PO Box 680 Monument, Co 80132 719/488-9117 appliedweatherassociates.com Probable Maximum Precipitation (PMP)
Probable Maximum Precipitation Introduction Concept of Probable Maximum Precipitation (PMP) Definition i. Theoretical values ii. Maximum depth of precipitation iii. Physically possible iv. Geographic region v. Certain time of year History of Hydrometeorological Reports (HMRs) Concept of Probable Maximum Precipitation (PMP) Concept of an upper limit to precipitation potential Corps of Engineers requested charts of Maximum Possible Precipitation, MPP (1946) HMR 23 (1947) provided subjectively derived estimates Derived from the most effective combination of factors controlling rainfall intensity MPP changed to Probable Maximum Precipitation (PMP) Better reflects the uncertainty of estimating maximum precipitation PMP values are generally considered estimates of the upper limit of precipitation production
Probable Maximum Precipitation Definition: The theoretically greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographic location at a certain time of year (HMR 59, 1999) Probable Maximum Precipitation Definition: The theoretically greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographic location at a certain time of year (HMR 59, 1999)
Probable Maximum Precipitation Theoretically - No technology exists to compute theoretical maximum precipitation values - Historic storms are used as the basis for computing maximum rainfall values - Procedures are used to adjust the observed rainfall from historic large rainfall events to determine the theoretical maximum rainfall - Meteorological computer models are providing useful tools for evaluating orographic effects = Spatial distribution are provided = Not useful for determing upper limit magnitudes Probable Maximum Precipitation Definition: The theoretically greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographic location at a certain time of year (HMR 59, 1999)
Probable Maximum Precipitation greatest depth of precipitation -Greatest depth for various area sizes and various durations = Area sizes: 1 to 20,000 square miles* = Durations: 1 to 72 hours* -Can be for all seasons (all season PMP) or for times of the year when there may be snow cover or frozen ground (cool season PMP) -Can vary by storm type = Local Storm = General Storm * PMP studies are not limited to 20,000 sq mi and/or 72-hour duration Probable Maximum Precipitation Definition: The theoretically greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographic location at a certain time of year (HMR 59, 1999)
Probable Maximum Precipitation physically possible - The PMP design storm must have characteristics that can combine naturally to produce an extreme rainfall storm - Many worst case storm characteristics cannot co-occur within an extreme rainfall storm - Some historic PMP analyses have inappropriately combined worst case storm characteristics Probable Maximum Precipitation Definition: The theoretically greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographic location at a certain time of year (HMR 59, 1999)
Probable Maximum Precipitation geographic location The PMP design storm type varies depending on the geographic location The PMP design storm type depends on basin characteristics In general small rapid response basins have the convective storm type as the PMP storm In general large basins with large lag times have the general storm type as the PMP storm Gradients of PMP values are not homogeneous for various geographic locations Probable Maximum Precipitation HMR 51
Probable Maximum Precipitation HMR 59 Probable Maximum Precipitation Break #1 Questions
Probable Maximum Precipitation
Probable Maximum Precipitation HMR 51
Method for Computing PMP Values No physically based theoretical method (yet) Observed extreme rainfall events are used Standard procedures are applied to the storm rainfall Primary steps Storm Search Identification of most significant storms Maximization procedures are applied Transpositioning procedures are applied Enveloping procedures are applied
Probable Maximum Precipitation Storm search Short list of significant storms Storm rainfall analyses (Depth-Area-Duration) Rainfall timing (mass curves) Storm in-place maximization Storm transposition Moisture transposition Elevation moisture adjustment Orographic transposition Depth-Area envelopment Depth-Duration envelopment Key Tasks for Site-Specific PMP Studies Identify extreme storms for the region Identify recent extreme storms Add 40 years of storms for HMR 51 region Review older rainfall data records HMRs may not have included all significant storms
Updated Storm Search Locations New Storms Analyzed for PMP Development
New Storms Analyzed for PMP Development New Storms Analyzed for PMP Development
Key Tasks for Site-Specific PMP Studies Identify extreme storm types Evaluate the use of HMR procedures for each storm type Identify unique topography Moisture depletion by upwind barriers Precipitation enhancement/decrease Effects on storm center location Review HMR procedures used for the basin location Identify inconsistent assumptions Probable Maximum Precipitation Break #2 Questions
Method for Computing PMP Values Maximization Storm rainfall production depends on Storm rainfall production efficiency Available atmospheric moisture Assumptions For a sufficiently long period of record, storms will be identified that have approached maximum rainfall production efficiencies The storm rainfall could have been greater is more atmospheric moisture had been available Procedure Determine the amount of atmospheric moisture that was available to the storm Determine the maximum amount of moisture that could have been available to the storm Increase the rainfall by a ratio of maximum moisture to actual moisture Called the storm maximization factor Use of a trajectory model
Updated Maximum Dew Point Climatology Updated Maximum Dew Point Climatology Map
Updated 2 Sigma SST Map Method for Computing PMP Values Transpositioning The largest storms over a region are identified The transposition region contains storm that are similar Meteorologically Climatologically Topographically The rainfall from each storm is adjusted to other parts of the region The adjustment is the transpositioning factor
Method for Computing PMP Values Enveloping For any location in a region The maximized and transpositioned Depth-Area (D-A) rainfall is plotted for each storm for each duration For each duration, an envelop curve is constructed that envelopes the rainfall values at each area size The D-A envelop curve procedure insures continuity in space i.e. The rainfall at each area size has continuity with smaller and larger area sizes The same procedure is followed for the Depth-Duration (D-D) rainfall plots The D-D envelop curve procedure insures continuity in time i.e. The rainfall at each duration has continuity with shorter and longer durations Area Enveloping
Duration Enveloping Probable Maximum Precipitation Break #3 Questions
Probable Maximum Precipitation Generalized PMP values (HMRs) Site-Specific PMP values Regional Statewide Individual basins Examples of Site-Specific PMP Study Findings Storm Maximization Storm representative dew point (SST) Historic analyses have selected dew points (SST) Hundreds of miles away Occurred during or after the rainfall event Time period of observations were not consistent with the rainfall duration Maximum dew point (SST) climatology used was unreliable Maximization ratios of 2.0 or greater were used
Examples of Site-Specific PMP Study Findings Storm Maximization, Dew point Analyses 12-hour vs 6-hour persisting dew points 12-hour persisting vs 6-hour average Observed dew point values Hour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Td 68 67 69 69 72 75 75 74 75 73 70 69 68 65 66 65 65 65 67 66! Rainfall Event! 12-hour persisting: 65 6-hour persisting: 72 6-hour average: 74 Examples of Site-Specific PMP Study Findings Maximum Dew point Climatology Published NOAA Climatology Combination of several analyses Not well documented Represented maximum observed values New climatology can be produced Long period of record of quality controlled data Return frequencies can be computed Climatologies of persisting and average values can be provided
Updated Dew Point Climatologies Examples of Site-Specific PMP Study Findings Homogeneous Climate Regions Select storms for each region Transposition Maximize Envelop
Probable Maximum Precipitation HMR 51 Recalculation of the Distance from the Coast curve in HMR 51
Statewide PMP Study Results vshmr 51 Statewide PMP Study Results vs HMR 51
Statewide PMP Study Results vs HMR 51 Statewide PMP Study Results vs HMR 51
Probable Maximum Precipitation Types of PMP studies: Generalized (Hydrometeorological Reports) Regional (EPRI Michigan/Wisconsin 1993) Statewide (Nebraska 2008, Arizona, Ohio, Wyoming) Site-Specific Results from Selected Site-Specific PMP Studies Wisconsin/Michigan Accepted by FERC Great Miami River, Ohio Accepted by Ohio State Engineer Catawba-Wateree Rivers, Carolinas Not accepted by FERC Williams Fork River, Colorado Accepted by FERC & Colorado State Engineer
Results from Selected Site-Specific PMP Studies Muddy Creek, Colorado Accepted by Colorado State Engineer Elkhead Creek, Colorado Accepted by Colorado State Engineer Broomfield Reservoir, Colorado Accepted by Colorado State Engineer Chelan Reservoir, Washington Study suspended Results from Selected Site-Specific PMP Studies Nebraska Statewide Accepted by Nebraska Dam Safety office FERC acceptance pending Brassua Dam drainage basin, Maine Accepted by the FERC Bleinheim-Gilboa Reservoir, New York Accepted by the FERC Lewis River drainage basin, Washington Accepted by the FERC
Recently Completed PMP Studies Do Regional/Statewide/Site-Specific PMP Studies Provide Improved PMP Values? Attributes: More storms considered New technologies used Recognized problems in the HMRs corrected Topographic features addressed
Do Regional/Statewide/Site-Specific PMP Studies Provide Improved PMP Values? Primary Question: Do the PMP values resulting from a sitespecific/statewide PMP study better represent the upper physical limit of extreme rainfall as required by the definition of PMP? Should a Particular Regional/Statewide/Site- Specific PMP Study be Accepted by a Regulator? Problem: Most regulators do not have access to a staff hydrometeorologist Potential Solution: Review by a government hydrometeorology office Review by a peer reviewer Review by an independent Board of Consultants (BOC) FERC BOC Meteorologist Hydrologist Civil Engineer
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