The conditions of drought are difficult to define, particularly as one is unfolding. Sunny, warm weather is almost always welcomed following a period of precipitation. And a stretch of dry weather is not usually remarkable.
In his 1947 book, Drought and Its Causes and Affects, I. R. Tannehill notes:
The first rainless day in a spell of fine weather contributes as much to the drought as the last, but no one knows how serious it will be until the last dry day is gone and the rains have come again...
The Palmer Drought Severity Index (PDSI) is the most commonly used drought index in the U.S. It was developed to measure intensity, duration, and spatial extent of drought. Measurements of precipitation, air temperature, and local soil moisture, are collected and compared to prior values of these measures. Values range from -6.0 (extreme drought) to +6.0 (extreme wet conditions), and have been standardized to facilitate comparisons from region to region.
Drought conditions in Montana are a major seasonal concern. Studies show that dry weather conditions are the driving factor behind uncontrolled fire events and certain insect outbreaks. The drought years between 1928 and 1939 remain the driest on record for Montana. Montana experienced extended drought conditions for the seven-year period from 1999 to 2006, and and the state may have remained in a drought cycle for several years beyond, (2010 Report).
The National Weather Service Great Falls, MT (PDF 91 KB) offers an excellent overview of weather extremes and events in Montana: The Montana Climate Office exhibits 50-year weather trends for five Montana cities. The general trend appears to be toward slight warming. The March mean temperatures seem markedly higher, however. A century of Montana’s precipitation patterns east of the Continental Divide is available at Montana's Web and Dry Spells as well as through The Governor’s Drought Information Advisory.
In Montana, the availability of adequate late-summer instream flows is an important consideration to the health and viability of certain fish species. Over the past two decades, Department of Fish, Wildlife & Parks managers and fisheries biologists have called for an alarming number of mid- and late-summer fishing closures on certain streams to protect the resources. Drought conditions in recent seasons have led to diminished access to other forms of water recreation as well, such as boating and rafting. Drought conditions also affect agricultural production. The allocation of water becomes more contentious as the resource becomes scarce. Drought conditions put government and tribal holders of instream rights at odds with historic consumptive uses for agricultural purposes. Low stream flows and warmer temperatures also may allow invasive species to advance while forcing native species to retreat to higher elevation waters.