This weekend, a full moon will occur in conjunction with perigee (the point at which the moon is closest to the Earth). While this occurence is unusual, it is not expected to result in extreme or abnormally high and low tides.
A rare lunar/solar alignment will occur this weekend, March 19-20. This event will coincide with lunar perigee, the point when the moon is closest to Earth. This year's perigee is notable because the moon will be closer to Earth than it has been since 1993.
While these events are rare, NOAA's tide experts from the Center for Operational Oceanographic Products and Services predict that U.S coastal areas will not experience extreme or abnormally high and low tides.
Here's why. The fact that the perigee event on March 19 is one in which the moon is slightly closer to the Earth than normal adds only minimally to an increased tidal range. The timing with monthly equatorial tides and the Vernal Equinox (defined below) on Mar. 20 also do not add to increased ranges of tide during this event. Finally, monthly mean sea levels for most U.S. coasts are lower in the spring season than in the fall, and this is also contributing to predicted tides not being extremely high for this event.
Given these factors, the predicted tide elevations over the coming weekend will not be extreme in relationship to other similar Perigean-Spring Tides this year. Predicted elevations over the coming weekend are not even expected to be the highest or lowest in 2011 (see the graph in the above image).
Perigee: The monthly orbital path of the moon around the Earth is irregular—it's not a circle with the Earth at the center. The actual path is an elliptical in shape and the Earth is not at the center of the ellipse. Perigee occurs at the time at which the moon is closest to the Earth in the monthly orbit. Apogee occurs at the time when the moon is furthest away. Tide-producing forces due to the moon are greater during perigee than at apogee because the moon is closer to Earth. The average distance of the moon from the Earth is 384,401 kilometers (238,856 miles). The moon during the perigee on March 19 will be 356,577 km (221,566 m) from the Earth.
Full moon: Full moon occurs when the Earth, moon, and sun are in alignment each lunar month, with the moon opposite the Earth from the sun. New moon occurs in a similar alignment each month, but when the moon is in between the sun and the Earth. The times of full and new moon are often referred to as Spring Tides, or times of syzygy. 'Neap Tides' occur at the first and third quarter moons when the moon is a right angles with respect to the Earth and sun. Tide-producing forces are greater during Spring Tides than at Neap Tides because the lunar and solar tide-producing forces are in alignment.
Equatorial Tides: Tides occurring semi-monthly as the result of the moon being over the equator. At times of equatorial tides, the moon is at minimum declination causing minimums in the lunar diurnal tide producing forces. This lunar diurnal component of tide producing force is maximum during the times of maximum north and south declination each month.
Vernal Equinox: The plane of the Earth’s equator is not the same plane as the Earth’s yearly orbit around the sun (the Ecliptic). There is maximum declination at the summer and winter solstices and minimum declination at the Vernal and Autumnal Equinoxes (that is the two times each year that the Earth’s equatorial plane is aligned with the plane of the Ecliptic). The Vernal Equinox, also known as the first day of the spring season each year, occurs on March 20, just one day after the Perigean-Spring Tides on Mar. 19. This yearly declination cycle affects the solar diurnal (once per day) component of the tide-producing forces with the solar diurnal tides being greater during the solstices.