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

Part III: Radar Analysis

In this next section we will discuss in-depth radar signatures and provide analysis for several storms, which produced significant wind damage and large hail across the region. In addition, we will examine closely the strong rotating supercells across eastern Vermont and try to explain why no tornados were produced. Other areas we will examine are the straight-line wind damage from Johnson to Island Pond Vermont, and the baseball size hail in South Duxbury, Vermont. We will first show the northeast radar mosaic (figure 11 below) of the widespread areal coverage and intensity of the storms, followed by a detailed examination of individual storms, with reflectivity and velocity cross-sections, vertical integrated liquid (VIL), and low-level plain view storm relative velocity displays.


Radar Mosaic Overview
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Figure 11 is a Northeast regional composite reflectivity mosaic from 1806 UTC to 2354 UTC on 26 May 2011, along with surface observations plotted in white. This loop shows the widespread areal coverage and intensity of the storms from eastern Lake Ontario into much of northern New York and Vermont during this severe weather outbreak.

In addition, the radar loop shows multiple long-tracked supercell thunderstorms across the region, several of which affected the WFO BTV forecast area. The brighter yellows, reds, and purple colors in the radar loop indicate very strong thunderstorms with intense rainfall rates, along with the capability of producing severe hail.


Caledonia/Orange Counties (Straight Line Wind)
The first storm we will examine will be the central and southern Caledonia County supercell, which was very well organized with some low-level rotation. However, a NWS storm survey showed no tornado developed, only straight-line wind damage occurred. Given the 60 to 70 mile distance from the radar, and the lowest two elevation scans being blocked by the Green Mountains, data quality in the lowest several elevation scans is very limited. This makes storm interrogation very difficult across eastern Vermont and parts of the Northeast Kingdom.
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Figure 12 shows the KCXX 0.9° storm relative velocity at 2349 UTC on 26 May 2011, with the Doppler radar tornado vortex signature indicated by a white triangle. The radar indicated a broad cyclonic (counter-clockwise) circulation with inbound velocities of 30 to 40 knots and outbound winds of 25 to 35 knots.

The TVS was indicating low level delta velocities (LLDV) of 50 knots and mid level delta velocities (MDV) of 56 knots at this time, which is relative strong. As the distance from the radar increases, the beam spreads out, making tight low level couplet circulations difficult to detect, and much broader than they may appear in reality.
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In addition, to a strong rotational signature, the storm near Passumpsic, Vermont also had a "kidney bean" like reflectivity structure. Figure 13 shows this signature very well, which represents good storm organization, and the potential for the storm to produce a tornado.

However, based on a damage survey, only straight-line damaging winds occurred from eastern Washington County into extreme southern Essex County, Vermont with this supercell. The brighter red and purple cores show a very intense storm core, which produced very heavy rainfall and some golf ball size hail near Lunenburg, Vermont.
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Figure 14 shows a vertical cross section of velocity from the KCXX at 2344 UTC on 26 May 2011. This shows the limited vertical rotation of the storm near Passumpsic, Vermont. However, the storm did have a storm top divergence signature of70 to 80 knots, along with a descending rear flank downdraft velocity of 40 to 50 knots.

A mid level cyclonic couplet was present around 700 hPa, but the vertical depth of rotation was limited. Due to the distance away from the radar, velocity data is very poor below 4500 feet above ground level.
Click to enlarge
Figure 15 shows a well defined kidney bean reflectivity structure near Orange, Vermont on 27 May 2011 at 0004 UTC. This reflectivity structure suggests the potential for a tornado, within the hook like structure. This storm developed along a surface boundary, which helped to enhance the low-level turning of the wind fields and provided a storm-scale environment favorable for supercells capable of producing a tornado.

However, no tornado was cited and the survey determined that the damage resulted from straight-line winds. Also, 1 inch hail was reported near Barre, Vermont. We see many times in the past several years good radar signatures of a possible tornado, but find the broad circulation at 4000 to 6000 feet above the ground is unable to reach the surface and many times quickly weakens across the higher terrain of the Green Mountains.


Washington County near South Duxbury, Vermont (Baseball Hail)
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The next supercell thunderstorm we will examine tracked from near Duxbury to Barre to northern Orange County, Vermont on the evening of May 26th and produced baseball size hail in South Duxbury and several areas of thunderstorm wind damage, along with rainfall amounts of 1 to 3 inches. We will show a reflectivity cross section to determine the vertical depth of the storm, along with a vertical integrated liquid product, which is a good indicator of the severity of the hail. Figure 16 shows a vertical reflectivity cross section taken near South Duxbury, Vermont on 26 May 2011 at 2243 UTC. This cross section shows a very deep and well-developed supercell thunderstorm with a storm top up to 50,000 feet. In addition, the reflectivity structure showed 50 dBZ to 45,000 feet and 60 dBZ to 37,000 feet, indicating very strong storm updraft capable of producing very large hail. Also, note the weaker 20 to 30 dBZ returns on the upper right part of the image, representing forward flank anvil and suggesting very strong jet stream winds aloft, helping to enhance upper level divergence over the storm. These factors all came together, along with a moist and unstable environment to promote supercell thunderstorms capable of large hail and damaging winds on the afternoon and evening of May 26th.
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Figure 17 shows the KCXX Vertical Integrated Liquid (VIL) at 2248 UTC on 26 May 2011. 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 stormís 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 17 shows VIL (pink/purple color) values between 65 and 70 kg/m2 near South Duxbury, Vermont. This indicates a very well developed updraft, capable of producing large hail and damaging winds, especially when the storm collapses and weakens, which occurred between South Duxbury and Northfield, Vermont on 26 May 2011.


Lamoille County Storm near Johnson, Vermont (Damaging Winds)
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In addition, to the supercell thunderstorms which occurred across our region on 26 May 2011, further north across central and northern Vermont, a bow-like line segment developed and created damaging straight-line winds from near Johnson to North Hyde Park.

Figure 18 is the KCXX 1.3° base reflectivity near Johnson, Vermont at 2218 UTC on 26 May 2011, which clear shows a bow like reflectivity structure. The weaker 20 to 30 dBZ returns (light green) near North Hyde Park, Vermont suggests a descending rear flank downdraft jet is present and capable of producing damaging thunderstorm wind gusts of 60 to 70 mph, based on the velocity values.
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Figure 19 is the KCXX 1.3° velocity near Johnson, Vermont at 2218 UTC on 26 May 2011, which clearly shows the descending rear flank downdraft jet of 55 to 65 knots. The highest velocity values at this time were located near North Hyde Park, Vermont, just behind the bow like reflectivity structure.

Damaging winds did occur from this storm from Johnson, Vermont into North Hyde Park, and continued toward Island Pond as the storm raced northeast at 40 to 50 mph. Also, when storms move very fast, they have a higher potential to produce wind gusts greater than 60 mph, as a rule of thumb.
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Figure 20 is a special 4-dimensional look into a supercell thunderstorm near Newport, Vermont on 26 May 2011 at 2218 UTC, using GR2Anlyst. This software program used by NWS meteorologists, shows the 4-dimensional structures of storms, and helps in the warning decision process.

This storm near Newport was very well organized with a defined Bounded Weak Echo Region (BWER), suggesting very strong updraft and the potential for large hail, especially when the storm collapses. This storm did produce 1 inch diameter hail near Newport, along with some gusty winds. Also, from Figure 20 below you can see a strong west to east tilt, suggesting very strong mid/upper level winds were impacting the storm and displacing the core of the downwind of the updraft. The storm top was near 50,000 feet, with both a forward and back anvil structure present, along with a >50 dBZ core to 35,000 feet.

Figure 11: Northeast Composite Reflectivity from 1806 UTC to 2354 UTC on 26 May 2011 with Surface Observations plotted (white)
Figure 12: KCXX 0.9° Storm Relative Velocity at 2349 UTC on 26 May 2011, along with Tornado Vortex Signature (white triangle)
Figure 13: KCXX 1.3° Base Reflectivity at 2349 UTC on 26 May 2011 and TVS (white triangle)
Figure 14: KCXX Velocity Cross Section near Passumpsic, Vermont at 2344 UTC on 26 May 2011
Figure 15: KCXX 1.3° Base Reflectivity 0004 UTC on 27 May 2011
Figure 16: KCXX Reflectivity Cross Section near South Duxbury, Vermont at 2243 UTC on 26 May 2011
Figure 17: KCXX Vertical Integrated Liquid near South Duxbury, Vermont at 2248 UTC on 26 May 2011
Figure 18: KCXX 1.3° Base Reflectivity near Johnson, Vermont at 2218 UTC on 26 May 2011
Figure 19: KCXX 1.3° Base Velocity near Johnson, Vermont at 2218 UTC on 26 May 2011
Figure 20: 4-Dimensional Reflectivity Cross Section near Newport, Vermont at 1846 UTC on 26 May 2011


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Page last modified: June 4, 2011
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