Weather service warns of temperatures pushing 100F all week for millions of people from Great Lakes to New York City

An excessive heat warning was issued in parts of the US east coast on Monday as the National Weather Service warned that a heatwave will create dangerous conditions for millions of people.

Cities in Delaware, New Jersey, New York and Pennsylvania were subject to the warning, while a heat advisory was in place for New York City and much of Connecticut and Rhode Island. Cities as far west as Chicago and Detroit will also see high temperatures.

Residents have been warned against working in the sun or spending time in non-air conditioned cars as the temperature is expected to rise through the week. The most severe conditions are predicted for Thursday and Friday.

Monday and Wednesday could see effective temperatures of 103F (39.4C) in parts of the north-east, the NWS said. That could rise to 105F (40.6C) by the end of the week.

Philadelphia, Wilmington in Delaware and New York's Westchester are among the cities expected to be most impacted by the heat, caused by high pressure to the south.

"The excessive heat will create dangerous conditions ... especially for those working in the sun, or living in non-air conditioned homes," the NWS said in the excessive heat warning issued early Monday.

It said the most adversely affected are young children and adults over 50, and warned that leaving pets or children in non-air conditioned cars is "very dangerous and can quickly become fatal".

The NWS said Monday and Wednesday afternoons will see a heat index – a combination of air temperature and humidity to give a relative temperature – of between 100F and 103F (39.4C) in parts of the north-east. It said people in impacted areas should prepare for muggy evenings and drink plenty of water.

In the north-east a heatwave is defined as three or more consecutive days of temperatures in the upper 90s. An excessive heat warning means that a prolonged period of "dangerously hot temperatures" can occur. The less severe heat advisory means hot temperatures are expected, which could cause heat stroke and other illnesses.

The prolonged heat is due to a "large dome of high pressure" over the north-east, Ohio Valley and Great Lakes, according to the Weather Channel.

Temperatures are already above normal for July, the Weather Channel reported. In the south of New England, the average temperature is five to six degrees hotter than the recorded average. In New York City, Newark and Philadelphia the start of July has been three or four degrees above the recorded average.

© Guardian News and Media 2013

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