Dunes, Fetch and the Power of Water
[Monday November 19 update: new photo taken today on Outer Banks added at the end of this entry.]
The past few weeks have been a blur, with Hurricane/Superstorm Sandy and its aftermath and additional stormy weather since then. I’ve been wanting to write a blog entry with my reflections on Sandy but haven’t had time; I will endeavor to do so but meanwhile there’s now another East Coast system coming up.
To follow up on the post by Dr. Masters a couple evenings ago which included a photo from the Outer Banks, here is that image compared with a Google Maps street view one at the same vantage point from 2007. Notice, in addition to the road damage done by Sandy, the huge protective sand dunes that are now completely gone!
I’m not sure how much of that extreme erosion was from Sandy, or other hurricanes in the past five years, or just a general combination of occasional strong onshore flows and sea level rise; regardless the present situation is that Route 12, which was breached along with the barrier island itself by Isabel and Irene, and is the only road access between the northern Banks (Nags Head, Kitty Hawk) and Hatteras, is hanging on by a thread near Rodanthe.
Last week’s nor’easter (the storm that TWC named Athena) and then an onshore flow earlier this week following a cold front have slowed post-Sandy recovery efforts on the Outer Banks. What's seen in that photo above taken earlier this week involved tides which were near normal astronomical levels, and those levels were only slightly elevated in association with the new moon.
So the setup wasn't anything particularly anomalous, yet with no protection whatsoever from dunes the water came right over sandbags onto the road, and through the pilings under the oceanfront houses.
Now, another weather system is coming with another brisk onshore flow pointed toward the Outer Banks and thereabouts. That we’re confident of. It will create high surf and a rise in water above normal astronomical levels.
Computer forecast models have been vacillating in the specifics, though, in the latest saga of the ECMWF (“European”) vs. GFS (American model). First the ECMWF was threatening, showing two troughs phasing together for a powerful storm directly affecting the coast from the Mid-Atlantic to New England, while the GFS (American model) had the second trough kicking the first one, and the primary cyclone, far out to sea. The ECMWF came around to that; then, in a role reversal, yesterday the GFS started showing a cyclone developing much closer to the coast, not from a phasing but from the second (upstream) trough sharpening up, and now the ECMWF is coming around toward that too.
This is all yet another example of how while these days confidence can be high in the generalities of the pattern quite far in advance, there is typically inherent uncertainty in the details beyond the first few days.
A key ingredient to the evolution next week is a core of jet stream energy still waaay upstream over the Pacific near the Aleutians, not even having reached the domain of the dense network of upper air observations over North America which are ingested into model "initializations."
300 millibar (approximately jet stream level) wind speed contours overlaid on an infrared satellite image; the Aleutians are located in the northern part of the map.
While recognizing such uncertainties, in the interest of examining the latest information available and noting a possible scenario here is a look at the WAVEWATCH III model forecast of significant wave heights (average of the top 1/3) based on the latest (as of this posting Friday afternoon) GFS model wind forecast, showing a modest increase aimed toward the southeast U.S. coast including North Carolina this weekend into early next week as the pressure gradient tightens up and winds increase.
Then there’s a more significant increase in wave size Wednesday into Thanksgiving, with the highest offshore but some of that wave energy reaching the coast, especially the Outer Banks which stick out farthest to the east.
Credit: NOAA (data), The Weather Channel (graphic)
For wave generation, what’s important is the fetch of wind and components thereof: its velocity, length and duration. The upcoming one will be blowing across a long distance, and for a long time. Velocity-wise, the pressure in the developing cyclone is not indicated to be too low, and the high pressure system to the north which will be over New England this weekend will weaken and move off to the east; thus the pressure gradient and wind speeds won’t be too strong unless the storm’s central pressure ends up being much lower than what models are currently showing.
Those initial ECMWF model runs evoked some aspects of the meteorological situation present during the March 1962 Ash Wednesday storm (see graphic below), the benchmark for Mid-Atlantic coastal flooding from non-tropical nor’easters. Such an outcome needed to be considered plausible given the remarkable accuracy of ECMWF model forecasts for Sandy approximately the same time (~eight days) in advance; fortunately the latest scenario portrayed by models is not nearly as potent, including for beaches in places such as New Jersey and New York which are still reeling from Sandy. Hopefully future model runs won't portray a more threatening trend.
Image credits: Hans Rosendal and Art Cooperman, Mariners Weather Log (1962, left); WSI (ECMWF model forecast, right)
In regard to that vulnerable situation on the Outer Banks, the magnitude of effects upon Route 12 and beachfront properties in Rodanthe and elsewhere will depend on how the meteorological and oceanic details of the upcoming situation play out. But even if major impacts from this system are dodged, there will be other storms and strong onshore flows on the Outer Banks and elsewhere, and the storm surge and waves and erosion from Sandy and other storms represent the perilous convergence of people & buildings with the inexorable power of water.
A web page maintained by the Mirlo Beach Home Owners Association begins thusly:
“Mirlo Beach is a family-oriented community on the outer banks of North Carolina located at the north end of the village of Rodanthe -- the gateway to Hatteras Island. It is a beautiful place to live and vacation. Right now, however, we are simply trying to survive!”
Update Monday November 19
The long, persistent, brisk ENE onshore flow which developed on schedule this weekend has resulted in a new round of closures of Route 12 (sometimes partially passable near times of low astronomical tide, then closed again as high tide approaches). The photo below was taken today, showing the ocean pouring across the road at Mirlo Beach.
The fetch/waves might abate a bit Tuesday into Wednesday, then will likely increase again by Thanksgiving as the new system referenced in the original blog entry develops. Then as that gets kicked out by this weekend the wind flow will finally turn around and blow offshore.
Image credit: North Carolina Department of Transportation
Updated: 9:30 PM GMT on November 19, 2012
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