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Top 5 Weather Events of 2011 across the North Country
Main 5 4 3 2 1

#3 Weather Event: May 26th-27th Flash Flooding and Severe Weather
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 most 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 transition into a significant flash flooding 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. Meanwhile, locations across central and northern Vermont were hardest hit with flash flooding on the evening of 26 May 2011. Some of the most significant damage from flash flooding occurred from Barre to Saint Johnsbury, Vermont.
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Figure 14 shows a Google Map display of the damaging wind, hail, and flash flooding which occurred during this event. The white balls indicate hail reports, the orange trees show areas of thunderstorm wind damage, and the green water icons represent areas of flash flooding. The yellow parallelograms in the image 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. Click here for a list of 24 hour rainfall amounts across northern New York and most of central and northern Vermont.
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Figure 15 shows an eastern United States water vapor loop from 1640 UTC (Universal Time Constant: i.e., EDT plus 4 hours) to 2310 UTC on 26 May 2011, along with 500 hPa (20,000 feet above the ground level) heights (blue lines), wind speeds 50 knots or greater at 500 hPa (yellow lines), and 5 minute lightning (red). This water vapor loop clearly shows several rounds of abundant deep layer moisture moving from the Ohio Valley into the Northeast, which helped to produce convection with localized very heavy rainfall. In addition this shows a strong shortwave trough across the western Great Lakes and northern Mississippi River Valley, along with a right rear quadrant of a 500 hPa jet streak across western New York. These two features and associated cool pool aloft helped enhance upper level divergence and aided in the vertical development of thunderstorms. The closed height contours over the western Great Lakes into the Mississippi Valley, suggests very strong jet stream winds aloft, helping to promote strong updrafts for long-lived thunderstorms.
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Figure 16 shows a surface analysis along with a radar composite image on 26 May 2011 at 2100 UTC. This analysis indicates a stationary boundary from western New York into northern Vermont, with several waves of low pressure riding along this boundary. This boundary combined with these areas of low pressure helped to produce several rounds of thunderstorms, with localized very heavy rainfall. The stationary boundary separated a warm moist air mass to the south, compared to a cool and drier air mass to the north. This strong baroclinic zone helped to enhance numerous rounds of training showers and strong to severe thunderstorms across the North County on 26 May 2011.
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Figure 17 is a Northeast region 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.


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 clearly 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 was 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° base 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 you can see a strong west to east tilt, a result of strong westerly mid and upper-level winds. The storm top was near 50,000 feet, with both a forward and back-sheared anvil structure present, along with a >50 dBZ core to 35,000 feet.


Hydro:

As the threat for severe thunderstorms dissipated on the evening of May 26th, our attention quickly turned to the potential for life-threatening flash flooding across the eastern Adirondacks into most of central and northern Vermont. The combination of a stationary boundary across the region and numerous thunderstorms training over the same areas, produced widespread rainfall amounts of 2 to 4 inches with isolated amounts over 6 inches in a 6 to 8 hour period. This rainfall in a short period of time caused extensive widespread flash flooding from Barre, to Saint Johnsbury, Vermont and many communities in between. Numerous roads and culverts were washed out and many residents and businesses received significant flood damage during the event, especially in the city of Barre. In addition, several rivers including the Winooski, Passumpsic, and Mad had very sharp rises and quickly rose above flood stage.
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Figure 21 shows the 24 hour Northeast Mosaic storm total precipitation from 12 UTC on 26th to 12 UTC on 27 May 2011. The yellow area is storm total radar rainfall estimates of 2.5 to 5 inches, while the red suggests rainfall amounts between 5 and 7 inches. From the image you can see the highest radar estimate rainfall occurred from Essex County, New York into Addison, Washington, Caledonia, Orange, and Essex Counties in Vermont during the event.
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Figure 22 shows the 24 hour precipitation (inches) observed from 12 UTC on 26th to 12 UTC on the 27th of May across our forecast area. The highest official observation came from the Plainfield, Vermont Coop at 5.22 inches in 24 hours located in eastern Washington County, Vermont. From the map you can see many locations receiving 2 to 4 inches (purple/pink) in the 24 hour period across most of central Vermont and parts of the Champlain Valley from the training thunderstorms. Click here for a complete listing of 24 hour rainfall reports across the region. A couple of the higher 24 hour rainfall amounts included 4.75 inches at the Saint Johnsbury Museum in Caledonia County, 4.15 inches 1 mile north of Northfield, and 4.06 inches in Danville, Vermont on 27 May 2011. This heavy rainfall in a short period of time associated with powerful thunderstorms caused major flash flooding across central Vermont.
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The heavy rainfall quickly produced flash flooding and flowed into many of the streams and rivers across the region. Figure 23 shows the hydro graph on the Passumpsic River, at Passumpsic, Vermont. This clearly shows a significant rate of rise between 00 UTC and 12 UTC on 27 May 2011 at the gage of over 10 feet. The gage started with a reading of 3.3 feet, but quick rose to a maximum reading of 18.95 feet at 10 UTC on 27 May 2011, which is just below major flood stage of 19 feet. This sharp rise caused significant flooding in the Passumpsic River Valley during the event. Also, the hydro graph showed very sharp rises on the Mad River at Moretown, Vermont. In addition, the Winooski at Essex Junction reached flood stage during the event, which caused some flooding in the Winooski River Valley from Montpelier to Colchester, Vermont.


Conclusion/Pictures:

The high instability values along with moderate deep shear produced an environment favorable for thunderstorm development during the afternoon and evening hours on 26 May 2011 across the WFO BTV forecast area. In addition, the amount of available moisture in the atmosphere and placement of the low-level and upper level jet features along with the present of a stationary boundary, helped to produce vertically tall thunderstorms capable of very heavy rainfall. Several rounds of thunderstorms first developed across the Saint Lawrence Valley and northern Adirondack Mountains, and then tracked east into the Champlain Valley, then into central and northern Vermont. These long-tracked supercells produced large diameter hail at several locations across the North Country, along with numerous reports of trees and power-lines down, from winds up to 70 mph. The highest concentration of damage occurred from Johnson to Island Pond, Vermont, with another maximum area of damage in the South Duxbury to Plainfield to Lunenburg, Vermont areas. Meanwhile, the greatest flash flooding occurred from Barre to Saint Johnsbury, Vermont. The following pictures were taken by Scott Whittier from WFO BTV during a storm damage survey in the areas of maximum damage.
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Figure 14: Google Map Plot of Severe Weather and Flash Flood Reports, along with the Warning Polygons.
Figure 15: Water Vapor Loop from 1640 UTC to 2310 UTC on 26 May 2011 with RUC80 500hPa Heights (light blue), RUC80 500hPa Wind Speed>50 Knots (yellow), and 5 Minute Lightning (red).
Figure 16: Surface analysis and composite radar on 26 May 2011 at 2100 UTC.
Figure 17: Northeast Composite Reflectivity from 1806 UTC to 2354 UTC on 26 May 2011 with Surface Observations plotted (white).
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.
Figure 21: Northeast Mosaic of Storm Total Precipitation from 12 UTC on 26 May to 12 UTC on 27 May 2011.
Figure 22: 24 Hour Precipitation (Inches) from 12 UTC on 26 May to 11 UTC on 27 May 2011.
Figure 23: Hydro Graph of the Passumpsic River Gage at Passumpsic, Vermont.
Figure 24: Photo by Citizen of Island Pond near Island Pond, Vermont
Figure 25: Photo by Scott Whittier of Damage Caused by Flash Flooding near Cabot, Vermont
Figure 26: Photo by Scott Whittier of Damage Caused by Flash Flooding near Cabot, Vermont.


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Burlington
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S. Burlington VT 05403
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