WILLIS RE ANALYTICS eVENT™ 2014 Atlantic Hurricane Season Outlook: Aug 15, 2014 This seasonal forecast briefing is the third and final update. In this briefing, we provide a summary of how the remainder of the hurricane season appears to be shaping up. Current situation Forecasts The 2014 North Atlantic hurricane has been Forecast skill is generally higher for forecasts issued part way through fairly average so far. The season got off to an the hurricane season due to shorter lead-time and the fact that some of early start with Tropical Storm Arthur the season has already passed. The major centers remain unanimous in attaining hurricane status over a month continuing to forecast below average activity (Table 1) with a slight before the long-term average date of the first nudge downward in forecast activity from their forecasts issued in early hurricane. Hurricane Bertha followed in early July. August and remained out over the open ocean where it re-curved toward Europe and caused The CSU forecast is based on their new early-August statistical significant flooding across the U.K. prediction scheme that is then moderated by the average activity in five analog years and some qualitative adjustment for factors not directly As we enter the peak of the hurricane season considered in their calculations. Their forecast for a quiet season is the storms to watch will be those that develop guided by cooler than normal ocean temperatures and higher than in the far eastern Atlantic just off the African normal wind shear over the tropical North Atlantic, and a forecast for coast since these have greater potential to weak El Niño conditions throughout the remainder of the season. become major hurricanes on their long track towards the U.S. However, so far the cooler The NOAA forecast, based on information from an array of dynamical than normal sea surface temperatures and dry simulations, is also nudged downwards from their pre-season forecast, air aloft has been suppressing development. again reflecting the unfavorable conditions over the tropical North Unfavorable wind shear has also been far Atlantic. In addition to high vertical wind shear, cool ocean stronger than normal – the highest in the temperatures and dry warm air at mid-levels, they also cite a southward historical 45-year record (NOAA, 2014). The shift in a core of winds at about 3km above the surface known as the likelihood of weak to moderate El Niño African Easterly Jet. This southward shift of the jet reduces the amount conditions developing for the remainder of of cyclonic spin in the background environment for hurricane the season will lead to further deterioration of formation. this already unfavorable environment. The range in the forecast numbers of named storms across the major Given that these unfavorable conditions are forecasting centers (Table 1) is 8.6 to 12.0 with an average of 10.2 forecast to persist and possibly grow stronger, (compared to a typical number of 12). The range in the forecast forecast centers are unanimous in predicting a numbers of hurricanes is between 3.6 and 6 with an average of 4.6 year of low activity. However, we caution that (compared to a typical number of 6.5), and the range in the forecast this does not necessarily correspond to a low- numbers of major hurricanes (Saffir-Simpson category 3-5) is between impact year. NOAA/CPC finds that the 1 and 2 with an average of 1.5 (compared with a typical number of 2). likelihood of at least one U.S. hurricane To give an idea of typical error bars on these forecasts, CSU states that landfall is the same under El Niño, neutral they expect two-thirds of forecasts to verify within 1 standard deviation and La Niña conditions. of the forecast number. This year, that range is between 6.7 and 11.3 named storms for the remainder of the season. Page 1 of 4 CSU predicts the likelihood of a major U.S. hurricane landfall during the remainder of the season at 73% of the average likelihood. CSU issues landfall probabilities that leverage skill from the long-term relationship between total basin activity and landfall. We caution that recent years saw exceptions to this rule; this may suggest a temporary breakdown in the relationship that may persist into this year. Analog Years An alternative view to forecast models is provided by hurricane activity in past years that had similar climate conditions to June and July this year. CSU uses this approach to qualitatively correct the output from their statistical forecast technique. Average activity over their 5 analog years of 1957, 1986, 1993, 2002 and 2009 (Table 1) indicates a below average number of named storms with slightly more major hurricanes than their official forecast. Table 1: Atlantic seasonal hurricane forecasts as of August 15, 2014 Summary of 2014 Atlantic Seasonal Hurricane Forecasts Date # Named Storms # Hurricanes # Major Hurricanes ACE1 Data Source Issued (% of normal) (% of normal) (% of normal) (% of normal) 1981-2010 Median Value (Source CSU) 12.0 6.5 2.0 92 Average of 5 Analogue Years (Source CSU) Jul 31 2014 8.6 (72%) 3.6 (55%) 1.4 (70%) 56 (61%) Colorado State University Jul 31 2014 10.0 (83%) 4.0 (62%) 1.0 (50%) 65 (60%) NOAA/CPC Aug 7 2014 9.5 (79%) 4.5 (69%) 1.0 (50%) 60 (65%) Tropical Storm Risk Aug 5 2014 12.0 (100%) 6.0 (92%) 2.0 (100%) 70 (76%) Weather Services International July 23 2014 11.0 (92%) 5.0 (77%) 2.0 (100%) n/a Average of the all of the above Aug 15 2014 10.2 (85%) 4.6 (71%) 1.5 (75%) 63 (68%) Climate signals: Sea surface temperatures The formation and development of hurricanes is highly dependent on the available energy in the upper layers of the ocean. That’s why sea surface temperatures are often used as a proxy for available energy as a major factor in determining seasonal activity. This year, waters over the breeding ground of the most intense hurricanes remain slightly cooler than normal (Figure 1). Given the tremendous inertia of the oceans, it is likely that these cooler waters will persist throughout the hurricane season and suppress intense hurricane development. In contrast, the East Pacific has seen warmer than normal ocean temperatures (Figure 1) and this played a major role in supporting the very active East Pacific hurricane season this year. Figure 1: Departure of sea surface temperature from a long- term average (°C) on Aug 11 2014. NOAA/NESDIS The cool North Atlantic waters are not only cooler than normal http://www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.h but also cooler than the average tropical ocean temperatures. tmlhttp://www.nhc.noaa.gov/tafb/atl_anom.gif) Recent research has suggested an important role for this relative measure of ocean temperature in driving hurricane activity. A North Atlantic that is warmer than the rest of the tropical oceans will tend to produce a high-level of hurricane activity whereas a cool North Atlantic relative to the rest of the tropical oceans, as we are seeing this season, will tend to suppress hurricane activity. 1Accumulated Cyclone Energy (ACE) is a combined measure of hurricane intensity, duration and frequency. ACE is calculated as the sum of the square of the maximum wind speed in each 6-hour period during the life of a tropical cyclone from the time it reaches tropical storm strength (wind speeds ≥ 65 kmph (39 mph)) in units of 104; ACE = 10-4∑v2max, where v is measured in knots. Page 2 of 4 Saharan Air Layer Seasonal forecasts based on large-scale predictors such as ocean temperatures can miss important intra-seasonal features. One such phenomenon is dry and dusty air at mid-levels that can blow off North Africa and track across the tropical North Atlantic (Dunion and Velden 2004), forming what is known as the Saharan Air Layer (SAL). Tropical cyclones that are embedded in this SAL can struggle to attain named storm status as the dry warm air suffocates the initial thunderstorm clusters. It is also thought these bursts of dry dusty air play a major role in reducing hurricane activity over the North Atlantic compared to the North Pacific. This year has seen a succession of SAL events that have further suppressed tropical cyclone activity on top of an already unfavorable background environment. Currently dry air extends across the entire North Figure 2: Satellite-derived snapshot of dry and dusty air (orange colors) tracking off North Africa out Atlantic (Figure 2) effectively over the tropical North Atlantic interfering with the development of thunderstorm clusters (grey putting a temporary lid on the clouds) on Aug 14, 2014. Source: Cooperative Institute for Meteorological Satellite Studies / main region of hurricane University of Wisconsin-Madison. development. Climate signals: El Niño During El Niño a strong overturning of the atmosphere results in strong winds aloft over the tropical North Atlantic. Despite their strength, hurricanes have a weak spot that when exposed to these strong winds can result in rapid weakening and can even prevent hurricane formation altogether. These strong winds can blow intermittently throughout an El Niño year, meaning a strong hurricane can sneak through an El Niño year (as we saw with Hurricane Andrew). The evolution of the climate system this year has not followed a smooth slide towards El Niño conditions. Rather, the atmosphere and ocean have failed to agree. First, the atmosphere did not follow the ocean’s lead and now the ocean is backtracking on its initial push towards El Niño.