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ERO High Risks can foreshadow extreme flash-flood impacts...

 

Excessive Rainfall Outlook "High Risk Days" (defined here) indicate potential for extreme impacts from flash flooding. These days correspond to about two out of every five flood-related fatalities and a significant majority of all flood-related property losses in the contiguous United States (CONUS). High Risk Days typically occur with slow-moving tropical systems, atmospheric rivers, or substantial mid-latitude storm systems. This page provides historical context to ERO High Risk areas and insights on ERO trends over the past 5-10 years. Statistics (including dates and tallies) for High Risk Days affecting NWS County Warning Areas, U.S. States, and U.S. Metropolitan Areas are located below and also provided in downloadable formats. For any questions about this content, visit our feedback page or contact Ashton Robinson Cook or Alex Lamers.



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CWAs and High Risk Days: 2016-2021


  • Since 2016, high risk areas have most frequently occurred across the western Gulf Coast, Lower Mississippi Valley, Carolinas, southern California, and from the Mid-Atlantic into southern New York.

  • Many of the high risks have been attributed to landfalling tropical systems or their remnants across the southern and eastern U.S.

  • Hurricane Harvey's influence across the western Gulf Coast is especially apparent. Harvey resulted in seven High Risk Days during its trek from southeast Texas to western Tennessee.

  • Atmospheric rivers are the primary impetus for heavier rainfall in California.

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For a list of High Risk Days that included all or part of each NWSFO CWA dating back to 2016, click here.

For a list of High Risk Days including all or part of each U.S. state dating back to 2016, click here.






High Risk Hotspots: 2016-2021


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  • High Risk "Hotspots", or "Capitals" were calculated by determining the number of days that an ERO High Risk covered all or part of a Metropolitan area (as defined by the U.S. Census Bureau) since 2016.

  • Lake Charles, LA and Beaumont/Port Arthur, TX led with 15 High Risk Days - mostly due to landfalling tropical systems.

  • Houston, New Orleans, the Carolinas, and the Mid-Atlantic are also sensitive to tropical cyclone activity.

  • Southern California high risks are typically due to strong atmospheric rivers.

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For a list of High Risk Days that included all or part of each U.S. Metropolitan Statistical Area dating back to 2016, click here.






High Risk Day Casualties and Reported Property Losses: January 2010 - September 2020

Downloadable in the following formats: .csv .tsv (tab-separated) .xlsx





A few notes about ERO product changes over time...

  • The issuance and valid times of the ERO have changed over the time frame of this database. The spreadsheet included here is arranged to sort data in a way that is consistent with the current forecast methodology. Fatalities, injuries, and damages are from NWS Storm Data, and from the High Risk areas, or contiguous Slight and Moderate Risk areas.

  • Up to May 2015, the Day 1 ERO was issued four times per day, at 06Z, 15Z, 18Z, and 03Z. However, the valid time was not fixed to a 12Z to 12Z period, as it is now. The 06Z issuance was valid for 30 hours, or until 12Z the next day, and is most similar to the initial ERO issuance time in the early morning hours now. The 15Z outlook was valid for 21 hours, also until 12Z the next day.

  • However, the 18Z outlook and 03Z outlook were valid for 30 and 21 hours respectively, until 00Z the next day. This created a rolling valid time, where every 12 hours a new 12 hour chunk of the forecast was added. It causes some complexity in assigning a High Risk to a particular 12Z to 12Z period.
  • For High Risks on the 06Z outlook, it was assigned to the subsequent 12Z to 12Z period and labeled as "Initial" in column E. The exception to that rule was if the High Risk was removed on the next 15Z outlook. In that case, the High Risk was valid mostly on the previous day, and was assigned as such.

  • High Risks issued on the 15Z, 18Z, and 03Z outlooks were assigned to the 12Z to 12Z period in which they were issued, and labeled as "Update" in column E.

  • Under the current issuance methodology, the 01Z Day 1 ERO update (issued about halfway through the Day 1 period) is labeled as "Day 0" in Column D, beginning with the change in methodology in 2015.

  • High Risks that were only issued on a special update are not included in these lists. These cases are rare, and would involve a High Risk only valid for a short period of time, or a High Risk that was issued after 01Z.


  • Slight, Moderate, and High Risk Day Trends: 2016-2021

    Charts created by Marc Chenard. Underlying dataset created by Ashton Robinson Cook. Click image to enlarge...





    Downloads and Helpful Links


    For a presentation on High Risk Days since 2010 (in .pdf format), click here.

    For some things you can do to prepare on a High Risk Day, click here.

    For flash flood preparedness tips, click here.

    For tips on interpreting the Excessive Rainfall Outlook, click here.

    For Excessive Rainfall Outlook categorical descriptions, click here.

    For Excessive Rainfall Outlook categorical descriptions en Español, click here.


    For a list of High Risk Days that included all or part of each NWSFO CWA dating back to 2016, click here.

    For a list of Moderate Risk Days that included all or part of each NWSFO CWA dating back to 2016, click here.

    For a list of Slight Risk Days that included all or part of each NWSFO CWA dating back to 2016, click here.


    For a list of High Risk Days including all or part of each U.S. state dating back to 2016, click here.

    For a list of Moderate Risk Days including all or part of each U.S. state dating back to 2016, click here.

    For a list of Slight Risk Days including all or part of each U.S. state dating back to 2016, click here.


    For a list of High Risk Days that included all or part of each U.S. Metropolitan Statistical Area dating back to 2016, click here.

    For a list of Moderate Risk Days that included all or part of each U.S. Metropolitan Statistical Area dating back to 2016, click here.

    For a list of Slight Risk Days that included all or part of each U.S. Metropolitan Statistical Area dating back to 2016, click here.