Hurricane Gert 12-17 August 2017 Maxwell E. Torres Force-13 March 5, 2019 1 1. Synoptic History Late on August 2, the National Hurricane Center (NHC) began tracking a tropical wave that was emerging off the west coast of Africa. Early on August 3, the wave emerged over water. Initially in a favorable environment, the wave coalesced rapidly and began to show signs of sufficient organization to be called a tropical depression. However, this period was short-lived as the wave entered a more hostile environment on August 5, resulting in its structure and convection severely diminishing. After this, the wave began moving westward at around 20kt and became elongated. This motion continued for the next five days. The eventually slowed down and turned towards the northwest and regained its compact, circular structure on August 10. On August 11, the wave passed to the north of the Leeward Islands and Puerto Rico. At this point, the wave’s organization increased, and curved convective bands developed around a center of circulation that was getting better defined. At 0000 UTC on August 12, a surface low had formed within the wave around 275 n mi east of the Turks and Caicos Islands and continued to gain convection over the next twelve hours. At 1800 UTC on August 12, while the low was about 230 n mi northeast of the Turks and Caicos Islands, it became well-defined enough to be declared a tropical depression. Within 6 hours, the depression had strengthened into a tropical storm and was given the name “Gert” around 215 n mi northeast of the southeastern Bahamas. Despite conducive sea surface temperatures (SSTs) and low wind-shear, Gert failed to intensify for the first 18 hours due to relatively dry air in its environment. By 0000 UTC on August 14, Gert had expelled the dry air and begun to intensify. This steady intensification would continue for the next 3 days. At 0600 UTC on August 15, Gert became a hurricane while located roughly 380 n mi west of Bermuda. Shortly after becoming a hurricane, Gert began to accelerate towards the northeast, having come under the influence of a mid-latitude shortwave trough that was moving off the United States’ east coast. At 1800 UTC on August 16, Gert reached its peak intensity estimated at 100kt while located 385 n mi southeast of Halifax, at which time, its eye measured 10 n mi in diameter. Within 6 hours of reaching its peak, Gert began to move over much cooler SSTs an encountered over 30kt of vertical wind shear. As a result, Gert’s convective cloud pattern quickly eroded, and it weakened to a tropical storm by 1200 UTC on August 17. 6 hours after weakening to a tropical storm, Gert fully transitioned into an extratropical cyclone 290 n mi southeast of St. John’s. the extratropical remnants of Gert moved steadily in a northeasterly direction for over a day while continuing to weaken. At 0000 UTC on August 19, the system dissipated completely over cold waters in the far northern Atlantic Ocean. 2 Meteorological Statistics Wind and Pressure Gert’s estimated peak wind speed of 100kt is based on several satellite estimates from the NHC’s Tropical Analysis and Forecast Branch (TAFB), the Advanced Microwave Sounding Unit (AMSU) and Advanced Dvorak Technique estimate. These estimates give a range of 90kt to 115kt, most of which are near, at, or above 100kt. Averaging these values gives a value between 100kt and 105kt. Because of its small eye, it was likely at the lower end of the averaged range. Thus, a value of 100kt has assigned as Gert’s peak intensity. The estimated minimum central pressure of 960mb is based on the NHC’s wind- pressure relationship estimate of 962mb accounting for the slight increase of wind speed from their 95kt value. Casualty and damage Statistics There are no reports of damage associated with Gert. However, two drownings in high surf are believed to be associated with Gert. Best Track Below is the best track chart for Hurricane Gert Date Time (UTC) Lat. Lon. Wind (kt) Pressure SSHWS (mb) 8/12 0000 22.4 66.8 25 1013 LOW 8/12 0600 22.7 67.8 25 1013 LOW 8/12 1200 23.2 68.7 25 1013 LOW 8/12 1800 24.0 69.4 30 1013 TD 8/13 0000 24.9 70.0 35 1012 TS 8/13 0600 25.8 70.6 35 1012 TS 8/13 1200 26.8 71.2 35 1012 TS 8/13 1800 27.7 71.6 35 1012 TS 8/14 0000 28.5 71.9 40 1009 TS 8/14 0600 29.2 72.1 45 1006 TS 8/14 1200 29.7 72.2 50 1002 TS 8/14 1800 30.2 72.3 55 996 TS 8/15 0000 30.8 72.3 60 992 TS 8/15 0600 31.5 72.3 65 986 1 8/15 1200 32.3 72.1 70 982 1 8/15 1800 33.2 71.8 70 980 1 3 8/16 0000 34.2 71.0 70 976 1 8/16 0600 35.4 69.5 80 973 1 8/16 1200 36.8 67.1 90 969 2 8/16 1800 38.2 64.1 100 960 3 8/17 0000 39.4 60.4 90 964 2 8/17 0600 40.7 56.2 80 968 1 8/17 1200 42.2 52.0 60 981 TS 8/17 1800 44.0 48.0 50 988 EXTRATROP. 8/18 0000 45.9 44.3 40 991 EXTRATROP. 8/18 0600 47.7 40.5 35 994 EXTRATROP. 8/18 1200 48.8 37.8 35 996 EXTRATROP. 8/18 1800 50.5 36.5 35 998 EXTRATROP. 8/19 0000 DISSIPATED 4 Below is the best-track map for Hurricane Gert 5 Forecasting and Warning Critique The precursor to Gert was never well forecast, having fluctuated between low, medium, and high chances of formation twice on the 48-hour and 5-day outlooks from the NHC before it finally formed. With the 48-hour outlook reaching a medium chance (40-60%) twice and the 5-day outlook reaching a high chance (>60%) twice prior to formation. After formation, Gert was forecast much better, with the only critique being the fact that it became a hurricane around a day before it was originally forecast to. Because it never got close to land as a tropical cyclone and was never expected to, no tropical cyclone watches or warnings were issued in association with Gert. 6 Cyclone Destruction Potential Scale since January of 2017, Force Thirteen has rated storms used the Cyclone Destruction Potential Scale (CDPS), a scale developed by member Devon Williams with the intention of determining how destructive a storm will be using multiple factors and presenting it to the public in an easily understandable way. The result is a ten-tiered scale that encompasses tropical storms as well as hurricanes. Hurricane Gert is estimated to have reached stage 5 on the old CDPS used from January of 2017 to December of 2018. A stage 5 is described as a powerful storm that will cause devastating damage. Because Gert never impacted land and rainfall estimates weren’t available at the time, no estimate has been or will be made using the new CDPS that was first used in January of 2019. 7 Other Data The following are sets of miscellaneous data from Gert throughout its life time. 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 .