The Center for Operational Oceanographic Products and Services with the National Centers of Environmental Information has released an experimental outlook on coastal high tide flooding every year since 2014. This year’s State of High Tide Flooding and 2018 Outlook assesses coastal high tide flooding based on new, national flooding thresholds. It focuses on more impactful, deeper floods at some locations and expands the outlook to 98 coastal locations.
High tide flooding, sometimes referred to as "nuisance" flooding, is coastal flooding that occurs at high tide when water levels measured at NOAA tide gauges exceed heights associated with minor impacts, leading to public inconveniences. These floods creep into busy city centers and streets. Decades ago, coastal flooding mostly occurred during strong storms. Today, it occurs more frequently during high-tide cycles and calmer weather. Though high tide flooding today is rarely life threatening, it is a serious concern in several communities.
During 2017, the U.S. average frequency for high tide flooding was the highest ever measured by 98 NOAA tide gauges, and more than a quarter of the coastal locations outside of Alaska tied or broke their individual records for high tide flood frequencies.
Water reached a flooding threshold at NOAA tide gauges a record-breaking number of times in the Northeast and Gulf of Mexico due to a combination of active nor’easter and hurricane seasons and sea level rise which has made these events more impactful. The top five cities that saw the highest number of flood days across the U.S. and broke records include Boston, MA; Atlantic City, NJ; Sandy Hook, NJ; Sabine Pass, TX; and Galveston, TX.
The Southeast Atlantic Coast is currently experiencing the fastest rate of increase in annual high tide flood frequencies--more than a 150 percent increase since 2000 at most locations.
In 2018, high tide flood frequencies are predicted to be 60% higher across U.S. coastlines as compared to frequencies typical in 2000, due primarily to local sea level rise. El Niño conditions that are predicted to develop in 2018 may bring even higher sea levels and more wind storms that will increase the reach and frequency of storm surges and high tide flooding.
Along the Northeast Atlantic and Northwest Pacific Coasts (where steeper topography often buffers the extent of impacts), 5-6 days (median values) are predicted, which represents a 100% and 10% greater frequency (median values).
Along the flatter and more-vulnerable Southeast Atlantic and Gulf Coasts, about 2-3 days are predicted in 2018, which represents increases of over 160% (fastest changing region) and about 50%, respectively, since 2000.
Along the Southwest Pacific Coast about 2 days of high tide flooding are predicted (70% increase) with little or no flooding expected (as in all cases, not considering wave runup or rainfall effects) within the Caribbean, Hawaii or U.S. Pacific Islands (Kwajalein Island being an exception).
The West Coast and northern portions of the Northeast Atlantic Coast are most vulnerable in the winter, while other areas will see increased high tide flooding in the fall when mean sea level cycles peak often around the time of highest spring tides or during wind events. NOAA provides a seasonal High Tide Bulletin that provides specific days and areas where seasonally high tides may cause flooding.
An El Nino is increasingly likely during late 2017 and early 2018, which historically increases the number of days with high tide flooding along both the U.S. East and West Coasts.
Based on past trends and the moderately strong El Niño that is predicted to develop, the U.S. East Coast is projected to have the greatest number of days with high tide flooding this fall.
Along the Northeast and Southeast Atlantic and Northwest Pacific Coasts, a 10% median increase in days with high tide flooding is projected due to El Niño (e.g., Norfolk is predicted to have 11 days with El Niño vs. 10 days without it)
Along the Southwest Pacific Coast a 20% median increase in days with high tide flooding is projected due to El Niño (e.g., San Diego has a chance of 3 days without El Niño vs. 4 days with El Niño)
Kwajalein Island, part of the U.S. Marshall Islands is projected to have less flood days with El Niño (4 days with El Niño vs. 7 days without El Niño) since it is in the Western Equatorial Pacific where sea levels drop during El Nino. El Nino typically causes sea levels to rise in the Eastern equatorial Pacific and U.S. West Coast.