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The July 21, 2010 Severe Weather Event across
Vermont and Northern New York

Part II: Pre-Storm Environment
Water Vapor Loop From 1131 UTC to 2253 UTC on 21 July 2010
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Figure 4 shows an eastern United States water vapor loop from 1131 UTC (Universal Time Constant: i.e., EDT plus 4 hours) to 2253 UTC on 21 July 2010, along with 500 hPa (20,000 feet above the ground level) heights (yellow lines) and wind speeds 50 knots or greater at 500 hPa (red lines). This shows a strong short wave trough across the eastern Great Lakes, along with a left front 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 central Great Lakes into the Ohio Valley, suggests very strong jet stream winds aloft, helping to promote strong updrafts for long-lived thunderstorms. Also, plotted in the image below are 5, 10, 15, 20, 25 and 30 minute lightning strikes on 21 July 2010.

Upper Air Analysis
In this section we will discuss the pre-storm upper air conditions, which helped to produce severe weather across the Weather Forecast Office Burlington county warning area (CWA).
Click to enlarge
Figure 5 shows the 250 hPa (35,000 feet above the ground level) upper air analysis on 21 July 2010 at 12 UTC. Isotach, are lines of equal wind speeds (blue contours), streamlines (black lines), and temperatures (red) in Figure 5 below.

This shows a 75 to 125 knot westerly jet across the northern tier of the United States the left front quadrant of the jet approaching central New York into southern Vermont. This is a region of upper level divergence which aids in the development of tall thunderstorms with the potential to produce severe weather.

Sounding Data
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The 18 UTC May 21st rawinsonde observation at Albany, NY (Figure 6) shows modest instability, and strong deep layer shear, due to the placement of the strong mid to upper level winds across western New York into southern New England (as shown in the previous section). The combination of surface temperatures in the 80s and dewpoints in the mid to upper 60s created surface based convective available potential energy (CAPE) values of 1971 J/kg, with a lifted index (LI) of -6C (Celsius). CAPE values greater than 1500 J/kg, suggests moderately unstable environment, favorable for thunderstorm development. The large CAPE profile and very high equilibrium levels indicated thunderstorms would extend 35,000 to 45,000 feet into the atmosphere, and be capable of producing severe winds or large hail. The equilibrium level is the level at which the rising parcel equals the actual air temperature at that given height, and results in the rising parcel now becoming stable; it no longer accelerates upward. In addition, the Albany sounding showed a very deep and well mixed layer from the surface through 700 hPa. This deep mixed layer produced an environment favorable for transporting strong winds to the surface associated with thunderstorm convection.

In addition, the Albany sounding showed surface to 6km shear of 45 knots. This shear was a result of the approaching mid/upper level trough and the embedded jet streaks. Thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells and organized convection, such as squall lines and derechos are commonly associated with vertical shear values of 35-40 knots and greater through this depth. Finally, the 18 UTC Albany sounding showed a precipitable water value of 1.47 inches, which suggests the potential for thunderstorms to produce very heavy rainfall. Precipitable water is the depth of the amount of water in a column of the atmosphere if all the water in that column were precipitated as rain. Values greater than 1.2 inches, suggests a greater potential for heavy rainfall, especially during the summertime.

Surface Analysis
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The surface analysis at 18 UTC on 21 July 2010 (Figure 7) from the Mesoscale Surface Assimilation System (MSAS), showed a weak pre-frontal trough (dotted brown line) across the Champlain Valley, with a surface cold front (blue line with triangles) approaching the Saint Lawrence River Valley. This cold front and pre-frontal trough helped to enhance low level convergence for the development of strong to severe thunderstorms. In addition, the surface observations (yellow), showed temperatures ahead of these features well into the upper 70s to lower 80s with dewpoints in the 60s, helping to create an unstable environment for thunderstorm development. The dewpoint is the temperature at which a given parcel of air is cooled to form condensation, and when the temperature and dewpoint are equal the air is saturated and the relative humidity value would be 100%. The white lines in figure 7 below are the National Weather Service mean sea-level pressure analysis, which identifies a weak surface low pressure near Rouses Point, New York and a meso-high approaching Montreal, Quebec.

Severe Weather Parameters
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Figure 8 shows the 19 UTC RUC (Rapid Update Cycle) CAPE values across the North Country. The yellow color-filled image below indicates CAPE values greater than 1500 J/kg and suggests moderate instability. The RUC analysis indicated two maximum areas of CAPE; the first across the Saint Lawrence River Valley and the second across the Champlain Valley, where values approached 2500 J/kg. The potential instability was contributed to by insolational heating (surface temperatures in 80s), and surface dewpoint values in the mid to upper 60s, while southerly winds were 5 to 15 mph.
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The surface to 6 km bulk shear (Figure 9) from the RUC at 19 UTC on 21 July 2010 revealed an area of increasing shear approaching the region from the southwest. The analysis showed values between 40 and 50 knots (pink color fill in Figure 9) approaching northern New York at 19 UTC. These strong values support organized and persistent thunderstorms, with a greater potential for supercells, capable of producing large hail and damaging winds. This increased shear was a result of the approaching mid/upper level trough and associated embedded upper level jet streak, which was highlighted in the 12 UTC upper air data.

Precipitable Water
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Figure 10 shows the RUC 40 km precipitable water values across the northeast United States at 18 UTC on 21 July 2010. This analysis shows values between 1.75" and 2.0" across much of the North Country, and provides an excellent indicator that thunderstorms would have the potential to produce very heavy downpours. The deep southerly flow ahead of the approaching surface features and mid/upper level trough helped to transport this very moist/humid air mass into our region. As a result, thunderstorms developed, which produced hourly rainfall rates of 1 to 2 inches across the region, and caused some flash flooding.

<<< Back to Part I Continue to Part III >>>
Figure 4: Water Vapor Loop from 1131 UTC to 2253 UTC on 21 July 2010, along with Rapid Update Cycle (RUC) Wind Speeds >50 Knots (red lines), 500 hPa Heights (yellow lines), and Lightning
Figure 5: 250hPa Upper level jet on 21 July 2010 at 12 UTC Isotach (dark blue >75 knots and lighter blue >100 knots), Streamlines (black), and Temperatures (red)
Figure 6: Albany, New York 18 UTC Sounding on 21 July 2010
Figure 7: Surface analysis at 18 UTC on 21 July 2010 with Mesoscale Surface Assimilation System (MSAS) National Weather Service mean sea-level pressure (white), surface observations (yellow), and cold front (blue line with triangles), and surface trough (dotted brown line)
Figure 8: RUC Surface-based Convective Available Potential Energy (CAPE) at 19 UTC on 21 July 2010
Figure 9: RUC 0-6 km bulk shear at 19 UTC on 21 July 2010
Figure 10: Figure 10: RUC Precipitable Water (inches)

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