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Heavy Precipitation Supercells of 29 May 2012

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Part I: Introduction

On 29 May 2012, the first significant severe weather outbreak of the 2012 season occurred, along with major flash flooding across portions of central and northern Vermont and parts of the eastern Adirondack Mountains. 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 upper 60s to lower 70s. This very unstable environment along with a warm front draped across central Vermont helped to produce strong to severe thunderstorms across the North Country during the afternoon hours on 29 May 2012, which transitioned into a significant flash flood event during the late afternoon and evening hours.

This severe weather event included a confirmed EF0 (on the Enhanced Fujita Scale) tornado near Glover, Vermont, along with numerous reports of large hail and damaging thunderstorm winds. Click here for the public information statement, regarding the confirmed tornado. Hail up to baseball size was reported near Crown Point, New York and trees and power lines were down in Milton, Vermont from the powerful storms. Furthermore, the thunderstorms were accompanied by very heavy rainfall amounts of 2 to 4 inches with localized radar estimates near 6 inches across the southern Champlain Valley, which caused significant flash flooding. Many roads were washed out and numerous rivers and streams had sharp rises, as a result of the heavy rainfall. Click here for the local storm report, which shows the locations of the severe weather and flash flood damage.

Many of the severe weather and flooding reports were concentrated from the eastern Adirondack Mountains in northern New York into central and northern Vermont. These reports were associated with several long-tracked heavy precipitation supercells, which continued into parts of New Hampshire. Click here for a complete plot of all the severe weather reports on 29 May 2012 across the Northeast and Mid-Atlantic States. Figures 1 and 2 below show a Google background map with severe thunderstorm and tornado warning polygons and a plot of the associated severe weather reports on 29 May 2012 across the North Country. The green "H" represents severe hail reports one inch or greater in diameter, the blue "W" represents thunderstorm wind damage or gusts 58 mph or more, and the red "T" is the tornado touch down location in Orleans County, Vermont.
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Storm Prediction Center Products (Day 1 Outlook, Probability of Tornados, and Mesoscale Discussions)
The NOAA/NWS Storm Prediction Center (SPC)'s Day 1 Convective Outlook (not shown) on the morning of Tuesday May 29th indicated a "Slight Risk" of severe thunderstorms across the North Country. As the morning unfolded, it became increasingly clear that a significant severe weather event was likely. At that time, SPC issued a Mesoscale Discussion (MCD, not shown), a product used to call attention to a particular area of thunderstorm threat. MCDs are often issued when the SPC is considering Severe Thunderstorm and/or Tornado Watch products, and/or to alert forecasters of potential changes in upcoming Convective Outlooks. With this MCD, SPC chose to upgrade to a "Moderate Risk" of Severe Thunderstorms across portions of central and northern New York and extreme southwestern sections of Vermont. "Moderate Risk" forecasts in the North Country are rather infrequent (occurring perhaps once or twice per year). When such forecasts are issued, it often indicates the potential for a more enhanced severe weather threat.
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Figure 3 above shows the early afternoon Day 1 Convective Outlook and the associated probabilities of tornadoes, damaging winds, and severe hail. The "Moderate Risk" area highlighted in the late morning Mesoscale Discussion is shown in the left-most panel, with a broad area of "Slight Risk" surrounding this area including much of New England. Tornado probabilities ranged between 2% and 5% -- with the 5% probabilities (shaded in brown) forecast for all of Vermont and the Champlain Valley region of eastern New York. This meant that as high as a 5% probability of tornadoes occurring with 25 miles of any point were expected in the 5% probability area. Severe hail probabilities were up to 30% across the entire North Country (i.e., there was a 30% probability of at least 1 severe hail report occurring within 25 miles of any point in the 30% probability area, shaded in red). The Severe Wind Outlook forecast called for 30% severe wind probabilities (red shading), though probabilities as high as 45% (indicative of a potential significant wind damage threat, purple shading) were forecast across north-central New York and into extreme southwestern sections of Vermont.

Shortly after the early afternoon Day 1 Convective Outlook was issued, the SPC issued a Tornado Watch for most of the North Country, effective through 9:00 PM local time. This is shown in Figure 4 below. At 17 UTC (12:00 PM local), SPC issued another Mesoscale Discussion, addressing areas in the Tornado Watch. The Mesoscale Discussion graphic is shown in Figure 5. SPC forecasters pointed to a region of more enhanced tornado threat extending along and north of the position of the surface warm front (shown by the red line with red half-circles in the graphic). In addition, surface winds enhanced by channeling effects in valley locations across central and northern Vermont helped to increase low-level wind shear that can be favorable for tornado formation.
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Click to enlarge

Figure 1: Google Map plot of severe weather reports and severe thunderstorm warning polygons (blue shapes) on 29 May 2012.
Figure 2: Google map background with tornado warning polygons (red shapes) and severe weather reports on 29 May 2012.
Figure 3: Storm Prediction Center Day 1 Severe Weather Outlook at 1630 UTC on 29 May 2012.
Figure 4: Tornado Watch #313 issued by the Storm Prediction Center on 29 May 2012 at 1631 UTC.
Figure 5: Mesoscale Discussion issued by the Storm Prediction Center on 29 May 2012 at 17 UTC.

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