Transatlantic differences? UK and US Atlantic tropical storm forecasts

July 31, 2007 at 6:02 pm | Posted in America, Atlantic, Britain, Forecasts, Question, Tropical storms, UK, US | 2 Comments

Answers welcome. Here is the situation, followed by my question. (Update: After writing my post, and doing a little more skim-reading around the subject, I just found this post on Weather Underground, which seems to answer my question, but your comments are still encouraged.)

This is the UK’s forecast:

Met Office forecasts Atlantic tropical storm season

which can be summarised very briefly as:

(Met Office North Atlantic Tropical Storm) Forecast for July to November 2007
Issued 19 June 2007

Ten tropical storms are predicted as the most likely number to occur in the North Atlantic during the July to November period, with a 70% chance that the number will be in the range 7 to 13. This represents below normal activity relative to the 1990-2005 long-term average of 12.4.

The method is explained:

The forecast is based on GloSea representation of dynamical and physical processes characteristic of tropical storms. This is done by counting the frequency of tropical storms in the model forecasts. However, as the dynamical model grid does not fully resolve tropical storms, numbers are calibrated using tropical storm behaviour in past forecasts. The forecast process also includes predictions of the sea surface temperature (SST) anomalies. This season, a cooling trend in SST is expected in the tropical North Atlantic, and this favours fewer tropical storms than seen in recent years prior to 2006.

That is clear to me, and for this season some significance seems to hinge on the SST cooling trend.

Meanwhile, the Climate Prediction Center of the National Weather Service gives this summary for the Atlantic Hurricane Outlook:

SUMMARY

NOAA’s 2007 Atlantic hurricane season outlook indicates a very high 75% chance of an above-normal hurricane season, a 20% chance of a near-normal season, and only a 5% chance of a below-normal season. This outlook is produced by scientists at the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC), National Hurricane Center (NHC), Hurricane Research Division (HRD), and Hydrometeorological Prediction Center (HPC). See NOAA definitions of above-, near-, and below-normal seasons.

The outlook calls for a very high likelihood of an above-normal hurricane season, with 13-17 named storms, 7-10 hurricanes, and 3-5 major hurricanes. The likely range of the ACE index is 125% to 210% of the median. This prediction signifies an expected sharp increase in activity from the near-normal season observed in 2006.

The prediction for an above-normal 2007 hurricane season reflects the expected combination of two main climate factors: 1) the continuation of conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995, and 2) the strong likelihood of either ENSO-neutral or La Niña conditions in the tropical Pacific Ocean.

An updated Atlantic hurricane outlook will be issued in early August, which begins the peak (August-October) of the hurricane season.

That seems to me to be a convoluted way of explaining things. The forecast depends on two conditions, numbered 1 and 2. (So, now I wonder: why did 2006 have a low number of only nine storms if factor 1—which I take to be an ongoing warmer Atlantic SST—has applied since 1995? Did factor 2 oppose factor 1 last year, yet reinforce it in previous years? Or did some other factor operate to reduce the number of storms last year?)

Needless to say, these are quite different forecasts! (Not to mention the fact that the Framing Science guys could go to town on these two examples. The NOAA summary requires page-changing to see the figure, and page-changing to read definitions, so cannot stand alone as an explanation.)

For me, the British one is much easier to understand. To begin with, the Met Office state clearly the average with which they compare forecast numbers over a specified period. The American forecast seems overly complex without telling me that basic piece of information (the average for a specific period against which we are comparing the current forecast) up front. However, I have taken a quick look at this graph of NOAA’s Accumulate Cyclone Energy Index, and can see that the information can be found buried within this chart:

Historical Atlantic Seasonal Activity

I know the methodologies are different. The Met Office explain theirs is a dynamical seasonal prediction computer model of the global atmosphere-ocean system that represents dynamical and physical processes characteristic of tropical storms (and I think perhaps other European models are similar). By contrast, it sounds like the NOAA forecasts are based on statistical prediction methods that have been developed over the years, and require more human (possibly subjective?) input. Am I right?

It will be interesting to see what plays out in real life, but also would be nice to hear from someone who could explain in fairly simple terms why there is such a noticeable disparity between these forecasts from each side of the Atlantic🙂

Yes, I found this National Hurricane Center tropical cyclone forecast intercomparison – North Atlantic too, with graphs such as this most recent comparison:

National Hurricane Center tropical cyclone forecast intercomparison - North Atlantic 2005

Are you bilingual in these tropical storm systems methodologies, and if so, can you translate between them and explain the main differences for me, please?

2 Comments »

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  1. http://www.wunderground.com/hurricane/seasonal2007.asp

    has something to say about this. I personally don’t know. The seasonal forecast methods are far from what I consider physics-based climatology. They are based on heuristics, (observed correlations with certain large scale spatial patterns in the weather and ocean surface). I suspect that in an increasingly unstable climate heuristics are of decreasing value.

    I gather the UKMO is a new approach, more physics based than heuristic. It will be interesting to see if they get better results, but it will take at least a decade to tell the tale. One success or failure will not be (or at least, should not be considered) conclusive.

  2. Thanks, Michael. I am glad you pointed out that these models need to be put to the test for ten years or so in order to have sufficient information to assess the trend for accuracy against observations.

    By the way, have you read about the newest Hadley Centre climate model, announced today? The Decadal Climate Prediction System is intriguing. (Though, there are already some pretty weird ‘interpretations’ of what it tells us: like ‘Global Warming to Slow Down, Then Speed Up Again’ which could confuse people, or even delay action by making people think global warming has gone away … because most people are not aware of lags in the system, nor the duration of effectiveness of GHG’s, nor the difference between external climate forcings and internal variability, and the list goes on …)


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