A one-month SPI map is very similar to a map displaying the percent of normal precipitation for a month. It is actually a more accurate representation of monthly precipitation because the distribution has been normalized (see detailed discussion of the Percent of Normal). Because the 1-month SPI reflects relatively short-term conditions, its application can be related closely with short-term soil moisture and crop stress, especially during the growing season. The 1-month SPI may approximate conditions represented by the Crop Moisture Index (CMI).
Interpretation of the 1-month SPI may be misleading unless climatology is understood. In regions where rainfall is normally low during a month, large negative or positive SPIs may result even though the departure from the mean is relatively small. Several specific examples demonstrate this limitation.
Examine the 1-month SPI for June 1995 as a first example. The South Coast Drainage Climate Division in California has an SPI of +3.11, giving the impression that the month of June was very wet. Indeed, the very large positive SPI means that the amount of rain that did fall during that June was greater than the third standard deviation and that this occurs approximately once in 100 years. However, the normal June precipitation for this climate division is 2.5 mm (0.10 in.) (see Los Angeles climograph), and the actual precipitation total for June 1995 was 15.2 mm (0.60 in.).Thus, it was highly unusual but hardly a flooding event. In comparison, the Willamette Valley Climate Division in Oregon had a 1-month SPI in February 1996 of +1.97. In this climate division, 371.9 mm (14.64 in.) of precipitation fell in February, which was 211.6 mm (8.33 in.) above the normal total for the month. Major flooding occurred along the Willamette and Columbia Rivers as a result of these high rainfall amounts. This comparison emphasizes the necessity of understanding the climatology of an area when examining the 1-month SPI.
A second example of how the 1-month SPI may be misleading is demonstrated by the 1-month January 1996 SPI for the Central Climate Division in Nebraska. Normally, the North American Great Plains get very little precipitation during the mid-winter months (see the Lincoln, Nebraska, Lubbock, Texas, and Bismarck, North Dakota climographs), and the January normal for this climate division is only 10.4 mm (0.41 in.). In 1996, the January precipitation total was 24.9 mm (0.98 in.). This is 239% of normal, and it gives an SPI of +1.43. However unusual the January precipitation totals were for this climate division, as well as others in southeastern Nebraska, these totals still provided a relatively small amount of moisture and did little to diminish the longer-term dry period in the region (see 9-month SPI for the end of February 1996).
The 1-month SPI can also be misleading with precipitation values less than the normal in regions with a small normal precipitation total for a month. This can be seen in the 1-month SPI for the end of February 1996 for the Southeastern Plains Climate Division in New Mexico. The SPI is -1.76. This large negative SPI gives the impression that February was very dry in this climate division. It was indeed very dry (zero precipitation fell during the month), but February is normally a dry month in this location (see climograph for Lubbock, Texas), so the departure from normal was only 10.4 mm (0.41 in.).
These are only a few of many possible examples. As with a percent of normal map, useful information is contained in the 1-month SPI maps, but caution must be observed when analyzing these maps.