We have new normals released which is a new set of data from the previous 30 years by which we will judge whether any day will be cooler or hotter, wetter or dryer than normal. Please note there are some formatting issues between coding differences between the NWS and myself – I corrected many of them but there is some letter cutoff on some of the paragraphs. I will continue to fix these as the day goes by…
Most of the U.S. was warmer, and the eastern two-thirds of the contiguous U.S. was wetter, from 1991–2020 than the previous normals period, 1981–2010. The Southwest was considerably drier on an annual basis, while the central northern U.S. has cooled somewhat.
There are twenty years of overlap between the current normals and the previous iteration (1991–2010), which makes the annual changes between these two datasets somewhat muted compared to trends over the same period. Monthly and seasonal changes are more dynamic. For example, the current normals for the north-central U.S. are cooler in the spring, while much of the Southeast is now warmer in October, cooler in November, and warmer again in December. Atmospheric circulation dynamics and surface feedbacks result in substantial differences from month to month and region-to-region.
Core Month Differences
Changes during the core months of each season (January, April, July, and October) can be representative of broader differences between different normals iterations. The following examples illustrate the percent change in precipitation and the change in degrees Fahrenheit in maximum temperature for each core month.
January temperatures have risen in the new normals by 0.5–1.5°F across most of the country. The north-central region was a notable outlier that cooled by more than 1.0° F in some places but generally follows annual cooling patterns. Outliers aside, this winter warming correlates with increased precipitation normals throughout much of the country, as warmer air can hold more water vapor during winter. This is especially true on the western and eastern edges of the cooler north-central zone, where precipitation increased by 10–25% in January. Texas and Florida were also notably wetter during 1991–2020.
April is the most dynamic core month and exhibits a variety of significant changes in maximum temperature and precipitation. The maximum temperature normals are considerably lower in the north-central U.S. compared to the previous normals period. The entire northern tier is cooler, but changes reach more than -2°F in parts of the Dakotas. Cooling patches reach all the way to Louisiana, although the areas to the west and east of the Mississippi Valley have warmed considerably. The entire southeastern quarter of the U.S. is now considerably wetter in April, while the southwest is drier. Additional precipitation is also seen in the lee of the northern Rocky Mountains and the Great Lakes.
The Northwest is considerably drier as a percentage of the previous normal during what is already a dry season. The rest of the West is pockmarked with wetter and drier zones. The eastern two-thirds of the U.S. has an indistinct pattern of changes in the precipitation normals. Most of the East also remained near the same temperature levels, except for persistent cooling in the north-central U.S. and warming in the Northeast. However, the entire West and lee of the Rocky Mountains and Texas are all considerably (up to 2°) warmer.
A strong zonal pattern in the western two-thirds of the northern U.S. marked both wetter and cooler normals for 1991–2020 than 1981–2010. The rest of the country was consistently warmer, but the southwest and south-central U.S. were also drier. The East was warm and displayed alternating zones of wetter and drier conditions that were reminiscent of shifting storm tracks.
Overall, the central month of each season displayed cooling normals in the north-central U.S. to varying degrees, most distinctly in April. Precipitation normals were generally wetter east of the Rockies and drier in the central and southern West. While annual normals changes were generally consistent, the single months showed pattern shifts that indicate a relationship to repeated preference of certain upper air wind patterns and storm tracks that shifted north or south with the seasons. The patterns of normals changes are following spatial trends in climate circulation dynamics and surface feedbacks, but still demonstrate the warmer conditions expected in the U.S. overall and the reduction of precipitation in the West and increase in precipitation in the East as anticipated by models of future climate change.