Louisiana Heavy Rainfall and Flooding

Heavy Rainfall and Flooding

Data Limitations

There is a relatively long historical record of precipitation data. However, there can be gaps between station locations, so some rainfall events, including high rainfall amounts, may not be adequately represented in the data. Also, flood risk depends on a precipitation event, preceding events, the built environment, and flood mitigation techniques. Flooding can and does occur outside of the Federal Emergency Management Agency (FEMA) Special Flood Hazard Areas. Flood impacts are often extremely localized, so the data listed below may not adequately represent a single community or neighborhood flood risk or history.

Definition and Description

Heavy Rainfall and Flooding

Heavy rainfall is rain with a rate of accumulation exceeding a specific value that is geographically dependent (AMS 2012). Flooding is any high flow, overflow, or inundation by water which causes or threatens damage (NWS 2009).


Heavy rainfall is a subjective term, but is rain falling at a rate more than the underlying surface can handle, causing runoff, inundation of low-lying areas, and flooding. This may include short-duration thunderstorms lasting a few hours or rainfall accumulating over several days. Flooding is the result of heavy rainfall but also the underlying surface. The rate of infiltration (how quickly it is absorbed by the soil), how quickly runoff reaches the creeks and rivers, if there had been prior rainfall, if the ground is frozen, and other local factors affect runoff and flooding. Consequently, a rainfall of a given rate and amount may cause flooding in one circumstance but not in another. Flooding is most likely in low-lying areas, along the edges of water bodies (ponds, lakes, rivers), and over impermeable surfaces (such as streets and parking lots). Primary causes include slow-moving thunderstorms and storms that track over a location in rapid succession, or tropical systems. Flash flooding may occur with intense thunderstorms while river flooding usually requires rainfall accumulated over a longer duration.

Rainfall accumulations may be compared against previous occurrences through the concept of “return-period values”. This is a statistical assessment of the frequency with which similar amounts have been recorded in the past at a specific location. These return periods, such as 1 in 25 years (a 4% chance of occurring in any given year), are not predictive tools – a large event occurring in the recent record does not prevent another similar or larger event occurring shortly after. Also, if heavy rainfall has already occurred, stormwater retention systems may be filled to capacity allowing a smaller event to cause flooding impacts similar to a much larger event. Rainfall rates and accumulations are usually greatest during the spring, summer, and fall, when warm air can hold more water vapor to produce greater rain rates. River flooding is most likely in spring or fall, when fronts may stall giving a focus for thunderstorm development. Tropical systems, either from the Gulf of Mexico or from the Pacific Ocean, can produce among the highest rainfall rates in the state.

Historical Data

Climate Extremes Tool – Precipitation

(Period of record varies by station; up to ~130 years) Southern Regional Climate Center

This interactive map shows daily precipitation extremes at airport weather stations, which can be used to show some previous heavy rainfall occurrences (i.e., the highest rainfall totals do not necessarily occur at airport weather stations).

1. Pan and zoom to the location of interest. 2. To obtain High precipitation records by month: On the left side of the screen, select Records For A MonthHigh PrecipitationMonth of interestSubmit. 3. The measurement unit is inches. Mouse over the icon on the map for record details (date of occurrence and station record). 4. To obtain All-time records: Select All-Time RecordsHigh PrecipitationSubmit.

NOAA Atlas 14 Precipitation Frequency Data Server

(Last updated in 2013) NOAA Hydrometeorological Design Studies Center

This interactive tool shows rainfall frequency estimates for select durations (e.g., 3-, 12-, and 24-hours) and recurrence intervals (e.g., 100-, 500-, and 1000-years) with 90% confidence intervals. Probable maximum precipitation (PMP) values are not represented in this tool.

1. Click on Louisiana from the map. A new tab will open. 2. To select a location, either enter the desired location, station, or address manually OR double-click the interactive map. 3. Precipitation frequency estimates will be displayed in both table and graph forms below. 4. For additional help, select FAQ from the left-hand menu, then refer to the Section 5 link under section 1.1.

Historical Climate Trends Tool

(1895-present)Southern Climate Impacts Planning Program

This tool shows precipitation trends by state and climate division, annually, seasonally, and monthly. Years, seasons, or months with high precipitation totals may be indicative as years with flood events during one or more parts of the year, however, this must be correlated with other data.

1. On the left side of the screen, select Louisiana. 2. Select the climate division of interest or the entire state. 3. Select annual, season, or month of interest. 4. Select Precipitation. 5. For more information on how to interpret the chart, click on Chart Info on the bottom left and hover over points to view individual precipitation values.

Climate Explorer – Extreme Events Tool

(Period of record varies by station; up to ~130 years) NOAA Climate Program Office and National Environmental Modeling and Analysis Center

The Climate Explorer is an interactive tool that allows you to view the number of extreme rainfall events per year for a station’s period of record.

1. Type in the city or parish you are interested in. 2. Click Extreme Events. 3. From the top menu, select a Threshold in inches (e.g., 1 in). 4. You can choose whether or not to select an option from the Percentiles drop-down menu, from the 10th to 90th percentile. 5. Choose at least (>=) from the next drop-down menu. 6. Select a Duration in days (e.g., 1 day). 7. Choose a station (red dot) on the map. 8. The resulting plot displays a bar chart with days per year (or multiday duration you chose) with precipitation reaching your selected threshold. Hover your mouse over the bars to view yearly information. Under the chart on the right side is a Plain-language description that summarizes the number of times the threshold was reached over the station’s period of record and the yearly average occurrence.

Flood Impacts by River Crest Height

(Period of record varies by gauge; up to ~110+ years)National Weather Service Advanced Hydrologic Prediction Service

This interactive tool shows a summary of flood impacts for location of interest. It can be used to show the extent of flood events.

1. Under the NWS River Forecast Centers drop-down menu to the right of the map, choose between the Lower Mississippi River (covers most of LA), or West Gulf River (a few stations near the LA/TX border) Forecast Center to access stations for your region. 2. On the map, pan and zoom to the area of interest. Click on the stream gauge of interest on the map. 3. Click River at a Glance tab near the top of the page. 4. Left column: Select gauge of interest. Right column: At a minimum, select Flood Impacts, Location Map, Record Crest History. 5. Click Make my River Page! 6. Information you selected will be displayed on a new page with flood categories and historic crests at the bottom of the page.

FEMA Flood Map Service Center

Federal Emergency Management Agency

This website can locate and identify flood hazard zones in a jurisdiction and produce maps for use in a hazard mitigation plan. Combined with other map layers, it can provide a spatial relationship between flood hazard zones and critical facilities and infrastructure. Note that the 100-yr floodplain is an estimate used for insurance and regulatory purposes. Floods can and do occur outside of the areas depicted. Note: This tool is a little more involved than some of the others and it is helpful to use a larger computer screen because of the amount of data shown.

1. Enter an address, place, or coordinates in the search bar. 2. Click Search. 3. Click Streets view in upper right corner. 4. The panel of land outlined in light blue is what will be mapped. If you need another panel, click on the one of interest. (Zoom out if needed. It may take a few seconds.) 5. Zoom in to view details. Note the legend below the map and effective date in bold above the map. 6. To download a black and white static image of full original FIRM panel, click on the Map Image icon. To access a colored map, click on the Dynamic Map icon. You may need to disable your browser’s pop-up blocker.

Historical Flood Risk and Costs

(1996-present) Federal Emergency Management Agency

This map visualization and graph shows state and parish flood events that are documented in NOAA’s Storm Events Database. It shows the number of flood events by parish and costs of flooding based on average National Flood Insurance Program and FEMA’s Individual and Household Program payments.

1. Under Choose a State, select Louisiana. 2. Louisiana statistics will be displayed on the page. 3. If you wish to view statistics by parish, click on a parish on the map.

Louisiana FloodMaps Portal: The Base Flood – on the map

LSU AgCenter, LA Dept. of Transportation & Development

This tool provides information about flood risk at a particular location and shows how FEMA’s Flood Insurance Rate Maps (FIRM) have changed over time.

1. Click Continue. 2. Enter an address/coordinate or select a parish from the drop-down menu. Note: if you select a parish, the page may bring you back to the welcome page. If so, just click Continue again, then click the X to exit the location pop-up menu on the mapping page. 3. The map shows the Effective FIRM, floodway areas, and other layers. 4. Click the Layers button on the top right to select various FIRM layers and ABFE (Advisory Base Flood Elevation; if available). Click the question mark next to any layer to read more. 5. Click the Legend button on the top right to interpret all map features. 6. Zoom in and click on the map for point-specific information. On the resulting pop-up box, click Community Info or What Does This Mean? to learn more. Note: Click the “i” on the top right of the screen for additional guidance.

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