Decadal Precipitation Trends in Pennsylvania

 

Three geographically separate climate regions were selected to examine the trends of precipitation during the past 100 years in Pennsylvania. While these climate regions (divisions 7, 3 and 9) have had a differing number of reporting sites during this period, the longer term averages of seasonal and annual precipitation are representative of climate trends rather than instrument changes.

 

Climate Divison 7

 

Winter (Dec-Jan-Feb) - The past 30 years have shown a little trend in winter precipitation, but the variability in winter precipitation has been at a maximum compared to the past century.

 

Spring (Mar-Apr-May) - The spring period has shown a steady rise during the past 30 years, but is still 10-15% below the very wet period during the three decades 1931-1960.

 

Summer (Jun-Jul-Aug) - Despite a steady decline in precipitation during the past 3 decades, there is an apparent cycle that points to a wetter summer period ahead in the 2001-2010 decade. The variability is nearly 20%.

 

 

Fall (Sep-Oct-Nov) - The last 30 years have seen a return to the wet times noted during the period 1931-1960. Autumn rainfall now averages about 15-20% higher than it did at the beginning of the 20th century.

 

 

Annual: Between the beginning of the 20th century and the end, there has been a nearly 15% increase in annual precipitation with values rising from 34" to about 40" by the 1990's. The increase seems to fairly uniform, though mostly concentrated in the autumn.

 

 

Climate Divison 3

 

Winter (Dec-Jan-Feb) - The past 30 years have shown a trend toward wetter winters with about a 10% increase in this time period.

 

 

Spring (Mar-Apr-May) - The spring period has shown a cycle that would favor a 10% decrease in precipitation during this next decade.

 

 

Summer (Jun-Jul-Aug) - An apparent cycle points to a drier summer period ahead in the 2001-2010 decade. The variability is 15%.

 

 

Fall (Sep-Oct-Nov) - The last 30 years have seen a nearly 20% increase in average autumn rainfall. The rainfall from 1941-1970 was 9.5", while it is now (1971-2000) about 11.1".

 

 

Annual: Between the beginning of the 20th century and the end, there has been a nearly 15% increase in annual precipitation with values rising from 38.5" to about 44" by the 1990's. The trend in this division points to drier springs and summers and wetter falls and winters in the next decade.

 

 

Climate Divison 9

 

Winter (Dec-Jan-Feb) - The past 30 years have shown a slight trend toward wetter winters, but the variability in winter precipitation is less than 15% by decade averages.

 

 

Spring (Mar-Apr-May) - The spring period has become decidedly wetter during recent decades as interdecadal variability remains at about 15%.

 

 

Summer (Jun-Jul-Aug) - An apparent cycle points to a wetter summer period ahead in the 2001-2010 decade. The variability is nearly 20%.

 

 

Fall (Sep-Oct-Nov) - The last 30 years have seen a nearly 25% increase in average autumn rainfall. The rainfall from 1941-1970 was only 8.2", while it is now (1971-2000) about 10".

 

 

Annual: Between the beginning of the 20th century and the end, there has been a nearly 15% increase in annual precipitation with values rising from 37" to about 42" by the 1990's. The increase seems to fairly uniform, though mostly concentrated in the autumn.