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Heavy Precipitation Supercells of 29 May 2012

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Part II: Pre Storm Environment

One of the more interesting factors behind this severe weather and flash flooding episode occurred during the early morning hours of May 29th. Severe thunderstorms had initially developed across western Clinton and Essex Counties in New York as well as across central and northeastern Vermont. Hail up to half-dollar size (1.25" diameter) fell across parts of Washington and Windsor Counties in Vermont. However, thunderstorms "training" over the same locations resulted in 1-2 inches of rain across central and northeastern Vermont. These antecedent wet conditions would help set the stage for localized flash flooding later in the afternoon.

The early-afternoon surface map (not shown) showed a cold front stretched across much of eastern Ontario, with a warm front extending from western Quebec southeastward across north-central and northeastern Vermont. Afternoon temperatures in the mid-80s and dewpoints in the mid to upper 60s would result in the development of a large amount of instability (CAPE values ranging from over 3000 J/kg across northern New York to around 1000-2000 J/kg across most of Vermont) which led to the possibility of another round of severe thunderstorms by the afternoon.
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Figure 6 is a water vapor satellite imagery loop, with 500hPa heights (solid yellow lines), 500hPa wind speeds greater than 50 knots (dashed red lines) and 250hPa wind barbs greater than 50 knots (green wind barbs) from the Rapid Refresh forecast model. Observed lightning strikes are also shown to get a sense as to where the most intense thunderstorms are located. In the image loop, we see a potent upper-level shortwave trough over the northern Great Lakes, with an upper-level ridge located just off the coast of Massachusetts into the Gulf of Maine. Two upper-level jets are evident in the loop: one approaching southern Ontario, and the other located over northeastern Maine. These jets helped to enhance upper-level divergence over the thunderstorms themselves, and helped maintain the persistence of the thunderstorm updrafts by enhancing vertical wind shear through a deep layer of the atmosphere.
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Figure 7 shows a high-resolution (250 meter) visible satellite image of the supercell thunderstorm that would go on to produce the EF0 tornado in West Glover, VT. The satellite image is taken from the MODIS Aqua satellite instrumentation platform at around 18 UTC May 29th. The high-resolution detail of the image illustrates several important details of the storm's structure and its near-storm environment. The cauliflower appearance in the cloud tops shows where the strongest updrafts are occurring, and in the center is an "overshooting top", which occurs when the strongest updraft are able to penetrate above the thunderstorm anvil and into the lower stratosphere. Near-storm environmental features shown include a west-to-east outflow boundary (which additional storms would develop upon) and the surface warm front (designated by the red line in the image). Also note the cumulus clouds that are aligned in parallel "streets" southeast of the storm - an indication that strong southeasterly unstable inflow air was being ingested into the storm's updraft, fueling its strength.

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 100 knot anticyclonic curved 250hPa was lifting across the eastern Great Lakes into central Canada and placing our region in a very favorable region of upper level divergence, which promoted deep thunderstorm convection.

This strong jet energy was associated with a digging mid/upper level trough and cold pool aloft located over the central Great Lakes. Figure 8 shows the 250hPa (35,000 feet above the ground level) upper air analysis on 30 May 2012 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).

Sounding and Surface Data
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In Figure 9, the early afternoon sounding taken above Albany, NY at 1700 UTC (1 PM) shows several very important features. One of the most important is the very large amount of CAPE and the lack of any CIN. The sounding shows over 3500 J/kg of surface-based CAPE, which is more than enough for development of thunderstorms.

The characteristic of the CAPE also plays a big role in thunderstorm features. This CAPE was quite wide which allowed for explosive updraft and thunderstorm development. The CAPE also had a significant amount contributed to it within the "hail-growth zone" which is the layer of the atmosphere between -10 C and -30 C, where hail growth is optimized.
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All of the features mentioned above combined with potent surface features to create a volatile day across the North Country. Figure 10 shows the surface warm front located along the Green Mountains of Vermont, separating warm and humid air from the cooler maritime air. This enhanced low level convergence (southeast winds to the east of the front, southwest winds to the west of the front) and turning of the winds in the low levels, leading to added helicity and directional wind shear. Notice that the location of the tornado was along the warm frontal boundary. This was the same warm front that was responsible for the development of heavy rain and thunderstorms the night before. Just west of the warm front was the feature that was the initial focus of thunderstorm development, a prefrontal trough. This was responsible for the first round of thunderstorms that affected the region between 1 and 4 PM. The cold front back off in Western New York was the focus for thunderstorm development across New York that later propagated eastward between 4 and 8 PM with a second round of thunderstorms.

Severe Weather Parameters
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Figure 11 shows the values of the Supercell Composite Parameter (SCP). Supercell thunderstorms tend to form in unstable environments with large amounts of vertical wind shear. The SCP shows a quick representation of the "composite" effects of both instability and shear.

Areas in which values are at or greater than 1 indicate areas where supercells may be more likely, as the combination of vertical wind shear and instability in these areas are sufficiently high enough to support their formation. SCP values are greater than 1 across interior New England; however, SCP values are as high as 12 across the Champlain Valley. The environment clearly supported supercells across most of interior New England; but especially so across the Champlain Valley.

Precipitable Water
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Figure 12 shows precipitable water values across the North Country during the afternoon on May 29th. Precipitable water is the depth of moisture in a column of the atmosphere. This is a strong indicator of the potential for heavy rain at any given location. A sharp moisture gradient is visible across Ontario, Canada associated with the surface cold front approaching the Saint Lawrence Valley. Air ahead of the approaching cold front in the warm sector was very saturated with precipitable water values approaching 2 inches. This is two standard deviations above normal for our region for the month of May. With precipitable water values this high it became clear to forecasters that heavy rain would be possible in any thunderstorms. This threat was recognized and a flash flood watch was issued around 11 UTC, following widespread 1 to 2 inches rainfall totals during the overnight hours and ending early Tuesday morning.

Figure 6: Water Vapor Loop from 1640 UTC to 2255 UTC on 29 May 2012 with RUC40 500hPa Heights (yellow), RUC40 500hPa Wind Speed>50 Knots (red), and Lightning (plotted).
Figure 7: MODIS Aqua visible satellite 250 meter resolution around 18 UTC on 29 May 2012.
Figure 8: The 250 hPa (35,000 feet above the ground level) upper air analysis on 30 May 2012 at 00 UTC. Isotach, (dark blue >75 knots, lighter blue >100 knot, and purple>150 knots), streamlines (black lines), and temperatures (red)
Figure 9: Albany, New York 1700 UTC Sounding on 29 May 2012
Figure 10: Local Analysis and Prediction System surface base CAPE (image) and surface observations at 18 UTC on 29 May 2012.
Figure 11: Supercell Composite Parameter and Bunkers storm motion knots (wind barbs) at 20 UTC on 29 May 2012
Figure 12: RAP 40 Precipitable Water Analysis at 22 UTC on 29 May 2012

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