Hurricane Ophelia; a detailed analysis

 

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“How now, Ophelia? What’s the matter?”

A famous line from Shakespeare’s masterpiece, Hamlet. Interestingly, an anagram of Hamlet is “Halt ’em!”, and halt us she did, ex-hurricane Ophelia causing Ireland to close down for a day under a Red Warning issued by our meteorological service, Met Éireann. Schools, public transport and businesses closed down as people were warned to stay indoors and to not make ‘unnecessary journeys’ until the strong extratropical storm, originating south of the Azores, had passed well to the north.

The strongest hurricane ever recorded so far east in the Atlantic, Ophelia reached Category 3 (Major) hurricane strength against the odds southwest of the Azores and continued as a major hurricane as far east as 26.6 °W, further east than any other since records began in 1851. Frances in 1980 did come close (29.9 °W, some 190 nautical miles (350 km) further west), so while it’s not completely unheard of, Ophelia was still a record-breaker.

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NASA Modis Aqua visible satellite image of Hurricane Ophelia at 1540 UTC on October 12th, 2017.

Development

Ophelia seemed to take the National Hurricane Center a bit by surprise, becoming a hurricane in a region that doesn’t normally spawn them, especially this late in the season. Starting off as a disturbance several hundred miles southwest of the Azores, it gained enough organisation to become Tropical Depression Seventeen early on October 9th and then strengthened to Tropical Storm Ophelia later the same day. Sea surface temperatures (SSTs) were borderline, just on the 26-27 °C threshold necessary for a hurricane to sustain itself. As the storm moved further east, however, it was moving over increasingly colder waters (though still about 2 degrees warmer than average for the area), so you would expect it to quickly lose intensity, but that’s not what happened.

The air at upper levels in that area was a couple of degrees cooler than average. This colder air offset the effect of the cooler water by just enough that the overall vertical temperature profile of the atmosphere was still unstable enough to promote continued deep convection, which is what hurricanes love. It was like a shot in the arm for Ophelia, an adrenalin drip pulling her through where she otherwise would not have made it. Also key was the marked ventilation of the storm by a trough in the north Atlantic, as seen by the northward flow of cold cirrus cloud from the centre of the storm. A trough to the northwest of a hurricane is known to be an important feature in helping poleward mass flow at upper levels, which helps to maintain low pressure at the surface

 

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Climatological map showing where tropical cyclones have originated in the period October 1-10th, 1851-2015. The majority of cyclones originate in the western tropical Atlantic, including the Caribbean and Gulf of Mexico, with very few in the east Atlantic. Image: National Hurricane Center.

 

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Ocean Heat Content chart, showing Ophelia’s location and forecast track (hours) on October 9th, just as it was declared a Tropical Depression. Ocean Heat Content (OHC) is a measure of the depth of the 26-degree isotherm and hence how much heat energy is available in the sea to fuel a tropical cyclone. Values of <50 kJ/cm² are deemed too low to cause a storm to intensify significantly, and values near 0 should cause weakening. However, the low OHC under Ophelia’s track was offset by the colder air at upper levels. Image: Regional and Mesoscale Meteorology Branch (RAMMB), Colorado State University.

 

Ophelia meandered slowly eastwards to the south of the Azores over the next few days. On the 14th (IR water vapour satellite image at the top of this page) we can see the large north Atlantic trough (the U-shaped dark area) digging southwards from New Foundland in the direction of Ophelia, which at this stage had reached Category 3 status, puffing out maximum 1-minute mean winds of 100 knots (185 kph) and higher gusts. Around the base of this trough flows the jet stream, which is usually one of the nails in the coffin of tropical systems. These strong winds at upper levels increase what is called windshear, a difference in wind with height. Windshear tears the vertical structure of hurricanes apart, with the upper part usually getting displaced to the north and the eye losing its shape and symmetry.

By late Sunday (15th) we can see that this is what’s happening (below). The trough is interacting with Ophelia (by now weakened to 80 knots (150 kph) and losing its eye), its strong winds and drier, cooler air attacking from the west as it steers the system northwards past Iberia, next stop Ireland. Ophelia continued to lose its tropical signature overnight, with a long stream of jet cirrus connecting it with the north as it became embedded in the polar front. The southwestern sector is devoid of high cloud, the cold dry westerly airflow inhibiting deep convection and exposing the eye. It is now in its transition from tropical hurricane to an extratropical low.

 

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GFS analysis chart for 0000 UTC on the 14th, showing 500 hPa heights (black), sea-level pressure (white) and 500-1000 hPa thickness (coloured). Hurricane Ophelia is visible southwest of the Azores (bottom left) as the large trough approaches from the north. Image: http://www.wetter3.de
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Same chart as above, but for 1800 UTC on the 15th. Ophelia has been intercepted by the trough and is starting extratropical transition. Image: http://www.wetter3.de

 

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Thickness advection chart, showing a thermal dipole forming in Ophelia’s circulation. Colder air is becoming entrained from the west, signalling the start of extratropical transition. Image: http://www.wetter3.de
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Infrared satellite image from 1800 UTC on the 15th, showing Ophelia losing its tropical appearance. The eye has disappeared and the higher (whiter) cloud has been displaced to the north due to the effects of windshear from the trough. Image: Sat24.com
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Interesting Day/Night satellite image from 0247 UTC Monday 16th, with Ophelia’s cloud mass just about visible in the top left quadrant. Note also the lights of Iberia, and even the large area of wild fires through Portugal and Galicia. Smoke from these fires was carried far north in the strong southerly flow to the east of Ophelia. Image: RAMBB, Colorado State University.

At this stage the Red Warning, intially issued for counties in the south and southeast, had been extended to cover the whole of the Republic of Ireland, and phones were vibrating with Whatsapp messages from bosses informing people not to come to work on the Monday morning. The National Hurricane Center was forecasting hurricane-force* winds arriving on the south coast Monday morning and spreading through the country during the day.

High-resolution models, which had been remarkably consistent for days, showed a strong core of hurricane-force (Beaufort 12) winds to the south and east of the centre as it approached the far south coast, with gusts of up to 90 knots (165 kph) possible in this area. However, timing would be crucial, as projections showed the by then extratropical warm secluded low starting a rapid weakening phase just after reaching the south coast (~51.5 °N). Winds over land would gradually become less severe further north and west as the filling low centre raced northeastwards through the west of Ireland. The worst winds would likely make it to the south coast but not much further.

 

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ARPEGE 0.1° high-resolution model maximum predicted gusts (knots) up to 0100 UTC Tuesday 17th, as forecast in the 1200 UTC run on Sunday 15th. The strongest gusts reach the extreme south coast, with lower values further north. Image: wxcharts.eu

Although it was initially stated by Met Éireann that much of the country away from the south and southeast coast (the original Red Warning area) would not receive winds strictly to Red Warning criteria (mean speeds >43 knots (80 kph) and/or gusts >70 knots (130 kph)), the fact that trees were still in leaf – and therefore more prone to damage – forced them to take the drastic decision to issue the countrywide Red Warning. In Ireland, such a warning means mandatory closure of schools, but public transport and many businesses closed too. The country slept and awaited Ophelia’s arrival early Monday morning.

On early morning radio, Met Éireann forecaster Joanna Donnelly stated that gusts of 150 kph were now possible anywhere in the country, as the system was likely to be forming a sting jet. It did indeed appear on satellite imagery to have the appearance of a sting jet while the low was still to the south, however the latest model forecasts were still bullish in confining the most severe winds to the extreme south coast.

Landfall in Ireland

This turned out to be the case, with only four out of the 35 stations on the island of Ireland reporting gusts above 130 kph. According to Met Éireann’s climate data, Roches Point, right on the south coast, measured a 10-minute mean windspeed of 62 knots (115 kph, Beaufort Force 11) for a time around 1100 UTC and a gust of 84 knots (156 kph). Sherkin Island, further southwest and more in line with the core of strongest winds (see the Met Éireann Tweet below), reported a maximum 10-minute mean windspeed of 57 knots (105 kph) and a gust of 74 knots (135 kph), although power was lost just as the strongest winds were arriving. Met Éireann data from the link above do show hourly reports continuing throughout the day, so the data was likely stored and since recovered, however it seems strange that Sherkin reported lower than Roches Point, given that it would appear to be more in line with the core of strongest winds. Fastnet Lighthouse, perched on a pile of rocks 50 metres above the sea some 6 kilometres offshore, recorded mean winds of 82 knots (152 kph) and a gust of 103 knots (191 kph). It’s likely that Sherkin Island received winds of strength somewhere between Fastnet Rock’s and Roches Point’s, however there is some question over it. The power also went at Cork Airport, but the data show a gust of 71 knots (131 kph). No maximum 10-minute mean speed is reported, however video footage of the roof of a stand collapsing at Cork City FC’s Turner’s Cross stadium and of a school roof flying through the air in Douglas verified the ferocity of the winds.

By mid afternoon the centre of the low lay over the west, with strong winds now affecting much of the east, including Dublin. However, these winds were a lot less than those experienced earlier in the south, however several trees did come down throughout the country, unfortunately causing the deaths of three people.

By late evening the low had past the north coast, introducing a strong gradient to its south. Several late-night flights into Dublin were diverted to Shannon as Ophelia made one last gasp before crossing Scotland and dying out over the North Sea and southern Scandinavia the next day.

Aftermath

In all, ex-hurricane Ophelia left a trail of destruction in its wake. Three people dead and almost 400,000 customers without electricity. It was an unusual storm in that it arrived from the south, losing its tropical characteristics only just before reaching the south coast. The only other storm in living memory to do the same was Debbie in September, 1961. It had been a hurricane near the Azores, though only Category 1, and only lost its tropical characteristics just south of Ireland (see here).

Comparing the two, Debbie was by far the more extreme. Its winds set records that still stand to this day. Eighteen died in Ireland as a result, as well as sixty others in a plane crash in Cape Verde. The chart below compares the maximum recorded gusts of the two storms. In many cases the reporting stations are the same for both storms, however four stations (Rosslare, Kilkenny, Birr and Clones) have been closed and replaced by other nearby stations (Johnstown Castle, Oak Park, Gurteen and Ballyhaise, respectively).

With the exception of Shannon Airport, the gusts from Ophelia were significantly less than those of Debbie. Rosslare, being on the coast, was a more exposed location nearby Johnstown Castle, but for the other stations, especially those well inland, Debbie was a remarkable event that may never be repeated.

 

Ophelia v Debbie gusts
Graph comparing the maximum reported gusts (kph) in Ireland during storms Ophelia (2017) and Debbie (1961). Debbie was a much more severe storm, setting wind records that still stand to this day. Image: Author

A lot has been said about the increased likelihood of future storms like Ophelia occurring this side of the Atlantic due to the warming oceans. On the question of whether Ophelia can be linked to this warming, a paper has already been published by the Royal Irish Academy.

The authors analysed the warm anomaly in the eastern Atlantic this October and found it to be part of the natural variability that occurs over longer time periods. Just three months previously, the same area of ocean was around two degrees cooler than average. Therefore, there is no evidence that Ophelia was linked to warming oceans. Similar situations have occurred in this region in the past.

*The winds quoted by the NHC are the maximum mean winds measured over a 1-minute period. This is different to the WMO standard 10-minute period, which is what all synoptic stations report. The 10-minute mean, which is an average of all the 1-minute means in that period, will be considerably less than the maximum 1-minute mean. This paper quotes a typical conversion factor of about 0.9, meaning the 10-minute mean is around 90% of the maximum 1-minute value. This should be kept in mind in situations like Ophelia, where the NHC is forecasting “hurricane-strength” winds inland over Ireland. They may not be Beaufort Hurricane Force 12 winds, which are based on 10-minute values and therefore about 10% (or about 1 Beaufort level) lower than the NHC’s winds. This is why Beaufort hurricane-force 12 was not reported at any land station (except maybe Sherkin Island).

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