MAweatherboy1's Blog

August Update: 2016 Atlantic Hurricane Season

By: MAweatherboy1, 8:06 PM GMT on August 07, 2016

With the calendar now turned to August, we are well on our way to the peak of hurricane season. With the dissipation of Hurricane Earl, the Atlantic has now seen 5 named storms and 2 non-major hurricanes, a little ahead of where we should be for named storms but very close to normal for ACE based on 1981-2010 climatology, via Dr. Ryan Maue's global ACE page. The National Hurricane Center is monitoring a couple of disturbances in the Atlantic that each have low probabilities of developing into tropical cyclones in the next few days, but odds favor no or limited development through the next 5 days.

With the basin largely quiet, the question many are asking, and many more will ask over the next week or two, is, "Where are the hurricanes?" Well, they aren't in the Atlantic. Instead, after a very slow start to the season in the West Pacific basin, we are entering into an active typhoon regime for that basin, and that is where the bulk of the tropical action will take place for the next 10+ days. We've seen the formation of Tropical Storm Omais, and that is likely just the appetizer with multiple typhoons forecast by most of the global models over the next 10 days. Even the East Pacific will take a backseat for awhile, as Tropical Storm Ivette and the newly minted Tropical Storm Javier fade out over the next few days.

What we are seeing now around the global tropics is largely the result of the Madden-Julian Oscillation, or "MJO". As described by NOAA's Climate Prediction Center, the MJO "is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days." In other words, it is akin to a large scale area characterized by rising motion and upper-level divergence, which aids in the formation of thunderstorms, which can in turn develop into tropical cyclones if other conditions permit. In the West Pacific in August, the "other conditions" (SSTs, shear, instability, etc.) almost always permit, and with the aid of the MJO it is no surprise that basin will turn very active. But as the saying goes, what goes up, must come down. All the rising air being produced in the West Pacific comes at the expense of sinking air on the opposite side of the world, which in this case is much of the Atlantic and East Pacific. The tropical Atlantic is truly a wasteland right now by August standards. The natural sinking air caused by the downward MJO phase is combining with dry, dusty Saharan Air Layer (SAL) activity to cause a massive plunge in Atlantic instability. By that metric, the conditions in the basin are more characteristic of late April or early May, well within the "off season" period of the Atlantic.


Figure 1: 6z GFS 200mb velocity potential anomaly at 18z on 8/7. Green represents the aforementioned rising air and divergence aloft, while oranges represent sinking air.


Figure 2: A graph of vertical instability in the tropical Atlantic. After an instability spike preceding the formation of Earl, this plunge proceeds at least a week of very quiet conditions across the tropical Atlantic.

Naturally, you are much more likely to get tropical cyclones forming in regions of rising air as opposed to sinking air. And this is the reason why, despite climatology arguing for increased tropical activity in the Atlantic, we are actually seeing a decrease in activity. It is true that, especially as the peak approaches, tropical activity in the Atlantic (and any basin) can occur independently of the MJO. But this is a rather strong pulse pushing into the West Pacific, and it stacks the odds heavily against development in the Atlantic, especially concerning stronger storms. I do not expect to see any hurricanes in the Atlantic through at least 8/20; I would put the odds of any named storms in that time at only around 30%.

So are we on our way to another below normal year of hurricane activity in the Atlantic? Not necessarily. We've explored what the next 10-14 days will look like, but that only takes us to around August 20. There's plenty of hurricane season to go after that. Make no mistake, we are now in the peak window, which I typically think of as August 1-October 15. Presuming little to no activity through August 20, the Atlantic will be digging out of a deepening hole to get to normal levels of activity for the season. But I go back to my previous hurricane season predictions for this year, and find that this season is really right where I thought it would be. La Nina, while fighting the influence of a warm PDO, is continuing to gradually settle in. This is contributing to near to below normal wind shear across the Caribbean and the rest of the tropical Atlantic, a positive for hurricane formation. Depending on your methodology, the Atlantic Multidecadal Oscillation (AMO) continues along in a neutral to positive state, with water temperatures generally above average in the tropics but with a classic strong +AMO signature still lacking. It's all going pretty much according to plan.


Figure 3: My take on the current Atlantic SST configuration.

My final preseason numbers released in May were 13 named storms, 7 hurricanes, and 2 major hurricanes, with ACE 90% of normal. In other words, a near normal season. A few days ago, Dr. Phil Klotzbach of Colorado State University issued CSU's August update of tropical cyclone activity. He is now calling for a season total of 15 named storms, 6 hurricanes, and 2 major hurricanes, with ACE of 100, which is very near the 1981-2010 normal. I am not changing my numbers today, and will stick with them now through the end of the season. I think they are a good compromise between the short term impulse to lower numbers and the clustering of above normal activity forecasts that now seem increasingly unlikely.

Some reasoning behind my numbers: We know we're in a lull now. However, if you look at almost any hurricane season, even active ones, save for perhaps 2005 and one or two others, very few saw near continuous activity throughout the entire August 1-October 15 peak. As we know, 2010 did not become active until after August 20. The years 1998 and 1988, both potential analogs for this year based on ENSO, saw little activity before August 20 as well but were near (1988) or above (1998) normal. Most other years as well see a couple weeks of break in the action during the peak months. The MJO will not remain in the West Pacific forever, and once it moves out of there, even if it doesn't move directly into the Atlantic, chances for activity will increase. Extrapolating out the timing, things should set up favorably for the peak 2-3 weeks of our season at the tail end of August and early-mid September. It will take some time, as the MJO will move only gradually, and the Atlantic will need time to re-prime itself in terms of instability and a more vigorous African wave train. I don't expect this to be another 2010 nor particularly close to it. The other extreme alternative would be a season like 2013 in which the basin never really becomes active. I don't see that happening either, given enough parameters that are otherwise favorable.

I've said several times this season, both during the early active period and the quiet July, that neither of those early season periods meant a whole lot. Climatology is unbending in its assertion that August, September, and October are always the months that determine whether a season is statistically above or below normal. We're in that window now, and the big question for the season has become whether we will balance a below normal start to the peak months with an above normal period of activity in late August, September, and/or October. I'm comfortable with where my numbers stand, and look forward to tracking whatever the rest of the season may bring.

Of course, I would be remiss if I didn't point out that I'm speaking heavily from a scientific and statistical viewpoint when I talk about "activity". As we all know, especially those who have experienced hurricanes directly, it only takes one storm to make a season memorable for millions of people in the path. In addition, I continue to believe the highest concentration of developments and ACE points will be closer to the US and Caribbean this year. We've already seen one case with Earl in which tropical waves struggle east of the Caribbean, then organize as they push west. This increases the risk of impacts in the US and Caribbean, especially given the warm waters in close to these regions. Very warm water also exists off the East Coast of the United States, and any cyclone or developing cyclone that enters the region near the Bahamas will have to be watched for quick strengthening. Finally, given the cooler ENSO state this year, the climatological increase in wind shear in the Caribbean that typically heralds the end of the real season may be delayed this year, and given the extreme heat potential in these waters, the odds of a late season hurricane in the Caribbean will likely be enhanced this year.


Figure 4: Tropical Cyclone Heat Potential (TCHP) in the Atlantic. TCHP values in the Caribbean are extremely high, and could aid in hurricane development later this season.

Summary of Main Points
*A generally quiet 10-14 days ahead
*Conditions should set up favorably for an active 2-3 weeks in late August through mid-September, with development opportunities lasting well into October.
*Continue to believe overall activity ends up near average (13-7-2).
*Continue to believe the US and Caribbean are at increased risk of hurricane impact, which we have already seen signs of with Earl.

For my thoughts from earlier in the year, which largely hold true now, see my original forecast from March and my May update.

Thank you as always for reading. This will be my final outlook for the 2016 season; future posts discussing overall activity will be of the postmortem variety. Depending on my time, which has been pretty stretched this summer and will remain that way as school begins in a few weeks, I may have entries on individual storms as well. Have a great week!

-MAweatherboy1


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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About MAweatherboy1

Average 20 year old weather nerd. Plymouth State University Meteorology, Class of 2018. NOAA Hollings scholar. Summer 2016 intern at NWS Boston.