National Weather Service United States Department of Commerce

The New 1991-2020 Climate Normals Have Been Released.


   The new 30-year climate normal values (the average conditions to be expected) have been released after careful calculations by NOAA's National Center for Environmental Information (NCEI). This information uses data collected between 1991 and 2020. Every 10 years, the normals are re-calculated based on the prior 30 years of data. This keeps the data fresh, helping to account for changes in the conditions near the observation sites. The previous 30-year time frame was 1981-2010.

What are these values used for?

   The normal values are referenced frequently in NWS forecasts and statements. We use the normal values to give perspective to how unusual some occurrence would be, and can answer questions like:

*  Is it warmer than normal?  * How much warmer?  * Is this an unusually wet month?  * Did we have more snow than normal last season?

   Each day, the NWS Forecast Office in State College issues detailed climate summaries for 5 locations around Central PA; Harrisburg, Williamsport, Johnstown, Altoona and Bradford. More climate summaries, including monthly data can be found on the NWS Climate information pages. Continuous, hourly data has been collected at these sites for many years, some for well over a century. In these summaries, and in the monthly, seasonal and annual summaries we periodically issue, we reference the normal maximum (high), minimum, and average (max + min / 2) temperatures, and the normal amount of precipitation and snowfall (where available). This can provide visibility and promote awareness to the trends and (ab)normality of the actual weather observed. 

   The normals are also used in academia for research purposes. They can be referenced by government and industry for a myriad of purposes. Some examples: If a local municipality wishes to plan for an event in the future, or decide on a budget for road salt/chemicals, they can look up these normals. If an industry wishes to plan on where to move materials during the year, they may check the normal temperatures if the material is sensitive to freezing temperatures. A company wishing to move it's office to a different city can use the normals to get an idea of how wet/rainy a location will be. You can use the normals to figure the amount of heating oil you may burn through during a winter with near-normal temperatures.

Key points:

  • NOAA's NCEI is the official source for Climate Normals for U.S. stations. NCEI stores the weather data from the U.S. and it's territories, and provides it to the public for an enormously wide range of purposes.
  • Climate Normals are not simple 30-year averages of monthly or daily observations; many stations do not have complete observations for the full 30-year period. For daily and monthly normals in particular, complex estimation and scaling procedures are used with temperature and precipitation normals to ensure they are representing the full 30-year period and are internally consistent.
  • The science behind the calculations of the new normals is the same as for the 1981-2010 edition, with only minor calculation technique updates.
  • Normals have changed from the 1981-2010 cycle to the 1991-2020 cycle, but two-thirds of the periods overlap (1991-2010), muting changes considerably.
  • Differences have been seen when comparing the 1981-2010 numbers to the 1991-2020 numbers. Many of these differences are increases in temperatures.
  • While many locations have trended warmer, not all areas of the U.S. have trended warmer from the previous 30-year normals, especially in spring. The normal temperatures have cooled in the north central U.S. Precipitation normals have also changed in complex ways, depending on season and location.
  • These values will be put into use immediately, and some of the calculations vs normal values may change very slightly, but abruptly on the climate summaries on the 4th of May.


Here are a few maps showing the change in normals between the old 1981-2010 values and the new 1991-2020 values:

Difference between new and old normal annual precipitation

Difference in Annual Precipitation (inches). Positive (negative) values mean that it is wetter (drier) in the new normals. It was slightly wetter in PA for the 30 year period from 1991-2020 vs the previous/old 30-year (1981-2010) period.

Takeaway: We had more rain & melted snow/ice in PA over the past 30 years than during 1981-2010.

Difference in average annual temperature between new and previous normals.

Difference in Annual Average Temperature. Positive (negative) values mean that it is warmer (cooler) in the new normals. The temperatures over PA through the whole year are warmer in the new 30-year (1991-2020) period than during the previous (1981-2010) period.

Takeaway: It was warmer in PA during the last 30 years than during the 1981-2010 period.


Additional Resources

Climate at a Glance mapping tool from NCEI :