The National Weather Service in Mount Holly, PA confirmed today that an EF-2 tornado touched down in Delaware County, PA last night. More specifically, they note that the tornado touched down in the Glens Mills, PA area.
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The National Weather Service has issued a Severe Thunderstorm Watch for the NYC Metro and surrounding areas of New York, New Jersey and Connecticut until 9:00pm. Severe thunderstorms are expected to develop this afternoon. In the presence of ample instability and wind shear, storms will be capable of organizing quickly.
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An EF-0 tornado touched down in Manorville, NY on Monday afternoon, producing winds up to 85 miles per hour. The tornado was on the ground for 1.6 miles and caused significant tree and structural damage, although it was only 50 yards in width.
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A complex of severe thunderstorms tracked from Northeast Pennsylvania into New York City on Thursday evening, producing widespread wind damage. The same complex of storms survived all the way to Montauk, almost 200 miles in total length, while still producing widespread damaging winds.
The storms formed in Northeast Pennsylvania during the middle afternoon hours on Wednesday, and quickly began shifting east/southeast toward New Jersey. The atmosphere ahead of the storms was moderately unstable. More notable was the presence of sufficient wind shear – this wind shear helps to keep strong thunderstorms organized and can also transport strong winds down to the surface.
By the early to mid evening hours, the very same storm complex was now pushing through New Jersey. Wind damage reports became more frequent, and as the storm moved through the NYC Metro Area it produced 58mph wind gusts in Tribeca and the Financial District.
The same complex of storms then tracked eastward through Queens and Brooklyn, while maintaining strength. While instability was lower in these areas, ample wind shear and a strengthening low level jet (winds in the lower levels of the atmosphere) helped it maintain intensity.
Over Long Island, the combination of the mentioned instability and strengthening low level jet stream helped the storm to maintain intensity, and also aided in downward transport of winds. This resulted in widespread wind damage reports across Nassau and Suffolk counties throughout the evening.
The storm caused a wall collapse and widespread tree damage in the area from East Farmingdale through Ronkonkoma including Islip and Brentwood. As of this morning, over 20,000 residents remained without power – likely from trees falling on power lines.
While not unheard of, a complex of storms moving from Northeast PA through the NYC Metro and all the way to Montauk is rare. The swath of wind damage reports with this single storm is quite impressive – arguably the most impressive of the summer so far.
This storm existed in an environment that was, at best, moderately favorable for severe weather. Instability was present but not extreme (around 1000j/kg of surface based cape). Wind shear was impressive (40 knots of mid level shear for storm organization). Low level wind fields were not overly favorable during the morning but became more favorable by evening. In the end, the storm took full advantage of its environment, riding along a gradient of favorable conditions well into the early evening.
Much of our region was placed within the right entrance region of an impressive jet stream on Thursday, which helped to enhance vertical motion during the evening. But more notable was likely the gradient of instability and the development of a stronger low level jet. This helped to enhance low level winds significantly and likely contributed to the uptick in wind damage reports on Long Island.
The low level jet is known to kick up after sunset (nocturnal low level jet), and in the Midwest and Plains states can help fuel evening outbreaks of tornadoes. In our area, the increasing low level jet helped foster an enhanced threat for wind damage – essentially giving the storm some extra “juice” to continue its trek eastward all the way to Montauk.