US

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British travellers to the US under the Visa Waiver Programme (VWP), which allows most British Citizen passport holders to visit for up to 90 days without a visa, must get an authorisation via the Electronic System for Travel Authorisation (ESTA) prior to their journey. Visitors should register through the ESTA website at esta.cbp.dhs.gov and are advised to do so at least 72 hours prior to travel. If you do not have an ESTA you will be refused travel to the USA. The US Customs and Border Protection department charge $14 per application for an ESTA. All payments must be made by credit card.
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UK Emergency Travel Documents (ETD) are not valid for entry into the United States under the Visa Waiver Program (VWP).  Holders of an ETD must apply for the appropriate US visa. See Entry Requirements.
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A number of devastating tornadoes have hit Alabama and the south of the United States in recent days. Travellers in the affected areas should monitor media reports and follow instructions from the local authorities.  
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Snow storms during winter can cause delays and cancellations throughout the major airline hubs in the USA. Be sure to contact your tour operator or airline for current flight information before travelling. To monitor airport conditions in the USA, please visit: http://www.fly.faa.gov/flyfaa/usmap.jsp
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The hurricane season normally runs from June to November and can affect the whole of the southern USA. See Natural Disasters. 
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Forest and brush fires (wild fires) are a danger in many dry areas in the US. See General - Wild Fires.
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There is a general threat from terrorism in the United States. Attacks could be indiscriminate, including in places frequented by expatriates and foreign travellers. See Safety and Security - Terrorism.
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Around 5.525 million British nationals visited the United States last year (Source: US Department of Homeland Security). See the General - Consular Assistance Statistics. You should be alert to the dangers of car and street crime in cities.
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Y ou should take out comprehensive travel and medical insurance before travelling. See General - Insurance.

http://www.fco.gov.uk/en/travel-and-living-abroad/travel-advice-by-country/north-central-america/united-states