State-by-State and Regional Analysis of Temperature and Precipitation Trends

State-by-State and Regional Analysis of Temperature and Precipitation Trends 1895-2011

I’ve been pouring over the statistical database that the NCDC maintains on temperature and precipitation trends and pulled together a brief state-by-state analysis. The database begins in 1895 (when the U.S. Weather Bureau began to homogenize its observation techniques) and is current up to February 2012. There are some surprises, but by and large the trend is certainly towards warmer and wetter conditions in most regions of the country.

Here is what the trends have been for each state (excluding Alaska and Hawaii). The complete analysis can be found here.

I’ve noted the overall trend for each state after the statistics and highlighted in bold the cases where the trend change has been significant; meaning a change in temperature greater than 1.0°F (up or down) or a change in precipitation greater than 5% (dryer or wetter) over the long-term mean. The percentage is change in precipitation over the past 116 years relative to the state’s long-term average:








The Case of Texas

Although last year was the 2nd warmest and 2nd driest year on record for the state of Texas, the long-term trend shows little change in either temperature or precipitation over the past 116 years for America’s largest state (ex-Alaska), one of the few states to experience such (others include North Carolina, Oregon, Florida, and Georgia).



There has been a spike of warmer than normal temperatures in Texas since 2000 but over the long-term this has been mitigated by a long period of cooler than normal temperatures between 1958-1997.



Extreme variability seems to by the ‘normal’ for Texas precipitation on a year-on-year basis, so in spite of last year’s record drought, the overall trend shows little change.

Some Conclusions

TEMPERATURE

Only six states have experienced a cooling trend (but none ‘significant’—cooler by 1°F or more). The states showing the greatest cooling trend are Alabama (-0.7°F), Georgia (-0.6°F), and Mississippi (-0.6°F). Forty states have experienced a warming trend (with 20 showing a significant 1°F or more increase in temperature. The states showing the greatest increase in temperature are North Dakota (+2.6°F), Nevada (+2.4°F), and Rhode Island (+2.4°F). Three states have shown no change: Texas, Maine, and West Virginia.



My crude attempt at mapping the temperature trends (I’m not a cartographer or Photoshop wiz, sorry!). The deep red indicates states that have seen a +1.0°F or more increase in temperature above the 116-year mean. The orange states have seen a minor warming (+0.1°-0.9°F) and the blue states have seen a decrease in temperature over the long-term mean. Maine, Texas and West Virginia showed no change in their temperature trends.

PRECIPITATION

Forty-one (41) states have trended wetter with 34 of these ‘significantly’ wetter (a change of more than 5% over the long-term mean). The greatest increase has been in Massachusetts where an amazing increase of 29.2% has occurred over the past 116 years. The other states showing the greatest increase in precipitation are Rhode Island (+28.3%), South Dakota (+25.7%), and Connecticut (+24.6%). Only 7 states have seen a drying trend and only 2 of those ‘significantly’ so (by more than 5%). The states showing the greatest drying trend are Wyoming (-8.5%), California (-7.7%), and Maine (-4.8%). The Maine figure is especially interesting since every other New England state has seen a significant increase in precipitation. Indeed, the Northeast region as a whole has seen a +12.4% increase in precipitation from the long-term average. The South Dakota trend seems strangely anomalous relative to its surrounding states. South Dakota has relatively few weather stations (COOP or otherwise) that may be a clue to the anomaly. Strange nonetheless (see more about this towards end of blog).



Another crude map illustrating the precipitation trends. The blue states represent where a ‘significant’ increase of 5% or more has occurred. The orange states have seen a decrease, but only California and Wyoming have seen a ‘significant’ decrease of 5% or more. The ‘white’ states have seen either no change or an insignificant change of less than +5% in their respective precipitation trend. Again, these are figures representing percentage change above or below the 116-year average.

Trends by Region

Here is the NCDC official map outlining their regional designations:



All the climatological regions have seen their temperatures increase except for the Southeast that has seen a slight cooling trend (-0.1°F). The Northern Rockies and Plains have seen the greatest temperature increase (+1.5°F), closely followed by the West and Southwest (both +1.4°F). All regions except the West have seen a trend towards wetter conditions with the Northeast leading the way (+12.4%), followed by the Upper Midwest (+10.7%). The West has trended drier by -4.2%.

Here is how the data breaks down by regions:



You can look at more detailed data for your specific location within a state by looking at this map:



Here is a map of all the U.S. Climatological Divisions.

You can see the data or each division by following the link here. This link, for instance, shows the trend for eastern Massachusetts (division 3 in the state). You can look at each and every year’s average annual precipitation for the division on the bar graph under the chart. Below is the precipitation trend for climate division 6 in South Dakota (central part of the state) where the increase has been 35.7%. This is the most anomalous of any division in the U.S. I have yet identified.






For the entire contiguous U.S. the temperature trend has been +1.2°F and +6.1% wetter compared to the mean over the 116 years since 1895.


Christopher C. Burt

Weather Historian