Questions and Answers

Here are some of the questions from our readers regarding the WetSand WaveCast Guide to Surf Forecasting, and some answers from the author and Chief Forecaster for WetSand WaveCast, Nathan Todd Cool.

If you have a question you’d like to ask Nathan, please send an e-mail to


Questions:


Q: In chapter 11, you calculated hurricane Hernan to have 70-foot swell heights, resulting in 22 face heights in SoCal. Was it really this big?

A: I admit that was an extreme measurement. But on Sep. 4, breaks like “the wedge” were seeing faces in the 15-20 foot zone, yet many breaks saw much less. Looking through the NHC archives, the NWS reported swell height measurements of 57 feet, with some models estimating swell heights over 60 feet. Most models will go off the scale at 45 feet, making it difficult to use many of the WAM’s. But that’s also an indicator for potentially heavy, damaging surf. So I like to overestimate the size in these cases. I mentioned briefly in the Hernan example that you may want to use a 50-foot measurement since the course of the hurricane can shift rather quickly; thus, making it better to use a 4-hour duration (50 feet) instead of the 8 hour (70 feet). Also, the 70-foot estimate used a 40% loss for angular spreading, yet as mentioned in chapter 8, the 70-degree angle could have resulted in only a 30% loss. Using the 57-foot estimate from the NWS, this would result in 16.5-foot face heights for a 40% loss of angular spreading, and 19.2-foot face heights for a 30% loss.


Q: Wasn’t Hernan a category 5 hurricane?

A: Yes, but for only about 6-10 hours. Also, in the book I gauged the forecast from Sep. 2, although Hernan actually reached a cat 5 status on the Sep. 1. But during this time (on the 1st), Hernan was located around 110W, just reaching the swell window for Southern California. By the 2nd, Hernan was near 115W, a more ideal swell window for Southern California, and had an average wind speed of 120 kt. By the end of the day on the 2nd, Hernan had dropped its winds to only about 100 kt. This is all hindsight now as well :) When we were tracking Hernan, we had to gauge the estimates on 48 hour projections, which had a 28 kt margin of error—this per archival reports from the NHC. For more information on Hernan, you can check out the NHC archive at http://www.nhc.noaa.gov/2002hernan.shtml Also, there are some really cool satellite photos by NASA, which you can see by clicking here.


Q: Is that really your last name [Cool]?

A: Yes :) I kid you not, I was born with it. Back in the mid 90s when I started WaveCast, I got some media attention from the Associated Press, LA Daily News, and the Ventura County Star, since WaveCast was a breakthrough “web surfing” site (in the literal sense). I had a hard time convincing the reporters that Cool was my real name, and I didn’t make it up. And my Dad, his Dad, and his Dad, having the same last name didn’t make it up either. :) The best we can see from our lineage is that coming from the old world, our ancestors may have spelled it Kuhl, or Kule; but when asked at the US port of entry, immigration officers may have used the English rendition: Cool.


Q: You mentioned the Coriolis Effect a few times, but you didn’t go in depth. Can you elaborate?

A: I’ll give it a shot. :) When talking about hurricanes and large mid-latitude storms, the Coriolis Effect causes the air to rotate around a low pressure center in the same direction as the underlying Earth. So the air flowing around a low-pressure system, like a hurricane, spins counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere (as does the Earth, itself). Many people refer to this as the reason water spirals in a draining sink or flushing toilet in opposite directions, depending on which hemisphere you are in. But this is not really true. Sinks and toilets typically are too small to detect the weak force of Coriolis. Also, note that Coriolis is near zero at the equator, and stronger at increasing latitudes.


Q: In chapter 5 you mentioned a pattern for Santa Ana winds. Can you show an example?

A: Absolutely. There are archives available from http://virga.sfsu.edu, and I've drawn an example of one pattern you can see by clicking here. Note the high pressure located inland in northern Nevada, and the low pressure system located off the California coast. As the wind barbs show, the wind is blowing in an offshore pattern.


Q: My question isn't listed here.

A: Please send an email to and ask us your question.


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