What Does 12.3 Foot Tide at Boston Mean?

We always talk about tide heights, but what does that number mean?
At 11:48 am Thursday October 27, 2011 the astronomical high tide at Boston Massachusetts is 12.3 feet. What is this number in relation to? That is a question I have been meaning to get to the bottom of for a couple decades now. Today is the day.
Eleanor Vallier-Talbot of the National Weather Service Office in Taunton taught me the definition.
Eleanor says that number is the distance of the water level at high tide relative to Mean Lower Low Water. Yeah.. ok.
That’s helpful, but whats Mean Lower Low Water?
Eleanor’s Colleague at NWS Taunton, Nicole Belk, sent a follow up note.
Thank you Eleanor & Nicole.
Here is a photo of the Boston Tide Gauge. Does anyone know where it is? I’ll Find it tomorrow.

I am tide chasing today, tonight, and tomorrow. (Snow chasing too, if given opportunity here south of Boston.)
The High Tide at Boston Tomorrow is 12.4′, the highest for the year.
We have an onshore wind gusting to 30 mph today, but offshore tomorrow, so flooding ‘should be’ minor.
Here is link to potential flooding in and around southern New England.. and the note from Nicole follows.


Hello- thank you very much for the footage of coastal flooding in the Scituate area(from Sept 29, 2011 new moon tide). I took a look at our database and although the Boston Harbor tide gage did not go to flood stage at the end of September, the Scituate tide gage did- just over its 12 foot flood stage level.

My understanding is that you had a question regarding the definition of Mean Lower Low Water? Every day on our coastal waters, we typically have 2 high tides, and 2 low tides. One high/low tide is typically higher than the other high/low tide. Zero feet MLLW is the average tide level that occurs during the lower of the 2 low tides. So when tide forecasts are displayed in AHPS, and displayed by default on the NOS Tides and Currents web site, the observed and predicted water levels are in feet with respect to MLLW.

I am forwarding you a couple of links to check out- this first one has the definitions of various datums as they relate to tide gages:
You’ll note that on this web site’s definition of MLLW, it refers to an average over a National Tidal Datum Epoch. According to the NOS web site for Boston Harbor (copied below), the last epoch was calculated for the period of 1983-2001.

This second web site is detailed information for the Boston Harbor tide gage. Note that at the bottom of the page, last sentence in “Location”, the exact location of the tide gage is mentioned:
http://www.tidesandcurrents.noaa.gov/station_info.shtml?stn=8443970 Boston, MA

If you have additional questions, feel free to e-mail me or call the office. If your questions are beyond what I can answer, I’ll find an appropriate contact (probably at the NOAA National Ocean Service or National Geodetic Survey) who can provide you additional information. Next week I’m out of the office, but Frank Nocera (cc’d on this message) will be able to help you out or direct you to the appropriate NOAA agency if you have any questions at that point.


Thank you Nicole!

4 pm Update, Thursday 10/27/11
Here is Video of Today’s Minor Flooding in Scituate


What Does 12.3 Foot Tide at Boston Mean? — 4 Comments

  1. Tim – My friend Larry and I flyfish for Striped Bass by first looking up the tide charts. A strong out-going tide seems to result in the best fishing for us. Through the years we have been fascinated by the variety and strengths of our New England tides. Thanks for the stories and info on your site! Lyle

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