Wed Jun 14 2017 04:49 PM
2017: 'Active Era' + Busy Season?

Those of us who know a little bit about Atlantic basin history have been paying a little more attention to 2017.

First some background.

A few years ago speculation began in earnest that the most recent "active phase" of the Atlantic which is agreed by most to have begun in 1995, may have been winding down, or have even come to an end. As the argument went, numbers across the board started to fall apart in 2012 (despite late-season Sandy), and really turned inactive by 2013, subsequently remaining soft in both 2014 and 2015, as well. (See below - YEAR, NAMED STORMS, HURRICANES, MAJORS, ACCUMULATED CYCLONE ENERGY).

1995 19 11 5 228
1996 13 9 6 166
1997 8 3 1 41
1998 14 10 3 182
1999 12 8 5 177
2000 15 8 3 119
2001 15 9 4 110
2002 12 4 2 67
2003 16 7 3 176
2004 15 9 6 227
2005 28 15 7 250
2006 10 5 2 79
2007 15 6 2 74
2008 16 8 5 146
2009 9 3 2 53
2010 19 12 5 165
2011 19 7 4 126
2012 19 10 2 129

2013 14 2 0 36
2014 8 6 2 67
2015 11 4 2 63

2016, despite doing battle with lingering effects of a record El Niño, hinted that the 2013-2015 downswing may have been more related to short-term anomalies, such as the major El Niño in 2015, the driest mid-to-lower atmospheric conditions during the Aug. 1 to Sept. 25 period since reliable records began during 2013 (Klotzbach), and a related or unrelated (possibly temporary) downshift in the Atlantic Multi-decadal Oscillation (AMO), than anything else.

'El Niño favors stronger hurricane activity in the central and eastern Pacific basins, and suppresses it in the Atlantic basin.' -

The AMO is suspected by many to have alternating long-term negative/positive impacts on tropical cyclones in the Atlantic basin on time scales spanning several decades. The "postive" (Warm) phase of the AMO 'is associated with warmer than average oceans in the north and tropical Atlantic Ocean and an enhanced West African monsoon season, both of which boost hurricane development' - NHC.



2012 19 10 2 129
2013 14 2 0 36
2014 8 6 2 67
2015 11 4 2 63
2016 15 7 4 141
2017* 14 7 3 n/a

Looking at all the stats above from 1995 on, 2016, and now the average NHC forecast for this year, certainly does not look much at all like what we might expect to see if the active era had truly come to a close. Maybe it's very possible that it is coming to a close, but just.. has not done so... yet (See image below). And if the active era is still in effect, the potential for a year with favorable conditions overall (favorable for tropical cyclones regardless of active or inactive phase) to work in tandem with the active era could mean a year that over achieves. 2017 could be such a year.

Below: AMO Indices 1950-2016

As Eric Blake pointed out in a tweet earlier this week, SST anomalies in the Atlantic indeed support the notion that 2017 could end up looking very much like an active season which occurs during the +AMO phase.

Credit: Eric Blake/Twitter

So it now looks like 2017 will at the very least not clearly be in the negative (low activity) phase of the AMO, and could still be in the active phase. Clues to then watch for indications that we may also be in an active season would include:

1. The state of El Niño–Southern Oscillation (ENSO). ENSO 'is an irregularly periodical variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting much of the tropics and subtropics.'

At present ENSO-neutral or weak El Niño conditions are expected. If verified, these would tend to favor an average to active season, more so an inactive one.

2. SSTs across the MDR (Main Development Region). Warmer SSTs in this area strongly correspond with more active seasons.

According to the May 25 update from CPC, 'Currently, SSTs are above average across the MDR, with the largest departures of between +0.5° and +1°C observed in the Caribbean Sea. For the ASO season, SSTs have been above average in the MDR since 1995. However, there is typically low skill and considerable spread in model predictions of Atlantic SSTs this far ahead of the ASO season. Complicating this situation is the possible continuance of a strong cold bias in forecasts from NOAA's CFS high-resolution model that was evident during past two hurricane seasons. This model is again predicting well below-average SSTs in the MDR, while the lower-resolution CFS runs are predicting overall above-average SSTs in that region.'

3. Wind shear, and especially in the MDR. Weaker wind shear is more supportive for tropical cyclone development.

As of the May 25 update from CPC, 'At present, the model forecasts of vertical wind shear vary considerably from one model to the next, are generally dependent upon the model's predicted strength of El Niño. The CFS model is predicting anomalously weak shear in the MDR during ASO 2017. At present, there is no indication that the shear will be excessively weak so as to support an extremely active hurricane season, and also no confident indication that the shear will be exceptionally strong. The MDR will most likely experience near-average or below-average vertical wind shear during ASO 2017.'

4. Sea level pressures in the MDR. Lower pressures in this region are more supportive of development.

As of the May 25 update from CPC, 'pressures across the MDR are near average and forecasts from the various global models do not indicate large anomalies for either higher or lower pressure during ASO 2017.'

5. Saharan Air Layer (SAL) intrusions. SAL can cap convection. Additionally, SAL reduces the amount of sunlight reaching the ocean, and can thus lower SSTs. However, depending on the strength and relative location of the SAL at any given time, it can even aid in some tropical cyclone development situations.

'The Saharan Air Layer (SAL) is an elevated volume of warm, dry, and stable air, ranging in depth from generally 900 to 5500 m above the surface, that originates over the Sahara Desert in Northern Africa and propagates westward over the Atlantic basin, most often during summer months.' (AMS)

These intrusions of dry, subsident Saharan air are notoriously hard to predict, but early indications are not convincing that SAL will have a more negative or positive impact on development of storms this season.

Sat Jun 17 2017 04:06 AM
Re: 2017: 'Active Era' + Busy Season?

A look a bit more reminiscent of a typical August, or at the very least, other active seasons occurring during +AMO.

This most recent snapshot of the 2017 Atlantic Hurricane Season seems to have gotten a nice boost from a substantial MJO pulse, of which influence should start to wane in a week or two.

It will be interesting (and maybe a bit more telling) to see how the basin looks in July. A continued healthy monsoon trof and warming Main Development Region? ...

Tue Jun 20 2017 03:30 PM
Re: 2017: 'Active Era' + Busy Season?

Above: 20 Jun 2017 17:45 UTC (8-km resolution) IR Image centered at Latitude 21.35° N Longitude 60.42° W
Left to Right: Tropical Storm Cindy, Tropical Storm Bret, Wave, Wave, Wave, W Africa Thunderstorm Complex

Here are some very noteworthy statistics about our storms and this season so far.

Tropical Storm #Bret's named storm formation latitude of 9.4°N is the lowest latitude June named storm formation since 1933. - Philip Klotzbach

Only 2 years w/ June Atlantic MDR TC formations: 1933 and 1979. 1933 most active Atlantic season on record based on ACE, 1979 near-average - Philip Klotzbach

Tropical Storm #Bret has formed - earliest Atlantic MDR (<20°N, E of 70°W) named storm on record - prior record was Ana on 6/22/1979 - Philip Klotzbach

Only 3 (prior) Atlantic hurricane seasons on record have had 2 concurrent named storms in June: 1909, 1959 and 1968 - Philip Klotzbach

Only four prior years have had 3 named storms in Atlantic by June 20: 1887, 1959, 2012 and 2016. - Philip Klotzbach

Wed Jul 05 2017 10:04 PM
Re: 2017: 'Active Era' + Busy Season?

Tropical Meteorology Project (Colorado State's Philip J. Klotzbach, Michael M. Bell and, in memoriam, William M. Gray) released their July update today, and while only a step up from their June release, it is a jump from their April issue.

Incidentally, if you are not already following Philip Klotzbach on Twitter, you really need to do so. Much of the information I share here comes directly from his shares, and by following him, I guarantee you will glean invaluable information and insight into this and many a season, spanning oceans and decades.

The Colorado State team now anticipates a total of 15 named storms (NS), 8 hurricanes and 3 major hurricanes in the Atlantic basin this year. That would include the three named storms we have seen already. Their justification for the increase is as follows

We now anticipate an above-average Atlantic basin hurricane season.The odds of a significant El Niño in 2017 have continued to diminish, and most of the tropical and subtropical Atlantic remains anomalously warm.The lack of El Niño conditions typically leads to a lower shear environment in the tropical Atlantic, while a warmer than normal tropical Atlantic provides more fuel for developing tropical cyclones. In addition, a warmer than normal tropical Atlantic is generally associated with lower surface pressures, increased mid-level moisture and weaker trade winds, creating a more conducive dynamic and thermodynamic environment for hurricane formation and intensification.

With the July update came their best analog years, which are as follows: 1953, 1969, 1979, 2004, 2006 and 2012. The average season totals of these years is 14.2 Named Storms, 8.2 Hurricanes and 3.3 Major Hurricanes, with an ACE (Accumulated Cyclone Energy) value of 133, which also implies an above-average season.

The complete July update is available here (PDF).

Mon Jul 10 2017 01:40 AM
Re: 2017: 'Active Era' + Busy Season?

As of July 10, 2017, the Atlantic basin continues running well ahead of climatology, with three named storms to date.

As a forecasting tool for the season as a whole, pre-season named storms, such as Arlene, tend to mean little. But the wide view of the year-to-date, with already three names before August, is inescapable. 2017 has the 'look and feel' of an active season, with the Tropical Atlantic especially supportive of development. As noted above, case-in-point, Tropical Storm Bret being the earliest storm formation on record in the MDR (Main Development Region).

Image below: Progress of the average Atlantic season (1966-2009). Date upon which the following number of events would normally have occurred. (Credit: NHC).

Sun Jul 16 2017 07:49 PM
Re: 2017: 'Active Era' + Busy Season?

July 16 2017 Update

Since the initial post a month ago on June 14, more things have become clear, and will be covered.

Thanks in advance to the many experts who have taken the time to pour over data and create several of the maps, tweets, etc which I will be including below.

The question, 2017: 'Active Era' + Busy Season?, is an important one, because while 'It only takes one,' and there have been plenty of seasons with below-average totals, but which produced one serious storm with a trajectory that had devastating impacts, it is also more than academic to ponder whether or not 2017 will likely produce more named storms and hurricanes, and whether or not the 'Active Era' is still ongoing.

AMO Phase still unclear

The SST anomaly and weather patterns during the three consecutive years of 2013, 2014 and 2015 do stand out as having looked very much like what one might expect going into a negative phase, but since '16, and now with what we are seeing this year, calls of the end of the active era have been put in doubt. In fact, 2016 saw more Named Storms and Majors in the Atlantic basin than in any year during the last negative phase (1970-1994), and the ACE (Accumulated Cyclone Energy) of 141 was greater than all but one year during that time. (h/t Daulton Bahm).

To me, the current SST and sea level pressure patterns in the Atlantic might best be described as mixed/neutral, with a slight, yet sloppy, positive bias, when considering all of the competing AMO definitions most experts consider. Thus, if my own take on this 'hot topic' (see what I did there?) is right, the AMO should at the very least not have a suppressing effect heading into the peak Atlantic hurricane months of August-September-October, and could, potentially, be supportive of development.

Medium-term Patterns

So with the longer-term state of the AMO still a bit up in the air, although probably not yet negative, what can be said of current trends and forecast conditions for the rest of the season. Here's where things really start to get interesting... I am just cutting and pasting a series of tweets or stories from a variety of experts for your consideration.

With regard to the Saharan Air Layer (dry, dusty, convection-inhibiting low to mid-level air from Africa that travels west across the Atlantic), after a much above average spring, SAL has been running very low so far this summer (h/t Michael Lowry)

JMA's forecast precipitation anomaly for peak Atlantic hurricane months suggest an active Aug-Oct in the tropics, as well as potentially along the US south and east coasts. (h/t Ben Noll)

SSTAs in the Main Development Region (and elsewhere) in the Atlantic have definitely been trending up this year (compare with 2014 and 2015!) .. Source: Michael Lowry

Potential for a peak hurricane season with below normal shear values across the MDR per JMA forecast. Source: Ben Noll

Two other factors that are outsized contributors to Atlantic hurricane seasons are ENSO and Atlantic MDR sea level pressure anomalies.

Sea level pressures in the MDR should be lower than average, if the anomalous warmth verifies.

As for ENSO (potential for a disruptive El Niño event in the Pacific), most forecasts are still looking for either a very mild El Niño, or ENSO neutral conditions to prevail.

The latest CPC update as follows

13 July 2017

ENSO Alert System Status: Not Active

Synopsis: ENSO-neutral is favored (~50 to 55% chance) into the Northern Hemisphere winter 2017-18.

During June, ENSO-neutral continued, although equatorial sea surface temperatures (SSTs) remained above average in the central and east-central Pacific Ocean [Fig. 1]. The latest weekly Niño index values were near +0.5°C in the Niño-4 and Niño-3.4 regions, and closer to zero in the Niño-3 and Niño-1+2 regions [Fig. 2]. The upper-ocean heat content anomaly was above average during June [Fig. 3], reflecting above-average sub-surface temperatures across the central and eastern Pacific [Fig. 4]. In the atmosphere, tropical convection was suppressed over the west-central tropical Pacific and enhanced over the Maritime Continent [Fig. 5]. The lower-level and upper-level winds were near average over most of the tropical Pacific, and the Southern Oscillation Index (SOI) and Equatorial SOI were slightly negative to near-zero. Overall, the ocean and atmosphere system remains consistent with ENSO-neutral.

Some models predict the onset of El Niño (3-month average Niño-3.4 index at or greater than 0.5°C) during the Northern Hemisphere summer [Fig. 6]. However, more than half of the models favor ENSO-neutral through the remainder of 2017. These predictions, along with the near-average atmospheric conditions over the Pacific, lead forecasters to favor ENSO-neutral into the winter (~50 to 55% chance). However, chances for El Niño remain elevated (~35-45%) relative to the long-term average. In summary, ENSO-neutral is favored (~50 to 55% chance) into the Northern Hemisphere winter 2017-18 (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period)

Tue Jul 18 2017 09:19 PM
Re: 2017: 'Active Era' + Busy Season?

With the formation of Tropical Storm Don in the Main Development Region, the question has been asked, how often have there been two tropical storm formations in the MDR prior to August. According to Tropical Meteorology Project's Philip Klotzbach, there have only been eleven. Not a lot. And only one of those years, 2013, ended up obviously, in fact well below normal.

Here's how those other years with two MDR storms prior to August ended up. I have indicated seasons occurring during the Positive AMO phase with a "+" sign before their respective year, and also placed that respective +AMO year in bold. Question marks have been placed before two years that aren't entirely clear to have been in the positive or negative phase (1901 and 2013), and only one year, 1979, was undeniably in a negative phase.


+1887 Hyperactive 19 - 11 - 2

(?) 1901 Near Average 13 - 6 - 0

+1926 Hyperactive 11 - 8 - 6

+1933 Hyperactive 20 - 11 - 6

+1944 Active 14 - 8 - 3

+1966 Active* 11 - 7 - 3

1979 Near Average 9 - 5 - 2

+1995 Hyperactive 19 - 11 - 5

+1996 Hyperactive 13 - 9 - 6

+2005 Hyperactive (Record) 28 - 15 - 7

(?) 2013 Below Average 14 - 2 - 0

An "average" season in the Atlantic basin, based on 1981 - 2010 data, has about 12 Named Storms, 6 Hurricanes, and 3 Majors.

* Despite near-normal season totals, 1966 is being counted active, owing to having had an ACE of 145.

Tue Jul 18 2017 09:47 PM
Re: 2017: 'Active Era' + Busy Season?

The Weather Channel is also keeping an eye on this trend for early MDR tropical cyclones, and covering it on More here:
Does an Active June and July Point to an Above-Average Atlantic Hurricane Season Overall?

Image Credit:

Thu Jul 20 2017 04:34 PM
Re: 2017: 'Active Era' + Busy Season?

With Don, the Atlantic has now had our fourth named storm of 2017. How this compares, as of July 20th, to the 1966-2009 average as compiled by NHC can be seen below.
(Base map and data courtesy NHC).

Thu Jul 20 2017 05:34 PM
Re: 2017: 'Active Era' + Busy Season?

The image below is a screenshot of the Tropical Cyclone Formation Probability Product, developed by the Regional and Mesoscale Meteorology Branch at CIRA.

This product, widely used by tropical cyclone forecasters, assigns a tropical cyclone genesis probability within 0-48 hours for a given area. Input parameters include 850 hPa Relative Vorticity (pre-existing low to mid level spin), Percent Pixels Colder than -40°C (indicative of deep convection), 850-200 hPa Vertical Shear (less deep layer shear is usually more conducive for TC development), Vertical Instability (is convection suppressed or unrestrained), Cloud-cleared Water Vapor Brightness Temperature (a measure of mid to upper level water vapor), 850 hPa Horizontal Divergence and Mean Sea Level Pressure, among a few others.

Even though this is a product used to aid in forecasts out to 48 hours, it can also serve as a bit of a clue as to the state of various basins and their sub-regions, when looked at over time. For the following exercise, I have looked at each main input parameter, save a few that would not apply so much here, for this season. Specifically, in the image below, we can look at the state of each Atlantic region during the first two months so far of the 2017 hurricane season, and try to ferret out any possible trends. And there have been some, which I will explain below.

First, it is important to point out those input parameters that are largely considered most useful in determining the potential for very near-term TC genesis. The Atlantic basin's top three non-climatology 48-Hour Genesis predictors are: RVOR (Relative vorticity), MSLP (Mean sea level pressure), PCCD (Percent pixels colder than -40°C). How often have these been favorable (positive), this season so far?

It is also instructive to look at the wider view inputs, such as Shear, Water Vapor, Sea Surface Temps, and so on.

For the input parameters in the screen shot below, if they have averaged above climatology since June 1, I have marked them with a plus (+) sign. For those that have averaged below climatology since June 1, I have marked them with a minus (-).

So, on to the discernible possible trends, relative to climatology. The regions and their respective inputs break down as follows:

Atlantic basin top three non-climatology 48-Hour Genesis predictors relative to climo:
GOM: One negative, two neutral
Caribbean: One positive, one neutral, one negative
East Coast: All three negative
Tropical Atlantic: One positive, two neutral
Sub-tropical Atlantic: Two negative, one neutral

Eight NESDIS Atlantic basin non-climatology 48-Hour Genesis predictors relative to climo:
GOM: One positive, three negative, four neutral
Caribbean: Three positive, three negative, two neutral
East Coast: One positive, four negative, three neutral
Tropical Atlantic: Four positive, two negative, two neutral
Sub-tropical Atlantic: One positive, five negative, two neutral

Keep in mind that this only helps understand where TCs have been more likely to form, and not necessarily where they will be likely to travel, survive, intensify, weaken, or dissipate. With that not insignificant caveat in mind, it is probably safe to conclude the following

A) The Tropical Atlantic has been the most favored region for TC genesis this season relative to climatology.
B) The Caribbean has been the second most favored region for TC genesis this season relative to climatology.
C) The Gulf of Mexico has generally been unfavorable for genesis so far relative to climo (although we did have one formation there, Cindy).
D) Both the East Coast and Sub-Tropical Atlantic have been hostile to development so far in the official season, overall, relative to climatology.

Should things remain stead-state for a while, then we might expect to continue this season to look to the Main Development Region in the Tropical Atlantic first and foremost heading into the peak Aug-Sep-Oct for the best and above-climo chances of formation, with respectable attention paid to the Caribbean - having been roughly in line with climatology, and still certainly the GOM, given that it is landlocked and anything forming there is very likely to affect land. Development off the East Coast and in the Sub-Tropical Atlantic might be less than average, save changes going forward.

As always, all opinions here are my own, and are not meant in any way, shape, or form to be used for any planning, critical or otherwise. Here at Flhurricane we usually suggest paying close attention to official forecasts from official agencies for that.

Thu Jul 20 2017 05:52 PM
Re: 2017: 'Active Era' + Busy Season?

A significant MJO pulse is anticipated by many models' runs to be crossing the Atlantic in August, which could readily flip the basin into a very active mode as soon as later in July, or early next month. The precise timing, location and outline of this anticipated region of enhanced convection remains unclear, but there are some hints that it could favor development in the Tropical Atlantic, Caribbean and/or Gulf of Mexico, in that order, and starting as early as late July.

Image credit: Levi Cowan/Tropical Tidbits

Thu Jul 20 2017 07:04 PM
Re: 2017: 'Active Era' + Busy Season?

Tropical Meteorology Project's Philip Klotzbach points out that there is indeed a difference in where US landfalling hurricanes form, and where in the US they hit (and I would add, how strong they get in the aggregate - with more Majors forming from development in the MDR, as well as in/near the Caribbean), based on where their genesis occurred.

Another facet of 2017 to think about as we head into the peak of the season.

Philip Klotzbach on Twitter:
"Hurricanes making landfall along the Gulf Coast tend to form further west. MDR hurricanes more likely to hit FL + U.S. East Coast."

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