A Little Too Much Excitement On Narragansett Bay
Introduction
Thunderstorms and sailboat racing don’t mix. Sometimes the rain from a thunderstorm results in a prolonged period of light and variable winds, or perhaps no wind at all. At other times, a storm wreaks havoc on the racecourse by producing incredibly strong winds, blinding rain, and steep waves. Neither version promotes finishing a race in a fast or safe manner. Sailors competing in the final race of the Jamestown Yacht Club’s (JYC) Summer Series experienced the “furious winds” version of a thunderstorm when a cold front rolled over Narragansett Bay (Rhode Island) on Tuesday, August 25, 2020. Before we delve into the details of that storm, let’s cover a couple of thunderstorm basics.
A Little Background
During their mature phase, all thunderstorms have an updraft and downdraft (Figure 1). The updraft sustains the storm by delivering warm, moist air to its upper levels. The speed of an updraft – a measure of the storm’s strength – is determined by the temperature difference between the warmer air inside the updraft and the cooler air surrounding it. It is not uncommon for an updraft to reach 100 mph. The larger the temperature difference, the faster the air ascends within the updraft.
A storm’s downdraft is the descending flow of rain-cooled air. In contrast to an updraft, air within a downdraft is colder and denser than the surrounding air. Downdrafts form when precipitation particles such as water droplets, ice crystals, and hail become too heavy to remain suspended by the storm’s updraft and begin to fall. As they fall, some of the particles evaporate before reaching the ground, which further cools and increases the density of the air within the downdraft. Since the speed of a downdraft is determined by the difference in temperature between the air within the downdraft and the surrounding air, evaporational cooling can dramatically increase the speed and destructive potential of the downdraft.
The National Weather Service (NWS) does not have an official threshold for a downburst, but a downburst is simply a strong downdraft. Downbursts may be further subdivided into microbursts and macrobursts depending on the duration of the strong winds and the size of the affected area. A microburst impacts an area less than 2.5 miles wide for fewer than five minutes. A macroburst affects an area greater than 2.5 miles wide and lasts more than five minutes.
The Downburst Arrives
The long-lived thunderstorm reached the fleet a little after 7:00 pm as many of the racers were approaching the finish line (click here for a map of Newport Harbor). Based on observations from competitors, wind speeds rapidly increased from 5 knots to 50 knots in just a few seconds. A J/109 observed at least 50 knots (the limit of the boat’s anemometer) before dangerously heeling to nearly 90 degrees. The maximum wind observation for the event -- 75.3 knots – was recorded by a J/120 at 7:04 pm. The winds quickly decreased to less than 10 knots a couple of minutes later, but the damage was done.
Since the fleet was actively racing and under full sail, the sudden and unexpected arrival of the downburst caused several boats to be knocked down. Some smaller boats were swamped and at least two boats were sunk. Crew members from many boats slipped overboard during knockdowns or while struggling to lower sails and gain control of the boat. Damage to sails and rigging was widespread. Fortunately, rescue efforts got underway immediately and there were no serious injuries. The relatively protected location of the racecourse and the short duration of the high winds also helped recovery efforts.
The automated weather stations in the area (click here for map) failed to capture the full force of the wind. The storm’s wrath completely missed station QPTR1 (Quonset Point) where the maximum wind during the race was a mere 6.9 knots (click here for observations). A maximum sustained wind of 26.4 knots and gust of 34.4 knots was observed at Station NWPR1 (Newport Harbor) at 7:06 pm (Figure 2). The Newport Tidal Station observed sustained winds of 23.3 knots and gust of 29.9 knots also at 7:06 pm (Figure 3). Both of these stations are approximately 1.25 nm northeast of the course. The wind at nearby Newport State Airport didn’t reach 25 knots during the event. Station NWPR1 and the tidal station also observed the steep drop in air temperature and increase in barometric pressure announcing the downburst's arrival -- click here for the observations from NWPR1 and here for the tidal station. The tidal station also captured a small, but noticeable, increase in water level caused by the downburst (click here).
The failure of the downburst to be observed by an automated weather station is fairly common. The area impacted by a downburst is relatively small and weather stations are widely distributed. Fortunately, many sailboats are essentially floating weather stations, capturing data that allow us to review and learn from high wind events. What are the lessons from the storm on Narragansett Bay?
Forecasting the possible impacts of an approaching thunderstorm with 100% certainty is a challenging task, but there are many readily-available resources that can offer clues regarding a storm’s potential for life-threatening behavior. Such clues can promote shortening sail or returning to the dock prior to a storm’s arrival, and the detective work begins with the forecast.
The Forecast
Assessing the possibility of thunderstorms should always start with a review of the large-scale weather pattern. The Daily Weather Map issued for August 25, 2020 showed a cold front extending from a low pressure system in eastern Canada approaching the northeastern US (Figure 4). The approach and passage of the cold front was expected to promote thunderstorms in the area highlighted by yellow diagonal lines, which included Narragansett Bay. Not all cold fronts produce thunderstorms, but severe weather is frequently associated with the passage of a frontal boundary. A weather-savvy sailor always considers the possibility of thunderstorms and closely monitors conditions whenever a frontal boundary is in the neighborhood.
The Storm Prediction Center (SPC) is a branch of the NWS that specializes in forecasting and monitoring thunderstorms and organized severe weather. The NWS defines a severe thunderstorm as capable of producing a wind gust greater than 50 knots, a hailstone at least 1” in diameter, or a tornado. Severe thunderstorms and organized thunderstorms such as squall lines and convective clusters present an enhanced risk to sailors because of their propensity to produce damaging winds. The likelihood that a thunderstorm will reach severe status increases the longer it persists. The SPC issues a daily Convective Outlook outlining those regions with a potential for severe thunderstorm development. The Convective Outlook for August 25 (Figure 5) placed Narragansett Bay under a Slight (SLGT) risk of severe thunderstorms.
In my opinion, the Slight category downplays the risk of severe weather and increases the likelihood that sailors will diminish or disregard the risk of thunderstorms. In common usage, the word slight suggests that the potential for something to occur is very low or essentially zero. Ironically, the common definition of slight isn’t consistent with its use by the SPC. The SPC’s definition of Slight --“Scattered severe storms possible. Short-lived and/or not widespread, isolated intense storms possible.” – is far more descriptive. (Click here for a chart explaining the SPC Risk Categories.) The bottom line is this – anytime your sailing area is under a Slight risk (or greater) you are at a much higher risk of encountering severe thunderstorms during your outing.
The SPC also issues Thunderstorm Outlooks which outline the probability for the development of any type of thunderstorm (severe and non-severe). Each Outlooks covers a 4-hour period, so six images are required to assess the potential for thunderstorms for a single day. The Thunderstorm Outlooks for August 25 covering the periods from 12:00 pm to 4:00 pm (Figure 6) and 4:00 pm to 8:00 pm (Figure 7) placed Narragansett Bay under a 40% chance of thunderstorm development.
The local NWS forecast office in Boston also publishes products designed to help sailors determine the threat of thunderstorms. The marine forecast for Narragansett Bay issued at 4:16 pm reminded mariners that a severe thunderstorm watch was in effect until 10:00 pm and mentioned the potential for damaging winds.
416 PM EDT Tue Aug 25 2020
...SEVERE THUNDERSTORM WATCH 450 IN EFFECT UNTIL 10 PM EDT THIS EVENING...TONIGHT...NW winds 10 to 15 kt. Gusts up to 20 kt after midnight. Waves 2 to 3 ft. A chance of showers and thunderstorms this evening. Some thunderstorms may be severe with damaging winds and large hail.
In addition to marine forecasts, NWS Boston publishes a Hazardous Weather Outlook (HWO) several times a day. HWOs discuss the potential for the development of severe weather, along with details regarding the geographic area at risk, timing, and nature of the severe weather threat. They are written in non-technical language and are extraordinarily beneficial for sailors.
The HWO issued at 4:28 am on Tuesday morning included the following:
A few strong to severe thunderstorms are possible this afternoon. Damaging wind gusts are the primary threat but large hail is also possible. The greatest risk for severe weather may end up south of the Mass Pike and especially across eastern Connecticut, Rhode Island, and southeast Massachusetts.
HWOs are a great decision-making resource that help fill in the blanks, particularly on those days when the marine forecast includes “showers and thunderstorms are possible…”.
Monitoring the Thunderstorm’s Approach
Once thunderstorms are underway, the most effective resource for monitoring their location, intensity, evolution, and movement is Doppler weather radar. (Click here if you would like to read a primer on using Doppler weather radar.) In addition, staying current on official severe weather watches, warnings, and local storm reports are part of the recipe for staying weather-wise and safe on the water.
Based on the approaching cold front and the favorable environment for thunderstorm development, the SPC issued Severe Thunderstorm Watch #450 at 3:10 pm for much of the northeastern US, including Narragansett Bay (click here for the Watch coverage area). The Watch mentioned that severe thunderstorms moving southeast across the Watch area would bring a potential of damaging wind gusts to 70 mph.
The thunderstorm that ravaged the JYC fleet formed inside the Watch area near southwestern Massachusetts at approximately 4:00 pm (Figure 8). Steadily picking up speed, the fledgling storm covered the 110 nm to Newport in just 3 hours (Figures 9, 10 and 11).
In addition to collecting information on the storm’s size, intensity, and evolution, NWS radar stations apply an algorithm to the data that predicts a thunderstorm’s movement. Although it typically requires an optional data subscription, many popular radar applications allow a user to overlay the projected track on a radar image (Figures 9 and 10). The line shows the storm’s projected track for the next 60 minutes with small perpendicular lines marking 15-minute intervals. Clicking on the cell often displays the compass heading and speed of the storm. While the algorithm is not perfect and often shifts the track slightly with each image update, the projected track is an invaluable resource to sailors.
On its journey to Newport, the thunderstorm was closely monitored by NWS Boston. At 6:23 pm, the storm’s history of damaging wind gusts prompted the office to issue a Severe Thunderstorm Warning for a large area of southwestern Rhode Island (Figure 12) (yellow polygon). The Warning advised the storm was moving southeast at 40 mph and was capable of producing 60 mph wind gusts. At the time the Warning was issued, the thunderstorm was approximately 25 nm northwest of Jamestown and predicted to pass directly over the racecourse in less than an hour.
As shown on Figure 12, many radar applications allow NWS Watch and Warning polygons to be overlaid on the radar image. Reading the text of a Watch or Warning is as simple as clicking on the polygon’s outline (click here). When severe weather threatens, trained volunteer spotters support decision-making by relaying information and damage observations to their local NWS office as Local Storm Reports (LSR). Clicking on an LSR icon opens a new window that reveals the details of the report. There were several LSRs indicating damaging wind gusts and downed trees as the storm moved through eastern Connecticut on its way to Newport. This additional data significantly improves situational awareness by allowing a sailor to quickly determine if they are inside the warning polygon, assess the storm’s potential hazards, and review recent impacts.
At 6:38 pm, a Special Marine Warning (blue polygon) was issued for Narragansett Bay and the offshore waters south of Newport (Figure 13). The Marine Warning advised boaters that the storm, which was 16 nm northwest of the racecourse, was capable of producing wind gusts to 50 knots and advised boaters to “move to safe harbor”.
The last Severe Thunderstorm Warning for the approaching storm (which was only 1.5 nm northwest of the racecourse) was issued at 6:59 pm and included Narragansett Bay, Newport, and southern Newport County (Figure 14). The Warning, issued just a few minutes prior to the arrival of downburst winds, advised that the storm was capable of producing ping-pong ball-sized hail and gusts to 60 mph.
The thunderstorm reached the fleet a little after 7:00 pm and lived up to the text of the Warnings by unleashing winds to 70 knots accompanied by blinding rain. Fortunately, large injury-producing hail did not occur.
Conclusion
Managing severe weather is part of the challenge of sailboat racing. Fortunately, the widespread use of smartphones, cellular internet access, improvements in weather forecast models, and the development of easy-to-use forecasting apps has made the process of monitoring severe weather much easier. Read the forecast, check the radar, keep an eye to the sky, and enjoy your sailing.
Additional Reading
Thunderstorms have dramatically affected many sailboat races over the past several years. Below are summaries of a few of more notable events.
