Atlantic Hurricane Season Outlook for August 2020 (free)

Atlantic Hurricane Season Outlook for August 2020

by Dr. Ryan Truchelut – issued 6 August 2020

Key Points

  • WeatherTiger’s August outlook for the 2020 hurricane season is for a 85-90% chance of an above average year, with a 10-15% chance of near-normal and almost no chance of below-normal cumulative activity. The median of our forecast remains about 180% of average hurricane season activity, or around 21-24 named storms, 9-12 hurricanes and 4-6 major hurricanes. 
  • Overall, model skill improves notably at this range, and many convergent lines of evidence from our objective modeling and seasonal analogs support an active season, with the potential for a risky steering current regime in the peak months. Warm sea surface temperatures in the Atlantic, a developing La Nina, and a favorable thermodynamic environment for tropical waves over the MDR are all leading indicators of hyperactivity in the peak months of August, September, and October.

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Each year’s list of Atlantic tropical cyclone names has twenty-one entries: one for all twenty-six letters of the alphabet, less Q, U, X, Y, and Z.

If there are more than twenty-one named storms in a season, the back-up names are the twenty-four letters of the Greek alphabet. In the vanishingly remote case that additional storms develop beyond the surely world-ending Hurricane Omega, it is unknown what naming convention would be used. I humbly submit that it be a steadily escalating number of poop emojis.

WeatherTiger’s updated seasonal forecast for August finds despite a 2020 tropical rollercoaster already exceeding the total U.S. impacts of many full seasons, our punishment must yet be more severe. We predict a roughly two-thirds chance of twenty-two or more named storms this year, along with total tropical activity 80% above normal.

In other words, it’s going to be a big fat Greek hurricane season.

This forecast is a reflection of both a harrowing first two months of the season, and signals for a hyperactive August, September, and October from historical leading indicators. Through early August, U.S. landfall activity in 2020 is higher than what occurred in nearly half of all seasons since 1900 for the entire year. This is despite only about 20% of the hurricane season being over, on average, from a U.S. landfall energy perspective.

Thus far, two hurricanes and three tropical storms have struck the U.S. coastline. This equals and exceeds, respectively, the number of total U.S. impacts in 2019. And while both Hanna and Isaias were category 1 intensity at landfall, they punched above weight in terms of wind and flooding impacts.

This week, Hurricane Isaias fumbled its approach to Florida but spread four to eight inches of flooding rains and damaging wind gusts exceeding 70 mph across the rest of the Eastern Seaboard. At some point, Isaias required watches or warnings for every inch of the mainland U.S. Atlantic coast, the most expansive advisory coverage since Donna in 1960.

While 2020’s nine tropical storms and two hurricanes have made their presence known, there is not a statistically significant relationship between activity or U.S. landfalls prior to early August and that occurring in the rest of the season. Rather, with 80% of the historical risk ahead, the concern going forward is a likelihood of favorable conditions for hurricane development in the busiest eight weeks of the season from mid-August to mid-October.

This window is the most active—50% of historical tropical cyclone activity occurs in September alone—because it is usually the only time when low-level instability, mid-level moisture, and upper-level wind conditions all align to support storm development over the Atlantic’s Main Development Region stretching from West Africa into the Caribbean.

Because oceans change slowly, looking at sea surface temperature (SST) anomalies in the spring and early summer offers some modest predictive power for the year ahead. However, seasonal forecasting skill improves from about 15% better than climatology in early June to a 40% improvement by early August, as mid-summer weather patterns in the Tropical Atlantic tend to lock into place through early fall. Uncertainties in ENSO trajectory are also much diminished in August vis-a-vis June.

This July, the full slate of leading indicators, including SSTs, trade winds, vertical shear, and average sea-level pressure, unanimously point to elevated peak season activity.

Qualitatively, the story goes like this: the near-record ocean warmth across the entire Atlantic Basin is increasing the latent heat energy available to storms and destabilizing the lower atmosphere; a developing La Nina in the central and eastern Pacific is keeping easterly trade winds in check and diminishing upper lows to reduce western Atlantic wind shear; persistent convection over East Africa is generating vigorous tropical waves, contributing both ample moisture and plentiful seeds from which hurricanes can grow.

Let’s look at the components of our predictor set one-by-one:

Any story about seasonal activity begins with Atlantic SST anomalies. While spring and early summer anomalies were modestly warmer than average, there was a significant warming of the MDR relative to normal in mid-July on the order of 0.5C. This warming was concentrated in the areas of the central Atlantic and Caribbean shown at left to have the strongest historical relationship with future activity, and the monthly mean SSTAs for July closely match the spatial distribution of the SSTA-ACE correlation coefficient. This is never a good sign.

Also not a good sign is that the top analog match to weighted Atlantic SSTAs is 2005, the second most-active hurricane season of all-time. The rest of the analog set also has some heavy hitters (1998, 1969), though also mixed in are some quieter years in the 50s.

Historically, the strength of July low-level zonal winds in the Caribbean is the best single predictor of August-October activity. Weaker easterly trades are more favorable for storm development as they allow tropical waves to more easily closing off of an earth-relative circulation, and also diminish one half of the vertical shear equation. Weaker easterlies than normal in July 2020 are shown in red above, and again fit neatly into the region where weaker easterlies have the strongest positive correlations with upcoming Atlantic ACE. The weighted analog set is once again mixed, but with a preponderance of active and hyperactive years (1995 included).

The 1995 analog is dominant in matching to July MSLP and lower-level humidity anomalies, taking the leading position in the optimized analog set for each. While the MSLP, easterly trades, and vertical shear were much lower than normal across the Atlantic this July, moisture availability lagged, particularly over the central Atlantic. This lack of moisture likely held the number of tropical waves developing the the MDR or Caribbean to “just” two– which is actually a very early start to the Cape Verde season.

However, as shown at left, the observed dryness in the Atlantic in July (in blue) is not actually in a location where those anomalies are statistically meaningful for the season going forward.

Rather, humidity was above normal over west Africa and the far eastern Atlantic, where there is a stronger positive historical relationship, indicative of a very strong easterly wave train emanating from an Indian Ocean standing wave. Mid-level moisture may have been a drag on July preventing a 2005-like outbreak of major hurricanes, but it should not be expected to continue to act as a brake on activity in the peak season.

Quantitatively, WeatherTiger’s predictive algorithm weighs these inputs to make a probabilistic forecast of seasonal activity. For August 2020, our projection is a 50% chance of between 145 and 220 total units of Accumulated Cyclone Energy and a most likely outcome of 180. Around 100 ACE units constitutes a normal season. The chance of 2020 breaking the all-time ACE record of 258, set in 1933, is roughly 1-in-10. Full probabilities are shown in the exceedance plot at right.

These numbers are broadly in agreement with other forecast groups, including Colorado State University, and translates into a 50% confidence interval forecast of four to six major hurricanes, nine to twelve hurricanes, and twenty-one to twenty-four named storms. (Nothing screams going Greek like shotgunning a 24-pack.) Our ACE forecast is unchanged since the June update, reflecting that these highly favorable conditions were generally expected to develop by our algorithm earlier this summer. However, because of the record nine named storms through early August, and two hurricanes, these numbers have been revised slightly higher since June.

A couple caveats to bear in mind here: the rapid warming of Atlantic SSTs mean that July monthly anomalies may not fairly represent SSTs going forward, should the state at the end of the month persist into August and beyond. Early August SSTs so far have not given back any of these late gains. Additionally, the persistent rising branch of the Walker circulation favoring east African convection is predicted by run after run of monthly modeling to persist through September, a configuration closely matching that observed in the maximally hyperactive month of September 2017. Additionally, MJO phase looks favorable for late August and early September Atlantic activity. So even though our ACE numbers are much higher than average, there are still some important factors to which our model is partially, if not fully blind. If I was to make a qualitative adjustment to this forecast, it would be up, not down.

So, a warning. The Tropics are likely to be fairly quiet for the next seven to ten days, though a tropical wave may try to organize in the central Atlantic by the middle of next week. Given everything I’m looking at, that’s not likely to be true much longer than that. Please take this opportunity to review your hurricane plan and replenish your hurricane kit in this reprieve, because life is going to come at us fast in the near future, 2020 style.

In conclusion, WeatherTiger’s seasonal outlook for August calls for continued hyperactivity in the Atlantic. And while we’re not forecasting the need for any emoji-based names, the peak of the season still looks, well, suspiciously like the one I referenced above.

It’s going to be a long, long, long three months. Keep watching the skies.

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P.S.: GIF version of this forecast:

Atlantic Hurricane Season Outlook for June 2020

by Dr. Ryan Truchelut – issued 8 June 2020

Key Points

  • WeatherTiger’s June outlook for the 2020 hurricane season is for a 75-80% chance of an above average year, with a 15-20% chance of near-normal and just a 5% chance of below-normal cumulative activity. The median of our forecast is about 180% of average hurricane season activity, or around 19 named storms, nine hurricanes and five major hurricanes. 
  • This is slightly above the consensus of some other recently issued forecasts, likely due to our model’s bullishness towards a La Niña developing by late summer or early fall. However, almost all forecast groups project above normal activity.
  • Overall, while model skill remains limited at this range, convergent lines of evidence from our objective modeling and seasonal analogs support an active season, with the potential for a risky steering current regime in the peak months.

Enjoy this free sample of WeatherTiger’s comprehensive hurricane coverage. If you like what you read, click here to sign up for our Hurricane Forecasting subscription service to have all of WeatherTiger’s seasonal hurricane outlooks, weekly tropical columns and video discussions, and individual storm forecasts sent right to your inbox as soon as they are released, plus lots of subscriber-only forecasts and insights.

On a lighter note: hurricane season.

The devil you know is back. Unlike a pandemic, at least taking down the shutters after a storm won’t make it return.

This doesn’t imply that hurricane season 2020 will be reasonable. WeatherTiger’s updated seasonal forecast and a consensus of other guidance favors an abnormally active year. We expect net activity of about three-quarters more than the average season, with 95% odds of more storms than Dr. Birx has scarves.

I’ll get into the forecast rationale and what it may mean for Florida, but first, a quick orientation for new readers: I’m Dr. Ryan Truchelut, President and Chief Meteorologist at WeatherTiger, a Tallahassee-based weather analytics and forensic meteorology firm. I have a doctorate in meteorology from Florida State University and 15 years of commodity weather and tropical forecasting experience. I provide hurricane analysis for USA Today Florida, and am running our hurricane forecasting subscription service for a fifth year, with nary a quiet season yet to the WeatherTiger name. Expect a column and Facebook Live discussion video each week, with more frequent forecasts as the situation warrants at this page.

As an aside, I’m also dad to a one- and three-year-old, so recently I’ve become half-time operator of a two-student preschool, R-Naught Academy (Mascot: The Fightin’ Fomites; Motto: “All your plans r-naught.”). If I’m slow returning e-mail this year, I’m probably pondering how the VeggieTales gang pick things up without hands while Minnie Mouse stickers are lovingly applied to my face. Send wine.

The fundamentals underpinning our June outlook are mostly unchanged since April. (You can read our earlier forecast here.) However, in the intervening time, significant uncertainty has come out of the forecast, requiring an upward revision to our forecast numbers. Water temperatures between west Africa and northeastern Brazil remain about 1°C above normal; more than any other region, warm spring anomalies there presage busy seasons. This oceanic signal is confirmed by robust spring heat in the lower atmosphere over the eastern Atlantic (above), with warmth in the regions with the strongest positive associations with Atlantic TC activity. As May continued, the Atlantic took on more of a classic positive AMO look, with a meathook of increasing warmth arcing from Brazil to Europe to Greenland. This tendency has become even sharper in the first 10 days of June, and gradual MDR warming looks to persist for the next few weeks.

Weighting the regions of the Atlantic that have a strongest relationship with ACE in the upcoming year, the top analog years for Atlantic SSTs to May 2020 are a quartet of active seasons: 1989 (137 ACE), 1961 (205 ACE), 2017 (225 ACE), and 1964 (170 ACE). In each of these seasons, May SSTs had a “bridge” of subtropical warmth extending from the Caribbean to Europe that merged with Equatorial warmth over the MDR over the summer months, leading to a favorable configuration of SSTs for the peak season. They also share cool-neutral or La Nina morphologies in the Pacific, so none of those analogs are immediately invalidated for ENSO reasons. Average ACE in these four seasons was 180-185, which agrees closely with our objective algorithm’s projection.

Alarmingly, the equatorial Pacific Ocean also cooled markedly in May. Our previous outlook highlighted the possibility of summer La Nina development, but clear evidence of an ongoing transition from neutral to weak La Nina conditions in the Pacific raises those chances. La Nina is linked with less vertical wind shear in the Caribbean, Gulf, and western Atlantic. More wind shear means weaker and fewer storms, so La Nina-influenced hurricane seasons are more active and destructive than their El Nino or ENSO-neutral counterparts. WeatherTiger’s latest ENSO modeling for the month of June (below right) has shifted dramatically cooler for ASO than previous runs, with a 60%+ chance of La Nina by October. This is especially significant as real-time model skill improves tremendously between May and June runs due the uncertainty associated with the ENSO spring predictability barrier beginning to diminish.

One piece of good news: the Kirkland Signature three-pack of tropical storms that have affected the U.S. coast so far this year (Arthur, Bertha, Cristobal) have no enduring predictive value for the year ahead. Hurricane season 2020 is running hot so far (~95th percentile), but prior TC activity has no correlation with the rest of the season until early August. Of the eleven fastest starts since 1950, five seasons ended above normal, two near normal, and four below normal. This early season activity is a shot over the bow, but nothing more.

Plugging the data into WeatherTiger’s forecast algorithm, the most likely outcome for 2020 is around 180 units of accumulated cyclone energy (ACE), with 19 tropical storms, 8 hurricanes, and 4 major hurricanes. That forecast is significantly above the median season, which has around 95 ACE units. Per our replicated real-time model errors, there is a 50% chance of the season ending between 140-210 ACE units, 16-21 tropical storms, 6-9 hurricanes, and 4-6 major hurricanes. Our exceedance curve for predicted ACE is shown below at left to give a sense of the odds of topping any given level of activity. Historical forecast skill for ACE over 1985-2019 using this methodology is about a 15-20% reduction in forecast error relative to climatology.

Let’s translate that forecast into the chances of a below, near, and above normal season, with examples of how Florida fared in each type of year.

Below normal: 5% chance

If a below normal (<70 ACE units) season doesn’t ring a bell, that’s because there hasn’t been one since three quiet years ended in 2015. Well below normal seasons are often associated with El Nino conditions. As such, strong shear near the U.S. usually puts the kibosh on significant Florida landfalls, and only one tropical storm hit the state over 2013-2015. Sometimes a season can unexpectedly flame out for non-El Nino idiosyncratic reasons, as in 2013. But I certainly wouldn’t count on that in 2020.

Near normal: 15-20% chance

The middle third (70-125 ACE) demonstrates a surprising truth: how many tropical storms develop in a season explains only about 20% of the variability of how many hurricanes strike the U.S., with development location, intensification favorability, and steering wind more important factors. Thus, a busy-normal season like 2011 yielded zero Florida landfalls, while a quiet-normal year like 1992 brought Category 5 Hurricane Andrew. Remember that an average hurricane season has one to two U.S. hurricane landfalls, and it really, truly, only takes one. Overall, if we land near normal in 2020, this too would be a highly fortunate outcome given the initial conditions.

Above normal: 75-80% chance

The top third of hurricane seasons (ACE>125) contains the multitudes, ranging from a third more than to two-and-a-half times normal activity. Our model suggests that 2020 tilts towards the deep end of the pool, with 60% odds of ACE over 160, and about a 10-15% chance of exceeding the record of 250 ACE units.

Steering still counts for a lot. The least worst outcome is something like 2010, which despite ACE of 165, saw five major hurricanes curve harmlessly into the open Atlantic, with only two modest tropical storms reaching the United States. The worst-case scenario is 2004, when overall hyperactivity (225 ACE) and a strong, west-based Bermuda high bludgeoned Florida with four hurricane landfalls in six weeks. Above normal years usually have at least one U.S. major hurricane landfall.

A subscriber-only word on landfall risks:

Clearly, active years can have modest impacts, and vice versa. Our subscribers received a version of this forecast with an extended section here defining steering analogs and U.S. landfall risks for the 2020 season. To get access to that analysis, click here to sign up for our hurricane forecasting subscription package.

In the meantime, a plea to prospective 2020 hurricanes: please exercise rigorous social distancing from the U.S. this year. Trust me, you don’t want to come here.

And away we go. Next seasonal update will be out in early August. Until then, keep watching the skies.

If you liked this forecast, sign up for our Hurricane Forecasting subscription service to have all of WeatherTiger’s seasonal hurricane outlooks, weekly tropical columns and video discussions, and individual storm forecasts sent right to your inbox as soon as they are released, plus subscriber-only forecasts and insights.

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