NWS Boston - Coastal Flood Threat and Inundation Mapping

Inundation Extent at 10 ft MLLW

Total Water Level Forecast

Inundation Overlay
Total Water Level (MLLW)
18 ft

4 ft

What is...?

...Total Water Level?

Astronomical Tide
+ Surge
= Total Water Level

Note: Surge is the departure from the astronomical tide and can be positive or negative.

...A Velocity Zone?
Areas along coasts subject to inundation by the 1-percent-annual-chance flood event with additional hazards associated with storm-induced waves (>3 Feet).

...MLLW?
Mean Lower Low Water (MLLW) is a tidal datum referenced by the National Weather Service for forecasting coastal flooding. MLLW represents the observed average of the lower low water height of each tidal day. For more information on tidal datums click here.

Coastal Inundation Mapping Scenarios Disclaimer

This product is based on the creation of inundation polygons and depth grids from static water surface elevations. The layers assume no components of near shore wave modeling. The depths associated with each layer were calculated using a 5 meter resolution digital elevation model for the pilot study area. Not to be used for navigation or in place of official National Weather Service flood warning and watch forecasts.

NOTE: During times when the area is under a Tropical Storm/Hurricane Watch or Warning, the storm tide or total water level forecast will reflect a plausible worst case scenario (versus the usual most likely scenario).