Chapter 07 2011 Rev2:Layout 1 12/21/11 10:18 PM Page 132 Tropical Weather and Storms 001 Mid-latitude weather and storms, the two • Be familiar with the distinctions between related subjects covered in the previous chap- watches and warnings ter, are quite different from tropical weather • Know what to do when a tropical storm or and storms. This chapter deals with the differ- hurricane is approaching ences, but its main focus is on the life cycle, tracks and destructiveness of hurricanes and tropical storms—weather events of great sig- National Weather nificance for those who live on the East and Service References: Gulf Coasts. 002 Upon Completion of this Section the stu- 003 NWS JetStream Homepage dent should: http://www.srh.noaa.gov/jetstream • Be familiar with the characteristics of tropi- 004 Tropical Weather cal and subtropical weather http://www.srh.noaa.gov/jetstream/ • Understand the life cycle of tropical storms tropics/tropics_intro.htm and hurricanes 005 National Hurricane Center • Know the structure of a hurricane www.nhc.noaa.gov • Be familiar with Atlantic hurricane storm 006 Atlantic Storm Activity tracks http://www.nhc.noaa.gov/index.shtml • Understand the potential damage from a 007 Eastern Pacific Storm Activity hurricane and its storm surge http://www.nhc.noaa.gov/index.shtml?epac • Be able to access information from the National Hurricane Center 132 Chapter 07 2011 Rev2:Layout 1 12/21/11 10:18 PM Page 133 Tropical Weather and Storms 133 Tropical/ the tropics there is a permanent (albeit oscil- Subtropical lating) low-pressure belt and in the subtropics there is a semi-permanent high. Storms in the Weather tropics are (and storms in the subtropics typi- cally are) single air mass without fronts. 008 The technical definition of the tropics is the There, however, also are hybrid storms called region of the Earth where the sun is overhead “Subtropical Storms” that are described later at least once a year (i.e., the band between the in this chapter. Some “ subtropical” regions, Tropic of Cancer—23.4° N and the Tropic of moreover, experience mid-latitude weather Capricorn—23.4° S). Over the oceans the with frontal storms during parts of the year weather is hot and muggy with calms or light (e.g., Florida in the Winter). variable winds—the infamous “ Doldrums.” Unlike the mid-latitudes where the dominant seasonal change is from warm to cold, the Intertropical dominant seasonal change in the tropics is Convergence from dry to wet. In the Northern Hemisphere Zone (ITCZ) the wet season is April through September and the dry season is October through March. 011 Recall from Chapter 2 that a broad low This seasonal cycle results from the oscillation pressure belt of rising air—the Equatorial northward and southward of a topical rain belt Trough—encircles the Earth roughly 5° to 10° that follows the sun. north and south of the equator, but it shifts 009 The subtropics are commonly described as north and south with the seasons. Within this the two zones lying north and south of the region the northeast trade winds and the south- tropics between 20° and 35° north and south east trade winds converge in a zone less than latitudes. The winters are relatively warm but 300 miles wide called the Intertropical Conver- not as hot as the summers. The regions over gence Zone (ITCZ). In this zone low-level air is the oceans are dominated by semi-permanent pushed aloft by surface convergence as part of subtropical highs with generally fair weather the planetary-scale process of transferring heat and easterly trade winds on their equator-side from the equatorial region to the poles. edges. 012 Deep convection and showers are features 010 Tropical and subtropical weather is strik- of the ITCZ. The resulting cloud bands are visi- ingly different from mid-latitude weather with ble in satellite images. The ITCZ, however, its succession of highs and lows with fronts. In does not have an exclusive on clouds and rain ITCZ Clouds Courtesy NOAA Chapter 07 2011 Rev2:Layout 1 12/21/11 10:18 PM Page 134 134 Weather bands. They exist throughout the tropics where 014 Bermuda High: In the subtropical North there is sufficient convective activity. Atlantic the semi-permanent Bermuda High 012a Like the tropical rain belt, the ITCZ follows (also known as the “Bermuda-Azores the sun. In the Atlantic Ocean, for example, the High” ) with its subsiding air, stable atmos- ITCZ can migrate as far northward as 15° north pheric conditions, and generally fair weather in the Northern Hemisphere’s summer and make Caribbean islands favorite vacation southward to just north of the Equator in the destinations. During the storm season, how- Northern Hemisphere’s winter. Over continen- ever, northeasterly trade winds along the tal land areas the ITCZ is less continuous southern portion of the Bermuda High can because trade winds exist mainly over the bring tropical storm systems that dramati- oceans. The position of the ITCZ can vary from cally transform this “tropical paradise”. The day to day. As the ITCZ migrates north over largest of these storms, the hurricanes, tend the Atlantic Ocean the increasing Coriolis to form in the eastern Atlantic off the west Effect—the Coriolis Effect increases with lati- coast of Africa (Cape Verde) and then tude— makes the formation of tropical storms migrate across the Atlantic steered by the more likely. northeastern trades of the Bermuda High’s southern edge, growing in strength as they Subtropical Highs migrate. 015 The Bermuda High is larger in the sum- 013 For most of the year, the weather in sub- mer than in the winter because Atlantic tropical waters is favorable for vacationing and waters remain cooler relative to the tempera- boating. Warm, moist air prevails over the ture of continental land masses. This oceans. The weather in each of the five major increased size combined with the Bermuda ocean basins (North and South Atlantic, North High’s clockwise circulation affects the south- and South Pacific and South Indian) is domi- eastern United States as storm systems nated by continental-sized, subtropical, high- migrating across the country are pushed fur- pressure regions centered at about 25 to 30 ther north by the high’s clockwise circulation. degrees north and south latitude. For example, Florida’s weather in the summer Tropical Storm/Hurricane Source Regions © 1992, USA Today. Reprinted with permission Chapter 07 2011 Rev2:Layout 1 12/21/11 10:18 PM Page 135 Tropical Weather and Storms 135 becomes completely subtropical—no mid-lati- Carib god of evil. In other parts of the world tude storms with fronts and single air mass they are given different names: “Typhoon” in thunderstorms are the only source of rain. the western North Pacific; “ Tropical Cyclone” in the Indian Ocean and northeast- Storm Source Regions ern Australia; and “Willy Willy” in northwest- ern Australia. 016 Except for the South Atlantic Ocean and 020 Tropical type storms including hurricanes eastern South Pacific Ocean where the waters are very different from mid-latitude storms. are too cool, hurricanes and other tropical To emphasize the difference, meteorologists storms form in all the major oceans between 5° refer to synoptic mid-latitude storms as and 20° north or between 5° and 20° south of “extratropical cyclones” because they develop the equator. Hurricanes do not form further outside the tropics. Mid-latitude storms get poleward of these two bands because ocean their energy primarily from horizontal tem- waters tend to be too cold. They also do not perature differences involving two or three air form within 5° of the equator where the Corio- masses and the fronts between them. In sharp lis Effect is too weak to generate the necessary contrast, tropical-type storms have little or no rotary circulation required for storm develop- horizontal temperature differences and get ment. their energy primarily from the release of 017 Hurricanes rarely threaten the West latent heat in a single warm moist air mass. Coast of the United States because (i) the 021 While the official Atlantic Hurricane sea- northeast trade winds push tropical storms son is 1 June to 30 November the season is that form west of Mexico and Central America very peaked with most tropical storms and away from the California Coast, and (ii) sur- hurricanes occurring from August through face waters off the coast of California and Baja October. California are normally too cold to sustain 022 As compared to extratropical (mid- tropical storms. Just the opposite conditions latitude) storms, tropical storms have different exist on the East Coast, however, where (i) the requisite atmospheric conditions, life cycles, northeast trade winds push storms towards and structures. the coast and (ii) the Gulf Stream warms coastal waters. 018 The tropical/subtropical waters of the North Atlantic, the Caribbean Sea, and the Gulf of Mexico are the source regions for the tropical storms including hurricanes that impact the East and Gulf Coasts of the United States. Extratropical Storms Hurricanes 019 While hurricanes are not the largest storm systems in our atmosphere (synoptic scale mid-latitudes storms are far larger in horizon- tal scale), or the most violent (major tornadoes have much higher winds), their combination of size and strength make them the most destruc- Tropical Storms/Hurricaner tive. They are aptly named after “Hurakan”, a © 1992, USA Today. Reprinted with permission Chapter 07 2011 Rev2:Layout 1 12/21/11 10:18 PM Page 136 136 Weather Necessary • humid air up to about 18,000 feet so water Conditions vapor can continue to be a source of latent heat at higher levels; 023 The critical ingredients necessary for a • locations far enough north of the equator so tropical storm to form are: there is sufficient Coriolis Effect to initiate a circulatory airflow; • ocean water above 80°F for sufficient evapo- • pre-existing winds (those not created by the ration; storm) coming from the same direction at • warm water about 200 feet deep or more so about the same speed at all altitudes to avoid storms do not stir up cold water from below.