Thursday, February 19, 2015

Breaking Down the Weather - Tampa Bay Edition

I try to pick one topic each year to learn a lot about (at least a lot more than I knew previously).  This year it's sail design.  Last year it was foils and everything underwater.  Two years ago it was weather and how it affects our type of sailboat racing.

The result of that is a set of tools that are fairly simple and allow you to have a good idea of what will happen and the order of changes to expect, though not necessarily timing.  For MWE Mike & I will provide a daily weather report each morning on what to expect that day based on the tools we have.

Since I will be racing in Tampa bay this weekend, I'm going to introduce the tools we use, provide a daily briefing for our J24 Midwinters event, and update it with a reflection based on expectations.

Necessary Information
A few key factors go into the matrix we use to determine what should happen with the weather.  The items we will use are:

  • Is there heated land (by sun) within 2 miles to windward of where you are sailing?
  • What is the air temperature?
  • What is the water temperature?
  • Are you in a high pressure, low pressure, or straight isobar system?
  • Are you in a gradient breeze, a sea breeze, or a funneling breeze?
  • Is it light air (under 6 knots) or breeze (6 knots and over)?
By answering these questions and plugging them into our matrix we get a simple expectation for the day.  Here is the matrix:

The Stability Index
Least Stable (1)
Neutral (2)
Most Stable (3)
Convergence or Subsidence
Low Dominant
Straight Isobars
High Dominant
Boundary Layer Thickness
Gradient wind
Sea Breeze
Funneling Wind
Surface Heating
Air Cooler than Surface
No Difference
Air Warmer than Surface
You score each row and add up the sum.  With that sum you look up the expected outcome from this list:

Scoring: 1 for Least Stable, 2 for Neutral, 3 for Most Stable
Score 3-4
  • Expect major unpredictable changes in wind speed & direction
  • Assume no prediction is possible, sail for pressure
Score 5-7
  • If air colder than water expect modest & unpredictable changes in direction & speed
  • If air warmer than water expect modest & unpredictable changes in direction w/ little change in speed
  • If some regular veers or backs occur expect them to break into irregularity - stay defensive
Score 8-9
  • Expect uniform wind speed
  • Expect pattern to be predictable
  • If there is land heated by the air and/or sun within 2 miles the gust mechanism will kick in and cause unpredictable and often harsh puffs with shifts
  • If there is land within 2 miles of your position the land is effecting your air. It will tend to shift towards more perpendicular as it gets closer to shore.
  • If you are sailing in light air (less than 6) then these rules do not apply - sail for pressure ALWAYS
  • A gradient breeze often has shift intervals longer than a leg (20-60 minutes), which causes them to appear as persistent shifts. Don't take that as gospel next time around.
  • In a sea breeze the shifts are short (2-6 minutes) so you can attack them more aggressively.

Let's look at Tampa Bay today, February 19.
  • Air temp: 52 degrees
  • Water temp: 64 degrees
  • Wind type: Breeze (20+)
  • Breeze Type: Gradient
  • Pressure System: High Pressure
The result is High Pressure (3), Gradient (1), Air Cooler than Water (1), which is a score of 5. That score says:

5: If air colder than water expect modest & unpredictable changes in direction & speed

So what does that mean? The breeze should be up and down and back and forth, but not with extremes. Expect some gusts to be harsh and be prepared to make big adjustments quickly.

This is just a small part of the weather analysis we do, so please ask Mike Ingham or myself if you have questions about the content or would like more thorough explanations. We'll see how well it holds true over the next 3 days on Tampa Bay....

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