Understanding Weather & Passage Planning

21 June 2016

“Based on the forecast, the best window for vessels to depart will be to leave in the early morning hours, near 0300-0400 EDT on the 22nd. This will allow for the best timing between fronts, and to arrive in Cape May on the 23rd before the next front. It will be important for vessels to motor (if needed) to maintain speed in order to arrive in Cape May by midday or so on the 23rd, as the next front with more showers/squalls moves through in the evening.”

That was the advice from our weather forecasters WRI late yesterday afternoon, and with that, the start of Leg 2 was postponed.

There was a mix of disappointment and relief at yesterday’s Skipper’s Briefing. For some, the chance to sail in a brisk southwesterly, downwind towards Cape May in 4-6 foot seas was exactly what they’d signed up for. Indeed, one boat set off on their own this morning - Scarecrow, a Jeanneau 53, one of the more experienced crews in the fleet.

From my perspective as organizer, making a decision that would be best for the fleet as a whole, I primarily was concerned about two things:
  • The potential for the development of strong to severe thunderstorms as the front passed (and at night, to boot).
  • The fact that once offshore, it would be very difficult to return against a 20-knot headwind if any of the fleet had problems.
So let’s look at both of those factors in detail, and examine in general what this forecast means for the actual conditions offshore.

Cold Fronts & Thunderstorm Potential

We had almost the exact same forecast in 2015 when preparing to depart from Portsmouth - a weak cold front would pass over the fleet on the first night at sea, following brisk south-westerlies ahead of it. After a day of fabulous downwind sailing, with a handful of boats even flying spinnakers, the front approached just after sunset, and with it, the strongest thunderstorms of the summer. Lightening flashed all around the fleet and several boats recorded wind gusts in the mid-40s.

It was over in a matter of hours, but was certainly something we’d have avoided if we knew it was coming. And we were lucky - only 50 miles to the north, the wind gusts were topping out at 70 knots. The front page of the Cape May newspaper the next day simply read ‘What the Heck Was That!’

To understand what caused those storms last year, and why the potential exists again this year, we have to look at the weather in the big picture. A cold front is a mass of colder air, usually formed along the leading edge of an approaching low pressure system. Low’s spin counter-clockwise around their center, and in this neck of the woods, typically approach from the midwest and head out to sea to the north.

As the Low and it’s associated front approaches, the wind tends to increase from the southwest ahead of the front. These winds can get rather boisterous, touching high twenties. It’s a great ride if you’re going with it, but if anybody would have to turn back for any reason, they’d have a tough time of it going upwind in strong headwinds, with approaching front on the way to hamper their progress even more.

Early signs of an approaching front are high cirrus clouds, sometimes called ‘mares tails’ or ‘mackeral scales.’ This indicated ice crystals forming high in the atmosphere as the very leading edge of the front pushes up the warmer air ahead of it. Temperatures are hot and humidity is up as the hot air is dragged up from the south. As the cold air behind the front marches along, it pushes the warm air vertically ‘up’ in the atmosphere ahead of it, creating convection, which in turn, creates the potential for rain showers, squalls and thunderstorms. The more convective energy there is, the stronger the squalls and storms. Towering cumulous clouds are a definite real-world hint at approaching squalls. Sometimes they come with rain and wind, sometimes thunder and lightening. The temperature differences between the cold air mass behind the front and the warm air in front of it can dictate the strength of any developing storms. The greater the difference, the more energy involved.

Furthermore, closer the to coast, the potential for even stronger storms exists as the land heats up much faster than the ocean, creating more convective energy and the potential for stronger storms.

The formation of thunderstorms along a cold front is different than that of your typical late-afternoon summertime thunderstorm. In the latter case, as the land heats up in the hot afternoon sun, that creates it’s own convective energy - storms can form more randomly in these cases and are often isolated to a very small localized area. Once the storm moves on, the air remains hot and humid behind it, with no real change in the overall weather.

A frontal passage has more potential to create an entire line of thunderstorms all along the front, as it did last year. That cold air moving behind the front is what’s causing the convection - it’s less localized and more spread out. As the front passes, there will also be a distinct change in the weather - humidity will drop dramatically, the wind will shift into the west-northwest, and the skies will clear.

Finding a Weather Window

All these things and more must be taken into account when planning an offshore passage, whether as an event manager or on your own boat. I look at a variety of sources as well, from NOAA marine weather to shoreside forecasts from places like Weather Underground. And of course, the consultation from WRI. It’s not just windspeed and direction.

In this case, the experience of the fleet was the major deciding factor. If this were the Caribbean 1500, for example, a forecast of thunderstorms would be less of a concern, as the fleet will have dealt with similar conditions previously. Plus, on a long passage you ‘never waste a fair wind,’ as the old saying goes - downwind sailing is always preferable to upwind!

The point of DelMarVa is to encourage people to expand their horizons, not to scare them away. Last year’s overnight thunderstorms were an excellent learning opportunity, in hindsight, but in the moment was a real challenge.

The challenge we have now that the decision to postpone has been made official, is squeaking out in the next good weather window. There is a second cold front approaching from the midwest, extending off a Low that is currently forming over the Great Lakes. Timing-wise, we want to get out immediately after the first front clears the skies and heads offshore, but before the second front sweeps over the region producing similar conditions to the first, with more potential for thunderstorms and a building south-westerly wind later on Thursday.

That window, at the moment at least, looks to be opening up sometime in the wee hours of Wednesday morning. It’s almost exactly 162 miles from marina-to-marina, and timing is key. This means that some boats will leave earlier than others, as the smaller and slower boats will want to get a head start. The goal is to get into Canyon Club Marina by midday on Thursday, ahead of the second front which is forecast to approach later Thursday evening. If you can average 5 knots, for example, that’s about a 33-hour trip, meaning a departure time of 0500 Wednesday would put you in Cape May by 1500 Thursday afternoon, just in time.

Our window looks good - clear skies and cooler temperatures are in the cards for Wednesday, with a nice westerly wind - the shift behind the first front - coming off the DelMarVa peninsula, meaning lower seas (less ‘fetch’ for the seas to build) and excellent reaching conditions. We may get some light and variable conditions Wednesday night as the interim high pressure builds over the region - meaning we’ll encourage the fleet to turn the motor on to keep moving - but by Thursday morning the south-westerlies will be building again as the next front approaches, which should make for great downwind sailing on the last third of the ocean leg into Cape May.

Weather Planning Resources

The following resources are excellent places to get weather and make your own decisions about going offshore in the future:
  • passageweather.com - for wind and wave forecasts - note, this will NOT discuss thunderstorm potential
  • wunderground.com - excellent for shoreside forecasts that focus more on thunderstorm and rain potential, great if you’re planning coastal passages
  • http://www.nws.noaa.gov/om/marine/zone/east/ermzn.htm NOAA’s coastal marine forecasts - again, best for wind and waves, not very specific with regards to thunderstorms and squalls
  • GRIB forecasts - available through saildocs.com, zygrib.org or Pocket Grib for iOS or Android. These offer more advanced big-picture weather patterns, that show pressure gradients and wind direction. They take some getting used to but are invaluable for longer passages. They also do not look at squall activity, but give you an idea on the big picture Highs and Lows that create the weather.
  • wriwx.com - our professional weather forecasters for the rally.