The HMR 52 Storm method generates a probable maximum precipitation (PMP) hypothetical storm as detailed in Hydrometeorological Report No. 52 (HMR 52) (Hansen, Schreiner, and Miller, 1982). Hydrometeorological Report No. 51 (HMR 51) contains the PMP index maps for the Eastern U.S., and HMR 52 contains information about the application of the PMP depths to a watershed. HMR 51 and HMR 52 apply to those areas of the United States east of the 105th meridian, with some exclusions, as shown in the figure below.
HMR 52 describes a procedure for developing temporal and spatial storm patterns for a 72-hour PMP estimate provided by HMR 51. The HMR 52 method computes a storm area and represents it as elliptical rings of decreasing rainfall intensity. These rings are referenced with a storm center (X and Y coordinates) and basin boundaries. Specifications for the temporal pattern, spatial pattern, storm location, storm area, and storm orientation parameters are applied in accordance with the criteria specified in HMR 52. Initial estimates of each of these parameters are entered by the user. Thereafter, HMS will optimize the parameters to maximize the basin-average precipitation.
Concentric idealized ellipses are used to construct the storm spatial pattern where each ellipse represents an isohyet of precipitation depth. The storm is located over the watershed by specifying the center of the pattern and the angle of the major axis of the ellipses. Total precipitation depth is computed using a specified storm area and area-duration precipitation curves. The total precipitation depth is converted to a temporal pattern based on the selected placement of the peak intensity within the storm duration. The most intense 6-hour period of the storm is constructed using the ratio of precipitation depth between the largest and sixth-largest hours.
The X and Y coordinates specify the location of the storm center, and are entered using the same coordinate system as the geometric data for the subbasin polygons. An initial estimate of the basin centroid provides a good starting point. The orientation is measured in degrees increasing clockwise from north (HMR 52 Section 4). If the actual orientation deviates from the preferred orientation by more than 40 degrees, a reduction is applied. According to HMR 52, the storm orientation should be bounded by 135 and 315 degrees. The peak intensity parameter specifies the time at which the precipitation intensity will be greatest within the 72-hour storm period (HMR 52 Section 2.3). The depth of rain falling during the period of peak intensity is subdivided into 1-hour increments using the 1 to 6 Ratio parameter (HMR 52 Section 6.5). Finally, the total storm area must be specified. The storm area represents the area of maximum intensity and produces the largest runoff.
The basin model must be geo-referenced to use the HMR 52 storm method.
