Friday, May 16, 2008

2008 Hurricane Season

May 15th is the official start of the East Pacific Hurricane season…or what I sometimes like to call “special happy time” but more frequently refer to it as “a royal pain in the ass for surf forecasters”. In celebration of the start of the season I thought that I would throw together a little info on East Pacific Hurricanes and how they affect the surf in Southern California.



Unless you have been living under a rock in the Himalayas (that doesn’t have cable or satellite TV) you probably have at least an idea of what a hurricane is…so I won’t spend a ton of time going over the storm itself…here is the official version from the National Hurricane Center…

“The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation

Tropical cyclones with maximum sustained surface winds of less than 34 knots are called "tropical depressions" Once the tropical cyclone reaches winds of at least 34 knots they are typically called a "tropical storm" and assigned a name. From there when winds reach 64-knots then they called “hurricanes” (or cyclones or typhoons…depending on what geographic region you are in).”

The official version is a little dry considering they are trying to describe potentially one of the most destructive releases of latent heat energy that can occur in our atmosphere…but hey that is government for you…I am sure that they could describe a nuclear explosion in a way that would make you fall asleep after the third paragraph.

For this little lesson…first I am going to throw down a little geography and terminology, because that is the way that I roll, and it will help our conversation about surf make more sense later, particularly when we start dealing with active storms. Anyway here is a little of the geography…

Our region is a little “special”
When you look around the world there are generally 7 areas that have consistent cyclone activity but our special little corner in the east North Pacific actually boasts an extra-bonus feature…our storms have a tendency to move away from land and generally pose less of a danger to life/limb/and property. Don’t get me wrong…they are still ass-kicking weather systems and will sometimes spin back and wreak havoc through Mexico and Central America...but they are quite a bit less likely to do so than the other tropical regions.

This is sort of a catch-22 from a surf standpoint…hurricanes have a tendency to send the strongest swell along their movement path, which means that while we don’t have storms make landfall, (and come along and stick it in sideways like they do to the East Coast sometimes), we also don’t always get the best swell that we could from these systems.




The Layout
From a geographic standpoint the East Pacific tropical region runs from the west Coast of Central America, down to the equator, and then out to the 140W longitude line. The northern border is a little more flexible since storms have a tendency to die off as they hit cold water.



The incubator
The Gulf of Tehuantepec and the surrounding areas have a tendency to be the storm incubator of the EPAC tropics. The mix of coastal geography, local wind patterns, and extremely warm water provide a good catalyst for storm formation. In most tropical seasons you can track a number of storms back to this little caldron that is located down along the coast of southern Mainland Mexico.



The ITCZ
The ITCZ is short for “Intertropical Convergence Zone”, which is good that they gave it an acronym because writing the full term gets old fast. The ITCZ is an area where two different cells of air circulation meet along the surface of the ocean and push skyward. This convergence has a tendency to create a band of thunderstorms and tropical systems that are the early stages of tropical waves (and eventually full tropical cyclones). Anyway the ITCZ is more of a fluid entity than a fixed weather feature…oh it always exists…but it can move around and change intensity as different factors influence it. The ITCZ is important in the fact that it provides a low-wind zone for tropical storms to start their cyclonic rotation without being disrupted. From a forecast standpoint it is important to keep track of the ITCZ…the further north it drifts the better of a chance you have a storm formation…to close to the equator and you lose the Coriolis Effect.



Sea Surface Temps
When it comes to forecasting surf from hurricanes it is always important to keep an eye on sea-surface temperatures. Hurricanes need some very specific conditions to form and maintain circulation. One of the biggest factors is the ocean temps. The general rule of thumb is that a storm needs sea-surface temps to be at least 80-degrees Fahrenheit (or around 27-degrees Celsius)...and that temperature needs to extend down about 50-meters below the ocean surface. The storm doesn't actually draw that much energy from the water but it is more about the water temperature's effect on the air-mass directly above it. (this sort of gives me a headache...so I leave the heavy mental lifting to the NHC's big brains)

Due to limitations of public satellites it is hard to get a read on ocean temps beyond what you can see on surface but you can sort of guestimate where the pockets of storm potential water is.

Once a storm has moved out of the warmer waters it starts to lose power as its convection fails. This also means a lot of the storms winds start to lift up off the surface of the ocean and swell production is cut off, which is obviously a good thing to keep an eye on if you are trying to score surf from a hurricane.

Upper level steerage and sheering winds
On some levels hurricanes are actually pretty fragile weather systems...we already talked about their need for warm water/airmass which affects them from a surface standpoint...well they also need specific conditions to occur in the upper level of the atmosphere in order to start circulation. In particular they need some light/moderate winds blowing through the higher altitudes to sort of spark up the circulation that eventually becomes the full-scale cyclone convection. If these winds are too light then the storm won't spin up. The adverse is true as well...if the winds are too strong they will shear the top of the storm off, breaking the balance needed to maintain rotation.

One thing to watch is how the large scale wind patterns are moving through the tropical region...sometimes a storm will start in a favorable area only to move into a region that has more wind moving in the upper levels and it will begin to shear...and again once the storm starts to unbalance the surface winds get disrupted and swell production is shut down.

La Nina and El Nino (its Spanish for "the nino"!)
You hear a lot about El Nino/La Nina patterns in connection to hurricanes...and they do have a strong influence on the season as a whole...but it is good to think of it in terms of "potential" rather than a guaranteed stellar surf season. Both of these patterns represent the difference in SST's for the East Pacific region. An El Nino year means that the SST's are above average and the La Nina means that they are below average. The thing to keep in mind that there is quite a difference between an El Nino that is 0.5 degrees warmer than average compared to an El Nino that is a whopping 4+ degrees warmer. The amount of energy that it takes to heat a large area of the ocean is staggering and the more heat that is poured into it the more energy it will have to release later to equalize itself.

Here is the official definition from the Climate Prediction Center...

El Niño - El Niño, a phase of ENSO, is a periodic warming of surface ocean waters in the eastern tropical Pacific along with a shift in convection in the western Pacific further east than the climatological average. These conditions affect weather patterns around the world. El Niño episodes occur roughly every four-to-five years and can last up to 12-to-18 months. The preliminary CPC definition of El Niño is a phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the 1971-2000 base period), averaged over three months, greater than or equal in magnitude to 0.5oC in a region defined by 120oW-170oW and 5oN-5oS (commonly referred to as Niño 3.4). El Niño, which would appear off the coast of Peru around Christmas time, is Spanish for "the boy" referring to the Christ child.

Basically when you break it down...an El Nino year means warmer water in the East Pacific Tropical region...more warm water means more potential hurricanes. La Nina means cooler water and less potential hurricanes…(Funny thing about that is the opposite is actually true for the US East Coast. El Nino means a less active tropical season while a La Nina means a more intense one. Damn can’t they make anything easy.)

And finally...this picture sums up the El Nino perfectly...



Ok enough science...lets get on to the surf.
Does your head hurt...yeah mine too...lets talk about the fun stuff...the surf that a hurricane can kick out.



Hurricane surf is pretty special...it has a tendency to be punchy, stacked up, and at the right spots...really hollow. It also can sneak in from swell angles that we could never get from normal southern hemi storms, which in turn lets it hit spots that are normally small and sheltered.

If you have been surfing for a few years you probably have a hurricane surf story...everyone does...they sort of go "remember hurricane 'blank'? Man I surfed this longboard-spot/reef/harbor-entrance/evil-lair-point-break...it was like double overhead and reeling!"

The thing is that hurricane swells feel magical...they usually hit over the summer when the water is warm and clear and the swell is so consistent that you surf until your arms want to fall off. When you start thinking about Hurricane swells in the terms of the waves that you will surf of course you want to score more and more of it.

Getting Hurricane Surf
When you get down to brass tacks hurricanes are similar, but smaller, versions of mid and upper latitude storms...so the same principles apply to them when it comes time for them to generate swell.

You may want to brush up on how waves are made...you can read my barely coherent ramblings on that here.

Here are the basic things that you need to keep in mind when forecasting surf from hurricanes.

Storm Size - The bigger the storm the larger the fetch, the more fetch the more potential swell.

Intensity - The greater the wind speeds the bigger the surf...generally goes hand in hand with storm size since the bigger storms have a tendency to have more intense wind speeds than the smaller systems.

Movement track - You want a storm to be moving toward you. Hurricanes that are moving are sort of like flashlights...the swell is sent out the along the movement track. So the longer the storm is moving toward your location the more swell (and bigger waves) you can expect.

Movement Speed - This is tied to movement track...a storm can move too fast and sort of outrun the swell it is producing, which limits both the quality and quantity. A slower moving storm will have time to build a better sea-state, which lets it build a better swell. A perfectly paced storm will actually create a "travelling fetch" which will let the storm pour more energy into waves that it has already created...basically building the swell without the need for faster winds.

Storm Track vs Storm Speed
Ideally if you want to get great hurricane surf you want the storm to be moving toward you at a slow to moderate pace. Now actually getting a hurricane to head your direction isn't all that probable...it is sort of like herding cats, (well if the cat was 300 miles across, didn’t respond to the slightest human stimuli, and could smash everything that you care about into tiny pieces…then yeah it would be just like that…where was I? oh storm movement) this is where good storm speed can help compensate for a bad storm track.

If we go back to the flashlight analogy...the faster a storm moves the tighter the beam of swell becomes. If a storm is stalled or moving at a very slow rate it is sending out energy in all directions but as the storm speed picks up the swell energy tightens along the movement path. This image is a good illustration of what I mean.



You can still get swell from a hurricane even if it isn't headed directly towards your location but the system needs to be moving at the correct speed in regard to your position.

These are good rules of thumb concerning storm speed.

1. The Storm is moving away from your location: You want the speed to below 2-knots
2. Storm is moving along a path 90-degrees perpendicular to your location: You want the speed to below 5-7 knots (depends on the storm size...a wider storm can be moving a touch faster).
3. Storm generally toward your location: You want the movement speed to be below 8-10 knots.
4. Storm is coming to punch your ticket and drive up your homeowners insurance: Then you can have storm speed up to 15-17 knots and still get swell...any faster and the system starts to outrun the swell energy…and probably your ability to escape to safety. (Man aren’t I cheery today)

Seasonal expectations
While you can get hurricane swell at almost any time during the tropical season... Southern California definitely sees better hurricane swell activity as we move through the middle to end of the season. The reason for this is more due to the nature of the seasonal wind patterns than anything.

During the "Early Season", which runs from the spring into early summer, tropical systems have a tendency to track straight from east-to-west and move out into the open ocean.



As you get into the middle of the season, which is summer into early fall, the storms start to make a slight jog northward and eventually hook back toward land. This hook starts to line up the movement track with SoCal making it more likely for us to get waves.



Finally at the end of the season most storms are performing the “hook” sometimes right after they have formed. This is one of the more dangerous times to be along the Pacific side of Baja since storms can spin back toward land relatively quickly. They can cause a lot of damage even making landfall as a tropical wave or depression…remember there is a lot of dry land in baja that can’t hold a lot of water…so even a couple of inches of rain (or say the 30 inches a tropical storm can drop in a short period) can cause major flooding.



Naturally you want to be somewhere in the middle/end of the season where the storms have a chance of "aiming" towards SoCal and the Pacific side of Baja.

Swell Windows and Swell Directions
Swell windows are pretty darn important to Hurricane swells...more so than the bigger frontal systems. First off a hurricane is a smaller storm so the fetch is narrower and the swell is more focused...this compounded by the fact that hurricane swells generally have shorter swell-periods which don't wrap around corners as well as long-period swells. So if the storm is out of your swell window you are sort of out of luck...I have seen swells where the angle of the swell will cut off surf like a knife-blade as you move a 1/4 mile up a beach.

Generally Orange County and LA have the most SE’erly swell windows and can take in swells from 155-160 degrees…some of the other spots like north San Diego, Santa Barbara, and Ventura start seeing swell around 170. Other very protected areas like the South Bay need a more traditional SW swell around 200 degrees (which isn’t all that common for a tropical systems.

My half-assed opinion on the 2008 tropical season.
Ok this post is way to freaking long...so if you made it all the way down to this part then you are probably at work, in class, or prison...somewhere where you have a lot of time on your hands.

Anyway I will try and sum up quickly...looking the long-range climate data from the CPC and the NHC it looks we may have a slower than average season. The CPC (Climate Prediction Center) is calling for weak La Nina condition for the EPAC for May, June, and July, which means less warm water available for storm production. Dr Grey over at the University of Colorado is calling for a stronger season in the Atlantic which usually indicates a slower season in the Pacific as well. They do expect the La Nina to weaken later in the summer, potentially increasing tropical storm activity as we head into late summer and early fall.

Personally, and my opinion of long-range climate models isn't really work all that much...I would basically expect a slow start to the tropical storm season, less major hurricanes and fewer named storms for the next few months. As we reach late summer this should turn around and become more active...setting up better swell potential as we head into the second half of the tropical season.

That is all I got for now...make sure to keep checking the blog I will probably talk about each tropical system as it forms throughout the season...either that or I will drink more this summer...I would say the odds are about 50/50 right now.

My favorite Hurricane Links
Here are a few of my favorite links to hurricane related stuff...I will drop these on my link list as well.

Climate Prediction Center
http://www.cpc.ncep.noaa.gov/

National Hurricane Center
http://www.nhc.noaa.gov/

NHC Satellites
http://www.nhc.noaa.gov/satellite.shtml

Navy Research Laboratory Monterey
http://www.nrlmry.navy.mil/tc_pages/tc_home.html

University of Hawaii - Tropical Page
http://www.solar.ifa.hawaii.edu/Tropical/

FNMOC Ensemble Forecasts (good place to see upper level winds)
https://www.fnmoc.navy.mil/efs/efs.html

NOAA Sea-Surface Temps
http://www.osdpd.noaa.gov/PSB/EPS/SST/contour.html

5 comments:

Danimal said...

Damn. Very thorough.

Much appreciated.

omar said...

Excellent, thanks man!!

Anonymous said...

Magical = Hurricane Linda- 1997

Anonymous said...

i wasn't a surfer in '97, but i sure as hell remember when those waves hit vta

Anonymous said...

yes magical = Linda & Guerrmo....'97....