Leslie leaves Bermuda, heads to Newfoundland


Tropical Storm Leslie has all but cleared out of Bermuda leaving behind a few stray showers this morning, with further showers or rain later today from an approaching cold front; a thunderstorm advisory is in effect in association with this approaching front.

The large tropical storm lacked the core of deep convection that is typically associated with tropical cyclones; this meant that it remained weak as it passed Bermuda. In fact, it had one main outer band of convection in a ring with a large (100mile wide) area of calm winds near the center. It is interesting to note that most if not all tropical cyclones have lacked an inner core of convection in the Western Atlantic beginning in 2010 despite seemingly favorable atmospheric conditions.

OSCAT pass of Tropical Storm Leslie passing Bermuda on September 9th 2012. It shows winds generally between 35 and 50kts affecting Bermuda; this coincides with what was observed in Bermuda. Additionally it shows peak winds between 50 and 55kts – not too much higher than what was observed, and only found in isolated pockets.

Winds…You may think Bermuda dodged the worst of Leslie – they did – but not by much. The storm simply wasn’t that strong. The region of the storm that passed over Bermuda likely only had maximum sustained winds of 45-55mph so getting sustained winds between 40 and 50mph is essentially getting the worst of Leslie. The Bermuda Weather Service observed peak sustained winds at 35kts (40mph) and a peak gust to 48kts (55mph). An Automated Weather Observation Station (AWOS) at Commissioner’s point Dockyard recorded a peak wind gust to 62kts (71mph). This site is typically used to parallel what would be seen on the hill tops around Bermuda. My PWS recorded a peak wind gust to 47mph. Very little damage to report from the winds save for one stop light that was knocked down in the City of Hamilton.

Rainfall…Interestingly enough, the storm dumped huge amounts of rain on Bermuda – safely ranging 2-5inches. The Bermuda Weather Service reports 4.35″ of rain over the weekend; all attributable to Leslie. This makes Leslie a particularly wet tropical cyclone for Bermuda. The wettest was an unnamed hurricane from 1939 that produced 7.35″ of rain in October. This, along with abnormal tides led to some minor flooding in low-lying areas that typically flood during heavy rain events. Interestingly, my PWS only recorded 0.42″ of rain; this is probably because most of the time it was only a light rain, which the strong winds likely blew across the rain gauge skewing its observation. This again erases the short term drought for Bermuda, but in the long term drought still exists with the island remaining 7-8″ below normal for the year.

Seas…The Bermuda Weather Service forecasts seas quickly decreasing as Leslie continues away from Bermuda, falling below the dangerous 9feet level this evening and into tomorrow morning. A small craft warning remains in effect until then. A NOAA buoy at the Esso Pier in St. Georges recorded an unremarkable storm surge of 1.02ft at the height of the storm and a storm tide of 3.28ft. 

Radar images of Tropical Storms Bertha(left) and Leslie(right) show that they both have one main band of convection around a relatively large echo free region with light winds around the center. Bertha’s main band of convection is about half as big as Leslie’s, but Leslie passes twice as far away.

Other Meteorology… The minimum pressure observed in Bermuda was 1002.1mb. Compared to the estimated minimum central pressure of the storm, 988mb, this isn’t significantly low. I made the comparison to Tropical Storm Bertha from 2008; the storm’s exhibited similar structure and intensity so it was easy to draw parallels between them. Both were fairly large and took on what is termed the “tire” or “donut” shape with a band of convection surrounding a large area of light winds. Bertha had slightly stronger winds, but overall, the impacts were about the same.

National Hurricane Center plot of estimated wind radii and watches/warnings for Tropical Storm Lesli as of 11am ET September 10th 2012 (Advisory 45).

Leslie is still out there and is heading for Canada. It still has a small opportunity to strengthen into a hurricane before reaching Newfoundland, but it is moving into cooler waters and still lacks an inner core. However, sometimes the transition into a post-tropical storm can increase the wind speeds and the increased forward speed may make gusts higher.

Tropical Storm Warnings and Hurricane Watches are in effect for the Southeastern 1/3 of Newfoundland and tropical storm conditions are expected to begin tonight; expect tree damage and associated power outages. Heavy rains of 2-4″ with isolated areas receiving 6″ will likely lead to some dangerous flooding.

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