Potential swell for 2/19/09: Current calculations

[nathant...@gmail.com]

After posting my SoCal surf forecast on WetSand today, it didn't take
long for inquiries to come in regarding the potential swell event I
mentioned for the 18th or 19th. While trying my darnedest not to sound
the hype alarm, I did want to share what I'm seeing on the models, and
how I'm calculating it all out. Your input would be warmly welcomed.

First, the forecast is based off the extreme long range models. Here
are a few I archived:

FNMOC 168H
http://wavecast.com/wams/archive/ww3_b_npac_sig_wav_ht_168.gif

NOAA WW3 150H
http://wavecast.com/wams/archive/enp_f150h.gif

NOAA WW3 168H Near Shore
http://wavecast.com/wams/archive/enp_wc_f168h.gif


Nothing is more dubious than a long range model, especially when we're
talking about anything past 144 hours -- and these are 150 - 168 hours
out. Still, all the models seem to agree on something sizable coming
in.

So here's how I'm calculating it:

1. Distance. Looking at the NOAA WW3, 168h near shore, the leading
edge of the largest area of fetch would be located around 35N, 135W,
which from SoCal, is about 850 miles. You can calculate this as well
at:
http://wavecast.com/calculator/

2. This distance, given 16-second periods, would have the swell arrive
in 1.5 days, or sometime around the afternoon of the 18th, or morning
of the 19th.

3. The distance also would result in about 70% decay. The seas in the
fetch are shown to be 11-12 meters, or 33-36 feet. A 70% decay would
mean about 10-12 feet once the seas near the SoCal coast.

4. For SoCal, there would be some angular decay since this system is
coming in from the NW. Angled about 20-30 degrees from true west (290
degree for swell), we're looking at another 15% loss, taking the
minimum seas from 10 feet down to about 8 feet.

5. Periods would likely be long, about 16 seconds. This would provide
a shoaling factor of approximately 1.6, which, when multiplied against
the decayed seas, results in 13-foot face heights, minimum, and about
15 feet maximum.

I can't stress enough that this is all based on extreme long range
models. This could all change tomorrow. For now though, this is what
the models are saying.

Comments, questions, or observations?

-Nathan

[Nathan]

Just a quick update: this morning's models have dramatically
downgraded this system from a 36+ foot fetch to one that's only likely
to be 25 feet within 800-900 miles from SoCal. It's a stark contrast
on the models in just 24 hours, but, such is the nature of working
with such a long range simulation.

I've added this as well to the latest SwellScan video forecast at:
http://SwellScan.com

I'll take another look at Thursday's AM models, and see how this
looks. I'll be updating the SoCal WetSand report then as well.

-Nathan