Bay Sea Conditions
Chuck Tribolet, firstname.lastname@example.org
© Copyright, Chuck Tribolet, 1997, 1998, 2001, 2004
since March 4, 2002
Chuck's Tongue in Cheek Guide to Swell Heights8' - Typical for Monterey. May be diveable, but I'd rather sort slides.
10' - Go only if you've already paid for the boat. And they MAY cancel.
12' - Stay out of the water.
14' - Stay off the beach. Not uncommon in winter.
16' - Stay off Highway One. Occurs a couple of times each winter.
And on the good end:
6' - Passable Diving.
4' - May be worth a vacation day to dive midweek. Not uncommon. Four foot days are why I check these resources every day.
2' - Quit your job. Go diving. (The lowest I've ever
seen is 3').
Don't forget that swell direction is important. Remember a few years
ago when Santa Cruz was getting hammered by southerly swells from a hurricane
off Tahiti, and the TV news had tape of a fishing boat getting capsized
in the mouth of Santa Cruz harbor? Monterey Bay was flat — all that granite
was protecting the bay. But watch out for "northwest". If it's a bit north
of true northwest, Ano Nuevo protects Monterey Bay. And if it's a bit south
of true northwest, Pt. Pinos protects the bay. But if it's truly northwest,
there's no protection at all, and the Breakwater can look like Monastery.
Actually, this is really only a little bit tongue in
offshore Coastal and Marine forecast is the first thing I check.
a northwest swell, and 8' or less, I go diving. If it's a west
and 10' or less, I go diving. If it's a southwest swell (VERY
go diving. Simple, right? Note that your thresholds may
vary: I'm a photographer, and surge makes photography difficult, so
maybe I'm a tougher grader than some. And some folks can only
dive occasionally, and want to chery-pick the very best days.
But above all, these resources aren't perfect. They only advise when
to take your gear to the beach. When you get there, look at the conditions,
and decide whether to dive there that day. There are a fair number
of days when the best action is not to dive in the ocean but rather into
a pitcher of beer, pizza, and dive talk.
By popular demand, here are examples of some memorable days in Monterey,
and how to interpret the information available. I've shown a very
good day, and a very bad day. There's a whole range in between, and
each person's skill level will determine where their "don't dive" line
is. Over time you'll learn what's worth driving to Monterey for,
and when to stay home. Most importantly: these resources can help
you decide when to drive to Monterey, but the decision to dive shouldn't
be made until you are on the beach, and have watched the waves for a while. There are many times and places
to dive elsewhere or elsewhen.
The Times, They Are a' Changin'
Since I captured these examples, some changes have taken place. The
WAM model examples on this page are in Interactive Data Language (IDL)
4.0 format. IDL 5.0 is now used, and there are a number of changes:
In addition, the model is also available in Gridded Analysis and Display
System (GrADS) format. The differences from IDL 4.0 are:
The aesthetics of the images are greatly improved. They are quite
pretty and might be considered art by some.
The wave height scale is now continuous as opposed to having discrete colors
for 3-6, 6-9, etc. feet. And it's in meters. Three feet is
about one meter, OK? There are also no contour lines. I find
this makes it harder to read the wave height. I'd really like to
see continuous AND contour lines.
The arrows are bigger. I think they are about the right size.
Because of the continuous colors, the images don't compress as well, and
are about 3x bigger (90K vs. 30K).
The basic concepts are the same, and until I get another really good/bad
day with the new formats, I'm going to leave these examples in place.
A different progression of discrete colors is used for wave heights.
Light blue is good diving. Darker blue is OK diving. They are
still in feet for us Luddite Americans.
Some of the colors, esp. 3-6 and 6-9 feet, are too close together.
The arrows are bigger. I think they are about the right size.
I usually use the GrADS format.
Why Don't the Models Agree?
Well, first and foremost, forecasting is an extremely inexact science.
It has only recently gotten so that the models are more accurate than "tomorrow
will be just like today." If you ever wonder why all these supercomputers
can't do a better job of forecasting, read a book on Chaos Theory.
I recommend James Gleick, "Chaos", ISBN 0-14-00.9250-0.
Measurement error. You try measuring swell heights sometime.
Lawsuit prevention. It seems to me that the forecasts tend
to be pessimistic about the size of the forecasted swells because they
are unlikely to be named as a party in a suit if the swells are smaller
Different definitions. Some measurements specify that 15%
of the swells will be bigger than the prediction, and 15% will be smaller.
Others take the mean of the largest third of the swells. It only
take one really big one you weren't expecting to scare the stuffing out
Different point of measurement. Buoy 46042 is in one place.
"Pigeon Pt. to Pt. Piedras Blancas" is large area (which happens to contain
46042). The "three-day model" is done by taking the WAM forecast
for a single point, and applying the diffractions due to underwater topography
and shoreline reflections. That single point is farther out than
When Are the Various Models Updated?
The Coastal and Marine Forecast is updated four times daily (3:00 a.m.,
9:00 a.m., 3:00 p.m., and 9:00 p.m.) and appears
on the web shortly thereafter. Occasionally updates are made
The WAM model is usually updated twice daily, usually around noon and
The Monterey Bay Swell model is updated hourly, and usually lags real
time by two to four hours.
The Experimental Three-Day Forecast and the Offshore Wave Height Forecast
appear to be updated twice daily, but I haven't managed to puzzle out just
A Note on Swell Intervals
The longer the interval, the deeper the swells reach. The shorter
the interval, the nastier the ride in a small boat. I remember one
day on the Pinnacles when we'd had a nice flat ride down and there were
SW swells that were only about two feet high, but they had a real long
intervals (18+ seconds), and I couldn't get under the surge. The
only saving grace is that with the long interval, I could get a picture
or two off during the calm between the surges. On the other hand,
I remember another day with really short interval (six seconds) swells
of 6-8 feet. I'd decided I wasn't going to try to get down to Carmel
Bay before I even got to the red mile buoy. The ride was like a roller
coaster. But sites like Eric's and George's (Aumentos) were quite
diveable and had no surge.
If you would like to learn more about all this, see Willard Bascom,
and Beaches, Anchor Books, 1980, ISBN 0-385-14844-5. Amazon.com
lists it as "hard to find", but it's worth a shot. Bascom did a lot
of the basic surveys of California beaches in the late 1940's using a DUKW,
which is a big amphibious truck used by the military. The book includes
a picture of Bascom surfing the DUKW at Carmel River State Beach on a day
when I sure wouldn't dive. He's standing up taking depth measurements
with a lead line. The book is worth the price just for that picture.
Conditions Today Don't Mean Anything Tomorrow
In fact, conditions this morning don't mean anything this afternoon.
Conditions are highly variable. Just because it's nice today doesn't
mean it will be nice tomorrow. It could just as easily be bad tomorrow.
Conditions can go from flat glassy to very nasty in a couple of hours.
One morning I ran my whaler down to Carmel Bay in about 30 minutes under
quite nice conditions. We did a first dive a MonoLobo Wall.
During the surface interval, things started to pick up a bit. I did
a second dive while my girlfriend sacked out on the bow locker. She
was awakened by a wave breaking over the bow. We had an hour and a
half run home at a fast idle. It was the nastiest small boat conditions
Stay alert to the short-term trends in conditions.
Conditions Here Don't Mean Anything There
Conditions at home don't necessary reflect what's happening in Monterey.
First the microclimates are different.
Secondly, most of what's happening in the ocean is an effect of the
above-water conditions thousands of miles away and several days ago.
A REALLY Good Day for Monterey Divers
First, let's look at a truly wonderful day. Monday, October 27, 1997.
I dove North Monastery Beach that day. The BIG sets were about 12
inches high. There was 40' vis and no surge in the shallows, and
some friends diving the wall reported 50' vis deep. It was well worth
taking a half day of vacation.
Since the WAM model provides the earliest warning of impending good
conditions, let's look at it first. You are looking at a map (slightly
distorted in the northern regions because of the projection) of the Pacific
Ocean. (Note: I added the place names with PhotoShop to help the
navigationally impaired. They don't appear in the real WAM model.)
Dark Blue is good diving. Light Blue is OK diving. Note the
little arrows that show the direction of the swell. Now find San
Francisco Bay (it shows, just barely). Note that the swells are coming
from the WSW or SW, which means that Pt. Lobos will shelter Monastery (and
the Monterey Peninsula will shelter the bay, but why dive the bay when
Monastery is a walk-in, walk-out dive).
The "From:" time is when the model was run. The "Valid:" time
is when the model thinks the ocean will look like the picture. The
times are GMT, aka Zulu, which is 8 hours (7 hours during daylight savings
time) ahead of Pacific time. 12Z is 4 a.m. Pacific Time the same
day. 00Z is 4 p.m. Pacific Time the PREVIOUS day. That means
this picture was generated at 4 p.m. 26 Oct. 1997 Pacific Time, and represents
the ocean at 4 a.m. 27 Oct. 1997 Pacific Time.
I was watching the WAM model and knew a couple of days ahead that I'd
probably want to go diving on
Monday. Here's what it looked like:
Now it's Monday morning. To dive or not to dive. Let's look
at the Coastal and Marine Forecast.
Pigeon Point is half way between Half Moon Bay and Santa Cruz.
Point Piedras Blancas is by Hearst Castle.
Swells are 3' West! Let's go diving! But first let's checkout
the current buoy report on the Monterey Bay Swell Model. First, please
note that the color codes are not the same as on the WAM model and that
their definitions of wave height seem to disagree a bit. But
blue is still good, and there's lots of blue on the map; so that's real
NATIONAL WEATHER SERVICE [Image]
SAN FRANCISCO BAY AREA
TTAA00 KSFO 271043
COASTAL MARINE FORECAST
NATIONAL WEATHER SERVICE SAN FRANCISCO
300 AM PST MON OCT 27 1997
PIGEON POINT TO POINT PIEDRAS BLANCAS OUT 20 NM
PIGEON POINT TO POINT PIEDRAS BLANCAS 20-60 NM OFFSHORE
300 AM PST MON OCT 27 1997
.TODAY...LIGHT WIND BECOMING NW 15 KT IN THE AFTERNOON. WIND WAVES
2 FT. SWELL SW 3 FT.
.TONIGHT...WIND NW 10 KT. WIND WAVES 1 FT. SWELL W 3 FT.
.TUESDAY...WIND NW 15 KT. WIND WAVES 2 FT. SWELL W 4 FT.
Now lets look at some of the less obvious things here:
The "bulls eye" (well, two thirds of one) shows where the wave energy is
coming from and the period of the waves. The color coding is NOT
the same as used on the map part. Note that most of the energy is
coming from the SSE! Wow, protection from Pt. Sur. This is
getting better and better. The distance from the center of the bulls
eye is determined by the wave period (time between waves). The outer
edge is about eight seconds, the inner edge is about 24 seconds.
I don't find wave period to be very useful.
In the box with the numbers, "Hs" is the "significant height', which is
the average height of the highest third of the waves and is in feet.
"Tp" is "typical period", the average period of the waves. "Dp" is
the direction the waves are coming from. I find the bulls eye chart
much more useful than this text box.
The little magenta cross is the location of the buoy. It's about
20 miles out.
A REALLY Bad Day for Monterey Divers
Saturday, November 15, 1997. The Nikon Underwater Photography Tournament
is today. The swell reports on the web are really ugly. I'm
staying home and writing this article instead of taking my Nikonos to Monterey
and trying to win fame, fortune, and some cool prizes.
Again, we'll look at the WAM model first again. All that turquoise
(9' to 12' swells) is not good news, but the pale green (12' to 15') and
bright green (15' to 18') are even worse. This does not look good.
The only glimmer of hope is that the swells are from the West, and MAYBE
there's a little shelter in the bay. Copper Roof is probably getting
There's a little skinny band of blue just a couple of pixels wide right
along the coast. If you have young eyes or have a real good monitor,
and can see it, ignore it.
Here's the Coastal and Marine Forecast. It's just as ugly.
Now lets look at the Buoy Report:
TTAA00 KSFO 151250
COASTAL MARINE FORECAST--UPDATED CONDITIONS TODAY
NATIONAL WEATHER SERVICE SAN FRANCISCO
300 AM PST SAT NOV 15 1997
PIGEON POINT TO POINT PIEDRAS BLANCAS OUT 20 NM--UPDATED
450 AM PST SAT NOV 15 1997
...HEAVY SURF ADVISORY...
...SMALL CRAFT ADVISORY...
.TODAY...SWELL W 17 FT. WIND S 25 KT GUSTS TO 35 KT. WIND WAVES 4 FT.
RAIN...TURNING TO SHOWERS AND ISOLATED THUNDERSTORMS IN THE
.TONIGHT...SWELL W 15 FT. WIND SW 20 KT. WIND WAVES 3 FT. SCATTERED
SHOWERS AND THUNDERSTORMS.
.SUNDAY...SWELL W 13 FT. WIND W 10 TO 15 KT. WIND WAVES 2 TO 3 FT.
Note that the scale has changed to accommodate the big swells.
Red now means 16-18' not 8'-9'. The good news is that this looks
better than it did yesterday when it was ALL 16 feet or more.
Want to see what it looks like right now? Go to: http://www.garlic.com/~triblet/swell/wamglance.html.
For information about Internet resources for the Bay Area diver, see http://www.garlic.com/~triblet/swell/.
For information about using GPS for diving in Monterey, see http://www.garlic.com/~triblet/swell/gps.html.
For information about the Northern California Underwater Photographic Society
(NCUPS), see http://www.ncups.org.
Last updated: 06:23 PM, Tuesday, February 28, 2006.
Webmaster: Chuck Tribolet, email@example.com