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Huge swell and wind waves may produce flooding. Add that to the perigean tide, and we’re likely to see some very high water. Randy Bush gives the details:

As I was getting ready for work this morning, some clown on the news was hopping around on a jetty near L.A., spouting prognostications of dire flooding and coastal destruction. He smugly informed us that a “perigean tide” was to be the cause of this impending cataclysm. As sailors, we’re supposed to know about all things tidal, so my curiosity was piqued. Then I made the mistake of communicating on this subject with Capt. Dan, who promptly sent me some reference material and gave me the assignment to write this. Once a teacher, always a teacher, I guess.

So in defense of my 105 sticker, here goes: Right now we are experiencing a “spring tide,” which coincides with the full moon and the new moon. The centers of the sun, earth, and moon all form a straight line, and the gravitational forces of the sun and moon complement each other. Thus, the spring tides exhibit the greatest tidal oscillation, or the difference in height between the high and low tides. When the sun and moon form a right angle with respect to the earth (first and third quarters of the moon), their gravitational effects tend to cancel each other, and “neap tides” occur, where the height difference between the high and low tides is significantly less.

OK, so what about this “perigean tide?” It’s simply the tide that occurs when the moon is at its perigee, or lowest point in its orbit around the earth. Since the moon is closer to the earth, we would expect its gravitational pull to be stronger, which it is. But it’s really only significant when it coincides with a spring tide. Then we have the greatest tidal range possible from a purely astronomical standpoint, the “perigean spring tide.” This is precisely what is occurring right now.

But our TV reporter has overstated the importance of the perigean component. If you do the math (and I’ll take NOAA’s word for this), the proximity of the moon has much less of an effect than the celestial alignment mentioned previously. A perigean tide does increase the tidal range, but only by a few inches in most cases. Aw shucks, Mr. reporter, no tsunami-like event for the news cameras.

But we do have a threat of coastal flooding this weekend, not so much because of the perigean tide, but the combination of the approaching weather front and the current spring tide.

For more info on tide studies, see Chapt 9 of Bowditch American Practical Navigator