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

Part II: Pre Storm Environment

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Figure 4 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 shows a strong short wave 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.

Upper Air Analysis
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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). A strong 130 to 150 knot anticyclonic curved 250hPa was lifting across eastern Canada and placing our region in a very favorable region of upper level divergence, which promoted deep thunderstorm convection.

Figure 5 shows the 250hPa (35,000 feet above the ground level) upper air analysis on 27 May 2011 at 00 UTC. Isotachs are lines of equal wind speeds (blue contours). Also shown are streamlines (black lines) and temperatures (red numbers in station plots).
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Figure 6 shows the 850hPa (~4,500 feet above the ground level) upper air analysis on 27 May 2011 at 00 UTC. This upper air analysis shows a strong boundary approaching northern New York, with a strong low-level jet of 30 to 40 knots ahead of this boundary.

This 850hPa jet helped to aid in low level moisture advection and transport of a very humid and warm air mass north into our region, along the approaching boundary. These features combined with the 250hPa divergence, helped produce an environment favorable for severe thunderstorms and very heavy rainfall.

Sounding Data
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The 00 UTC May 27th rawinsonde observation at Albany, NY (Figure 7) shows high instability, and moderate deep-layer shear, due to the placement of the strong low to mid-level winds across central New York into southern Canada(as shown in the previous section). The combination of surface temperatures in the lower 80s and dewpoints in the mid to upper 60s created surface-based convective available potential energy (CAPE) values of 2403 J/kg, with a lifted index (LI) of -8C (Celsius). CAPE values greater than 1500 J/kg, suggest a moderately unstable environment, favorable for thunderstorm development. The large CAPE profile and very high equilibrium levels indicated thunderstorm tops would extend to 35,000 to 45,000 feet into the atmosphere, and be capable of producing severe winds or large hail, along with very heavy rainfall. 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 surface to 1km shear of 27 knots. This shear was a result of the approaching boundary and the embedded low level jet. 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, which was present across our region due to the jet stream winds aloft. Finally, the 00 UTC Albany sounding showed a precipitable water value of 1.45 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.

Severe Weather Parameters
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Figure 8 shows the 19 UTC RUC (Rapid Update Cycle) CAPE values across the North Country on 26 May 2011. The yellow color-filled image below indicates CAPE values greater than 2000 J/kg and suggests moderate instability.

The RUC analysis indicated a maximum area of CAPE; across the northern Adirondacks into central and southern Vermont, with values between 2000 J/kg and 3000 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 10 to 20 mph.
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The surface to 3 km bulk shear (Figure 9) from the RUC at 19 UTC on 26 May 2011 revealed an area of increasing shear approaching the region from the west across the Saint Lawrence Valley. The analysis showed values between 50 and 60 knots (pink color fill in Figure 9) across 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 mid to 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 26 May 2011. This analysis shows values between 1.5" and 1.8" pooling ahead of a surface boundary approaching the Saint Lawrence Valley, 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 feature 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 significant flash flooding.

Figure 4: 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 5: The 250 hPa (35,000 feet above the ground level) upper air analysis on 27 May 2011 at 00 UTC. Isotach, (dark blue >75 knots, lighter blue >100 knot, and purple>150 knots), streamlines (black lines), and temperatures (red).
Figure 6: The 850 hPa (4,500 feet above the ground level) upper air analysis on 27 May 2011 at 00 UTC. 850 hPa Wind Barbs (blue), 850 hPa Heights (black lines), 850 hPa Temperatures (isotherms: red>0C and Blue<0C), 850 hPa Dewpoint (green).
Figure 7: Albany, New York 00 UTC Sounding on 27 May 2011
Figure 8: RUC40 Surface-based Convective Available Potential Energy (CAPE) on 26 May 2011 at 19 UTC
Figure 9: RUC40 Surface to 3km Bulk Shear at 19 UTC on 26 May 2011
Figure 10: RUC40 Precipitable Water Analysis at 18 UTC on 26 May 2011

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