Memorial Day 2012: An Unexpected Wind
Boaters clearly understand the dangers of being in a thunderstorm, but what about being near a thunderstorm? How close is too close? Are you too close if you can see the storm? Are you too close if you can hear the thunder or see the lightning? Are you too close if you can feel the rain? On Sunday, May 27, 2012, Lake Erie boaters learned that even sixty miles can be too close.
Great Lakes boaters are familiar with the statement winds and waves higher in and near thunderstorms that concludes National Weather Service (NWS) marine forecasts any time thunderstorms are possible. For example, the forecast issued for the open waters of Lake Erie on Sunday, May 27, 2012 stated:
National Weather Service, Cleveland, OH
10:07 AM EDT SUN MAY 27 2012
This afternoon: east winds 10 knots or less increasing to 5 to 15 knots. A chance of showers and thunderstorms. Waves 2 feet or less.
Tonight: southeast winds 10 knots or less becoming south. Waves 2 feet or less.
Winds and waves higher in and near thunderstorms.
There are sound reasons why the marine forecast contains this cautionary statement, as Great Lakes thunderstorms may be capable of producing damaging winds at or above 60 knots. And where there are high winds there are usually big waves. Boaters are particularly vulnerable to bad weather as it isn’t always possible to reach safe harbor ahead of a fast-moving thunderstorm, leaving the crew no alternative but to endure the storm’s full fury. And thunderstorms have a knack for wreaking havoc on regattas, such as the 2010 Lake Ontario 300 Challenge (event summary) and the 2011 Chicago-Mackinac Race (event summary).
At 8:00 am on Sunday morning, the surface analysis chart (figure 1) showed a warm front extending into central Ohio from a low pressure system in South Dakota. This surface boundary was expected to move north throughout the day and reach southern Ontario by 8:00 pm (figure 2). As mentioned in the marine forecast (above), modest easterly winds were expected to transition to southeasterly with the passage of the front.
Click here for a larger version.
Click here for a larger version.
The inclusion of showers and thunderstorms in the marine forecast were related to concerns that ongoing convective activity over Michigan and the upper Great Lakes (click here for the regional radar image at 8:18 am) would move southeast and reach Lake Erie and northern Ohio by late morning or early afternoon. There was also a potential that the northward-drifting warm front could initiate thunderstorms as it pushed through northern Ohio on its way to southern Ontario. Figure 2 shows the expected location of the warm front at 8:00 pm on Sunday evening. The Storm Prediction Center (SPC) concurred with these observations and placed nearly all of the Great Lakes under an enhanced risk for the development of severe thunderstorms (click here for Convective Outlook) and damaging wind gusts (click here for Wind Outlook) greater than 50 knots.
A Pleasant Morning
By mid-morning, the combination of high pressure over northern Ontario and low pressure to the southwest was contributing to northeasterly to easterly winds in the range of ten to twelve knots across Lake Erie. Wave heights at NCBC buoy 45005, located in Lake Erie’s western basin, were one and half feet. Visible satellite (figure 3) and radar imagery (figure 4) indicated that skies over the lake, with the exception of the northeast corner, were generally clear. (The green area surrounding Cleveland on the radar image wasn't precipitation. Rather it was likely ground clutter, insects and/or birds.) Conditions were nearly perfect for a relaxing sail which, considering that it was Memorial Day weekend, would guarantee many boaters would venture onto the lake.
Click here for a larger version.
Click here for a larger version.
A Turn for the Worse
At 11:31 am, Doppler radar (click here for radar image) indicated that the first, and rather modest, cluster of thunderstorms had reached Lake Erie east of Erieau, Ontario. At the same time, a much larger cluster of thunderstorms to the north was approaching Long Point. And not far behind was a similarly-sized group of storms near Detroit moving to the southeast. During the next thirty minutes, the initial cluster of storms underwent significant development while the larger clusters to the north and northwest pressed towards Long Point (click here for radar image).
At 1:00 pm, the group of storms – now much larger than when it first reached Lake Erie – was approaching the shore west of Erie, Pennsylvania, while the two larger clusters were beginning their transit of the lake.
Click here for a larger image.
The base velocity radar image at 1:00 pm (figure 6) contains a series of linear returns (click here for annotated version) that suggests the approach of a downburst, the flow of rain-cooled air associated with a thunderstorm's downdraft racing towards the Ohio shore from the complex over southern Ontario. (Click here for a primer on thunderstorms and downdrafts.)
The appearance of the storms on radar prompted forecasters at the National Weather Service in Cleveland, Ohio to issue a Special Marine Warning at 1:05 pm announcing that a line of thunderstorms north of Ashtabula was producing strong winds 34 knots or greater and was moving east at 40 knots. Mariners can expect gusty winds … high waves … dangerous lightning … and heavy rains. Boaters should seek safe harbor immediately.
The winds at Conneaut, Ohio (station CBL01) (click here for map), at 1:00 pm were from the east-northeast (70°) at 9.2 knots with gusts to 10.3, observations that were consistent with the preceding three hours. A few minutes after the issuance of the Special Marine Warning, the thunderstorms produced a dramatic impact on the winds all along the Ohio shore. At 1:10 pm, the winds were northerly at 24.2 knots gusting to 35.8. Over the next eight minutes, sustained winds remained above 25 knots while gusts surged to 41 knots. A mere two minutes later (1:20pm), the sustained winds were 40.3 knots with gusts just shy of 50 knots.
During the next ninety minutes, the sustained winds at Conneaut averaged around 25 knots while a 51.9 knot gust (an event maximum) was recorded at 2:30 pm. The sustained wind didn’t settle in below 20 knots until after 3:00 pm, nearly two hours after the initial arrival of the outflow boundary.
Fairport Harbor, Ohio
The extraordinary winds at Conneaut were far from an isolated incident. At Fairport Harbor, Ohio (station FAI01), approximately 40 miles southwest of Conneaut, east northeast winds backed to north and increased from 4.7 to 16.1 knots from 1:06 pm to 1:12 pm while gusts during the same period increased from 8.1 to 26.4 knots. Six minutes later (1:18pm), the sustained wind reached 19.7 knots and a gust of 38 knots was observed (an event maximum). During the next ninety minutes, sustained winds at Fairport Harbor hovered between 25 and 30 knots with gusts into the low 30s.
Similar observations were captured at Cleveland, Ohio (station CND01), although owing to the greater distance from the storms, the impact occurred slightly later. Between 1:30 pm and 1:36 pm, winds backed from east northeast to north northeast and increased from 5.8 knots (gust 6.9) to 18.3 knots (23.0) as the leading edige of rain-cooled air reached the shore.
At approximately the same time (1:38 pm), an additional outflow boundary was readily apparent on base velocity imagery (figure 11) as it barreled south towards Cuyahoga and Lake County, Ohio from the thunderstorm cluster near Long Point, Ontario.
Although it had an impressive appearance on radar, the arrival of the outflow boundary a little after 2:10 pm produced only a modest increase in sustained wind speed at Cleveland from 23.0 to 27.0 knots and a corresponding increase in gusts from 26.4 to 33.3 knots. Overall, the sustained wind remained above 25 knots for nearly two hours with a maximum gust of 34.4 knots being observed at 2:48 pm.
Remarkably, the strong winds from the thunderstorms pushed all the way to Toledo, Ohio located at the southwestern corner of Lake Erie. The distance from the storms (approximately 100 miles) acted to delay the arrival of the strong winds, with sustained winds finally reaching 20 knots at 3:30 pm, nearly two hours after the strong winds reached Conneaut. A maximum gust of 30.0 knots was observed at Toledo at 4:00 pm. The observations at Toledo (figure 12) indicate that the increased distance traveled by the strong winds also acted to dampen the rate at which the winds increased. A comparison of the observations between the two locations (figures 12 and 13) shows the rather steady increase at Toledo compared to the abrupt rise at Conneaut.
And similar to the observations from surrounding locations, the higher wind speeds persisted at Toledo for several hours.
Why Were The Strong Winds So Persistent?
Strong winds produced by thunderstorms are a relatively common occurrence, but typically the fury builds and subsides in a matter of a few minutes – the calm before and after the storm. Uncharacteristically, the strong winds on May 27th persisted for nearly two hours across the western two-thirds of Lake Erie. Why was this event different?
Let's start with a refresher on the forces that control the wind. In the Northern Hemisphere, the surface wind generally flows clockwise and outward from an area of high pressure and counter-clockwise, and inward towards an area of low pressure (click here for a more detailed primer). The speed of the wind is governed by the difference in barometric pressure between highs and lows across a region – known as the pressure gradient force – with the largest pressure differences leading to the strongest winds. The pressure gradient force across a region changes constantly in response to building or weakening highs and lows, or highs and lows that are moving relative to one another.
The passage of a thunderstorm often leaves a pool of rain-cooled air (“cold pool”) in its wake. The air in this cold pool air is denser, and therefore heavier, leading to an area where the barometric pressure is relatively higher than the surrounding air which was unaffected by the storm's precipitation. This small-scale area of high pressure ("mesohigh") can have dramatic impacts on the flow of the large-scale wind. Many times, the cold pool suppresses the wind until the atmosphere erases the influence of the cold pool through warming. In other instances, the cold pool dramatically enhances the wind.
Prior to the arrival of the thunderstorms, the large-scale wind across the lower Great Lakes was easterly (figure 14) – flowing from the area of high pressure over eastern Ontario along the warm front towards the area of low pressure in South Dakota (figure 15).
Click here for a larger version.
The area of rain-cooled air from the parade of thunderstorms crossing Lake Erie during the early afternoon hours produced a localized area of increased barometric pressure ("mesohigh") over the lake near Erie, PA (click here for image). The pressure difference over Lake Erie was also enhanced by the steadily falling barometric pressure across northern Ohio associated with the approaching warm front and low pressure system (figure 15).
At 8 am, the barometric pressure at Erie, PA was 1019.5 mb and 1018.6 mb at Toledo, OH, resulting in a modest difference of .9 mb (figure 16). During the next two hours, the pressure difference between the two locations increased as Erie experienced a slight increase while the pressure at Toledo declined slightly before stabilizing. Beginning at 10 am, the pressure at both locations fell steadily, ending with a .9 mb difference at 1 pm. The modest pressure difference between the locations is consistent with the steady winds across the lake during the morning hours.
At 2 pm, the cold air produced by the thunderstorms increased the barometric pressure at Erie to 1020.6 mb, approximately 3.3 mb greater than the observation at Toledo (1017.3 mb). A mere forty-five minutes later, the pressure at Erie jumped to 1023.4 mb, 6.5 mb greater than Toledo (1016.9 mb) -- a seven fold increase in the pressure difference at 8 am. At 3 pm, the pressure at Erie had subsided to 1021.5 mb, but falling pressure at Toledo (1016.7 mb) resulted in a 4.8 mb difference. Between 3 pm and 4 pm, the pressure at Erie fell precipitously to 1017.2 mb, a mere .5 mb higher than the pressure at Toledo (1016.7 mb).
While the initial blast of wind across Lake Erie was entirely the result of thunderstorm dynamics (downdrafts), the prolonged nature of the higher winds (nearly two hours in some places) was the result of the increased barometric pressure differences across Lake Erie. The creation of a mesohigh to the northeast by the thunderstorms combined with the falling pressure to the southwest associated with the warm front led to very strong, prolonged winds west of the thunderstorm activity. In contrast, the area to the east, where the barometric pressure gradient was more modest, experienced much lower wind speeds, as indicated by the observations at Dublin, New York (click here) As the large pressure difference (pressure gradient) across the western half of the lake declined, the strong winds steadily subsided.
Waves: Adding Insult to Injury
But high winds weren’t the only hazard produced by the strong thunderstorms. Wave heights are a function of wind speed, wind direction and its duration. Localized enhanced wave heights are often associated with thunderstorms. Lake Erie, the shallowest of the Great Lakes, is particularly susceptible to wind-induced waves and fluctuations in the level of the water. Variations in the level of the water of several feet or more (known as displacements) occur periodically on Lake Erie, particularly when strong southwest or northeast winds sweep down the long axis of the Lake for a prolonged period. (Click here for a case study on a Lake Erie displacement.)
Water level observations from Buffalo, Cleveland, and Toledo (figure 16) indicate that the strong winds from the storms prompted a rapid decrease of nearly one foot at Buffalo beginning around 3:00 pm. While a corresponding increase was observed at Toledo, the change was far less abrupt than the decrease at Buffalo owing to the proximity of the storms to the eastern end of the Lake.
As the winds moderated after the initial blast, the relentless force of gravity steadily began to restore the level of the Lake. The observations, particularly at Cleveland (figure 16 in red), show the alternating and steadily diminishing increases and decreases in the water level as Lake Erie sloshed back and forth trying to reach equilibrium.
Large storm-induced waves can be very dangerous for unprepared boaters. Waves along the southern shore of Lake Erie, estimated at nearly six feet, caused a small boat containing a couple and their two daughters to capsize near Lakewood, Ohio at approximately 3:30 pm. The Coast Guard was called by an alert homeowner and although they arrived quickly, large waves and the nature of the shoreline prohibited them from reaching the family. Fortunately, Lakewood firefighters were able to rescue the family by hoisting them up an eroded cliff nearly 75 feet high (read more). The family was wet and cold, but otherwise uninjured.
As a Lake Erie sailor, I have heard many tales about the abrupt and long-lived increase in winds on the afternoon of May 27, 2012. The clear skies and gentle breezes during the morning hours created perfect conditions for a peaceful outing. And the approaching thunderstorms -- if they were noticed at all-- seemed too far away to raise any concerns. Holidays are popular times to entertain non-boating family members, which only increased the difficulty in coping with the dramatic deterioration in conditions. Although this event was unusual, it is an excellent example of the challenges associated with boating on the Great Lakes.