LakeErieWX



May 27, 2012: An Unexpected and Memorable Wind

Introduction

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 that the marine forecast contains this cautionary statement as Great Lakes thunderstorms regularly produce damaging winds of 60 knots or more. And where there are high winds there are frequently high 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 a regatta, such as the 2010 Lake Ontario 300 Challenge (event summary) and the 2011 Chicago-Mackinac Race (event summary).

Boaters clearly understand the dangers of being in a thunderstorm, but how near is too near? 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.

The Forecast

At 8:00 am on Sunday morning, the surface analysis chart 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. As mentioned in the marine forecast (above), modest easterly winds were expected to transition to southeasterly with the passage of the front.

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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 (link to 1218Z regional radar) would move southeast and reach Lake Erie and northern Ohio by late morning or early afternoon. There was also the risk that the northward drifting warm front could initiation thunderstorms. The Storm Prediction Center (SPC) concurred with this analysis and placed nearly all of the Great Lakes under an enhanced risk for the development of severe thunderstorms (Outlook hyperlink) and damaging wind gusts (Outlook hyperlink) greater than 50 knots.

A Pleasant Morning

By mid-morning, the combination of an area 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 buoy 45005, located in Lake Erie’s western basin, were one and half feet. (Show a Google Earth image of wind observation from around the lake.) Visible satellite and radar imagery indicated that skies over the Lake, with the exception of the northeast corner, were generally clear. 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.

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A Turn for the Worse

At 11:31 am, Doppler radar (hyperlink to 1531Z BR) indicated that the first, and rather modest, thunderstorm 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 cluster near Detroit moving to the southeast. During the next thirty minutes, the initial storm underwent significant development while the larger clusters to the north and northwest pressed towards Long Point (hyperlink to 1559Z BR). Comment on the wind.

At 1:00 pm, the initial storm – 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.

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BR and BV 1700Z

The base velocity radar image at 1:00 pm (hyperlink to annotated version) shows the signature of an outflow boundary, the flow of rain-cooled air associated with the storm’s downdraft, racing towards the Ohio shore from the thunderstorm 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 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.

Conneaut, Ohio

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. The winds at Conneaut, Ohio (Station CBL01) (show a google earth image with Conneaut shown), 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. 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 wind was 40.3 knots with gusts just shy of 50 knots. This dramatic increase in wind speeds coincided with the arrival of an outflow boundary, clearly visible on the base velocity radar image at 1:23 pm.

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BR & BV 1723Z

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For 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 gust front.

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). For the next ninety minutes, sustained winds at Fairport Harbor hovered between 25 and 30 knots with gusts into the low 30s.

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Cleveland, Ohio

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 built from 5.8 knots (gust 6.9) to 18.3 knots (23.0) as the leading edge of the outflow boundary reached the shore.

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BR & BV 1733Z

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At approximately the same time (1:38 pm), an additional outflow boundary was readily apparent on base velocity imagery (perhaps these images should be linked to instead of shown) as it barreled south towards Cuyahoga and Lake County from the thunderstorm cluster near Long Point.

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BR & BV 1738Z

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.

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BR & BV 1810Z.

Western Basin

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 indicate that the increased distance travelled by the strong winds also acted to dampen the rate at which the winds increased. A comparison of the observations between the two locations shows the rather steady increase at Toledo compared to the abrupt rise at Conneaut.

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And similar to the observations from other 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?

We’ll begin with a refresher on the forces controlling the wind. 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. As density and barometric pressure are associated with temperature, it should be no surprise that the barometric pressure associated with the cold pool is relatively higher than the surrounding air which was unaffected by the storm’s precipitation. This small-scale area of high pressure 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. But in this instance, the cold pool dramatically enhanced the wind.

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Prior to the influence of the thunderstorms, the large-scale wind was easterly – flowing from the area of high pressure over eastern Ontario along the warm front towards the area of low pressure in South Dakota. The large area of rain-cooled air from the parade of thunderstorms crossing Lake Erie on the afternoon of May 27th reinforced this existing large-scale easterly wind by increasing the barometric pressure over the northeastern portion of the Lake (link to some sort of image). The pressure gradient was additionally enhanced by the steadily falling barometric pressure across northern Ohio associated with the approaching warm front (link to image).

While the initial blast of wind across Lake Erie was entirely the result of thunderstorm dynamics, the prolonged nature of the higher winds (two hours in some places) was the result of the increased barometric pressure differences in the vicinity of Lake Erie. First, the reinforcement of high pressure to the northeast by the thunderstorms, and the falling pressure to the southwest prompted by the warm front.

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 and 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.

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Water level observations from Buffalo, Cleveland and Toledo 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, gravity set out to restore the level of the Lake. The observations, particularly at Cleveland, show the alternating and steadily diminishing increases and decreases in heights as Lake Erie sloshed back and forth trying to reach equilibrium.

Large storm-induced waves along the southern shore, 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 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.

Summary

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