National Weather Service Burlington, VT Twitter Page National Weather Service Burlington, VT Facebook Page
Local forecast by
"City, St" or zip code
 Current Hazards
 Current Conditions
 Model Data
 Weather Safety
 Contact Us
The July 21, 2010 Severe Weather Event across
Vermont and Northern New York

Part I: Introduction
On 21 July 2010 the second significant severe weather outbreak of the 2010 season occurred across northern New York and parts of Vermont. A very warm and moist air mass was in place across the North Country with temperatures in the 80s and surface dewpoints in the mid to upper 60s. This very unstable environment along with an approaching cold front helped to produce strong to severe thunderstorms across the North Country during the afternoon and evening hours on 21 July 2010.

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. Furthermore, the thunderstorms were accompanied by very heavy rainfall amounts of 3 to 4 inches, which caused some flooding. Most of the severe weather reports were concentrated along a line for near Brushton, New York in western Franklin County to Saint Johnsbury, Vermont in Caledonia County, associated with a long-tracked miniature supercell, 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 overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local climate up to 20 miles (32 km) away. The next area of damage and large hail occurred with a second long-tracked miniature supercell, which tracked from near Bristol to Brookfield, Vermont, then into western and central New Hampshire. As a result of the extensive wind damage, two National Weather Service (NWS) storm surveys were performed. The first survey was in Franklin County, New York, from Brushton to Malone, and found damage consistent with straight-line thunderstorm winds of 60 to 75 mph. The second survey near Brookfield, Vermont, also was a result of straight-line winds of 80 to 90 mph. Click here for the Public Information Statement associated with the storm surveys conducted by the NWS Burlington, Vermont.
Click to enlarge
Figure 1 shows a Google Map display of the damaging wind reports and hail. The green bubble indicates the size of the hail in inches, while the orange bubbles in the image shows where damaging thunderstorm winds occurred. The damage reports included trees and power lines down through the region, along with structural damage in Franklin County, New York and in Orange County, Vermont. In addition, 1.50 inch diameter hail was observed at Bristol, Vermont and near 2.0 inch hail was reported in Brookfield, Vermont. Also, several reports of large hail were received across northern New York near Potsdam and Bangor. Click here for a complete listing of all severe weather report 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 and shear 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 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.

SPC Products (Day 1 Outlook and Probability of Damaging Winds)
In this section we will examine the SPC Day 1 Convective Outlook (Figure 2), along with the probability of damaging winds (Figure 3).
Click to enlarge
From Figure 2 you can see the SPC Day 1 Outlook had the Champlain Valley and all of Vermont highlighted in a moderate risk for severe thunderstorms, which is very unusual for our area. 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. Meanwhile, the definition of a moderate risk day from SPC indicates a potential for a greater concentration of severe thunderstorms than the slight risk, and in most situations, greater magnitude of the severe weather. Typical moderate risk days include multiple tornadic supercells with very large hail, or intense squall lines with widespread damaging winds.
Click to enlarge
Meanwhile, Figure 3 shows the probabilistic forecast of severe thunderstorm winds to be 45% across our region on 21 July 2010. From SPC a probabilistic forecast represents the probability of one or more events occurring within 25 miles of any point during the outlook period. Therefore a 45% probabilistic forecast, means you have at least a 45% chance of receiving severe thunderstorm winds, within 25 miles of any point during the outlook period. It's very rare for SPC to place our region in a 45% or greater outlook for severe thunderstorm winds.

Continue to Part II >>>
Figure 1: Local Storm Report Plot of Damaging Winds (orange square s with tree) and Hail (green and orange bubble)
Figure 2: Storm Prediction Center Day 1 Convective Outlook at 1613 UTC on 21 July 2010
Figure 3: Storm Prediction Center Day 1 Damaging Wind Outlook at 1613 UTC on 21 July 2010

National Weather Service
1200 Airport Drive
S. Burlington VT 05403

Webmaster: Webmaster
Page last modified: August 7, 2010
About Us
Career Opportunities
Privacy Policy