Review of the past hurricane season reports of hurricanes, tropical storms, tropical disturbances and related flooding during

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WORLD METEOROLOGICAL ORGANIZATION

^{___________________________________________}

RA IV HURRICANE COMMITTEE

THIRTY-THIRD SESSION
GRAND CAYMAN, CAYMAN ISLANDS
8 TO 12 MARCH 2011

RA IV/HC-XXXIII/Doc. 4.2(3)

(03.II.2011)

________

ITEM 4.2

Original: ENGLISH

REVIEW OF THE PAST HURRICANE SEASON
REPORTS OF HURRICANES, TROPICAL STORMS, TROPICAL

DISTURBANCES AND RELATED FLOODING DURING 2010
(Submitted by Bermuda)

1. Introduction

The 2010 Atlantic Hurricane Season was one in which 5 storms threatened the island, with one (Igor) making a significant impact, whilst the others either dissipated nearby or veered away ‘on the doorstep’. This meant a few challenges in operations and maintaining a level of credibility in the eyes of the public. However, with our one impact this year, Bermuda benefited from a heightened sense of preparedness in advance of the onset of damaging conditions. Thankfully, no injuries or losses of life were reported this hurricane season, and only minimal impacts to property and infrastructure were evident. The four systems which came close, but produced minor impacts, are summarized below, whilst Hurricane Igor is described in more detail in the next section.

1.1Tropical Storm Colin

A Tropical Storm Warning was posted at 11:30am on Thursday 6 August, in anticipation of Colin producing winds in excess of 34 knot at times on the following weekend. However, the only real impact of Colin was building seas and swells on the South Shore. TS Colin continued to weaken through Saturday 7 August, with only a few bands of showers and one thunderstorm affecting the area. Seas and swells continued to build from the south, though area winds averaged 25 knots as Colin's slowing continued to delay the onset of tropical storm force winds, which had been forecast for the weekend from as early as Thursday. At noon on Sunday 8 August Colin was downgraded to a tropical depression and was declared “dissipated” at 6pm. As Colin continued to weaken, it produced maximum sustained winds of 28 knots with a peak gust of 40 knots at the Commissioner’s Point AWOS site at 18:30 UTC on 8 August (for metadata on this AWOS site, see item 2.3).

1.2Hurricane Danielle

Hurricane Danielle, a late August major hurricane, was never forecast to come close enough to Bermuda to even produce tropical storm force winds, let alone hurricane force winds. However, Danielle was treated with caution, as the position and direction of the system’s movement meant hat large swells were forecast for the south shore beaches. This prompted a Small Craft Warning to be posted and a Tropical Storm Watch was also issued on the 27August to cover the unlikely eventuality that Danielle’s peripheral winds would skirt our Marine Area (in addition to the large swell expected). Danielle made its closest point of approach to our east on the 28 August at a distance of more than 200nm. In the absence of wave recorders in Bermuda, we can only estimate the wave heights – 24-hour forecasts for that day called for significant waves and swells of up to 20 feet outside the reef.

1.3Tropical Storm Fiona

Almost exactly the same scenario as for Colin played itself out during the approach and dissipation of Tropical Storm Fiona, early in September. On Thursday 2 September, stable air under a Saharan Dust Layer provided a hazy day. Even though the next day was expected to remain similar, a Tropical Strom Warning was issued on Thursday night. Vertical wind shear separated the low centre from the deep convection and reduced it to a tropical depression; it was duly downgraded by the time it came within 100nm to the southeast of Bermuda. Early on the 4 September, weakening TD Fiona brushed by to the east, bringing light to moderate winds initially from the northeast, backing westerly then eventually southwesterly through the day, with moderate to heavy showers & thunderstorms.

1.4Tropical Storm Shary

Tropical Storm Shary was forecast to produce tropical storm force winds on the 29^th and 30^th of October, prompting a Tropical Storm Watch and later Warning. However, due its small size, the interaction with a cold front to our west and its subsequent direction change away from Bermuda, the local weather effects were minor. The center of Tropical Storm Shary passed 140 km (76 nm) southeast at 00UTC on the 30^th of October, based on RADAR fixes.

2. Hurricane Igor

All aspects of the storm’s approach, progress and departure went very well from the perspective of BWS. The NHC track forecast was sufficiently accurate from an early stage that we had adequate lead-time to prepare staffing levels, and even work out roughly when we would be issuing watches and warnings. The intensity forecast was fortuitous, in that we were able to justify the level of communications with the Emergency Measures Organization, Department of Airport Operations, the Media and the public regarding the extent of preparation, from an early stage. This in turn meant that most people took this storm seriously and took the appropriate measures. Preparations made in anticipation of a hurricane, which was a full two categories weaker than forecast when it actually hit us, meant that Bermuda was in a much more ready stance than had the forecast been for a Category 1. If the forecast had been as accurate in terms of intensity, would people & agencies have made the appropriate preparations? This is difficult to ascertain until and unless we have a Cat 1 which is accurately forecast as such.

Source www.nhc.noaa.gov - Infrared satellite image of Igor south of Bermuda on Friday, 17 September, 2010

2.1 Staffing & Operations

We had the ideal level of staffing to be able to run the operation – some staff lived in St David’s (the island on which BWS sits) and were able to go home Sunday night, returning Monday morning via car. Others stayed the entire weekend. We would usually only have 2 people per 12-hour day and night shifts, but we actually had 4 forecasters and 3 observers on hand, along with the Director (myself). We intentionally planned to have staff (including management) prepositioned on the other side of the Causeway for continuity reasons, and to ensure there was a ‘fresh’ crew to come in and relieve us all once the Causeway re-opened. The staffing roster was adjusted to allow those who had been here all weekend to have a rest period of at least 36 hours prior to their next shift.

Food and water was purchased early on, in anticipation of supporting the crew through the weekend who might have had no scope to leave. Our training room was set up as a sleeping area for those who were off shift, with cots, air mattresses, and blocked out windows to the other lit areas of the office. The storm shutters were all down from an early stage also. Vehicles were secured in the lee of the Air Traffic Control Tower and the alternate balloon launch/generator building early on. Bikes were secured indoors.
Power, phones and internet never went down during the storm (we have a generator and backups but never needed them), allowing us to continue to do our jobs unfettered by technological limitations. Instruments which had not been safeguarded against the effects of surge (most needed to stay in place for measurements e.g. wind towers, barometers) all functioned remarkably well, with the exception of the GOES Satellite dish, which (as expected) swung on its axis with the wind. Once the winds abated, our Ground Electronics Services (GES) crew put it back into alignment. GES had a technician and the manager on hand 24/7 to safeguard critical airport equipment and bring it back up once the storm had passed.
We launched 3 balloons during the storm itself, and only one was successful. This was due primarily to the trees bordering the balloon launch deck at BWS, which ‘ate’ 1 balloon and 2 sondes. Only when the winds became a bit more southerly were we able to successfully launch.

2.2 Storm Surge

Source: Bernews.com

This is a photograph from King’s Square in St. George’s, showing surge and wave action above ground level. We must discover a means by which we can make accurate storm surge predictions and observations. Currently there are no models on which to base accurate surge predictions on a case by case basis, and the one verifying point (Esso Pier) is in a location which is only helpful for the north side of St. George’s (where there are few properties near the water). Data from that Tide gauge is outlined in the following graph, indicating a departure from the predicted tide of approximately 1.75 ft at that site:

This may not be representative of the surge conditions in St. George’s Harbour (or indeed in any other enclosed bodies of water around Bermuda), signifying a need to examine this phenomenon more closely locally.
We did get some help from the NHC Storm surge unit, but the estimates provided are vague and all-encompassing, instead of specific to each storm and location. This is not a criticism of them (in fact they provided some guidance, when none was available from elsewhere). However, we are not part of their modeling effort, as is outlined in this figure:

(Source: http://www.nhc.noaa.gov/ssurge/ssurge_slosh.shtml)

Things that would need to happen in order to get to a better level of storm surge forecasting and observation:

Accurate and high resolution bathymetry and coastal mapping – this is vital to the effective modeling of surge. Without this, any accurate surge estimates that can be made will be so broad-brushed as to not be useful.

A real-time storm surge model which is configurable by storm, as opposed to a generalized model. All storms have their own characteristics, so comparing Igor to Emily or Florence would be inappropriate in term of surge, even though they all made direct impacts and were Category 1 storms.

Verification by way of measurement equipment – wave buoys and sea level gauges are necessary to give us some data on which to verify the output, allowing us to tweak the model appropriately.

2.3 Data & Impacts

Post Storm Country Report

Country: Bermuda

Tropical Cyclone: Hurricane Igor

Dates of data: 19-20 September 2010

Station

Maximum Sustained Wind

Maximum Wind Gust

Calm

Total Rainfall

Minimum Sea Level Pressure

Direction ° True

Velocity
Knots

UTC Date/Time

Direction
° True

Velocity
Knots

UTC Date/Time

mm

hPa

Bermuda Airport TXKF (10 min avg)

150

59

20/0223

81

20/0218

81

963.7

Fort Prospect^¹

127

51.5

20/0350

166

90

20/0530

St. David’s^²

166

79

20/0550

166

102

20/0550

Commisioner’s Point^³

133

70

20/0340

133

101

20/0340

Automated wind and pressure measurements around the island during Igor, as outlined in the chart above.

Source: Hinson’s Island automated weather station (privately owned)

It was noted from some more high resolution (1 minute) sea level pressure data, Bermuda’s pressure actually got down to 963.7 hPa at 2:51 UTC on the 20^th (see below).

This is the barometer on which all of our official METAR/SPECI and SYNOP observations are based. Also in the same screen shot are wind measurements from each end of the runway at LF Wade International Airport. All of this data is from a Vaisala MIDASIV airfield integrated weather observing system.

It was noted that the wind maximum associated with Igor didn't evenly bracket the pressure trough as it would in a symmetric system. We have come to the conclusion that we were probably experiencing some form of an extratropical Transition (ET) wind max at the same time as the closest point of approach. This conclusion is largely based on the satellite/radar signature, and Chris Fogarty's ET schematic, as shown below.

Meteosat IR Imagery for Igor’s passage near Bermuda at 0300UTC and Bermuda Radar & Sferics Lightning products from BWS for 2:56UTC.

Source: Dr. Chris Fogarty, Environment Canada Canadian Hurricane Center / Atlantic Storm Prediction Center / National Lab for Marine and Coastal Meteorology http://projects.novaweather.net/

1 32° 17' 57.6882"N, 64° 45' 53.4774"W Elevation 230ft AMSL - 10-minute average

2 32°21.825’ N 64°39.368’W, Elevation 159ft AMSL - 10-minute average

3 32° 19' 44.5584"N, 64° 49' 55.9596"W Elevation 262ft AMSL - 10-minute average

Directory: pages -> prog -> www -> tcp -> documents
documents -> Review of the ra IV hurricane operational plan
documents -> Cyclone programme
documents -> World meteorological organization technical document
documents -> World meteorological organization ra IV hurricane committee thirty-fourth session
documents -> World meteorological organization ra IV hurricane committee thirty-third session
documents -> Review of the past hurricane season
documents -> Ra IV hurricane committee thirty-fourth session ponte vedra beach, fl, usa
documents -> World meteorological organization ra IV hurricane committee thirty-second session
documents -> World meteorological organization ra IV hurricane committee thirty-fifth session
documents -> English only review of the tropical cyclone operational plan for the south pacific and south-east indian ocean

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