Forest Trail Explorer

US Forest Service and Southern Research Station

Precipitation Patterns

Seasonal Average Precipitation (March–August) Map
Seasonal Average Precipitation (September–February) Map
Palmer Hydrological Index for the Southern Mountains of NC
Palmer Hydrological Index for the Northern Mountains of NC

Seasonal Precipitation

The seasonal distribution or timing of precipitation is an important characteristic of climate. While average annual precipitation tells how much rainfall an area receives in a normal year, the seasonal distribution tells when it arrives. The timing of precipitation also indicates how often an area experiences drought and flooding. Both drought and flooding are natural occurrences in Western North Carolina, but changes in the frequency of these events have the potential to greatly impact water resources for forest ecosystems.

Western North Carolina receives most of its rainfall during the spring and summer months, while the fall and winter months are comparatively dry. This pattern in the distribution of rain throughout the year is essential to biological processes. Forests need more water in the warmer growing season. Deciduous trees, especially, take up much more water in spring and summer when leaves are on the trees due to evapotranspiration (the process of water coming through the leaves of trees and then evaporating into the air). The timing of precipitation in Western North Carolina allows trees to leaf out in the spring and sustains the flow of water in streams.

Wet and Dry Periods

The Palmer Hydrological Drought Index (PHDI) is influenced by precipitation and shows long-term dry and wet conditions. The PHDI reflects how precipitation affects groundwater, water tables, and reservoirs. On the graphs, the red indicates dry conditions while the green indicates wet conditions. Any green value above 2 indicates severe wet (flooding), while any red value below -2 indicates severe dry (drought). Uncharacteristically frequent or extended dry or wet periods indicate an interruption in the seasonal pattern of precipitation.

With climate change, there may not be any change in total precipitation, but there may be extended wet and dry periods that might result in more flooding or drought. The PHDI for Western North Carolina shows that the longest dry periods over the past century have occurred in the past three decades. Palmer Hydrological Index for the Northern Mountains of NCWhile flooding and drought are natural disturbances in Western North Carolina, increased frequency or severity of these disturbances will negatively impact the forests.

The above content is a part of the Western North Carolina Vitality Index. To view the full report, visit

Forest Inventory and Analysis Resources

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Oregon State University, PRISM Group. Accessed from:

Mera, R. PHDI Graphs. January 2008. Raleigh, NC: North Carolina State University.