MetStat® has teamed with Weather Decision Technologies (WDT) to produce the Extreme Precipitation Index (EPI), which is a new product that objectively conveys the rarity of precipitation in real-time.  More often than not, extreme precipitation result in flooding – the most frequent severe weather threat and the costliest natural disaster facing the U.S. (90% of all natural disasters in the U.S. involve flooding).  The EPI is a real-time measure of the Average Recurrence Interval (ARI) of precipitation; when the EPI is high, the likelihood of flooding is high.  Often referred to as the “return period”, the ARI represents a precipitation event (amount per unit time) as the average number of years (climatologically) between equivalent events for a specific location.  An ARI of 100 years is the same as a 1% probability of an event occurring in any given year (“a 100-year event”).  The general public as well as hydrologic engineers and emergency managers often have better sense of the consequences of a 100-year storm versus an absolute amount of precipitation, therefore making the EPI a powerful way to convey the magnitude of occurring or forecast precipitation events.

Precipitation frequencies have been calculated in terms of amount and period (e.g., how often 10inches of rain may fall in a 24 hour period). These frequencies are provided in precipitation frequency atlases such as NOAA Atlas 2 and Technical Paper 40, but undergoing revision at the NWS Hydrometeorological Design Studies Center (HDSC) as part of NOAA Atlas 14.

In 2009, MetStat® first demonstrated a real-time operational ARI product based on observed precipitation, but now MetStat® and WDT have teamed to provided forecast EPI maps.  Using WDT’s gridded national quantitative precipitation forecasts (QPF), EPI forecast maps are created for 6- and 24-hour time increments.  WDT’s Weather Research and Forecasting model (WRF) features an objective analysis system with the WRF Four Dimensional Data Assimilation (FDDA) scheme. The objective analysis system employed is the Local Analysis and Prediction System (LAPS) analysis, developed and maintained by the Global Systems Division of the NOAA Earth Systems Research Laboratory. With LAPS, WDT is able to assimilate the IR, water vapor, and visible satellite image channels from geostationary satellites as well as WDT’s quality controlled three-dimensional radar mosaics. These two data sources, combined with traditional in situ observations, provide a more accurate initialization of the initial model moisture field through a three-dimensional cloud analysis. This technique has been shown to improve forecasts of precipitation and reduced model spin-up time.

24-hour QPE for Hurricane Irene (ending Aug. 28, 2011) 24-hour Extreme Precipitation Index Forecast 24-hour Extreme Precipitation Index Observed

WDT is a private sector leader in the field of operational numerical weather prediction (NWP) and has been providing data services and decision tools based on locally run mesoscale NWP models since its foundation. A natural extension of its product suite is to apply the ARI technique to its automated quantitative precipitation forecasts (QPF) as a means to provide more useful guidance to decision makers. The forecast ARI product specifically uses WDT’s WRF domain.

WDT’s WRF domain and terrain.

This domain is updated four times per day and provides a 5-day forecast on an 11.7 km grid.

The Forecast Package includes EPI maps, updated four (0300, 0900, 1500 and 2100 UTC) times a day, based on the Quantitative Precipitation Forecasts (QPFs) from the Weather Research and Forecast (WRF) mesoscale numerical weather prediction model.  Advances in the science of numerical weather prediction have significantly increased skill and resolution of QPF over the last decade and mesoscale models such as WRF provide excellent forecast guidance, particularly for strongly forced events typical of those that lead to widespread heavy rainfall and flooding. The maximum products make it easy to identify areas of potential flood risk for the next five days by looking at one single map.

The EPI is a color-shaded map of the average number of years between the recurrence of a similar precipitation event, otherwise known as the Average Recurrence Interval (ARI) or “return period.” The EPI allows users to quickly ascertain areas with the most unusual precipitation and potential for flooding rather than using simple precipitation amounts, since what is deemed heavy rain in one part of the U.S. may be typical in another. EPI maps provide an objective, timely and accurate depiction of the magnitude and extent of high-impact precipitation and allow users to make appropriate decisions. The conversion of precipitation to a EPI removes the distraction of heavy, but not abnormal, precipitation thereby highlighting only the high-impact, most unusual precipitation.


Describing floods in terms of an Average Recurrence Internal (ARI) or “return period” (e.g. 100-year) has been used for decades to convey the rareness of flooding at stream gauges. However, describing the intensity of precipitation in a similar manner has not been done as routinely, but provides an equally objective perspective of extreme precipitation events. Official, gridded NOAA/NWS precipitation frequency estimates provide the statistical basis for translating observed or forecast precipitation into an equivalent ARI at any location in the U.S..

ARI is defined as the average, or expected, period of time between exceedances of a given rainfall amount over a given duration. For example, suppose five inches of precipitation at a location is equivalent to an ARI of 100 years. This means five inches of precipitation is only expected to occur, on average, every 100 years at this location. Since the ARI is an average, a similar or even larger precipitation amount could occur again this year, next year or any other year. It does NOT mean an event of 5 inches will not occur again for 100 years. The ARI can also be described as a probability or percent chance of occurring in any given year. The table below converts the different terminologies and provides some potential flooding consequences.

It is important to understand that the ARI of precipitation does not necessarily equate to a flood of the same ARI. Floods can be caused by heavy rain, spring snowmelt, dam/levee failure and/or limited soil absorption. The degree of flooding from heavy precipitation depends on the precipitation intensity, storm duration, topography, antecedent soil conditions, ground cover, basin size and infrastructure design. Precipitation associated with a ARIs as low as 1 to 5 years can cause significant urban flooding since most urban storm water systems are designed for 1 to 10 year ARI precipitation events, yet this may not equate to any flooding in well-drained rural areas. ARIs for the design of highway and other transportation infrastructure typically vary from 10 to 25 years. However, it is a near certainty that rainfall associated with ARIs greater than 100-year will cause major flooding, regardless of anything else. Dams and levees are generally designed for rainfall ARIs much larger than 500 years, but can be compromised during ARIs of 100-500+ year events.

Categorical description of potential flooding consequences (when EPI is rainfall)
EPI/ARI Probability of occurrence in any given year Percent chance of occurrence in any given year
Rivers at all-time peaks, potential dam/levee over-topping, catastrophic flooding possible 500 yr 1 in 500 0.2%
Rivers near all-time peaks flows, devastating flooding possible 100 yr 1 in 100 1%
Rivers above flood stage, major flooding possible 50 yr 1 in 50 2%
Rivers at/near bankful, low lying flooding 20 yr 1 in 20 5%
Streams at bankful, high river flows 10 yr 1 in 10 10%
Street flooding and small streams near bankful 5 yr 1 in 5 20%
Minor flooding 2 yr 1 in 2 50%
Little or no flooding 1 yr 1 in 1 100%


For a free demo of EPI forecast maps please contact us.