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2013 Season Review

Introductory comments

The Atlantic hurricane season officially runs from June 1 and ends November 30 and so is now officially over. This document takes a retrospective look at the season, and highlights key events and discusses pre and mid-season expectations in light of an unusually quiet season.

Overview High impact storms The 2013 Atlantic hurricane season got off to an early start with In 2013, it was that saw the worst of the season, Tropical Storm Andrea forming on June 5. The season followed being battered by three storms from the Atlantic, and with 13 named storms which is roughly consistent with the long- five from the North Pacific Basin (not included in this term median of 12 named storms. A well-spread season saw mid- summary). Three of these eight storms were November contributing a named storm, as Melissa formed on hurricanes. November 18 - a named storm forms in the Atlantic in November about three out of four years. There was one Tropical Depression Hurricane Ingrid is considered to be the worst storm of (Eight) that formed in early September but did not reach Tropical the season reaching well into Category 1 of the Saffir- Storm strength (34 knots or more) and so did not receive a name. Simpson Scale. It crossed the Yucatan Peninsula as a Tropical Depression before becoming a Tropical Storm Taking a closer look at Hurricanes, we see a different story. Based and then a Hurricane as it moved into the Gulf of on a 30 year climatology, which is the standard period to Mexico. Ingrid made landfall, on Mexico, near La represent a climate state, we normally see six of the twelve Pesca, , causing widespread damage named storms becoming hurricanes, and three intensify into estimated to be in excess of $1.5 billion and led to the major hurricanes (Saffir-Simpson Category 3-5). However, this deaths of 23 people, mainly through flooding. Mexico season’s hurricane counts were well below average with only two also took the lion’s share of the named storms that hit storms, Humberto and Ingrid, making it to hurricane status, and land during the season with Tropical Storms Barry and not a single major hurricane. The U.S. has not seen a major Fernand also affecting the country. hurricane make landfall since 2005 – an eight-year stretch. In contrast, the U.S. was neither affected frequently nor A season without a major hurricane hasn’t occurred since 1994 severely by tropical in 2013. Tropical Storm and the number of hurricanes this year was the lowest since Andrea, was the only storm to make landfall in the U.S., 1982. Accumulated Energy (ACE) is a measure of storm affecting Florida, Georgia and South Carolina with strength and duration that is used to quantify the total energy of attendant tornados, heavy rain and flooding. Andrea Tropical Storms and Hurricanes. The ACE value this season was took one life but did not cause significant insured the lowest since 1983 and activity was about 67 damage and consequently it was not identified by PCS % below the 1981-2010 average. as a catastrophic event.

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Figure 1: 2013 North Atlantic Tropical Cyclone Tracks by Category on Dec 12, 2013. (Source: http://www.nhc.noaa.gov/2013atlan.shtml)

Table 1: Summary of 2013 North Atlantic Hurricane Season Source: NOAA Type Name Start End Max wind (mph) TS ANDREA 05-Jun 07-Jun 65 TS BARRY 17-Jun 20-Jun 45 TS CHANTAL 07-Jul 10-Jul 65 TS DORIAN 23-Jul 03-Aug 60 TS ERIN 15-Aug 18-Aug 45 TS FERNAND 25-Aug 26-Aug 60 TS GABRIELLE 04-Sep 13-Sep 60 TD EIGHT 06-Sep 07-Sep 35 H HUMBERTO 08-Sep 19-Sep 85 H INGRID 12-Sep 16-Sep 85 TS JERRY 29-Sep 03-Oct 50 TS KAREN 03-Oct 06-Oct 65 TS LORENZO 21-Oct 24-Oct 50 TS MELISSA 18-Nov 21-Nov 65

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How did the Table 2: Summary of seasonal forecasts from 2013 forecast go? Date # Named # # Major Data source ACE Many forecasts issued at the Issued Storms Hurricanes Hurricanes beginning of the season, and Observed storms n/a 13 2 0 30 updated throughout, called for 1981-2010 Median Value n/a 12 6.5 2 92 an active or above average (Colorado State University) season (see summary Table 2). Average of 5 analogue 10-Apr 10.4 7.2 4.2 151 However, it was in fact one of years (Colorado State the quietest seasons observed in University) 03-Jun 17.2 9 5.2 164 the last 20 years and in many 02-Aug 12.4 7.2 3.8 124 ways was well below the Colorado State University 10-Apr 18 9 4 165 average, as there were: 03-Jun 18 9 4 165  No major hurricanes 02-Aug 18 8 3 142  Less than a third of the Tropical Storm Risk 05-Apr 15 8 3 131 normal number of 04-Jun 15.6 7.7 3.5 134 hurricanes 06-Aug 14.8 6.9 3 121  Lowest number of UK Met Office 20-May 14 9 n/a 130 hurricanes since 1982 Weather Services 08-Apr 16 9 5 n/a  Sixth-lowest activity in International terms of collective strength National Oceanic and 110- 23-May 13-20 7-11 3-6 and duration of named Atmospheric Administration 189 storms 110- 08-Aug 13-19 6-9 3-5 175  Lowest number of “Hurricane Hunter” aircraft Florida State University 30-May 15 8 n/a 135 reconnaissance hours spent on missions to investigate Nearly all of the centers that issued multiple forecasts through the season (a few of which storms since 1966 are sampled from previous Willis reports) reduced their estimates through the season. The initial higher estimates were based on some strong evidence for an active season There were above the average including: number of named storms which alone suggests that although the  Anomalously warm tropical Atlantic potential for storms to develop  Absence of El Nino conditions seemed present during the  Anomalously low Atlantic sea level pressures season, there were inhibiting factors when considering the development of the storms.

Lessons learned for forecasting The 2013 Atlantic Hurricane Season provides learning opportunities and will fuel debate for some months to come. One of the main surprises is that the ENSO forecast actually verified quite well with predictions for the cool side of neutral conditions which are typically more favorable for tropical cyclone development than warm (El Niňo) conditions. Most seasonal forecasts rely on the ENSO signal and generally call for suppressed activity during strong warm phases (El Niňos) and non-suppressed activity otherwise. So, with the season behaving like it was an El Niňo despite the fact it was not, forecasts for elevated tropical cyclone activity did poorly. One hypothesis considers the detailed characteristics of the sea surface areas used to derive the various ENSO indices, in particular NINO 3.4. There are some suggestions that the temperature anomalies were distributed in an unusual way which allowed for a neutral index value but actually contained some very warm water in the east Pacific perhaps resulting in deceptive perceptions of neutral conditions which were in reality more like an El Niňo. We also had numerous plumes of dry, dusty air from the Sahara Desert (Saharan Air Layer or SAL) and dry air from drought-plagued Brazil which are known to inhibit hurricane development (Dunion 2004). More study will have to be conducted to elucidate these theories.

Page 3 of 4 Summary The 2013 hurricane season was unexpectedly quiet. Most quiet years occur during El Niňo conditions but that was not the case this year as the ENSO state was neutral. There were a number of factors inhibiting cyclone genesis in the Atlantic which resulted in the low number of hurricanes observed. The main unanswered question about this season is why did so many inhibiting factors occur when the broad scale climate indicators that normally give us a strong indication about the oncoming season were guiding forecasters towards the view that 2013 would be an active season? This unusually inactive season will be an active subject of debate for some time and provides a learning opportunity to build improved and more dependable forecasts.

References

Klotzbach P.J., W.M. Gray, 2013: SUMMARY OF 2013 ATLANTIC TROPICAL CYCLONE ACTIVITY AND VERIFICATION OF AUTHORS' SEASONAL AND TWO-WEEK FORECASTS. http://typhoon.atmos.colostate.edu/forecasts/2013/nov2013/nov2013.pdf

Dunion, Jason P., Christopher S. Velden, 2004: The Impact of the Saharan Air Layer on Atlantic Tropical Cyclone Activity. Bull. Amer. Meteor. Soc., 85, 353–365.

National Hurricane Center: Monthly Tropical Cyclone Summary for the Atlantic http://www.nhc.noaa.gov/text/MIATWSAT.shtml

Contact us

Geoffrey Saville Dr. James Done WRN Atmospheric hub Leader Project Scientist and Willis Research Fellow Global Analytics & Willis Research Network National Center for Atmospheric Research Willis Group Earth System Laboratory The Willis Building P.O. Box 3000, Boulder, CO 80307 51 Lime Street Phone: +1 (303) 497-8209 EC3M 7DQ [email protected] London http://www.mmm.ucar.edu/people/done/ Phone: +44 20 3124 8858 [email protected]

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