TS Leslie developed some very deep convection over night with cloud tops reach an astonishing -88 degrees Celsius. Those cold cloud tops, however, did not wrap all the way around the center of the storm. This is due to the continuation of vertical wind shear which has been just enough to keep Leslie from strengthening much. I do think the burst of deep convection is a sign that Leslie will intensify given the right upper level conditions.
The current forecast track has not budged much from recent ones and it looks like a slow and steady course to the north-northwest throughout the week ahead makes the most sense considering the pattern. There will simply not be enough western Atlantic ridge to push Leslie back to the west enough to directly impact the U.S. That being said, I do believe that an increase in long period swells is coming – especially once Leslie becomes a hurricane. Its slow movement will allow for quite a build-up of energy in the ocean and this means a great surf weekend coming up for portions of the East Coast. It also means an increase in rip currents and as such, people heading to the beach need to be mindful of this hazard. We are still a few days away from the swells reaching the U.S. and I will address this again later in the week.
As for Bermuda, it looks as though Leslie could come fairly close to the island by the weekend as a strong hurricane. Some of the intensity models suggest that Leslie could approach or exceed category three intensity towards the end of the week. Interests in Bermuda need to pay close attention to the future track and strength of this storm. It also looks like Leslie will be quite large with an expanding wind field so even a brush with Bermuda could mean a period of high winds and very rough seas for the area.
Looking at the rest of the tropics, we have invest area 99L well out in the east-central Atlantic. It may develop some but is small in size and poses no threat to land areas.
In the eastern Pacific, TS John has formed to the southwest of the Baja peninsula and will move swiftly off to the northwest and not be a bother to Mexico.
Checking on the current state of the El Nino, we see that it has, in fact, backed off quite a bit in recent weeks. Comparing the SST anomaly maps from August 2 and the one from today, it is clear that the warming trend has stopped and even reversed in the eastern Pacific. This means that the threat of a significant El Nino event during the hurricane season is likely minimal at best. In other words, the negative conditions that a Pacific El Nino would typically bring to the Atlantic Basin are probably not going to be there.
The reason El Nino affects the Atlantic is similar to how a large hurricane, with its well established outflow, affects other tropical waves moving across the Atlantic. Remember TS Jean which formed east of Isaac? The large scale upward motion of Isaac led to strong outflow high in the atmosphere and this literally tore Jean apart and caused it to weaken and dissipate. An El Nino event, especially strong ones, causes tropical convection to persist across a good deal of the Pacific. This upward motion on a basin-wide scale acts to spread strong upper level winds across the deep tropics of the Atlantic, much like the outflow from a hurricane. This shear stops developing Atlantic systems in their tracks. Because we are not seeing a significant El Nino event taking shape right now, I suspect that September will be quite busy with a shift in activity more towards the Caribbean Sea and Gulf of Mexico. This is a natural evolution in any season when we see the tropical waves that come off of Africa lose their punch and the build up of low pressure in the western part of the Atlantic and Caribbean leads to development there.
The long range models, and we’re talking beyond 10 days here, indicate this pattern change is coming. Once Leslie moves out, we will probably only wait a few days, if that, before we have yet another named storm.
Curiously though, we have yet to have a major hurricane this season in the Atlantic. The layer of persistent dry air has really put a lid on things, literally. I think that Leslie has a chance to become a category three hurricane as it nears Bermuda as the global models indicate what looks like a favorable upper level wind pattern for it to develop. We may have had a lot of activity, but no intense hurricanes just yet.
I will go over a lot of what I have outlined here in today’s edition of the Hurricane Outlook and Discussion video for our iPhone app. I will also post an update here in the blog this evening to reflect the very latest on Leslie and whether or not I may be taking a trip to Bermuda later this week.