ENVIRONMENTAL CONDITIONS GIVE THE 2017 ATLANTIC HURRICANE SEASON SOME POTENTIAL, ALBEIT WITH CAVEATS
WHAT TO WATCH:
• Environmental conditions favor near to above normal Atlantic tropical cyclone activity during June-November
• ENSO is not a skillful indicator at this point but will become an important controlling factor if El Niño or La Niña develops that bears watching
• Gulf impacts should be lower than normal regardless of total basin hurricane activity based on analogs
2017 ATLANTIC ANALOGS THROW SOME COLD WATER ON THE LIKELIHOOD OF A VERY ACTIVE HURRICANE SEASON FROM JUNE-NOVEMBER.
Forecast indicators with the current lead-time on the Atlantic hurricane season, which begins officially on 01 June, are different from what they would be approaching the peak season. The importance of this cannot be overstated, as will be illustrated in examining the potential risks and uncertainty for the upcoming season. At this point, the Atlantic Multi-Decadal Oscillation (AMO) is the leading indicator, with contributions from the Scandinavian (SCA), previous season Eastern Pacific Accumulated Cyclone Energy (EP ACE), the Atlantic Meridional Mode (AMM), the Madden-Julian Oscillation (MJO), and the Pacific Transition (PT). The AMO and AMM represent the environment of the tropical Atlantic, most notably including sea surface temperature (SST). The PT and EP ACE are tied to Pacific conditions, and the MJO, a cluster of thunderstorms that moves around the equator every 30-60 days on average, propagates through the Pacific and Atlantic, bringing impacts to both. Finally, the SCA is related to mid-latitude blocking pressure patterns that relate to the intrusion of detrimental dry air to tropical cyclone formation into the tropics.
Based on the forecast indicators, tropical cyclone activity from the best analogs for the 2017 Atlantic hurricane season from June-November is shown in Figure 1, with specific analog years listed in the figure caption along with the interpretation of the plot. The main purpose in showing the total tropical cyclone activity from 5 of the best analogs is to understand the distribution of cyclones within the basin. Two noteworthy items stand out:
• First, only one hurricane, though it was major hurricane (Category 3-4), moved through the central Gulf of Mexico in making landfall in the U.S. One significant Gulf of Mexico hurricane in 5 years is not particularly active.
• Second, there is a lot of activity in the western Atlantic including major hurricanes hitting the Bahamas as well as two hurricanes making landfall along the U.S. East Coast, in addition to multiple other hurricanes tracking offshore.
Controlled by the typical steering flow for tropical cyclones in the analog years, this highlights the main landfalling risk area for 2017, which would appear to be the East Coast rather than the Gulf of Mexico. Having said that, it is also important to examine the total activity in the basin from the analog years, which can enhance or diminish the probability of landfalling impacts simply by altering the number of “swings at the plate”, so to speak considering that tracks ultimately rely on short-term weather patterns. Table 1 (not shown) highlights the monthly breakdown of tropical cyclone activity from the top analogs from 3 different angles, as described in the caption.
The key takeaway from the numbers in the table is that in total, the analog years displayed a near normal number of tropical cyclones, but featured the prevalence of weaker than normal storms. This illustrates the point that nothing in the analogs strongly favors a very active or inactive season from the perspective of storm number. The fact that weak storms were favored also introduces the uncertainty that exists in 2017- if short-term weather conditions, which are critical for tropical cyclone development, become favorable often enough to produce strong storms, a scenario where the June-November season has above normal activity is not out of the question at all based on the analogs.
There is one variable not included in the forecast indicators that could play a major role in the fate of the 2017 Atlantic hurricane season. The El Niño Southern Oscillation (ENSO), which modulates the vertical wind shear (VWS) that tears cyclones apart when strong and favors development when weak, could be a contributor if the upcoming season becomes very active or inactive. At the current lead-time on the peak hurricane season of August-October, ENSO is not a reliable indicator because it can trend toward El Niño or La Niña from this point by the time the peak season arrives based on its climatology. This will need to be monitored in the coming months and it will be included in our peak season outlook that will be released prior to August, at which time the direction of the ENSO state will become more clear. Our current ENSO forecast indicates the likelihood of a neutral state to persist through the June-August time period in terms of a mean state, but there is support for an El Niño or La Niña trend from opposing indicators. A near normal number of tropical cyclones from the analogs showed that there is not a strong lean on the probability of an active/inactive hurricane season at this time, and the ENSO state could be the main factor that ultimately moves the needle one direction or another. A trend toward El Niño (La Niña) favors strong (weak), and would indicate an inactive (active) season.
Figure 1: Accumulated Cyclone Energy (ACE) tracks for the 2017 Atlantic analogs (not listed). Tracks are color-shaded so that a cyclones of Tropical Storm intensity are shaded gray, Category 1 and 2 hurricanes are shaded blue, Category 3 and 4 hurricanes are shaded red, and Category 5 hurricanes are shaded black. This is done for ease of interpretation, and the maximum ACE value, which is directly tied to wind speed, is plotted so that two storms tracking over the same region will not create the false signal of a major hurricane.
AT LEAST A SOMEWHAT ACTIVE HURRICANE SEASON MAY LAY AHEAD IN 2017, BUT GULF IMPACTS COULD BE MINIMIZED BASED ON UNFAVORABLE CONDITIONS IN THE REGION.
The forecast indicators were used to create a statistical model in addition to the analogs, and it illuminates the potential that the 2017 Atlantic hurricane season may possess if some strong storms can develop. The model produced a forecasted ACE of 141 for June-November, which is above both the normal (105) and the analogs (93). ACE, the accumulated cyclone energy, reflects the power of each storm via wind speed added up throughout the lifetime of the storm. Therefore, ACE is a better measure of activity than storm number as it incorporates both intensity and lifetime to assess impacts. Using cross-validation techniques, the model showed a ~0.71 correlation coefficient to the test data in demonstrating a good overall skill level. This, combined with the analogs, provides a functional range of activity from slightly below normal to an above normal season.
Given the “noise” that can exist within tropical cyclone seasons, it is prudent to examine the large-scale environmental conditions that contribute to tropical cyclone development before making an official forecast. In order to accomplish this, 4 important variables were examined from August-October (during which ~90% of tropical cyclone activity takes place in the Atlantic):
• Sea Surface Temperatures: Figure 2 shows the SST anomalies from the analogs, demonstrating the favorable state of the AMO and AMM in supporting warm waters that can act as an energy source for tropical cyclones. Alone, this would favor an active hurricane season.
• Wind Shear: Figure 3 shows the VWS anomalies from the analogs, demonstrating a mixed picture with favorably low wind shear values over the main development region of the central Atlantic, while high shear that can tear storms apart dominates in the landfalling areas. This may offer some explanation as to why the landfalling impacts were relatively low in the Gulf of Mexico and Florida, as storms would have more often than not moved into an unfavorable environment approaching the coast. Alone, this is probably neutral, which makes sense with an ENSO neutral state.
• Water Vapor: Figure 4 shows the column water vapor (CWV) anomalies from the analogs, demonstrating high moisture values that help storms to be sustained in the early formation stages, and promote intensification. The exception to this is over the Gulf of Mexico, where frequent dry air was in place to “put the nail in the coffin” of the Gulf of Mexico storms when combined with the impacts from high VWS. The high CWV in the western Atlantic may also explain why a large number of storms formed there despite unfavorable shear. Alone, high CWV favors an active hurricane season.
• Sea-Level Pressure: Finally, Figure 5 shows the mean sea-level pressure (MSLP) anomalies from the analogs, demonstrating widespread favorably low pressure to promote the necessary moisture convergence and ascent for tropical cyclone formation. Alone, this favors an active hurricane season.
In summary, 3 out of 4 environmental variables support the notion of an active 2017 Atlantic hurricane season that is suggested by the statistical model, with the 4th variable likely to swing one direction or another based on the ENSO state.
Taking all information into consideration, our official forecast for the 2017 Atlantic hurricane season will call for activity that is ~115% of normal in a slightly active season overall. This translates into a total of 13 named storms, with 7 hurricanes and 3 major hurricanes. A comparison of this to a normal season as well as the analogs is shown in Table 2. The table shows a slightly active 2017 Atlantic hurricane season overall in the forecast, while the analogs show that the above normal storm number (13) could translate into a lot of weak storms. In terms of probability, this forecast means that there is a 50% chance of a near normal season, a 40% chance of an active season, and a 10% chance of an inactive hurricane season. The reasoning behind the forecast is that the environmental conditions from the analogs are generally conducive to tropical cyclone formation, supporting the active season projected by the statistical model. However, the slightly below normal activity during the analog years as well as the potential for El Niño formation were enough to hedge just a bit to arrive at the position of a slightly active season.
FIGURE 2: Mean Atlantic August-October SST anomalies (°C) from the 2017 Atlantic analogs (not listed) in the typical development region for tropical cyclones show favorably warm SSTs for development.
FIGURE 3: Mean Atlantic August-October VWS anomalies (m/s) from the 2017 Atlantic analogs (not listed) in the typical development region for tropical cyclones show favorably low shear in the development region of the central Atlantic, but less favorable conditions in the landfalling regions.
FIGURE 4: Mean Atlantic August-October CWV anomalies (mm) from the 2017 Atlantic analogs (not listed) in the typical development region for tropical cyclones show favorably high moisture for tropical cyclone formation except in the Gulf of Mexico.
FIGURE 5: Mean Atlantic August-October MSLP anomalies (mb) from the 2017 Atlantic analogs (not listed) in the typical development region for tropical cyclones show favorably low MSLP basin-wide, which promotes the moisture convergence and ascent required for tropical cyclone formation.
TABLE 2: Comparison of tropical storm number (excludes tropical depressions which explains the discrepancy from Table 1), hurricane number, and major hurricane (Category 3-5) number from average (left), forecasted (middle), and analog (right) years.
For a list of the analog years with weightings, Table 1, impacts on oil and natural gas operations, as well as short-term updates that now occur daily, please visit the Tropical Weather Dashboard.
I would like to thank Tom Walsh and Ed Whalen for their contributions to the production of this article.
