When assessing capability for severe convective weather (i.e. thunderstorms and heavy convective showers), two of the many atmospheric variables assessed by meteorologists involve both wind speed and direction, and how these two variables change with respect the vertical height in the atmosphere, and with time. These variables are known as directional wind shear (changing of wind direction in the vertical from southerly winds at the surface to more of a due westerly wind aloft in most cases of widespread severe weather outbreaks), and vertical speed shear (increasing wind speed with vertical height).
Directional wind shear is of particular importance to tornado and hail forecasting. This is due to the fact that storms moving into, or forming within, a highly ‘directionally sheared environment’ tend to have a higher propensity to develop rotation within them, which leads to higher potential for tornado development. In addition, the rotational aspect of the wind field can suspend water droplets in the actual thunderstorm (cumulonimbus) cloud, that would have fallen as rain, and keep them circulating within the cloud. The process causes these droplets to unfreeze, and refreeze repetitively as they travel vertically within the cloud to areas where the temperature is above freezing and then below freezing. In addition, during the ‘frozen’ stages, additional water droplets in the cloud may also collide and coalesce (merge or become part of) to the already frozen mass, causing it to increase in size and weight / mass. Eventually, the frozen mass (i.e. hailstone) becomes too heavy to be supported by the circulation of the storm and falls to the ground, making car and homeowner’s insurance companies very apprehensive, no doubt.
Speed shear is assessed when determining potential for damaging wind gusts and hail within thunderstorms. Thunderstorms tend to ‘mix down’ air in the mid and upper levels of the atmosphere, toward the surface of Earth. This is due to the convective process (in, up, and out motion). For instance, if review of atmospheric profiles indicates 50 knot or higher winds a couple thousand feet above the surface, with nearly light to calm winds at the surface, may raise a red flag to a forecaster, as storms may cause vertical mixing of the air aloft toward the surface (damaging winds).
Hail is also of concern, as the higher the speeds aloft, the more massive the hailstone will have to be to overcome the upward force of the wind keeping it suspended and circulating within the cloud. This is why such concern is echoed throughout the meteorological community when both speed and directional shear are noted, in tandem, over an area forecast to see thunderstorm development.
A strong cold front passed through the Capital Region during the daytime on Sunday, June 2, 2013. This cold front would be responsible for putting an end to a brief summer like period of heat and humidity that began on May 29, 2013 when tornado producing thunderstorms marked the entrance of the unseasonably warm airmass over the region.
Unlike its predecessor on May 29, this cold front would encounter a differing set of atmospheric conditions. The directional wind shear was not as pronounced over the region last Sunday, as it was in the event in the later part of May.
In advance of the actual cold front was a pre-frontal trough of low pressure which actually triggered the thunderstorm activity that Sunday afternoon. Slow moving, heavy rain producing, thunderstorms were the result as this trough of low pressure fed off of warm and increasingly moist air in place over the region. Radar estimated that as much as 4 to 6 inches of rain fell over portions of the Lake George Region, and southern Vermont, as well as 3 to 5 inches of rainfall over portions of the Capital Region and mid Hudson Valley. Actual weather spotter reports out of Bennington, Vermont reported as much as 8 to 10 inches of water across roadways in the area per National Weather Service documents.
In general, the storms dumped anywhere between one half to an inch and a half of rainfall over the region. The locally higher amounts mentioned above being in areas where storms redeveloped, and tracked, over the same general areas, a process known in weather jargon, as training.
Other higher rainfall amounts reported by co-op National Weather Service weather observers, and local media outlets, included 2 inches at Mechanicville, and 1.80 inches in Corinth, both in Saratoga County. 1.59 inches of rain was reported in the Washington county town of Hudson Falls, and 1.70 inches in the Bennington County, Vermont town of West Arlington.
Numerous reports of wind damage were also noted across the New York and Massachusetts borders, and the New York and southern Vermont borders, as the storms came through during the afternoon and early evening hours.
The data cited in this article was taken, in large part, from the National Weather Service storm summary that may be viewed by clicking here.
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