The state of knowledge of the effects of urban heat islands is advanced through investigation of a heat event in the highly complex coastal environment of New York City (NYC) by using the Weather Research and Forecasting (WRF) Model and surface observations in the NYC metropolitan area to evaluate heat retention at high- and low-temperature times during 18-20 July 2013. Urban surface air temperatures are 1°-2°C higher than rural temperatures throughout the daytime and increase to 3°-5°C higher during the night. Lack of a land-sea temperature gradient prevents development of a land breeze during the night. A land-sea temperature difference approaching 20°C leads to sea-breeze effects during 18 July that reduce daytime skin temperatures, but higher winds greatly reduce the sea breeze during 19 July. WRF Model data are generated using three urban parameterization schemes. The most sophisticated multilayer urban parameterization proves to be most accurate when compared with surface observation data. Errors between WRF Model data and surface observations are attributed to assigned coastal sea surface temperatures, excessive building drag, and too little urban heat retention. Adjustments to the input parameters to the multilayer scheme improved accuracy to lead to the control simulation used for urban heat island effects and land-sea-breeze analysis. NYC building interaction with the synoptic flow generates urban drag and wake effects, although relatively high winds limit their extent. Urban flow results and identified model errors support the development and deployment of the best urban parameterization scheme.