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Posted at 11:15 AM ET, 05/ 5/2010

2010 Hurricane season outlooks: Tropical trouble

By Greg Postel

* Nice weather with few flaws: Full Forecast | NatCast | UnitedCast *

Forecast Source
Named Storms
Hurricanes
Major Hurricanes
AccuWeather - Joe Bastardi
15
5
2 or 3
Colorado State University
15
8
4
Tropical Storm Risk Inc.
16
8-9
4
WeatherBug
12-17
6-9
3-4
 
Forecast Average
15-16
7-8
3
2009 Season
9
3
2
Long-term Average
10
6
2

Nearly unanimously, forecasts for the 2010 hurricane season are calling for a very active one. This shouldn't be too surprising given that we've seen this advertisement over and over again in recent years.

In general, the forecasts offered to us long before the season starts have so far shown little skill. However, they do offer a glimpse of what the atmosphere and ocean might look like during those critical months when hurricane development most sensitively depends on how the two are arranged. So rather than systematically disregard these early forecasts, a sensible approach is to use them with caution - just as many of the forecasters who make them advise.

Keep reading for more on the rationale for an active hurricane season...

The uncertainty in these forecasts is rooted in the fact that they depend on long-range forecasts of factors that, while statistically associated with hurricane development, can change with time. They are based on the expected state of the atmosphere several months down the road which don't always evolve in ways forecasters anticipate.

Of course what really counts is, for example, the strength of the wind shear (which tends to tear storms apart) and the structure of the temperature in the immediate storm environment, not necessarily what these variables are doing in May. After all, our present weak El Nino could unexpectedly intensify in the months leading up to the season, helping to strengthen the wind shear over the tropical Atlantic and hindering hurricane development (El Nino, in general, is associated with reduced hurricane activity in the tropical Atlantic).

This is what happened in 2009. El Nino strengthened more quickly during the summer than many predicted and was associated with an unfavorable change in the wind patterns over the Atlantic. As a result, many hurricane forecasters were forced to reduce their numbers in the outlooks posted later that fall.

With all of that said, this year's crop of early forecasts have identified clues to how the upcoming season may evolve that, in my opinion, make them more believable than usual. One of them is the rate of the demise of El Nino. El Nino is beginning to weaken rapidly from its winter 2010 maximum. It is forecast by a significant majority of the models to entirely dissipate by late summer. While there is no guarantee in this outcome, the consensus that El Nino will be gone and perhaps even replaced by La Nina (which would be even more favorable for hurricane development) lends credence to the elevated threat.

elnino-simulations.jpg
Model simulations for El Nino. These simulations show sea surface temperatures cooling in a region of the Pacific which would portend El Nino weakening. Image courtesy International Research Institute for Climate and Society.

The figure above is a summary of El Nino forecasts by various models for the next 9 months, in 3-month segments. During the heart of this year's hurricane season (between JAS and ASO on the bottom - which stands for July, August, September and August, September, October), the strength of El Nino (as measured by the left scale) is on average expected to be less than zero ... which is in fact a La Nina condition.

Another glaring sign that this may be an active season is the degree to which the sea-surface temperatures (SSTs) in the Main Development Region (MDR) of the Atlantic Basin (spanning the coast of Africa to Central America) are anomalously warm. Warm SSTs tend to fuel storm development. The values observed across a broad region of the tropical Atlantic are at record levels for this time of year. The figure below shows the current SST anomaly pattern over the Atlantic Basin. Colors shaded in yellow and red are temperatures warmer than average (defined as the 1971-2000 mean).

atlantic-ssts-2010.jpg
Atlantic sea surface temperature anomalies (departure from average). Courtesy National Hurricane Center.

While warmer water can provide more heat energy for a storm to ingest, the scale of these SST anomalies is so great that they are closely associated with additional large-scale atmospheric characteristics over the MDR that also favor growth: weaker than normal wind shear and more humidity in the atmosphere.

When viewed in the context of these extremely unusual conditions in the MDR, and in light of the confidence in the decline of El Nino, the early 2010 hurricane forecasts carry more weight than in years past. The key, of course, is if these symptoms persist through summer and fall. Only then can they give the annual parade of African easterly waves - precursors to the vast majority of Atlantic hurricanes- a better than average chance to actually produce...

By Greg Postel  | May 5, 2010; 11:15 AM ET
Categories:  Tropical Weather  
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Comments

Yep, we are already talking about the busy summer the Red Cross is likely to have...I have folks beginning to understand things like analog years (beware the 2004 and 2005, not to mention 1995 analogies) and the location of the Bermuda high. Most here have an idea about El Nino vs. La Nina, but dont know about the reasons why (shear, of course)...its an education process here. Im stressing that it would be a great idea to have our resources in FL especially ready to go...

Posted by: DullesARC | May 5, 2010 11:30 AM | Report abuse

I've seen a couple of discontinuities on the Atlantic ITCZ per the NHC tropical charts, but none has yet been labeled a "tropical wave".

The ITCZ itself is setting up over or north of the isthmus of Panama, a signal that the wet season has evidently begun. This is probably a relief for Panamanians who have been complaining of an extended dry season this past El Nino winter.

Posted by: Bombo47jea | May 5, 2010 12:51 PM | Report abuse


Every one of them will throw the oil clean-up back to day 1.

Posted by: blasmaic | May 5, 2010 1:01 PM | Report abuse

Even if these forecasts were perfect, they would have little value. The number of storms means nothing unless one can predict how many of those will affect land. One land falling storm can devastate a region, such as Florida with Andrew in 1992, even if there are very few storms in a given season. And, nobody claims to have skill in predicting the number land falling storms

Posted by: SteveT-CapitalWeatherGang | May 5, 2010 5:50 PM | Report abuse

Steve--Don't totally agree. The more active the season, the higher the probability of multiple landfalling storms and multiple areas being impacted. See 2004 and 2005. Sure, you can get a horrible storm in an otherwise inactive season...but if we know in advance odds are higher for significant storm activity and impacts, I think that helps planners -- even if we can't pinpoint locations.

Posted by: Jason-CapitalWeatherGang | May 5, 2010 8:28 PM | Report abuse

Hi SteveT,

Don't agree when you say, "If these forecasts were perfect" they would have little value. Most of these forecasts have landfall likelihood components to them (either directly or indirectly). Tell me who wouldn't take one of these if they were perfect, and why they would be useless?

As you say ... "nobody claims to have skill in predicting the number land falling storms". This wouldn't be true if the forecasts were perfect.

The upshot is that a little insight is better than none.

greg

Posted by: gregpostel | May 5, 2010 8:43 PM | Report abuse

Jason, Greg, I guess we'll have to agree to disagree.

Question: Would (should) there be differences in hurricane preparedness by vulnerable interests along the East and Gulf coasts if it were known for certain the hurricane season would be inactive versus active without knowledge of how many would actual cross the coast??

Of course, there is no certainty (little skill) in the seasonal predictions of storm counts and no skill in forecasting the number of landfalling storms. The odds might be higher for landfall in active seasons, but to my knowledge not significantly so.(Greg, I don't believe any, let alone most, of the forecasts have landfalling likelihood components. I'm open to be proven wrong).

Probably the only beneficiaries of the seasonal tropical storm forecasts are insurance (and reinsurance) companies which can profit by playing off the perceptions, rather than the realities on the information content of those forecasts.

Thanks for the opportunity to open the discussion on this important subject.

Posted by: SteveT-CapitalWeatherGang | May 6, 2010 9:06 AM | Report abuse

So just to be clear...

A named storm is a TS and up. TD's don't get names.
A hurricane is at least a Cat 1.
A major hurricane is a Cat 3 and up? (or Cat 4?)

I guess 2005 is the so-called gold standard when it comes to tropical action, and I've seen comparisons to that year. Not everything is in place yet, and even if it were, there's no guarantee for a rerun of that busy season -- which saw a Hurricane Zeta on New Year's Day 2006.

Posted by: ennepe68 | May 6, 2010 10:23 AM | Report abuse

Hi SteveT:


you say: Greg, I don't believe any, let alone most, of the forecasts have landfalling likelihood components. I'm open to be proven wrong.

I'll prove you wrong :)

(1) CSU's hurricane forecast
(2) TSR's hurricane forecast
(3) Accuweather.com foreacst

Aside from these links which we provided at the top of the blog, I'm aware of private vendors who do the same.


all in fun,

greg

Posted by: gregpostel | May 7, 2010 9:26 AM | Report abuse

Interesting discussion. Hurricane history suggests that a busy season is a greater threat to send Category 2 or stronger storms into the US coastline. However, for the DC area, the correlation is less clear. In the "Greek Alphabet Season" of 2005,for example, DC saw next to nothing. In 1896, the year of DC's worst hurricane-related windstorm, there were only six tropical cyclones. During 1972, the year of Hurricane Agnes, there were only four named tropical cyclones. Basically, Agnes was the season.

Posted by: ricschwartz | May 8, 2010 11:07 AM | Report abuse

Hurricane history suggests that an active season brings a greater risk to the US coastline, particularly to east Florida and the Gulf Coast. There is considerably less correlation for the DC area. In the Greek Alphabet season of 2005, for example, DC saw little. In 1972, a year with only four named tropical cyclones, it dealt with Agnes. In 1896, a year with only six tropical cyclones, the metropolitan area was struck by its worst hurricane-related windstorm.

Posted by: ricschwartz | May 8, 2010 11:19 AM | Report abuse

The comments to this entry are closed.

 
 
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