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Posted at 10:30 AM ET, 07/14/2009

Hurricane Season Off to Slow Start

By Capital Weather Gang

No signs yet of season heating up

* D.C. Weather to Heat Up: Full Forecast | New Weather Channel Show *
* CWG's Matt Rogers on El Nino | Hurricane Tracking Map *

Tropical sea surface temperatures are generally above average, but tropical storm activity is non-existent. Image courtesy NOAA's National Hurricane Center.

By Dr. Gregory Postel, Tropical Weather Expert

The long-range forecasts for the 2009 Atlantic Hurricane season have thus far come to a general consensus. They nearly unanimously agree on a near-average or slightly quieter-than-average season, with the number of named storms expected to be around 12 and the number of hurricanes about half that.

Now that these statistically based predictions have served their purpose by providing an estimation of what to expect, we can turn our attention to the shorter-term factors that strongly influence what we will actually get. While the developing El Nino may continue to play a significant role in creating a less-than-optimal setting over the Atlantic and Caribbean basins (see Andrew Freedman's post from July 8), there are other atmospheric processes we must watch on a daily/weekly basis that could ignite the otherwise sluggish start to this season.

Let's look at some of the factors that can affect tropical activity on time scales less than a month...

Certain phases of a pattern known as the Madden-Julian Oscillation (MJO), for example, are known to foster tropical cyclone development over the Atlantic. Capable of altering rainfall (and the vertical circulations that go along with it) and wind shear (a change in wind direction with height in the atmosphere that can rip apart developing tropical systems), the MJO's influence over the tropical Atlantic can be linked with upswings and downswings in storm frequency during hurricane season.

The North Atlantic Oscillation (NAO, a north-south oscillation in atmospheric pressure over the Atlantic Ocean) is another large-scale driver of tropical cyclone patterns over the Western Hemisphere. Often cited for its ability to steer hurricanes, the NAO can also at times create a dusty, highly sheared environment hostile to tropical storm formation.

One way to look at the structure and evolution of the MJO and NAO is through the lens of the weather prediction models; those very same models that forecast whether or not it will rain on your town tomorrow. While there is obviously much more to the complicated puzzle of hurricane formation than just knowing the phase of the MJO/NAO (or the status of El Nino, for that matter), the weather models are not yet showing a favorable configuration in any of these factors for Atlantic development.

Despite the fact that the sea-surface temperatures are warm enough for storms to form, and in fact warmer than "normal" over large portions of the basin, the environmental conditions are just too windy, dry and dusty. And they will probably remain that way for at least the next week -- perpetuating the already quiet July. Stay tuned ...

Dr. Gregory Postel is the lead meteorologist for a weather-risk management firm in Overland Park, KS. He earned his Ph.D. in atmospheric sciences from the University of Wisconsin-Madison, and conducted research on factors leading to the development of tropical cyclones. He's an avid hurricane chaser and has been known to drive more than a thousand miles to intercept land-falling hurricanes.

By Capital Weather Gang  | July 14, 2009; 10:30 AM ET
Categories:  Tropical Weather  
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Dr. Postel: My understanding is that computer models generally do a poor job of projecting phases of the NAO, especially at longer timescales. Can you comment on the reliability of projections for these different cycles during the coming months? Thanks!

Posted by: Andrew-CapitalWeatherGang | July 14, 2009 11:04 AM | Report abuse

Some models are better than others, and it might depend on your definition of "poor". Here is a site from CPC that tracks the GFS system's skill for NAO forecasts out to 2 weeks:

At 10 days, the (I assume linear) correlation is 0.65 from the ensemble mean. There's definitely utility there. But I would say that strictly relying on these forecasts at the 2-week leadtime could quickly lead you astray.

Posted by: gregpostel | July 14, 2009 12:00 PM | Report abuse

Dr. Postel,

What is your opinion on the controversy concerning warm water and stronger hurricanes? Some have expressed the belief that ocean surface temperatures will increase and that this will cause stronger hurricanes.

Do you think sea surface temperatures will rise in the next 100 years?

Do you think that increased sea surface temperatures will result in stronger hurricanes?

Do you think that increased sea surface temperatures will result in more frequent hurricanes?

Thanks in advance,
Mr. Q.

Posted by: Mr_Q | July 14, 2009 1:22 PM | Report abuse

The Australian Meteorological Service seems to have a pretty good site for tracking the MJO, but attempts to correlate MJO patterns with other large-scale atmospheric patterns seem to be a bit of a mystery.

Right now we appear to be in an MJO lull with no significant Kelvin waves or phase patterns. The ITCZ has also been holding in a normal pattern near or just south of the latitude of the Panama Canal [9 degrees North latitude].

Posted by: Bombo47jea | July 14, 2009 1:24 PM | Report abuse

Thanks for the clarification.

I was just a bit confused by the sentence in your post, "While there is obviously much more to the complicated puzzle of hurricane formation than just knowing the phase of the MJO/NAO (or ENSO for that matter), the weather models are not yet showing a favorable configuration in any of these for Atlantic development." It's unclear to me what the time frame is in that sentence. Are you saying that in the short-term, such as the next couple of weeks, the models aren't showing favorable conditions for storm development, or are you looking further ahead?

Nice explanations of the various factors that come into play in the North Atlantic Basin, btw. Hopefully readers found it helpful.

Posted by: Andrew-CapitalWeatherGang | July 14, 2009 1:29 PM | Report abuse

Mr. Q,
Thanks for your questions. Tough ones, no doubt. Whether or not the SSTs will rise in the future is not my area of expertise. I'll try to get at the SST/intensity relationship instead. And of course, there's no easy answer. There are lots of other factors involved in determining hurricane strength. The temperature and wind characteristics of the near-storm environment, as well as its own internal dynamics, play large roles there. For example, a drier and windier background flow could counter effects SST increases might otheriwse induce. And it's not just the SST that matters when considering a warmer lower boundary ... it's the vertical gradient of temperature that is the key. Higher SSTs coming with a warmer, more stable atmospheric lapse rate under a lower tropopause won't help either. But... I suppose all else being equal, warmer water gives the storm more internal CAPE (juice) and ultimately lower pressures/higher winds. But I really don't know what the atmospheric environment would give us if SSTs do indeed go up in the future.

Posted by: gregpostel | July 14, 2009 1:55 PM | Report abuse

Thank you for your reply Dr. Postel.

Mr. Q.

Posted by: Mr_Q | July 14, 2009 2:20 PM | Report abuse

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