We should all know by now that the official hurricane season is June 1 – November 30. That’s what the powers that be designated as the most likely time to see tropical storms and hurricanes form in the Atlantic Basin. Within that 6-month window (hard to believe hurricane season takes up half the year) there is a pronounced uptick in activity that many refer to as “the real hurricane season”. It usually begins around mid-August and lasts until roughly the end of October. Some years, like 2005, are exceptional for the amount of activity spread out over the entire season. Other years, like 2013, it seemingly never comes. On average, however, today is generally thought of as the beginning of the real hurricane season and if it’s going to be active, this is when we begin to find out.
Before jumping in to what to look for in the coming days and weeks, let’s examine a few of the larger pieces of the puzzle that could help to shape the peak months of the season.
Sea surface temperature anomalies showing the obvious difference between 2015 and 2016 with a much colder tropical Pacific this year along with a warmer tropical Atlantic as well
ENSO or El Niño Southern Oscillaion
This one is quite simple. El Niño tends to kill Atlantic hurricanes, especially in the deep tropics. It did a great job of that last year (Joaquin formed outside of the deep tropics from a non-tropical area of low pressure). The prediction was for there to be no El Niño this season and that has come to pass. In fact, the tropical Pacific is generally a lot colder now than it was last August. This is regarded as a plus for Atlantic hurricane development. While we are not seeing a true La Nina just yet, at least according to official standards, the Pacific has cooled significantly and you can see this clear as day in the two images I have posted here.
Atlantic Sea Surface Temperatures
While the Pacific has cooled, most of the tropical Atlantic, Caribbean and Gulf of Mexico has warmed to above normal levels. In fact, areas in the western Atlantic are as warm as I have ever seen them with anomalies (departure from normal) running several degrees above the long term average. This provides more fuel for hurricanes but not just at the surface. The upper ocean heat content is also very rich right now in areas close to land. The amount of available energy is somewhat astounding and if a hurricane comes along under the right atmospheric set-up, we could see intense development and not the weak, lop-sided storms we’ve observed in recent years.
Note that warm water alone does not yield hurricanes. It is certainly a must-have ingredient but it is not the catalyst by itself. Despite the record warm water temps, other factors need to be favorable or nothing of note happens. I’ll go over some of that as I continue on…
Shear analysis showing favorable (green) in the east Atlantic but quite unfavorable (red) in the Caribbean and Gulf of Mexico
Shear analysis showing favorable (green) in the east Atlantic but quite unfavorable (red) in the Caribbean and Gulf of Mexico (click to enlarge)
Wind shear or the change of wind direction and/or speed with height in the atmosphere is a very important piece of the hurricane puzzle. Too much strong wind blowing against a developing or already developed hurricane usually leads to its demise. In 2005, for example, shear values over the Gulf of Mexico were close to zero for much of the season, especially when Katrina and Rita formed. These were ideal conditions, hard to duplicate (thank goodness) but it does happen.
Right now, the signal is mixed. Shear values over the eastern Atlantic are favorable for development. However, for the time being, the farther west one looks, the more the shear increases, literally screaming out of the west across the Gulf and Caribbean. This is a deviation from what we’ve seen for a good chunk of the season as shear has generally been favorable until recently. The pattern right now is such that anything tracking west will run in to this shear zone and be torn apart – unless the pattern changes, which it can but so far, I don’t see it doing so right away. This is something we will have to monitor almost daily since things can evolve rather quickly and change the outcome in a hurry.
Overall, compared to 2015 especially, shear is down and for the most part, this is a favorable sign for Atlantic hurricanes.
Tropical Atlantic vertical instability (blue line) is quite below where it should be (black line)
The lack of an unstable atmosphere is something that has been in place for several years in a row. Simply put, warm, dry air has been capping thunderstorm development in the MDR or Main Development Region of the deep tropics. Moist air, filled with energy, needs to lift in to colder air, if warm air is present over warm air, then there is little in the way of convection and thus we don’t have showers and thunderstorms developing which are THE mechanism for developing hurricanes. No convection = no hurricanes. For some reason, dry air, even when the famed SAL or Saharan Air Layer is not present, has been dominant for the past several hurricane seasons. This mid-level layer of stability has resulted in very little action in the MDR – the area between Africa and the Lesser Antilles. If memory serves, I think that Ike in 2008 was the last of the long-track hurricanes. Others such as Irene in 2011 struggled despite forming in the MDR. It is something new that we’ve been seeing and needs further study since it seems to be rather persistent.
So far this season, instability has been generally lacking again across the tropics and unless this changes, tropical waves will struggle until reaching more unstable air farther west.
This brings up an interesting scenario. If the tropical waves struggle out in the open Atlantic, everyone seems happy and safe since hurricanes have a hard time in dry air. Ok, well, what happens when those same tropical waves struggle all the way to say, 70 degrees W longitude? At that point, they blossom under a much more favorable environment and become hurricanes much closer to land areas. Earl was a classic example of this. It “waited” until right before landfall and sprung to life. Ask the people in Belize and southern Mexico about the “struggles” of the parent tropical wave that became Earl. Yes, it could have developed much sooner and become an intense hurricane but my point is, the later they wait to develop, the better the odds they hit land. So while the dry air may be impeding development out in the deep tropics, don’t be fooled by people who say this is a uniformly good thing. Sometimes the opposite is true.
Sea Level Pressure
Air has weight. We call this air pressure. High pressure squashes cloud formation generally (especially convection or thunderstorms) and low pressure tends to foster storminess. In the tropics, lower surface pressures helps to make the air slow down and converge, allowing it to pile up and promote thunderstorm development. Higher surface pressures typically doesn’t allow this set up and is thus a negative factor for development. So far this season, surface pressure in the deep tropics has been higher than normal more so than lower than normal. It’s not outright terrible for development but it hasn’t helped much for sure. This, like shear values, needs to be monitored on a weekly basis as it can change and lead to a burst of activity when pressures lower.
Going forward – the next 60 days
If you add it all up, the ingredients are there for a busy hurricane season but not so much so that it will set records. There are still other aspects that are more difficult to go in to or that I myself don’t quite understand well enough yet. However, the larger, more obvious pieces to the puzzle indicate enough positives for an active season to warrant concern – more so than the past few seasons for sure.
For me, the single ingredient that worries me the most is the very warm water in the western Atlantic. While it is obvious that if no hurricanes track in to this warm water it won’t matter, what if they do? Remember what I said about the weak, struggling tropical waves? If even a few survive to form later and farther west, it could mean they make it in to the area of very warm water. It’s something we will just have to wait and see about.
So that brings me to the current set up.
Current area of interest, 98L, southwest of the Cape Verde Islands
Right now, we do have a strong tropical wave in the deep tropics. It emerged from the African coast a couple of days ago and is moving generally westward. The NHC gives it fairly low odds of developing further but some of the computer models are more favorable for continued development.
Warm water is plentiful but the dry air is not far away from the system. If it manages to survive the next few days, shear awaits unless it tracks north of the Caribbean. It’s going to be one of those “wait and see” situations. However, I find it ironic and perfectly timed since today is typically the beginning of when we look out across the tropical Atlantic for signs of activity. Right on cue, we have something to watch. Whether or not it becomes noteworthy remains to be seen.
The next 60 days will probably define the hurricane season. Will it end up being a non-event for U.S. interests once again? Or, will the 10+ year streak of no category three or higher hurricanes making landfall come to an end? The stage is set, maybe not for an award-winning performance so to speak, but certainly more so than in recent years. Right now, it is still quiet. There is time to prepare if you feel so inspired. The historic flooding in Louisiana should serve as a reminder that we’re not talking about only wind or surge here. It’s hurricane season and the peak months are just beginning. I’d say “be ready” because it sure seems like all that warm water won’t go untouched.
I’ll have more on the current goings on in the tropics with my video discussion to be posted later this afternoon.
M. Sudduth 9:20 AM ET Aug 15