Bell's Beer Bayview Mackinac Race
Managing Your Mac Weather Forecast

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Introduction
Experienced Mac sailors understand that developing a pre-race weather forecast and monitoring its evolution on the racecourse is an important strategic and safety aspect of participating in the Bell’s Beer Bayview Mackinac Race. Misjudging the wind forecast can quickly send you to the back of your fleet, while failing to properly assess the risk for severe weather can unnecessarily jeopardize the safety of the crew.

This article introduces several resources for developing your pre-race forecast, as well as, the more challenging task of monitoring development weather conditions after the starting gun. A companion web page containing hyperlinks to the resources presented in this article can be found here.

The Big Picture

Large-Scale Dynamics
Your Mac forecast should begin with an understanding of the large-scale weather features. Is a low pressure system expected to transit Lake Huron during the race? If so, when is the warm and/or cold front and accompanying wind shift expected to arrive? Is the system expected to produce strong winds, large waves, or thunderstorms? Or, will the Mac be dominated by the fluky, thermally-driven lake and land breezes associated with a high pressure system?

NOAA’s Weather Prediction Center (WPC) (click here) produces a variety of products that focus on predicting the development and evolution of large-scale weather features. The WPC’s surface forecasts are divided into short-term and long-term periods with the short-term period covering the first 2 ½ days of the forecast period (in 12-hour intervals) and the long-term period covering days three through seven at 24-hour intervals. By carefully reviewing the forecasts in chronological order, you can determine how large-scale weather features will affect the racecourse, and then use the data to chart the fastest course to Mackinac.

Graphics published by the WPC contain a variety of unique symbols and meteorological shorthand to show the anticipated position of high pressure systems, low pressure systems and their accompanying frontal boundaries, and additional features such as ridges, troughs and squall lines.  Sailors who are unfamiliar with this meteorological shorthand are encouraged to review the legend published by the WPC (click here).

Weather Prediction Center Surface Forecast

WPC products, along with the majority of weather forecasting materials available on the Internet, are based on Greenwich Mean Time (GMT) sometimes called Zulu Time (Z) or Universal Time Coordinate (UTC). Converting from GMT to Eastern Time during the summer is straightforward – simply subtract four hours from GMT. Below is the legend for a 24-hour surface forecast which provides an opportunity to review the GMT conversion process and other nomenclature used to describe the graphics.

WPC Surface Forecast Legend

The first line contains the acronyms for the various NOAA entities that contributed to the preparation of the forecast. The second and third lines describe the parameters that are included in the forecast product. The term Issued in the fourth line provides the time and date the graphic was published. In this example, the graphic was published at 1605Z or 12:05 am Eastern (1605Z minus 4 hours) on May 13, 2013. The fifth line indicates when the forecast is Valid, which is the date and time for which the forecast was prepared. The surface forecast shown above displays the surface weather features as they were expected to be at 1200Z / 8:00 am Central time on Tuesday, May 14, 2013.

Precipitation
The WPC also issues Quantitative Precipitation Forecast Graphics (QPF) (click here) which show the amount of liquid precipitation anticipated across the country. This product covers the first three days of the forecast period in 24-hour increments, and combines days four through five and six through seven into two separate 48-hour forecasts.


Thunderstorms

Storm Prediction Center
Thunderstorms, and the associated hazards of downburst winds, lightning, and locally higher wave heights, are a relatively common occurrence across the Great Lakes during the summer. Forecasting the location, timing and type of severe weather that is expected to develop across the United States is one of meteorology’s most challenging tasks – a task assigned to the staff of NOAA’s Storm Prediction Center (SPC) in Norman, Oklahoma.

It is important to understand that the NWS defines a severe thunderstorm as one that produces 1” diameter hail, a wind gust of at least 58 mph (50 knots), or a tornado. Although frequent lightning is an obvious hazard to sailors, it is not a factor in meeting the threshold of a severe thunderstorm.

SPC Convective Outlooks
Each day, the SPC issues Convective Outlooks for days 1, 2, and 3, and a combined Outlook for days 4 through 8 (click here). The Outlooks contain a graphic that identifies the areas of the country where organized storm development is expected, along with a narrative describing the details of the risk.

The Convective Outlooks present the risk by category – Marginal, Slight, Moderate and High – based upon the statistical probability of severe weather occurring within 25 miles of any given point in the highlighted area. A description of the risk categories used by the SPC can be found on its FAQ page (click here)


Because the risk of severe weather affecting a specific location on any given day is very small, even a Slight risk deserves a sailor’s respect, as it represents a significant increase in the potential for severe weather. It should be noted that even thunderstorms that fail to meet the NWS’s severe threshold often present a significant risk to mariners in the form of strong winds and frequent lightning.


In addition to the traditional Convective Outlooks, the SPC also publishes Thunderstorm Outlooks (click here) which identify the regions at risk for any strength of thunderstorms and the probability of development. In contrast to the one-day period covered by Convective Outlooks, each Thunderstorm Outlook covers only a 4-hour period and they are not issued beyond the current day.


Hazardous Weather Outlooks
In contrast to the Convective Outlooks from the SPC, Hazardous Weather Outlooks (HWO) are issued in text form by each local office of the National Weather Service (NWS) and describe the risk of severe weather for the general public. HWOs are issued each day and address the overall probability, geographic coverage, storm type (single cell storms, squall line, etc.), and timing of severe weather for the current day along with a combined summary of days 2 through 7.

While SPC Convective Outlooks cover the entire United States, Hazardous Weather Outlooks focus exclusively on the County Warning Area (CWA) assigned to each NWS office. There are two NWS offices with CWAs that cover a portion of Lake Huron – Detroit, MI and Gaylord, MI. In order to assess the risk of severe weather where you intend to sail, you must consult the HWO of the NWS office which has responsibility for that area of the lake. Click here for a map showing NWS responsibilities for Lake Huron.

MARINE FORECASTS

Text Forecasts
The Detroit NWS office is responsible for issuing off-shore (beyond 5 nautical miles) marine forecasts for all of Lake Huron. Several times each day, NWS Detroit publishes a text version of the marine forecast which begins with a synopsis of the current weather pattern and a forecast covering the next several days. This synopsis is followed by a forecast of wind and wave conditions covering a five-day period. Because conditions often vary dramatically across the expanse of Lake Huron, the text wind and wave forecast focuses on pre-determined regions that allow mariners to quickly obtain the forecast for their area. A chart showing these regions may be found here.

Graphic Forecast Products from the NWS
The NWS offices surrounding the Great Lakes cooperatively maintain an excellent website devoted to marine weather forecasting (click here). While the home page provides forecasts for the entire Great Lakes basin, the data for an individual lake can be accessed by clicking the appropriate hyperlink along the left-hand column of the home page. The graphics, which are designed for the recreational boating community, are easy to interpret and don’t require a conversion from GMT to local time.

The forecasts extend for 4 ½ days in three-hour increments for the following meteorological parameters:

Other Graphic Marine Forecast Products
Gridded Binary files (GRIBs) pack a lot of information into a small file size and are a convenient method of obtaining marine forecasts. Routing software, such as Expedition, while relatively expensive, integrate navigation, yacht performance and GRIB weather data into a single race-management interface.

There are many sources of computer model wind and wave forecasting resources on the Internet. While offering the advantages of higher resolution and shorter time intervals, these products require conversion from GMT to local time and are published without modification by a meteorologist. Several of these products are available on the BYC Mac Resource page (click here).

A Forecast Plan For The Mac
The month preceding the race is the perfect time to begin investigating and getting comfortable with the resources presented in this article and on the Mac resource web page (click here). There is no better way to learn a new skill than practicing. On each Wednesday in the month preceding the race, I encourage you to prepare a trial forecast for the upcoming weekend. In addition to analyzing the graphics and making the necessary GMT conversions, these practice forecasts will allow you to witness how high and low pressure systems evolve and how the speed and direction of the wind changes in response.

The accuracy of weather forecasts diminishes as the forecast period increases. For example, a forecast valid in 48 hours is typically more accurate than one valid in 7 days. For this reason, beginning the preparation of your official Mac forecast more than a week before the start isn’t recommended.

I typically start a daily review of the WPC surface forecasts and SPC Convective Outlooks a week before an event and will often save the graphics in a folder to compare to later forecasts. This allows me to track the movement of the large scale weather systems across the country and gauge the changes in the forecast from day to day. I typically begin a daily analysis of the wind and wave forecasts on Monday or Tuesday for a weekend event.

The final forecast should be based upon the most current forecast data available and is usually prepared the morning of the race. If you have been analyzing the forecast for the preceding week and using the data to steadily hone in on your strategy, the creation of your final forecast shouldn’t be too time-consuming. And hopefully it won’t reveal any surprises.

MANAGING WEATHER CONDITIONS AFTER THE START


Managing the forecast and keeping tabs on rapidly deteriorating conditions becomes far more difficult once you are offshore. Monitoring the evolution of weather patterns and reviewing updated forecast data after the race has started can help you adjust your strategy based upon whether an off-shore or in-shore route holds the best chance for a steady breeze. In addition to aiding your strategic goals, checking the weather regularly and remaining apprised of National Weather Service watches and warnings will spare you from being surprised by thunderstorms, a relatively common occurrence across the Great Lakes during July.

ACCESS TO DATA

Introduction
There is no shortage of weather data on the Internet. The sheer volume of data, combined with the ease of access, may contribute to a case of ‘data overload’ in the days preceding the Mac. However, once you are on the racecourse and your trusty Wi-Fi connection is no longer available, you will likely find yourself quickly transitioning from ‘data overload’ to ‘data deprivation’. There are technological solutions to address your data deprivation, but each possesses its own set of issues and problems.

VHF Radios
VHF radios are a reliable and relatively inexpensive method of obtaining basic weather information. Modern marine VHF radios contain a “WX” button used to select the seven channels (WX1 to WX7) dedicated to receiving weather observations and forecasts.

Through its NOAA Weather Radio (NWR) program, the National Weather Service (NWS) continuously transmits NWS general and marine forecasts, regional weather observations, and severe weather watches and warnings in the VHF range. The NWR transmissions are automatically rendered from typed text to audio and, unless interrupted by warnings of severe weather, cycle repeatedly through information of interest to mariners and non-mariners, these include:

Because this information is presented in a continuous loop, you may have to listen to the broadcast for several minutes or more until it returns to the marine-related portion of the program. I find that it is helpful to take notes as you listen to the forecast.

Several NWR transmitters, each operating on an assigned frequency so as not to interfere with nearby stations, serve the waters of Lake Huron. The range of each transmitter is approximately 25 miles, however, this range may be diminished by the height or placement of the antennae, operational integrity of the station’s equipment, or weather conditions. The distance at which you are able to receive a NWR broadcast is also dependent upon the equipment onboard your vessel. The height and quality of your antenna and condition of the coaxial cable and ancillary fittings connecting it to your fixed VHF radio all influence the quality of your reception. Handheld VHF radios have a much shorter effective range than fixed units.

There are two NWS offices with County Warning Areas (CWA) covering a portion of Lake Huron -- -- Detroit, MI and Gaylord, MI. Each NWS office is responsible for preparing the weather information that is broadcast by the NWRs located in their CWA. The marine observations and forecasts you hear on your VHF radio are specifically prepared for the portion of Lake Huron falling within the NWS’s warning area.


Since the NWR system broadcasts weather information for a specific portion of the Lake, and the operational limit of the transmission is approximately 25 miles, you will need to select the WX channel associated with the NWR transmitter closest to your location. This is particularly important if a severe thunderstorm prompts the issuance of a watch or warning for a portion of the Lake. A map showing the location and associated WX channel of the NWR transmitters serving Lake Huron can be found here, while a map showing the marine zones for each NWS office is located here.

Cellular Internet
Although your VHF radio is a reliable source of weather information, it does not provide the ability to access Doppler radar or the wide array of weather forecast graphics discussed above. Accessing the Internet or creating a Wi-Fi hotspot using a cellular connection, provides access to the weather resources and Doppler radar, but cellular access more than 10 miles offshore is typically unreliable. It is possible to improve cellular reception by installing an external antennae and signal booster; however this technology only improves an existing signal and therefore provides limited value in areas where shore-based cellular access is spotty – such as northern Lake Huron.

Cellular Internet should not be your primary source of weather information. However, when it is available, Internet access provides the ability to view Doppler radar imagery – a particularly useful resource if severe thunderstorms are expected to impact the racecourse.

Satellite Weather Data
Accessing weather information via XM satellite offers many advantages over VHF and cellular, but at a relatively high cost. A typical XM setup requires a satellite antennae, receiver, a display device (such as laptop or chartplotter), and a monthly subscription from a provider of meteorological data. The cost of the antennae, receiver and software typically exceeds $1,000, and data subscriptions can reach $50 per month.

If cost isn’t a barrier, satellite technology avoids the broadcast range limitations of both VHF and cellular Internet and assures reliable, consistent access to weather data – including Doppler radar -- regardless of your location on Lake Huron. You’ll be limited to the products and resources offered by your data provider, but this limitation is more than outweighed by reliable access.

MISCELLANEOUS TOPICS

Barometers
An accurate barometer, particularly a digital version, is a relatively-inexpensive, low-tech method of monitoring large-scale weather patterns and the potential for severe weather. Typically, falling barometric pressure, particularly rapidly-falling barometric pressure, announces the approach of a squall line or cluster of thunderstorms. Your on-board barometer may provide your first clue that the atmosphere is brewing some excitement. (Check out my article Feeling The Pressure: The Value of A Barometer by clicking here.)

National Weather Service Terminology
The National Weather Service uses a variety of terms to simplify the forecast process and warn the public about the potential for hazardous weather. Misunderstanding this terminology may lead to poor decision-making that places you and your crew at increased risk during inclement weather. Several terms of interest to the marine community are presented below:

Doppler Radar
There is no better tool for tracking the development and movement of thunderstorms than Doppler radar from the NWS. The NWS maintains the only nationwide network of radar stations and freely shares the raw data and imagery with the public. The stations are strategically located to ensure that all areas are served and to allow overlap in the event that a station fails. There are two NWS stations collecting data over Lake Huron – Detroit, MI and Gaylord, MI.  

A few important facts about radar imagery:

 

Have A Safe And Speedy Trip To The Island!


Contact Information
Mark A. Thornton
LakeErieWX@gmail.com