Every year just before the June 1st start of the Atlantic hurricane season, the NHC [National Hurricane Center]  and  Dr. Gray of CSU [Colo. State] and others put forth their forecast for the season. I had intended to write about Atlantic hurricane season forecasts at that time, but it started to Snowden (pun intended). Since then there have been intermittent blizzards followed by massive flooding that seems to bring the fecal matter right out of the sewers into the open — but that’s a different sort of weather report for other posts. I will stick to the actual planetary weather for the rest of this article.

The pre-season predictions this year were for a very active hurricane season.
The reasons given for the very active forecast were:

  • high ocean temperatures in the tropical Atlantic (south of 25N),
  • an active equatorial monsoon coming into the Atlantic off the west coast of Africa and
  • continued ENSO neutral conditions in the mid-Pacific (meaning no El Nino expected)

It can also be noted that the higher than average ocean temperature and the elevated equatorial humidity are both expected outcomes of global warming, although both are also associated with known multi-year global scale oscillation patterns as well.

Here are the predictions from NHC and CSU versus the averages over the period 1981-2010:

# of Tropical Storms (sustained wind >= 37 mph):
Avg……12
NHC…..13-20
CSU…..18

# of Hurricanes (sustained wind >= 74 mph):
Avg……6
NHC…..7-11
CSU…..9

# of Major Hurricanes (sustained wind >= 111 mph):
Avg……3
NHC…..3-6
CSU…..4

Percent of median ACE [Accumulated Cyclonic Energy]:
Avg……103%
NHC…..120%-205%
CSU…..175%

Here are links to the detailed official (NHC) and NHC recommended expert (CSU -PDF) reports predicting a very active 2013 Atlantic hurricane season.  CSU is good enough to publish an archive of all their past predictions.  They also predict a host of other measures, including a 140% long-term average chance of a US landfalling hurricane – I think that rounds to 2 (two).

So, how does Chantal validate these predictions?

It seems odd that just the third Atlantic storm of the season could validate the prediction of a very-active season. It might be worth noting that Chantal is over a month ahead of the average date of the third named Atlantic storm, except an active June & July has a slightly negative correlation with total season activity. So what gives? The answer comes from the excellent NHC Tropical Cyclone FAQ.

In the specific page titled: G7) Does an active June and July mean the rest of the season will be busy too? the correlations of early season and total season activity levels are given.  After the surprising weak negative correlation the following fact appears: If a named storm starts in the eastern Caribbean or further east in the central Atlantic during June and July then the season will likely be active.  The NHC calls it a sufficient condition to say the season will be at least average and most likely the season will be active.  ” According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often times above average.”  They also promise that the August forecast updates will mention Chantal (and?), because Chantal formed well east of the Caribbean in the Atlantic.  So, if this season turns out to have less than average activity, it would be a first.

I think it is significant that Chantal formed when the entire eastern and central tropical Atlantic was under the influence of the SAL – the Saharan Air Layer.  Chantal formed and survived under adverse conditions: the SAL air is dry and dusty.  As humid air mixes with dry, dusty SAL air the moisture is adsorbed onto the dust and the result is still dry, dusty air until the dust is largely saturated with water or washed out of the air.  A heavy, deep SAL area can suppress most storm activity and thereby the SAL can make tropical storm formation nearly impossible.  A storm forming on the edge of the SAL, like Chantal, will draw in SAL air and will often end up choking and sputtering on it, either remaining very poorly organized or just dissipating.

It is also interesting that Chantal formed within a strong tropical wave and its associated region of high humidity which rolled off the coast of Africa (tropical waves happen every few days).  Unsurprisingly, Chantal has begun to strengthen as it pulls away from the SAL and into the region of higher ocean temperature (>85 F) and higher humidity in the eastern Carribbean.

Chantal was possible because of the strength of the tropical wave and the amount of moisture carried with that wave. These were generalized predictors for an active season: here they combined in the creation of a mid-Atlantic early season storm, in itself a firm predictor of an active Atlantic hurricane season. If you’re in a hurricane prone area (especially SE Florida), get ready folks… hurricane season has begun with a bullet.  Even better, as the CSU report says:

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.

Chantal predictions

Chantal truly has been a bullet — moving at or faster than 25 mph for its entire life. One of today’s earlier forecast discussions contains these lines: “CHANTAL IS RACING TOWARD THE WEST-NORTHWEST OR 295 DEGREES AT 25 KNOTS. I AM SURPRISED THE SYSTEM EVEN HAS A CLOSED CIRCULATION MOVING AT THAT SPEED.” (the NHC and NWS are always shouting – perhaps a hold over from teletypes? – and they usually don’t talk of surprises at all ;~).

Chantal is expected to be near hurricane strength when it passes over Hispaniola (Haiti/Dominican Republic) and eastern Cuba. The passage over the mountainous islands will then almost certainly reduce the intensity, perhaps even below tropical storm levels. The current NHC predictions call for Chantal to fail to become a hurricane at any time in the next five days – the near unanimous prediction is that Chantal ends up weakened and in the Bahamas within a few days, possibly approaching the SE US coast (FL, GA, SC) within 4 or 5 days.

I put on my doomcaster hat

For Chantal, I will put on my doomcaster hat and say that this storm will be a hurricane before leaving the Caribbean within the next day or so. Here are my reasons:

  • sea surface temperatures: SST >= 29 C or SST > 85 F;
  • its solid alignment of lower level convergence and strong vorticity with upper level divergence;
  • moist air around nearly all of the storm (leaving SAL mostly behind);
  • the way Chantal has begun to draw moist hot air off of S. America;
  • the lack of shear or at least the failure of shear to prevent outflow;
  • the recent strengthening and expansion of the storm (up to and through some undefined, personal heuristic of ‘symmetric critical mass’ while moving over hotter water so that Chantal nearly fills the eastern end of the Caribbean == look out).

Chantal will still weaken in traversing the Greater Antilles, but I think it starts that weakening as a hurricane.

The NHC predictions are usually remarkably accurate though it seems to me they underestimate the speed of increasing intensity fairly often. The NHC says ~25% chance of hurricane strength before landfall – I would go much higher to >~75% and therefore say Chantal will be a hurricane before landfall. That’s just me, my heuristics, my reasons and hunches, my self-taught scraps of meteorology and, of course, my doomcaster hat. We will see how it turns out real soon – within 24 hrs or so.

Either way, the formation of tropical storm Chantal in early July, on the edge of the SAL, east of the Caribbean says:
All signs now point to a busy Atlantic hurricane season.

Public domain photo by NOAA.