TECH 5 for April 17, 2014 – Tropical Storm Forecasting

ben-harrisonGood afternoon and welcome to another Tech 5 program and a five minute trip into the amazing world of technology. This is Ben Harrison.

The June 1 start of the 2014 Atlantic hurricane season may seem a long way off, but respected forecasters Tropical Storm Risk (TSR) have focused on the likely weather conditions and have already revised their long-range forecast issued in December, 2013.

The good news is that Dr Adam Lea and his team at the Department of Space and Climate Physics at University College London are predicting a relatively mild hurricane season this year. TSR has lowered its forecast for North Atlantic and US hurricane activity in 2014.

The not-so-good-news is that TSR’s April forecast for 2013, along with those of virtually every other recognized forecaster, predicted above-average activity for a season that thankfully did not happen.
As Dr Lea pointed out, it should be stressed that extended range forecasts are uncertain at this early stage.

The revised forecast predicts North Atlantic basin tropical cyclone activity in 2014 to be 25 percent below the long-term (1950-2013) norm and 40 percent below the recent 2004-2013 10-year norm. It predicts 12 tropical storms including five hurricanes and two intense hurricanes, with three tropical storms and one hurricane landfall on the US mainland. This compares to long-term norms of 11, six and three respectively, and to 2004-2013 norms of 16, eight and four respectively.

The main reasons for the reduction in the TSR forecast the expectation that August/September 2014 sea surface temperatures in the tropical North Atlantic will be cooler than thought previously, and the increased expectation that a moderate El Nino event will develop August/September 2014. Both of these factors will moderate North Atlantic hurricane activity.

The methods through which tropical cyclones are forecast have changed with the passage of time. The first known forecasts in the Western Hemisphere were made by Lt. Col. William Reed of the Corps of Royal Engineers at Barbados in 1847. Reed mostly utilized barometric pressure measurements as the basis of his forecasts. Benito Vines introduced a forecast and warning system based on cloud cover changes in Havana during the 1870s. Forecasting hurricane motion was based on tide movements, as well as cloud and barometer changes over time. In 1895, it was noted that cool conditions with unusually high pressure preceded tropical cyclones in the West Indies by several days. Before the early 1900s, most forecasts were done by direct observations at weather stations, which were then relayed to forecast centers via telegraph. It was not until the advent of radio in the early twentieth century that observations from ships at sea were available to forecasters. Despite the issuance of hurricane watches and warnings for systems threatening the coast, forecasting the path of tropical cyclones did not occur until 1920.

The 1930s saw the usage of radiosondes in tropical cyclone forecasting. The next decade saw the advent of aircraft-based reconnaissance by the military, starting with the first dedicated flight into a hurricane in 1943, and the establishment of the Hurricane Hunters in 1944. In the 1950s, coastal weather radars began to be used in the United States, and research reconnaissance flights by the precursor of the Hurricane Research Division began in 1954.

The launch of the first weather satellite, TIROS-I, in 1960, introduced new forecasting techniques that remain important to tropical cyclone forecasting to the present. In the 1970s, buoys were introduced to improve the resolution of surface measurements, which until that point, were not available at all over sea surfaces.

About four days in advance of a typical tropical cyclone, an ocean swell of 1 metre (3.3 ft) in height will roll in about every 10 seconds, moving towards the coast from the direction of the tropical cyclone’s location. The ocean swell will slowly increase in height and frequency the closer a tropical cyclone gets to land. Two days in advance of the center’s passage, winds go calm as the tropical cyclone interrupts the environmental wind flow.

Within 36 hours of the center passage, the pressure begins to fall and a veil of white cirrus clouds approaches from the cyclone‘s direction.

Within 24 hours of the closest approach to the center, low clouds begin to move in, also known as the bar of a tropical cyclone, as the barometric pressure begins to fall more rapidly and the winds begin to increase.

Within 18 hours of the center’s approach, squally weather is common, with sudden increases in wind accompanied by rain showers or thunderstorms.

Winds increase within 12 hours of the center’s approach, occasionally reaching hurricane force. The ocean’s surface becomes whipped with foam. Small items begin flying in the wind.

Within 6 hours of the center’s arrival, rain becomes continuous and the storm surge begins to come inland.

Within an hour of the center, the rain becomes very heavy and the highest winds within the tropical cyclone are experienced. When the center arrives with a strong tropical cyclone, weather conditions improve and the sun becomes visible as the eye moves overhead. At this point, the pressure ceases to drop as the lowest pressure within the storm’s center is reached. This is also when the peak depth of the storm surge occurs.

Once the system departs, winds reverse and, along with the rain, suddenly increase. The storm surge retreats as the pressure suddenly rises in the wake of its center.

One day after the center’s passage, the low overcast is replaced with a higher overcast, and the rain becomes intermittent.

By 36 hours after the center’s passage, the high overcast breaks and the pressure begins to level off.

Well, there you have it: A mini course on the inaccurate profession of weather forecasting, which as satellite imagery and computer modeling will only get better: We hope!