A South Coast Special

A doctor at Brigham and Women's Hospital in Boston grabs a few chunks of 2.25
A doctor at Brigham and Women’s Hospital in Boston grabs a few chunks of 2.25″ diameter hail.

It’s hard to believe that after egg sized hail fell in Boston Tuesday afternoon it was not the most impressive severe weather event on Tuesday, August 4th. The morning damaging wind storm produced a swath of damage across Long Island and southern New England that is one of the most impressive I’ve seen in years.

Tuesday morning had all the hallmarks of a daybreak severe weather episode. Strong wind shear, steep lapse rates, and an impressive surge of low level moisture. Our computer models struggled with whether or not storms would develop but I decided to be fairly bullish on the severe weather potential anyway the night prior (as did some other local meteorologists). As it turns out – that was the right call. The morning sounding from Chatham (modified for the pre-storm environment at Warwick, RI) shows an uncapped, highly sheared, and highly unstable environment.

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With approximately 3,000 joules of CAPE and more than 60 knots of effective bulk shear when storms were able to develop they became beasts. Storm mode was messy with the clusters resembling giant high reflectivity blob, but on doppler radar velocity data showed several pockets of intense damaging wind.

The first such pocket moved across the North Shore of Long Island producing widespread wind damage. When it left the Island a second area of strong winds clipped Groton and Stonington while the core of the wind moved ashore in South County, Rhode Island.

okx11 fatality occurred in Mystic from a falling tree from the northern storm while the southern storm produced an incredible 71 mph wind gust on Great Gull Island (near The Race), a 61 mph wind gust on the Stonington Borough breakwater and widespread wind damage in far southeastern Stonington (Pawcatuck and Greenhaven) as well as Westerly and Charlestown. In Charlestown, an 83 mph wind gust was measured and falling trees injured several campers at Burlingame State Park.

Trees down on Greenhaven Road in Stonington
Trees down on Greenhaven Road in Stonington

KGON 041018Z AUTO 28027G43KT 1 1/2SM R05/4000VP6000FT +TSRA BR BKN014 OVC033 23/21 A2984 RMK AO2 PK WND 24043/1015 LTG DSNT ALQDS RAB10 TSE01B07 P0012 T02280211

KWST 041034Z AUTO 22016G48KT 1/2SM TSRA FG FEW004 BKN012 OVC018 21/19 A2987 RMK AO2 PK WND 23048/1028 VIS 1/4V1 3/4 LTG DSNT ALQDS RAB21 TSB20 P0030 T02110189

Meanwhile, a second cluster of thunderstorms was bringing an exceptional wind storm to portions of the Providence metro area. In terms of damaging straight-line winds the radar signature over Warwick/Cranston was one of the most impressive you’ll see in New England.

box1The first sign of trouble was over western Rhode Island in the town of Coventry. In about 5 minutes the core of the storm exploded – with 65 dBz over at 30,000 feet by 6:12 a.m. Within the next 20 minutes what starts as a relatively benign looking velocity signature in the low levels of the atmosphere turns into a monster with a large core of destructive winds (at least 5 miles wide with >50 knots).

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A closeup of the storm over Warwick and Cranston shows just how intense the storm was with a few pixels of radial velocity approaching 80 knots.

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As luck would have it this passed very close to TF Green airport where a wind gust of 67 mph was measured as the storm moved through.

KPVD 041034Z AUTO 30027G52KT +TSRA 17/16 A2985 RMK AO2 PK WND 29058/1028 LTG DSNT ALQDS RAB14B29 TSE01B11 P0057 T01720161 RVRNO $

What’s interesting is that this wind occurred about 2 miles south of where radar was picking up the strongest velocity. In fact, the velocity sampled over KPVD was 56 knots (at 1300 ft) which is almost exactly the peak gust measured at the airport was. It stands to reason that areas on the Warwick/Cranston line saw wind gusts up to 90 mph as the storm roared in and the atmosphere was able to efficiently mix to the surface.

Courtesy: WX1BOX Warwick/Cranston RI - N1EGS-John Buco
Courtesy: WX1BOX Warwick/Cranston RI – N1EGS-John Buco

A few takeaways from this event that really stand out.

One, The the Storm Prediction Center frequently doesn’t outlook or issue watches for these “south coast specials”. They’re characterized bya relatively narrow zone of high theta-e air advects north from the ocean resulting in a small area of substantial CAPE near the coast. What’s frustrating is that even though there’s only a small geographic area at risk – the population for that area is HUGE – including Long Island, southern New England, and even the New York City area. A severe thunderstorm watch would have been helpful to increase situational awareness of forecasters (both TV and NWS) around the region.

Two, even though reflectivity imagery didn’t have a classic signature for a widespread destructive wind event – velocity data from both OKX and BOX was spot on in identifying the areas hardest hit. Even a giant blob of reflectivity (as opposed to a well-defined squall line) can produce significant severe winds.

Three, this event shows that true severe thunderstorm events can be just as damaging – if not more than a tornado event. In fact, the 125,000 power outages from this thunderstorm event in Rhode Island was worse than Hurricane Sandy. This event was one of the few around here to verify Severe Thunderstorm Warnings with multiple 50 knot measured gusts. This storm was the real deal. It’s important to differentiate between the run of the mill severe storms that take down a couple diseased tree limbs from the ones that have the potential to produce widespread and life-threatening weather. This is one of our biggest challenges here in southern New England during convection season.

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