LIVE Extreme Precipitation Index maps!

The Extreme Precipitation Index (EPI) relates precipitation amounts to their average recurrence interval (i.e., “return periods”) providing an indication of the severity of an event.  The following maps provide real-time access to 6- and 24-hour Extreme Precipitation Index (EPI) maps via the Weather Decision Technologies (WDT)  interactive iMap interface.  These maps are based on precipitation from MetStormLIVE, a new cutting-edge Quantitative Precipitation Estimator that leverages thousands of precipitation gauges reports, high-resolution dual-polarization NEXRAD radar, satellite information and innovative algorithms.

Within iMap, select “Weather” then choose “Extreme Precipitation Index” in the control set at the top of the map. Toggling the legend key button in the lower left will show the color-coded legend.

Download archived 6- and 24-hr EPI/ARI maps (through 2/6/2017) based on National Weather Service gauge-adjusted radar-precipitation from our Download page.  Please contact us for more recent (post 2/6/2017) maps until we can upgrade our hardware for posting maps of these automatically.

Precipitation and Extreme Precipitation Index for Tropical Storm Hermine – Texas, Sept. 7, 2010


MetStat® has teamed with Weather Decision Technologies (WDT) to produce the Extreme Precipitation Index (EPI), which is a new product to objectively convey the rarity of precipitation in real-time. More often than not, extreme precipitation results 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.  In 2009, MetStat® and WDT first demonstrated a real-time operational EPI product based on observed precipitation.

Precipitation frequencies have been calculated in terms of amount and duration (e.g., how often 10 inches 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 are undergoing revision at the NWS Hydrometeorological Design Studies Center (HDSC) as part of NOAA Atlas 14.

Using WDT’s gridded national quantitative precipitation estimates (QPE), current maps of EPI based on recent precipitation over the past 6 and 24 hour periods are created. These maps are made available to clients on an interactive Google™ maps interface and now make it easy for anyone to interpret the significance of recent or ongoing precipitation events anywhere in the U.S., even without knowing anything about the precipitation climatology. This product proved to be quite popular with media outlets as a way of characterizing significant precipitation and potential flood events to the public. It should be noted that while floods are associated with heavy rain, the EPI product itself is not an indication of an equivalent flood occurrence or depth. In other words, a 100-year ARI for rainfall may or may not result in a 100-year flood event, since an ARI does not account for runoff, drainage capacity, etc. However, it clearly follows that when rainfall amounts have or are expected to significantly exceed climatological normals that flooding is likely to occur.

The EPI is a color-shaded map of the average number of years between the recurrence of a similar precipitation event allowing 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.

The EPI maps are based on the industry’s highest resolution and most accurate gauge-adjusted NEXRAD precipitation data from WDT and official precipitation frequency estimates published by the National Oceanic and Atmospheric Administration’s (NOAA) Hydrometeorological Design Studies Center.

Free Demo EPI maps can be viewed via WDT’s interactive map at and MetStat® GoogleTM Map interface.

Read our original article “Average Recurrence Interval of Extreme Rainfall in Real-time” at and an ARI map of Hurricane Irene’s rainfall featured in the NY Times.



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
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%

National Weather Service (upper right) flash flood watch/warning display on April 15, 2011 coincides with heavy precipitation (lower left) while the Extreme Precipitation Index (lower right) puts the precipitation into a statistical perspective — a 40-60 year event!

EPI Maps

 The EPI Analysis Package includes near real-time EPI maps, updated each hour, based on observed/analyzed precipitation.  In addition, maximum EPI maps are provided which provide a quick summary of areas impacted by rare precipitation during the past several hours and days via a single map.  The EPI  maps are based on the industry’s highest resolution (1-km2) and most accurate gauge-adjusted radar-based precipitation data from WDT.
Real-time EPI Analysis Package includes all of the following, which equates to 192 maps a day for less than $3/day:

  • 1-hr EPI (Full U.S.)
  • 3-hr EPI (Full U.S.)
  • 6-hr EPI (Full U.S.)
  • 24-hr EPI (Full U.S.)
  • Maximum* 1-hr EPI (Full U.S.)
  • Maximum* 3-hr EPI (Full U.S.)
  • Maximum* 6-hr EPI (Full U.S.)
  • Maximum* 24-hr EPI (Full U.S.)

* Maximum ARI – Maximum 1-, 3-, 6-, and 24-hour ARI over the past 3, 12-, 24 and 72-hours respectively.


We offer free EPI maps based on gauge-adjusted NEXRAD precipitation from Weather Decision Technologies (WDT) as well as the National Weather Service (NWS).  Here is a summary of differences between the free maps and those available with a premium paid subscription:

Free NWS-based EPI Maps
Free WDT EPI Maps
Premium EPI Maps from WDT or Marta
6-hour analysis
24-hour analysis
Available in an interactive map (e.g. Google and Bing)
1- and 3-hour analysis  
High resolution (1km x 1km)  
Low latency (update speed)
ARIs up to 1,000 years    
Maximum ARI maps    
6-hour ARI forecasts available every 3-hours out 5-days (updated 4X/day)    
24-hour ARI forecasts available every 12-hours out 5-days (updated 4X/day)    
Maximum ARI forecast maps    
Available in GIS formats