The above near real-time EPI maps are updated every 6-hours (0000 UTC, 0600 UTC, 1200 UTC & 1800 UTC), which coincides with updates of the gauge-adjusted radar-estimated precipitation from the National Weather Service (NWS) . These maps are neither official nor certified and only represent ARIs up to 100-years.
Download archived maps from our Download page.
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 http://imapweather.com and METSTATs GoogleTM Map interface.
Read our original article “Average Recurrence Interval of Extreme Rainfall in Real-time” at EarthZine.org 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
||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
||1 in 500
|Rivers near all-time peaks flows, devastating flooding possible
||1 in 100
|Rivers above flood stage, major flooding possible
||1 in 50
|Rivers at/near bankful, low lying flooding
||1 in 20
|Streams at bankful, high river flows
||1 in 10
|Street flooding and small streams near bankful
||1 in 5
||1 in 2
|Little or no flooding
||1 in 1