Another Active Atlantic Hurricane Season Being Predicted
“The 2021 hurricane season, starting June 1, will be more active than normal. This initial forecast sees a year in line with recent years but well below the record years of 2005 and 2020. We expect more hurricane landfalls. ”
Although it is only mid-April, the first seasonal forecast for Atlantic basin tropical storm activity has been released. Colorado State University’s Department of Atmospheric Science (CSU) released its April tropical storm forecast, which it subsequently will update in early June, July, and August. In the opening line of the report, the CSU forecasters state: “We anticipate that the 2021 Atlantic basin hurricane season will have above-normal activity.” That prediction is a continuation of the active storm condition that has existed in the Atlantic since 1995. Although the forecasters discuss the number of storms and the likelihood of one landing along the U.S. coastlines or in the Caribbean, they remind people in coastal locations to be prepared because it only takes one storm to devastate an area and put people’s lives at risk.
The CSU forecast calls for the 2021 hurricane season, which traditionally runs from June 1 to November 30, to experience 17 named storms, or storms with sustained wind speeds of 39 miles per hour (mph) or greater. The forecast expects eight of them to become hurricanes, with winds of 74 mph or greater, and four to develop into major hurricanes, with 111 mph or greater winds.
While an active storm season, this forecast calls for significantly fewer storms than experienced last year when there were 30 named storms, 13 hurricanes with six becoming major hurricanes, all records. Last year’s storm activity surpassed the prior record year of 2005 when there were 28 named storms, 15 hurricanes and seven major hurricanes.
Exhibit 8. Twenty Years Of Atlantic Tropical Storm Data SOURCE: Stormfax Weather Almanac, PPHB
Once can see how this year’s forecast is more in line with the storm counts experienced over the past 20 years. With the completion of the 2020 storm season, the National Oceanic and Atmospheric Administration (NOAA) has updated its long-term trend averages. The 1991-2020 average shows 14 named storms, seven hurricanes and three major hurricanes, which is an increase of two named storms, one hurricane and one major hurricane from the average for the 1981-2010 era. This new trend calculation lends support to the environmentalists who argue that climate change is making the world warmer, including seawater, which helps the formation and strengthening of storms. The problem with this analysis is that we have much better satellite and observational data today than in the past, so we are capturing more storms that previously would never have been known to exist.
This point about better data that can be used to distort conclusions about the frequency of tropical storms was highlighted in a series of charts from the web site for the Geophysical Fluid Dynamics Laboratory, which is a laboratory within the NOAA Office of Oceanic and Atmospheric Research. This is one of seven NOAA Research Laboratories around the country conducting research into various climate issues.
Exhibit 9. Atlantic Tropical Storm Record Shows No Increase SOURCE: GFDL/NOAA
The first chart shown on the GFDL web site displays the number of moderate-duration storms (lasting for more than two days) adjusted for an estimate of missing storms in the past. It also shows the number of short-duration storms (lasting less than two days). As these graphs show, we appear to be in a period of rising storm activity, at least since the 1980s for moderate-duration storms, and since the 1950s for short-duration storms. However, as the commentary on the chart from the web site states: “Figure 1: Atlantic tropical storms lasting more than 2 days have not increased in number. Storms lasting less than two days have increased sharply, but this is likely due to better observations. Figure adapted from Landsea, Vecchi, Bengtsson and Knutson (2009, J. Climate).”
For many people, the visual from the chart is sufficient to support the claim that climate change is increasing the number of Atlantic basin tropical storms. People would make that claim saying that because this chart ends with 2009, the recent record year for storms would certainly support their belief. Therefore, we must act to stop the warming. But when they read the commentary associated with the chart, which is covering the data for nearly 130 years, their view is not supported by the scientific evidence. The web site has another chart, which would seem to support the warming world narrative.
Exhibit 10. The Statistical Problem With Visual Impressions SOURCE: GFDL/NOAA
Again, the climate change advocates would point to this chart ending in 2006, and if extended to include the more recent data, it would bolster their view. What this chart shows is a long-term declining trend in the number of storms from 1880 to about 1930, using the adjusted annual storm count, after which the number trends higher until the 1950s, after which it declines until about 1980 before beginning another uptrend. While we are in a period of rising storm activity, we experienced a similar strong secular uptrend from about three storms in 1930 to 15 in the early 1950s.
Once again, the commentary accompanying the chart raises questions about the conclusion. “Figure 2: Atlantic tropical storm counts adjusted for likely missing storms. Once an estimate for likely missing storms is accounted for the increase in tropical storms in the Atlantic since the late-19th Century is not distinguishable from no change. Figure adapted from Vecchi and Knutson (2008, J. Climate).” The scientists found that the adjusted storm count shows essentially no change in trend.
The third, and last, chart shows the history of Atlantic basin tropical storm indices normalized for missing storms. The green, blue, and red charts all show rising trend lines. Only the yellow charts, which count the number of tropical storms making landfall on the U.S. coastlines, show declining long-term trendlines.
Exhibit 11. Trends Of Atlantic Basin Storm Activity SOURCE: GFDL/NOAA
Once again, the commentary associated with the chart on the web site states: “Figure 3: Normalized Atlantic Indices. Since the late-19th Century global (green) and tropical Atlantic (blue) temperatures have risen – an increase that was partly driven by increased greenhouse gases. If one does not account for possible missed storms (first red line) Atlantic tropical storms appear to have increased with temperature; however, once one accounts for possible missed storms (second and third red lines) basinwide storms have not exhibited a significant increase. When one focuses only on landfalling storms (yellow lines) the nominal trend has been for a decrease. Figure adapted from Vecchi and Knutson (2008, J. Climate).”
While the climate change advocates would again point to the fact that these charts are not reflective of the recent storm data, we would ask: Why would NOAA put this data on its web site if it was doubting the accuracy in reflecting long-term trends? Yes, the visuals would certainly suggest that there has been a steady uptrend in the frequency of Atlantic basin tropical storms. But over the 130+ years of data, there has been no statistically significant measurement that would support the visual conclusion. One should understand that climate scientists have been studying and writing about tropical storm trends for decades. A recent paper on the GFDL web site states that “Most climate model studies project the total number of TCs [tropical cyclones] each year to decrease or remain approximately the same.” At the same time, it says the intensity of storms is projected to continue to rise. In virtually every observation, the paper speculates on the possible impact from climate change beginning to emerge from natural climate variability. These conclusions come from models. This entire topic will continue to be debated, and another active storm season will add fuel to the debate. But as we have seen from the data, we have experienced prior periods of sharply rising storm numbers, during decades when data is totally observational.
Improving our storm forecasts is critical for minimizing damage and potential loss of lives. One of the areas of interest of William Gray, the originator in 1984 of the CSU hurricane-forecasting effort, was attempting to predict where storms might land. His efforts were motivated by his desire to make the storm forecasts of greater value to people. If they could be informed of highly likely landfall locations, people would have time to prepare and even evacuate, reducing the risk of storm-related deaths. The art of predicting landfall locations has improved, but it is still difficult to give people more than several days’ notice that they are within a storm’s target zone.
The CSU forecast provides probabilities for a least one major hurricane (Category 3-4-5) making landfall along the U.S. coastlines. This season, CSU estimates a 69% probability of a landfalling somewhere along the entire U.S. coastline, which compares to an average for the last century of 52%. Thus, we should expect and prepare for a storm making landfall. The U.S. East Coast, including the Florida peninsula, has a 45% probability, compared to an historical average of 31%. The Gulf Coast from the Florida Panhandle westward to Brownsville, Texas, has a 44% probability compared to 30% historically. Although the CSU forecasters expect a higher probability of landfall, they rate the likelihood of an East Coast versus Gulf Coast landing to be in line with the historical difference – one percentage point, although the overall probability this year is higher.
As the CSU forecasters put it: “Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much activity is predicted.” This is a great message and warning for coastal residents. We endorse it.