Boston Metro Microburst, Unexpected?
By Quincy Vagell on July 22, 2012, 4:45pm
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I recently heard some discussion that the atmosphere was not particularly favorable for severe weather in eastern Massachusetts on Wednesday.
Well, even though CAPE (potential energy) values were not terribly impressive, I do feel that several red flags went up.
A severe thunderstorm led to a microburst just northwest of Boston, with winds of 70 to 80 miles per hour. The NWS even issued a Tornado Warning for the storm. There were several reports of funnel clouds, although they could have been mistaken for shelf clouds or gust fronts.
Anyway, the image below shows some severe weather parameters that were huge red flags for severe weather, including wind damage and possible tornadoes:
1. Craven Significant Severe Parameter: I like this term, because it takes into account both CAPE and shear. Most severe storms happen when values exceed 20,000 and values over 60,000 favor extreme severe weather, supercells and tornadoes. Here, there's a maxima literally right over Boston, where values reach 90,000. This is extreme for southern New England.
2. Effective Bulk Shear: This ties back to the previous parameter. Even though strong shear was mostly north of Connecticut, it was northeastern Mass. that had some very strong shear. Shear makes thunderstorm development more favorable and strong shear, over 40 knots, favors severe thunderstorms. Here, the shear values were around 55 knots where the microburst hit.
3. Supercell Composite Parameter: This is similar to Craven's number, but it has a focus on supercell thunderstorms. Supercell thunderstorms are literally "super cells", where individual thunderstorm cells become intense storms. When values exceed 6-8, especially in the Northeast, it's usually a sign that supercells will develop. In this case, numbers are 10+ and approaching 12 near Boston.
4. Energy-Helicity Index: When values of this index reach the range of 1-2 or higher, the threat of wind damage and tornadoes increases significantly. In this case, there are once again some high values, on the order of 1+ in the vicinity of Boston. It is interesting to note that values reach 2 a bit further to the south, and there were numerous reports of damaging winds in that vicinity.
5. Normalized CAPE: I like to look at this value in addition to "standard" CAPE. It gives a better idea of energy within the atmosphere. As values exceed 0.25, this means that the area of CAPE is larger within an air parcel. In the image above, NCAPE values were around 0.25 in the Boston area.
6. Significant Tornado Parameter: For this parameter, any value over 1 indicates the potential for tornadoes. As numbers get over 2, the threat becomes even greater. Here, values were between 2.5 and 3, yet another indicator that particularly severe and possibly tornadic thunderstorms could form.
The moral of the story is, there are MANY factors to consider for severe weather.
It's not just about CAPE or about one particularly severe weather parameters. The big picture has to be considered. Once you start seeing a lot of these numbers line up, then the red flags go up.
Also, even with some pretty impressive values with some of these parameters, there were no confirmed tornadoes in the region on Wednesday.
For advanced severe weather watchers, the following mesoscale analysis page from SPC is particularly helpful in tracking and forecasting severe thunderstorms: http://www.spc.noaa.gov/exper/mesoanalysis/new/viewsector.php?sector=16#
Opening image courtesy of the Boston, MA NWS Facebook Page.
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