Timing of First Hot Day

Timing of First Hot Day

This tool is excerpted from Mid-Atlantic Regional Climate Impacts Summary and Outlook: Spring 2023.

Tool Background

This interactive data tool details an analysis of climate projections derived from global climate model output. In this tool, we focus on the timing of the first hot day (i.e., days with temperatures at or above 85 degrees F or warmer). In the Mid-Atlantic, the first day with temperatures at or above 85 degrees F generally occurs in late March to mid-April in the southern portions of the region and as late as mid-June at higher elevations or further north. The timing of the first hot day has important implications for public health, electricity demand, agriculture, among other sectors. The earlier hot days occur, the longer and warmer spring, summer and fall temperatures may be. The tool provides an analysis of the timing of the first hot day, defined as days with maximum temperatures at or above 85, 90, or 95 degrees F. It includes the timing of the first hot day for the climate normal, as well as for future periods.

Key Findings

  • The timing of the first hot day is projected to occur earlier in the year under both emissions scenarios.
  • Under a high emissions future, by the end of the century, much of the southern portion of the Mid-Atlantic could regularly see days above 85 degrees F in late March and days at or above 95 degress F in early May.

Figure 1. Timing of First Hot Day

How to Use the Tool

Selecting Temperature Thresholds, Time Periods and Future Emissions Scenarios: Use the filters to the right of the maps to adjust the temperature threshold and the 30-year time period. Users can also select the future emissions scenario (Low or High Emissions).Note that some locations are not projected to experience any days above 90 and/or 95 degree F. In these cases, the map does not show a value.

Viewing Data for a State or County: You can use the State and County filters to the right of the table to have the plot show data for a particular location of interest.

Technical Notes

Localized Constructed Analogs (LOCA) is a downscaled climate data product available at 1/16-degree (6-km) resolution over the continental United States. LOCA data sets1 include the 32 climate models available in the Coupled Model Intercomparison Project 5 (CMIP5) archive, for two future greenhouse gas concentration trajectories: a low emissions future, Representative Concentration Pathway (RCP) 4.5;2 and a high-emissions future, RCP 8.5.3 For this study, we used LOCA data over the Chesapeake Bay watershed from 1991–2100 (or 2099 for some models). Access LOCA datasets and learn more about the methodology.

Data were processed by the Northeast Regional Climate Center to calculate the number of days per year with low temperatures at the given thresholds.


  1. https://loca.ucsd.edu/ Return to text ⤴

  2. More information on RCP 4.5 can be found in: https://doi.org/10.1007/s10584-011-0151-4 Return to text ⤴

  3. More information on RCP 8.5 can be found in: https://doi.org/10.1007/s10584-011-0149-y Return to text ⤴