dust which will provide opportunities for named storm development.

12z NASA-GEOS5 dust forecast for next 5-days shows series of typical Saharan air layers heading west across tropical Atlantic from Africa.

These opportunities for storm development could arise with the passing of the lesser known phenomenon of the Madden Julian Oscillation (MJO). The MJO is long known to influence tropical convection as it traverses the planet’s tropic regions every 30 to 60 days. During times when El Niño neutral, like it is forecasted to be this year, this oscillation brings more positive atmospheric upward motion and can help us determine when the Atlantic Ocean might have more named storm activity. During the first passage of the MJO this year, the Atlantic saw the formation of tropical storms Cindy and Bret. Although TD4 is likely struggling because of the SAL, the Atlantic basin is also now in the suppressive phase of the MJO which limits large scale tropical development due to a large scale atmospheric sinking motion. The next phase of the MJO which could possibly enhance Atlantic named storm activity would occur near the end of July or first part of the August, followed by a suppressive phase in the middle to end of August. If the pattern holds, it could be followed by an active phase of the MJO again in mid-September which could enhance named storm activity at that time.

With the understanding that there could be pulses of activity in the Atlantic Basin associated with the MJO, where might these storms track? As highlighted in the May 15th update, the more important concept to grasp is where named storms may make landfall and not necessarily the total number of named storms that will develop. The climate forces this season are much more conducive to named storm development closer to the U.S. coastline. This, combined with the current placement of the Bermuda – Azores high pressure and the high pressure that has been dominating the central high plains states, would allow for storms to track toward the east coast of the U.S., putting the coastline at higher risk of landfall if storms were to develop there. Add in the fact that the sea surface temperatures off the east coast are warmer then average, and this has the potential to create interesting conditions for named storm activity later this season.


Long range models continue to develop tropical waves over western Africa into named storm. Could these thunderstorm over central Africa be the next system of interest?

With that being said, the long range forecast modeling always seems to try to spin up a named storm in the long range, with most of these scenarios often failing to come to fruition. In looking at the extreme long range forecast (next 46 days), the Atlantic basin looks to remain quiet with only a few hints at Cape Verde storm development through mid August.

Not much activity for the next 45 days as indicated by the ECMWF MEPS Cyclone Tracks by WeatherBell. However, a few Cape Verde storm development so need to keep as we get into the heart of the season.

This is interesting because globally named storm tropical activity in terms of Accumulated Cyclone Energy (ACE) is at record low levels. In fact, the North Pacific has had 0 typhoons so far in 2017, and this is only the seventh time since 1950 that this has happened through July 7th of a season. This also follows the quietest season on record for the southern hemisphere, which recently just ended. It should be noted that years with an ACE under 200 in the southern hemisphere have also managed to be below normal in the Atlantic, with a few exceptions like 1995. Based on the seasonal forecasts, this year looks like it could also potentially be an exception due to a higher risk of named storm activity near the east coast of the U.S., as storms struggle in the MDR.