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The 2011 Pre-Memorial Day Severe Weather Outbreak and Flash Flood Event across the North Country

Part I: Introduction

On 26 May 2011, the first significant severe weather outbreak of the 2011 season occurred, along with devastating flash flooding across portions of northern New York and much of central and northern Vermont. A very warm and moist air mass was in place across most of the North Country with temperatures in the 80s and surface dewpoints in the mid to upper 60s. This very unstable environment along with a stationary front draped across the region helped to produce strong to severe thunderstorms across the North Country during the afternoon and early evening hours on 26 May 2011, which transitioned into a significant flash flood event during the overnight hours.

This severe weather event included numerous reports of severe and damaging thunderstorm winds of greater than 60 mph, along with several report of large hail, with baseball size hail reported near Duxbury, Vermont. Furthermore, the thunderstorms were accompanied by very heavy rainfall amounts of 3 to 5 inches with localized radar estimates near 7 inches across central Vermont, which caused significant flash flooding. Many roads were washed out and several rivers reached moderate to major flood stage as a result of the heavy rainfall. Most of the severe weather reports were concentrated along and south of a Star Lake, New York to Canaan, Vermont line, associated with several long-tracked miniature supercells, which continued into New Hampshire and Maine. A supercell is a thunderstorm that is characterized by the presence of a mesocyclone; a deep, continuously-rotating updraft. Supercells are the least common type of thunderstorm and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate sensible weather conditions up to 20 miles (32 km) away. As a result of scattered wind damage, National Weather Service (NWS) storm surveys were performed. We surveyed damage near Johnson, Plainfield, and Lunenburg, Vermont, and through the survey it was concluded that the damage was the result of straight-line thunderstorm winds.
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Figure 1 shows a Google Map display of the damaging wind, hail, and flash flooding which occurred during this event. The white ball icons indicate hail reports, the orange tree icons show areas of thunderstorm wind damage, and the green water icons represent areas of flash flooding. The yellow parallelograms in the image below show thunderstorm warnings, while the green colored boxes are flash flood warnings, and the red color polygons are where tornado warnings were issued. Click here for a complete listing of all severe weather and flash flood reports across Weather Forecast Office (WFO) Burlington, VT forecast area.
In this review, we will investigate the pre-storm synoptic and mesoscale features that contributed to the severe weather outbreak, along with several products issued by the Storm Prediction Center (SPC). This includes examining area soundings for instability, shear, and moisture parameters, reviewing upper air data and water vapor for position of short waves and jet streaks, and surface data to indentify low-level boundaries and max instability as a focus for thunderstorm development. Finally, an in-depth radar review will be provided with detailed discussion about the reflectivity, velocity, and storm total precipitation signatures that contributed to producing the severe wind, hail and flooding reports.

Storm Prediction Center Products (Day 1 Outlook, Probability of Tornados, and Mesoscale Discussions)
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From Figure 2 you can see the SPC Day 1 Outlook had the Champlain Valley and all of Vermont highlighted in slight risk for severe thunderstorms. Click here for the Day 1 text product issued by SPC. From SPC a slight risk implies well-organized severe thunderstorms are expected, but in small numbers and/or low coverage.

Depending on the size of the area, approximately 5-25 reports of 1 inch or larger hail, and/or 5-25 wind events, and/or 1-5 tornadoes would be possible. Also, click here to see how well the Storm Prediction Center did in the forecasting of slight risk across the eastern United States versus actual reports of severe weather.
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Meanwhile, Figure 3 shows the probabilistic forecast of severe thunderstorms producing tornadoes within 25 miles of a given point during the outlook period. A portion of northeastern New York and Western Vermont had a greater than 10% probability, which is relatively rare for the local area. On occasion we will see probabilistic forecasts of 30 or 45% chances of large hail or damaging thunderstorm winds across our forecast area.

The combination of a well defined surface boundary and very favorable turning of the winds in the middle to upper levels of the atmosphere, created conditions favorable for supercell thunderstorms capable of producing isolated tornadoes. Click here to view the meso-discussion issued by the SPC and the associated tornado watch box information graphic here.

Figure 1: Google Map Plot of Severe Weather and Flash Flood Reports, along with the Warning Polygons
Figure 2: Storm Prediction Center Day 1 Outlook issued at 13 UTC on 26 May 2011
Figure 3: Storm Prediction Center Day 1 Tornado Outlook issued at 16 UTC on 26 May 2011.

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Page last modified: June 4, 2011
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