## 2013 Atlantic Hurricane Season: A Busted Forecast

Although the 2013 Atlantic hurricane season has not yet concluded and we still have November to contend with before the season will officially be over, all indicators are beginning to make it obvious that the end of the show is near. If you recall in my July hurricane season forecast , all signals were looking for an active season. All the criteria seemed to fit but for some unknown reason, a big piece of the puzzle has alluded forecasters as to why this years hurricane season was very, very below normal. Yes, the Atlantic basin did have a total of 13 named storms, so in one sense the season has been “active” but there were only 2 hurricanes and both were minimal category 1 hurricanes.  However, 13 named storms does really cannot tell the whole story.

So, how do we get a better feel of the hurricane season activity for this year? One method called Index. ACE Index is using a sum of the energy accumulated with all the cyclones (tropical storms, sub-tropical storms and hurricanes) that happened to form within the hurricane season. Calculating ACE is done by using the square of the the wind speed every six hours during the storm’s lifetime. Remember, a tropical depression is below 35 knots and it is not added. Hence, a cyclone that is longer lived will have a higher ACE indices verses a shorter lived cyclone which will have a lower ACE indices.  ACE indices of a single cyclone is windspeed (35 knots or higher) every six hour intervals  (0000, 0600, 1200, 1800 ) or ACE = 10kn2.

Total ACE:

$\Sigma = \frac{Vmax^2}{10^4} \ \$

* Vmax is estimated sustained wind speed in knots. *

An average seasonal ACE in the Atlantic is between 105 -115 or mean average of 110. Although technically we are in a Neutral state, this season “almost” seems more like a El-Niño year. An El Niño tends to hinder tropical cyclone development in the Atlantic, due to higher amounts shear. But in a true El-Niño year, you normally will have a very active season in the East Pacific (EPAC), with a few major hurricanes, and a subdued Atlantic.

2013 Atlantic ACE
Tropical Cyclone Name
Max Wind Speed (in Knots)
ACE 10kn2
Andrea
55
1.6175
Barry
40
0.7625
Chantal
55
2.4800
Dorian
50
2.5725
Erin
40
1.5450
Fernand
50
0.6550
Gabrielle
55
1.8150
Humberto
80
8.9375
Ingrid
75
4.9000
Jerry
45
1.9450
Karen
55
2.2675
Lorenzo
45
1.6200
Melissa
55
3.4725
No-Named Storm (AL152013)
45
1.4000
——–
ACE Total:
36.1200

As part of the post-season review, the low pressure system in December(AL152013) was classified as a subtropical cyclone and the ACE Index has been updated to reflect the ACE numbers during it’s short lived status as a subtropical cyclone.

Two notes of interest: One is that of the two hurricanes this year, Humberto was just hours from beating the record for the latest first Atlantic hurricane of the season. Secondly, this will be the 8th year in a row where the Atlantic hurricane season has not had a major hurricane (category 3 or higher). In fact, 10/24/2013 was the 8th anniversary when Hurricane Wilma made landfall on the southwestern side of Florida as a Category three hurricane and headed northeastward and the areas of the Miami-Dade, Broward, and Palm Beach counties were impacted with category two winds. If by at the end of this year hurricane season with no major hurricanes, we will tie the record.

So is there a last hurrah for one or two more storms or is it time to fold the tents and call the season over? Technically, no as the hurricane season does not oficially end until November 30. This does not mean a cyclone cannot develop. It is just that a cyclone developing after the official season is over, is a somewhat rare event, but it is not impossible. Usually any late season storms develop either in the Southwestern Atlantic or more likely in the Caribbean. The two late season cyclones late that come to mind were Sandy (2012) and Mitch (1998). As of yet, it appears that the rest of November will be quiet and although there is a chance development of in the Caribbean due to the , I am somewhat skeptical on that. I won’t rule it out yet but for the moment I just am not convinced this will happen, IMHO.

As always, remember to always keep an eye to the sky and to always stick with official information, either your local  or the NHC

## Atlantic Hurricane Season 2013. An Active Season??

The 2013 Hurricane season which began June 1st and both the NOAA and ENSO reports are out and the 2013 hurricane season will most likely be a much more active season than the previous years. This hurricane season may be a dangerous one for many in the East Coast of the US, especially the Florida East coast, Florida Keys and the Gulf coast of Florida and possibly even the Caribbean but we will get to that a bit later.

So lets start with the basics of why this season is expected to be different. Beginning with ENSO or El-Niño Southern Oscillation. ENSO is a climate pattern in the equatorial Pacific. Many might wonder what does the equatorial Pacific have to do with storms in the Atlantic. Globally, what happens in the Pacific can induce what happens in the Atlantic. Now with ENSO, there are basically three phases. One is a cool stage, another is a warm stage and there is also a neutral stage.

To have a cool stage, abnormal SST’s (Sea Surface Temperature) must have five consecutive overlapping 3-month periods: for example – (June, July, August), (July, August, September), (August, September, October) or more and the abnormal SST’s must be -0.5 degrees Celsius or lower. This also known as an . During an La Niña, abnormal SST’s can allow anticyclones to develop due to convergence (upward motion) which can help induce the possibility of development of a tropical cyclone.

Conversely, a warm stage must have the same five consecutive overlapping 3-month periods: or more and the abnormal SST’s must be at .05 degrees Celsius or higher. This is known as an . During an El-Niño, storms in the Pacific will have an impact in the Atlantic. The possibility of tropical cyclones in the Atlantic developing is reduced, usually due to vertical shear. The Neutral stage is stage between the two and normally will only last a short period of time. The Neutral stage basically has no impact on the Global Weather patterns.

Next up, the Atlantic Tripole. When there is an Atlantic Tripole, basically what is happening is that in the tropical area of the Atlantic Ocean you are have warm waters to the south, and then from the tropical area to approximately 40° north latitude you now have a large of much cooler waters.  From 40° north latitude northward, there would be an area of warms waters again. Since this is not the normal pattern, sometimes this can have an increase in the amount of convection and lift somewhere between Africa and the Leeward and Windward Islands. This can help the development of any tropical systems.

Hopefully, you are still with me as we talk about the NAO (North Atlantic Oscillation) and the the warm SST’s in the tropical area and the MDR (Main Development Region), MJO (Madden Julian Oscillation), and finally the PDO (Pacific Decadal Oscillation). Take a deep breath as we get into some more difficult reading. Please note: I have purposely have left out the  in this discussion. Although it can a contributing factor, I felt we have enough information as is and did not want to add any further possible confusion.

The  describes the strength of the surface pressure difference that is between the Azores High and the Icelandic Low. The NAO has two phases: a Positive phase and a Negative Phase. With a Positive NAO you have a stronger than average high across the Atlantic, this will increase the trade winds to the south of it. Those trade winds run across the eastern portions of the Atlantic and into the Caribbean. The stronger trade winds allow for increased evaporation cooling which then allows for cooler than average SST’s. Conversely, with a Negative NAO, the Azores High and the Icelandic Low are now weaker than average. So now with a weak high the trade winds will be much calmer allowing the SST’s to increase considerably. The NAO while it is a big player with the SST’s, the NAO also is a player in tropical cyclone development. Consider this, with quicker than average trade winds, any tropical waves will have very rapid movement as they come of the coast of Africa. The tropical waves will find it difficult to consolidate or strengthen if the forward motion is over 20 MPH or more. Not only can the tropical wave find it difficult to strengthen but this can lead to .

The NAO is also a player in the track of tropical cyclone. Recalling that during a Negative NAO, the Azores high is typically weaker but it is oriented west to east and closer to the US Southeast coastline. During a Negative NAO, any troughs on the East coast will typically be weaker and recurving of a tropical cyclone will be much more difficult to do. A Positive NAO on the other hand will allow recurvature much easier. Please note that the short term timing essentially will govern the track of a tropical cyclone, not the long term mean.

The abnormally above average   is another concern this year especially in the . The MDR is a very large area, extending from the Caribbean Sea to almost the coast of Africa. A warmer than normal MDR will provide the feeding fuel for the beginning of the development but also help nourish tropical cyclones. The minimum water temperature needed for a tropical cyclone to begin development  is approximately 26° C  or (79° F) and a depth of at least 160 feet but there also other factors needed.

Now let’s get to the . What is the MJO?  The MJO is an eastward progression of large regions of both enhanced(positive) and suppressed(negative) tropical rainfall. Simply put, there are two completely different phases with the MJO. One is a negative phase and the other is a positive phase. The MJO is different than ENSO as the MJO moves eastward at between speeds but anywhere from 8- 18 MPH across the different regions in the tropics, crossing the Earth in 30 to 60 days (with an average of 40 days). As it heads eastward and eventually into the East Pacific and then into the Atlantic the two phases are completely opposite of each other. In a Negative phase, lift is subdued and development of a cyclone is harder, but not impossible to have. In a Positive phase, now moisture and lift (upward motion) into the atmosphere allows easier development of a tropical cyclone (increased convection/thunderstorms activity).

Lastly, the . The PDO is a pattern of change in the Pacific Ocean’s climate. The PDO is easily recognized with cool or warm surface waters in the Pacific Ocean (north of 20° N. latitude). During a Negative or cool phase, the west Pacific  becomes warm and the eastern ocean is cool. Conversely, during a Positive or warm phase, the west Pacific becomes cool and the eastern ocean is now warm. So again you are wondering how does that affect you if you live on the US east coast? During a Negative phase and in the Atlantic basin, the rising of air helps upward lift and possibly allowing more convection which can help promote tropical cyclone development.

So now that you have had a short lesson in Meteorology 101, what does all this mean? In one sense, not a thing, but certain patterns may appear this season and may allow for possible stronger storms and also a much higher chance of landfall somewhere along the US Eastern coast. The past few years, many of the storms were recurved and they went out to sea.  would develop, head east and pull the storms up and keep the storms away. This year appears to be different. A more pronounced and elongated Bermuda high will remain allowing any tropical cyclones to be caught under the . If the Bermuda high weakened enough, and a storm was under the ridge of the high and near the southwestern (or the periphery) of the ridge. This may allow the storm may follow around the edge of the high. This may allow a more northwestern track instead of a western track and possibly towards the East Coast of the US. Any storms much further west and if the ridge is still strong would possibly be in the Caribbean and if a system in the Caribbean and if there were a trough over the Plains of the US, this may have a storm heading toward the  US Gulf Coast.

Using those different meteorological basics we talked about earlier, lets see why the possible reason why this may be a more active hurricane season.

ENSO

The ENSO is now in a neutral stage. Looking at the SST anomaly chart you must recall that for either an La Niña or El-Niño (either -.05° Celsius or .05° Celsius) for the last 3 months or more. Notice that is the chart both NINO 4 and NINO 3.4 have not reached the threshold needed. The short term forecast is that we will be in a neutral stage with a cold bias. Long term forecast is a bit cloudy but it is forecast that later in the season we may have a weak La Niña.

Atlantic Tripole

Now it is time again for the Atlantic Tripole. Although the chart below is not a true representative of an Atlantic Tripole, it does show the different areas. The black arrow designates the warm waters and the red arrow designates the cooler waters. In a non-Atlantic Tripole, the areas north of 40° are much cooler waters. A bit later in the season the southern Atlantic (the tropical area) is expected to be much warmer. With much warmer waters and tropical waves emerging from the coast of Africa those tropical waves have a better chance of possibly developing into something tropical.

NAO

Our attention is now focused in the NAO. All indicators reflect a Positive NAO. The Azores high is further south which can allow tropical systems to head further West or WNW. In the chart below is the latest NAO forecast both the Observed and Ensemble. The black line being the observed and the  smaller lines from the different ensemble members. It you note that nearly all are indicating a Positive NAO. If you may recall from your Meteorology 101 course, a Positive NAO allows the latitudinal displacement of the Azores high further southward. For the time being the NAO is in a Positive phase but it is expected later in the season for the NAO to go into a Negative phase.

SST’s In The MDR
The abnormal high SST’s in the MDR is something I cannot emphasize enough. Recalling that a tropical system’s primary energy source is the , higher SST’s may allow easier development of of tropical wave, although there are other factors needed.

MJO

As discussed earlier, the MJO progresses eastward with two different phases. At this time, the MJO is entering the East Pacific and Western Caribbean is a Positive phase. In the Positive phase, upward motion or lift and moisture allows easier development of convection/thunderstorms. The chart below can be a bit confusing but itself is a simple one once you comprehend how it works.

Looking at chart, you will notice that there 8 Octants with Octant 1 at the lower right and Octant 8 at the upper left. You will also notice near Octant 6 a legend for some of the different weather models with different a color for each. Next, you will notice a circle in the middle. You will also see (sometime difficult to read) numbers which correspond the different days (or index). For simplification, if the index is within the circle the MJO is considered weak. If the Index is outside the circle, then the MJO is considered a strong pulse. The MJO in the diagram moves counter-clockwise direction even though the MJO itself runs from west to east. You might have noticed the RMM1 and RMM2. Although this is something we need to worry about but the RMM1 and the RMM2 are simply mathematical methods which combine cloud amount along with winds at the upper and lower atmosphere. This provides a measure of the strength and location of the MJO.
To make things easier, we are only going to need octants 8 (East Pacific & Gulf of Mexico/Caribbean) and octant 1 which is the Atlantic basin. In the example above, near 28 in octant 1, you can see that several models (UKMET, GEFS, ECMF, CANM) all are outside the circle. Please note though, even if the MJO diagram is showing the different strong pulses within the different models, this does not always mean a development is imminent. This just means that there is usually upward motion or lift in the region and the possibility of easier thunderstorm activity.

PDO

Although not stated earlier, the  PDO is a long-term ocean fluctuation of the Pacific Ocean and wanes and waxes approximately every 20 to 30 years. When there are changes in the locations in the Pacific between the cold and warm water masses, they alter the path of the . The easiest way to state this is that the jet stream in the northern hemisphere delivers the the different storms across the US. In this case the PDO is in a Negative (or cool) phase and this will will try to steer the jet stream further north over the Western portion of the US but this also allows for possible  in the Atlantic.  The diagram below shows the different warm and cool phases.

Summary

Although no one (not even the Specialists at the NHC) can tell exactly what the 2013 hurricane season will be like, using some the factors I outlined in this post, it does appear that this hurricane season will be a much more active season. When you have a Negative PDO, a Neutral ENSO but with a cold bias, a Negative NAO, extremely warm SST’s in the MDR, along with a Atlantic Tripole, all these factors (along with others we have not discussed) do seem to favor a season of much higher activity.  It is unrealistic to even try as to where (if any) landfall will occur but just using the above factors along with some analogue years for comparison, areas of risk for possible cyclone landfall are higher in some locations whereas other will have a lower risk. The graphic below is a generalization and is not meant to alarm those in the higher risk areas.

Okay, now that wasn’t too bad was it. Hope you enjoyed your short class of Meteorology 101 and  that it hopefully paid out and I hope you learned a little in the meantime. Please note this was a generalization of what the 2013 hurricane season might like and many contributing factors may and will usually change throughout the hurricane season. If you are not sure on any of the items discussed, please feel free to Contact Me at the top of the page.

Lastly, and most importantly, please be sure to put together your Emergency and Hurricane Survival Kit if you have not already done so. If you have pets, please be sure  to have plans to care for them. Check your homeowners or renters insurance before any possible storms develop and ensure you are properly covered for property damages and I would highly recommend that you also take flood insurance.