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Heavy Precipitation Supercells of 29 May 2012

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Part III: Radar Analysis

Radar Mosaic Overview
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Figure 13 shows a composite radar mosaic image loop from 1736 to 2200 UTC May 29th. The strongest storms are those that show dark red and/or purple shading, corresponding to 60-70 dbZ reflectivity values. The loop shows that initial cells developed in the Champlain Valley of New York, just to the east of the Northern Adirondacks and migrated eastward into northern and central Vermont. By 1830 UTC, the strongest storms were the supercell moving across eastern Franklin, northern Lamoille and Orleans Counties, and the cluster of severe thunderstorms entering Addison County from Essex County in New York. By 1900 UTC, additional thunderstorms were developing in a line along an outflow boundary left from the northern Vermont supercell and would push into Chittenden and Lamoille Counties. From 2000-2200 UTC the main severe weather threat came from the line of thunderstorms moving from Warren County in New York across Rutland and Windsor Counties in Vermont.

Orleans Counties (EF0 Tornado near Glover, Vermont)
On 29 May 2012 a supercell thunderstorm tracked from near Altoona, NY across the northern Champlain Valley up the Missisquoi River basin, then southeast into southern Orleans County, before weakening across eastern Vermont. A supercell is a thunderstorm that is characterized by the presence of a mesocyclone; a deep, continuously-rotating updraft, which can and did produce a tornado during this event, along with very heavy rainfall. 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. This supercell produced golf ball size hail near Albany, Vermont, along with rainfall amounts of 2 to 4 inches in a very short period of time.
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Figure 14 below is a digitized damage path from Google Earth of the tornado 3 miles southwest of Glover, Vermont. The total tornado path length was a third of a mile and about 100 yards wide.

The lighter blue color in the image below suggests surface winds near 70 knots based on uprooted softwood trees and minor structural damage to a house. Meanwhile, while the outline color had damage consistent to winds around 50 knots with tree limbs broken off and some smaller 6 to 8 inch diameter trees blown over.
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Figure 15 shows the KCXX 1.3° reflectivity at 1926 UTC on 29 May 2012, along with the location of tornadic damage 3 miles southwest of Glover, Vermont, with Google Earth as the background image. This image clearly shows a hook-like reflectivity structure, which is very typical of tornadic producing supercells.

The locations of the tornado occurred within the hook-like structure. Given the distance from the radar and location east of the Green Mountains, sampling of this storm was very poor in the lowest elevation scans. During the damage survey, the team confirmed the tornado occurred near a ridge top, with a west to east valley helping to channel low level easterly winds into the storm and enhance the tornadic circulation.
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Figure 16 shows a reflectivity cross section at tornado touch down 3 southwest of Glover, Vermont at 1926 UTC on 29 May 2012. This 3-D image shows well-structured supercell thunderstorm with a 70 dBZ core to 20,000 feet and a storm top of 50,000 feet or close to 9 miles tall in the atmosphere.

The surface reflectivity gradient in the image shows a weak hook like signature, but beam blockage from the Green Mountains, resulted in poor data sampling. Also, from the image a bounded weak echo region is present, suggesting a very strong storm updraft, which helps in the development of large hail. The white triangle is the location of the tornado, in the southwest rear flank of the storm, very close to the descending rear flank downdraft and the enhanced reflectivity core.
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Figure 17 shows a velocity (left image) and reflectivity (right image) cross section at 1907 UTC on 29 May 2012, 23 minutes before confirmed tornado touch down. The velocity cross section clearly shows a deep mesocylone circulation thru 700 hPa. The green colors on the left image, indicate winds coming toward the radar at 20 to 30 knots, while the yellow colors show outbound winds of nearly 40 knots, creating a total storm rotation of 65 knots. You can also see how the mountains to the west of the storm, results in significant low level beam blockage in both the velocity and reflectivity data. The very strong storm updraft and well established upper level divergence pattern, helped to carry 50 dBZ reflectivity cores above 500 hPa (right image). The white triangle shows the position of the developing tornado, in the southwest flank of the storm, very close to a descending reflectivity core. The best cyclonic circulation is co-located with the descending rear flank downdraft and ascending forward flank updraft, which is very typical of well-established supercell thunderstorms.

Essex County, New York Storm near Moriah (Golf Ball Hail)
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In this section we will examine the supercell thunderstorm which produced golf ball size hail near Moriah, New York. Figure 18 shows a very tall heavy precipitation supercell, with a storm top near 50,000 feet at 1817 UTC on 29 May 2012. This storm also had a 70 dBZ core to 20,000 feet, which was well above the freezing level, and provided good indication of large hail, within the storm core.

Also, from the image you can see the bow echo reflectivity structure associated with the descending reflectivity core, which would suggest the potential for gusty thunderstorm winds. From the reflectivity structure a shelf cloud signature seems likely as the storm crossed the southern Champlain Valley, given the updraft/downdraft interaction.
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Figure 19 shows the KCXX Vertical Integrated Liquid (VIL) at 1817 UTC on 29 May 2012. VIL is an estimate of the total mass of precipitation in the clouds. The measurement is obtained by observing the reflectivity of the air as obtained by radar. This measurement is usually used in determining the size of hail, the potential amount of rain under a thunderstorm, and the potential downdraft strength when combined with the height of the echo tops. When VIL values quickly fall, it may mean that a downburst is imminent, resulting in the weakening of the storm's updraft and the storms inability to hold the copious amounts of moisture/hail within the storm's structure and a greater potential for the storm to produce damaging winds. Figure 19 shows VIL (pink/purple color) values between 65 and 70 kg/m2 near Moriah, NY. This indicates a very well developed updraft, which produced golf ball size hail at Moriah, NY. In addition, severe hail was observed in southern Addison County associated with VIL values between 60 and 65 kg/m2.

Rutland County, Vermont Storm near Brandon
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The final storm we will be discussing was located near Brandon, Vermont and was another well-established supercell with strong rotation, but never produced any damage. This supercell developed near Ticonderoga NY, then tracked east into Shoreham, VT, before weakening in western New Hampshire during the early evening hours. No thunderstorm wind damage occurred with this storm, but hail up to 1.25 inches in diameter occurred at Shoreham at 1945 UTC on 29 May 2012. Figure 20 shows the GR2 Analyst 50 dBZ or greater reflectivity near Brandon, VT at 2000 UTC. This shows the 50 dBZ core to 30,000 feet, with a large 60 dBZ or greater reflectivity core in the forward flank of the storm, associated with very heavy rainfall and large hail. The image also shows an elevated bounded weak echo region, associated with strong storm updraft, with a descending rear flank reflectivity core near the hook echo signature in the southern flank of the storm. This was collocated within an area of very strong gate-to-gate mid-level (8000 feet above ground level) rotation.
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This strong cyclonic rotation can be seen in Figure 21 which shows the KCXX 1.3° velocity (right image) and reflectivity (left image) at 2000 UTC on 29 May 2012 near Brandon, VT. The green colors show inbound winds toward the radar of 30 to 40 knots, while the red colors are outbound winds of 30 to 40 knots with a total mid-level rotation of 75 knots. The lower level data had poor data sampling and deliasing problems with the velocity data. The green triangle in figure 21 shows the KCXX radar was indicating a tornado vortex signature, suggesting a strong potential for a tornado, due to the gate-to-gate rotation within the storm. Also, the pink square was suggesting very large hail was possible within this supercell, due to the very tall reflectivity core. Finally, from the reflectivity image on the right, you can clearly see a well-defined hook echo to the reflectivity structure. This also would suggest the potential for a tornado, given this storm structure, but this feature was in the mid-levels around 8000 feet along with the velocity couplet and never reached the ground. The storm report shows the NWS at BTV only received large hail for this storm.

Figure 13: Northeast Composite Reflectivity from 1736 UTC to 2200 UTC on 29 May 2012 with Surface Observations plotted (white)
Figure 14: Digitized tornado damage path from Google Earth, with outline blue color winds of 50 knots and the interior blue winds of 70 knots based on damage from NWS survey.
Figure 15: KCXX 1.3° Reflectivity at 1926 UTC on 29 May 2012, along with location of tornadic damage near Glover, VT.
Figure 16: KCXX reflectivity cross section near Glover, Vermont at 1926 UTC on 29 May 2012.
Figure 17: KCXX Base velocity (left image) and reflectivity (right image) cross section near Glover, Vermont at 1907 UTC on 29 May 2012.
Figure 18: KCXX reflectivity cross section near Moriah, New York at 1817 UTC on 29 May 2012
Figure 19: KCXX Vertical Integrated Liquid (VIL) near Moriah, New York at 1817 UTC on 29 May 2012
Figure 20: KCXX Reflectivity Cross Section near Brandon, Vermont at 2000 UTC on 29 May 2012
Figure 21: KCXX 1.3° Velocity (left image) and Reflectivity (right image) at 2000 UTC on 29 May 2012, along with Tornado Vortex Signature (green triangle) and hail signature (pink icon).

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