17 NC counties and 1 VA county are under alert, including Halifax, Edgecombe, Mecklenburg, VA, and Northampton counties. Details
Published: 2007-06-19 09:29:41
Updated: 2007-06-19 09:29:41
Posted June 19, 2007
MIKE MOSS SAYS: Marty, You sent in your question on June 14, 2007, and when I did some poking around, it appears there were several factors that would have pushed tides above typical levels for that period. First, there was a new moon coming up on June 15, meaning that tides were approaching "spring tide" ranges due to the alignment of the sun, moon and earth all along a more or less single direction, which increases the magnitude of tides. This is a predictable effect. Similarly predictable is the cycle of the moon's distance from earth. When it reaches "perigee," the closest points in its orbit around the earth, its gravitational effect on tides is greater. There was a perigee on June 12th, so you had combined effects of the moon being both at the correct angle with the earth and sun for enhancing tides, and at a close approach to the earth, enough by themselves to produce a large tidal range.
The less predictable wild card was the occurence of several days of modest to fairly strong north-northeasterly winds along the coast. Because of a phenomenon called "Ekman Transport," in which the net movement of a fairly deep layer of water is at a 90-degree angle to the right of the wind direction, this persistent northeasterly wind was pushing water toward the west-northwest, which, given the orientation of the NC coastline, piles the water up not unlike a mild version of "storm surge." This resulted in tides that were above predicted levels for a few days, since long range tide predictions can account for regularly occuring astronomical factors like the moon and sun, but not for transient weather systems.