Saturday, August 27, 2011

A benefit of probabilistic surge forecasts - an example from Irene's impact on NYC

Here's a good site for surge predictions from SUNY Stonybrook.  The latest forecast for the Battery shows a good surge of 1-1.5 m above normal level in the early morning on Sunday.  You can see this forecast below where the dashed line is the expected forecast within a grey zone that represents the standard deviation of all the surge predictions at the Battery.  The red trace represents what the observed surge happens to be and the differences between observations and forecasts is represented in green.  Notice that the forecasts so far have been underestimating the observed water level most of the time.

The forecast surge height is one type of forecast but that doesn't tell how high the water will be relative to mean sea level.  Afterall, if the surge hits at low tide then the impact isn't as high.  So Stonybrook offers this second forecast timeseries superimposing the forecast tide on top of the surge and below is that forecast.

The timing appears bad if you don't want a flood.  The peak surge that you saw above happens to coincide with the morning high tide at the Battery yielding a water level 7 to 9 feet above mean low water.  This forecast is confirming why the New York City government placed mandatory evacuations of all low lying areas in their Zone A.  To see where Zone A exists, check out the city government site's interactive map at

What is nice about these forecasts from Stonybrook is the grey zone of uncertainty.  It allows us to plan for reasonable worst case scenarios because forecasts have uncertainty.  But is the zone broad enough?  Will there be a surge higher than the grey zone?  The answer is quite possibly yes but the probability may be somewhat low.

Another site is available that shows the probability at which a surge is forecast to exceed some height, and also the surge heights as a function of probability.  I like the latter because it allows me to set my threshold probability for taking action and then I can see if the surge height exceeds my elevation.  Getting the forecast is easy.  Just go to and then click on the storm (Irene) and storm surge probabilities are listed above.  I choose the probability of a 5' surge and you can see the results below.  The forecast shows a 50% probability of a surge exceeding 5 feet in the New York harbour, even more around Staten Island.

  Of course the problem is how cautious do you want to be?  If you were sitting at 5 feet above the water would you move if the probability of you going under is at least 50%?  That's probably playing it a little too aggressive in my opinion.  Fortunately there's a website related to the one at  the NHC that allows you to dial your own probability.  Go to and select the drop down menus until you find "20% exceedence height".  Now you see that the surge height forecasts are higher, 5 - 7 feet around the Battery to almost nine feet in western Staten Island.

So if you're a little more cautious, like me, perhaps you would dial in your personal probability threshold at 20% and you'll see that if you're below 5-7 feet above the water, you may want to move.  So to put it all together, here's what I'd do to decide for yourself if you need to move.  Note that I'm assuming you have no outside call for evacuation.

1.  Know my altitude above Mean Low Water
2.  Look at the tide forecasts closest to your site at  Click on the forecast point to get a menu and click on 'Tide Predictions'

3.  Determine the height above mean sea level (set at zero) for a set time.  Here I choose 8 am on Aug 28 and I get 5.5 feet.  That's high tide.  

4.  Then look at the probability of exceedence for at least 20%, preferably 10% from  We saw from above that 5-7 feet, so let's choose 7 feet.  

5.  Then add the value you got from 3 and 4.  In this forecast, 12 feet.

6.  Compare what you got from 5 with your altitude above mean low water.  If what you see from 5 is higher, then you make the decision to move.

Better yet, listen to the authorities because I do not espouse taking these steps above as the way to make a critical lifesaving decision when the authorities have so much more information at their disposal.  NYC produced a really nice evacuation map at

BTW, the surge at the Chesapeake Bay Bridge was about four feet at 6 pm today.  That compared to a 20% exceedence of 5-7 feet.  So yes, you may have stayed dry if you were at least 8 feet above the high tide today which was 3.5 feet.  But why take chances?

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