Outer Banks Swell Observations: “Sneaky Swells” – Dr. Jeff Hanson

Wave Man’s Notebook  - October 25, 2015 I spent this past week at the Oceans 2015 conference in Washington DC.  There were not enough discussions on waves, but I needed to catch up with old friends and important colleagues.  I quickly learned that I missed a fun OBX swell event – with 12-14 s swells sneaking in on Thursday and lasting until Friday morning.  Apparently I was not the only one to miss this event; the swell occurrence was completely absent from area surf forecasts, including ours! However, a quick check of our buoy wave system history charts shows that this swell did indeed occur.  OK, so what is a wave system history chart?  Just click on the WAVE SYSTEM HISTORY link above our forecast chart to bring up the latest...

wavehistory Open the Wave System History chart using the link above the forecast graphic.

Our custom Wave System History chart is made from local buoy observations and shows the evolution of our different wave systems over the past 4 days. wavehistorychart

Wave System History chart from Duck Pier buoy on October 22, 2015

There are two parts to the chart; an upper panel shows the swell evolution arrows (vectors), and the lower panel indicates the total wave height and how clean the surf is at any given time.  We realize that these charts are not for the faint at heart!  We have designed them for the dedicated surf junkies among us.  Let me explain how to read them… Upper Panel – Vector chart:    The upper panel provides the time history of swells for the past 4 days up to the present, as recorded by the buoy.  Dates are provided along the bottom of this panel.  The data for the current date is all the way to the right side of the graph.  The arrows represent the evolution of swells from a given wind source.  Black arrows represent wind swell, color arrows represent ground swell systems.  Wave height is represented by the length of the arrows, with the length scale given by the white arrows in the top left.  Wave period is represented by the origin of each arrow, using the scale to the far left. Wave direction is simply the direction the arrows are pointing, relative to the compass in the upper right. Lower Panel – Surf condition chart:  The lower panel shows the overall wave height over the past 4 days, using the same time scale as the upper panel and the wave height scale on the left. Furthermore this chart is broken up into time segments showing how clean the surf is, based on the presence of wind swell, mixed swells, or pure ground swell. Getting back to our sneaky swells, notice the arrows from Thursday 22 October, contained within the black oval on the Wave System History Chart? These swells, missed by the forecasts, came in at the 12-14 s period.  They grew in height and persisted into Friday morning.  One just has to ask, where did they come from?  Well, we have just the tools to answer that… We use the concept of ‘wave dispersion’ to estimate the source time and travel distance of observed swells. Wave dispersion simply describes how, in deep water, longer period waves travel faster than shorter period waves.  Combining wave dispersion with our measured wave directions allows us to back-trace from the measurement location to the estimated source of the swell.  The results of applying this swell tracking algorithm on the 22 October swell appears on the map below. To reach the Outer Banks, the 12-14 s period swells traveled from the central North Atlantic a total of 1095 miles over a 2-3 day period.

SwellGenerationEstimated Source of 12-14s OBX Swell on Thursday 22 October 2015.

OK, I suspect you are wondering about the accuracy of this!  How can we expect you to believe that the swell originated from some dot we plotted in the middle of the ocean? At this point we turn to official weather charts from our estimated time and location of swell generation, to see if there was a storm event that could have produced the swell.  Our estimated swell generation date was October 20, 2015.  Have a look at the official NOAA Atlantic weather chart for this day.

weatherserviceNorth Atlantic Weather Chart for October 20, 2015.

The computer-generated NOAA weather charts can be tricky to read – so we have provided some annotations to guide you to the important stuff.  The location of the Outer Banks along the US coastline is indicated by the small red circle marked OBX.  The large red circle shows a low pressure system, which rapidly developed and then dissipated, on October 20, 2015.  In fact, this low is not evident on the charts from 19 or 21 October.  It was a rapid-fire one-day event on the 20th.  Our estimated swell source location from above falls directly within this circle, confirming that our sneaky swells were produced by a sneaky North Atlantic low with a very short lifespan.  The quick evolution of this storm explains the absence of these swells from our local surf forecasts. So before you decide there are no waves for you today, check out our buoy Wave System History charts for the presence of any swell events that sneaked past the forecast models! Jeff Hanson WaveForce Technologies / AcuSea