Rare Severe CT Mesoscale Convective Vortex

Mesoscale Convective Vorticies aren’t unusual. The circulations that develop within clusters of convection occur frequently throughout the country. They’re not terribly unusual in the northeast either. What is unusual for southern New England is an MCV directly resulting in widespread severe weather including microbursts and a tornado on Long Island.

A beautiful looking MCV developed a surface mesolow and an enhanced low level jet on its eastern side Friday afternoon. Widespread wind damage occurred immediately to the east of the vortex as illustrated by OKX radial velocity images.

OKX 0.5 degree radial velocity and base reflectivity 2001 UTC

Radar imagery shows the strong rotation but not quite tight enough to classify as a tornado vortex signature. The rotation was strong but gate to gate shear was not enough to get me worried about a tornado touchdown.  Surprisingly, once the rotation left the jurisdiction of the National Weather Service in Upton, NY the Taunton office issued a tornado warning even though the rotation never tightened up.

A large swath of wind damage occurred east of the mesolow/vortex from Guilford and Madison north to Ellington. An impressively long swath of wind damage in this part of the country especially since damage was fairly widespread through those towns with 40-55 knot wind gusts. The most significant damage occurred in Glastonbury where the National Weather Service estimated microburst winds of between 85 and 100 mph.

OKX  2027Z 0.5 degree SRV shows weak gate-to-gate shear within broad (but strong) mesocyclone. This is approximately 4300 feet AGL using a storm motion of 190 degrees at 28 knots.

OKX 2027Z 0.5 degree base velocity. Bright pink pixels are outbound velocities of >60 knots approximately 4300 feet AGL.

While the radar velocities don’t show strong evidence of a tornadic circulation (too broad here) the strong winds on the east side of MCV were quite apparent with many >60 knot pixels at around 4,300 feet AGL. Impressive. These winds were readily transported to the surface with a microburst that started near Homestead Drive/Paddok Street and continued north to Hebron Avenue. The worst of the damage was near Butler Drive and Needletree Lane areas.

Here’s a look at the path of the center of the mesocyclone (where gate to gate shear was the strongest) and a path of the microburst.

Path of Glastonbury microburst and center of low level mesocyclone per OKX storm relative velocity around 4300 ft AGL

No surprise here that the microburst developed to the east of the cyclone’s center. I did notice that the core of the strongest winds at the ground was just west (by about 0.5 miles) of where the strongest winds 4,300 ft AGL winds were located on radar. This makes sense given the fact the core of the strongest outbound velocities tilted east with height when looking at other tilts available from OKX.

OKX 0.5 degree base reflectivity showing mature MCV at 2036Z

The base reflectivity images show a beautiful, mature mesoscale convective vortex. While of a smaller scale this is not too dissimilar from the 21 July 2003 MCV in Pennsylvania that  produced an F3 tornado in Potter County.

What is interesting is that most of the damage from the Connecticut MCV occurred near vortex itself with only sporadic damage on the feeder band immediately east of the vortex (wind damage was reported in Coventry and Windham). There was also little if any lightning from this severe event – it was convection but in most cases not even a thunderstorm!

OKX 0.5 degree base reflectivity. Note 2 inflow/feeder bands into the MCV – one over Middlesex County CT and the other from south of Montauk into Washington County RI

There was wind damage reported in the larger band well east of the vortex over the Atlantic Ocean, Block Island, and Washington County RI. Winds gusted to 52 mph in Block Island and trees were reported down in portions of Washington and Kent Counties in Rhode Island.

There were also several transient low level mesocyclones in this outer band that seemed to act quite similar to a landfalling tropical cyclone.  Here’s one just south of Block Island with 55 knots of outbound SRV and 36 knots inbound! Not a shabby TVS for Mohegan Bluffs! There weren’t any reports of a water spout from this couplet but we will likely never know what it actually produced. The 2003 MCV in Pennsylvania produced an F3 tornado in one of those feeder bands so we have seen northeast tornadoes from this type of setup. We lucked out with only sporadic wind damage in the 2 feeder bands and not a tornado.

The MCV over Connecticut did produce a mesolow that lead to a quick pressure drop near its center. HFD reported a sea level pressure of 1006.0 mb at 2053Z down from 1008.6 mb at 1953Z.  But take a look at this Glastonbury mesonet station barograph (hat tip to my friend Steve Gencarelle for pointing this out!).

Wow!! 1007.5mb to 1001.0mb in 40 minutes!!! Now that’s a mesolow! The strong isallobaric component of the wind likely provided an assist to the already strong winds.

This certainly was an unusual severe weather day across the state. While we know many of the signals for widespread/high end severe weather in this part of the country there always are exceptions. Yesterday was one of those exceptions and would be next to impossible to forecast well ahead of time.

The setup with modest CAPE but strong shear does occasionally produce significant severe weather. This time of year (July and August) I always say “beware the upper level low!” The 500mb 12z analysis from SPC shows a deep an anomalous closed low over the Great Lakes.

A strong unidirectional southerly flow lead to nearly 40 knots of 0-6km bulk shear on the afternoon of the 10th. The low level jet, near 25 knots, was not terribly strong given the setup. Modest CAPE around 1000 j/kg for surface based parcels was enough for surface based convection in a large area of ascent/QG lift ahead of the upper level low.

18z RUC analysis BUFKIT sounding for HFD

Hodographs weren’t terribly long given modest low level jet but there was enough turning with locally backed surface winds for spinups as we saw on Long Island.

While these setups aren’t classic significant severe setups, warm season cut-off lows can lead to pockets of sig severe. It’s difficult to forecast (mainly due to weak instability, ongoing convection, clouds cover etc.) but watch out for cut offs in August!

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7 thoughts on “Rare Severe CT Mesoscale Convective Vortex

  1. While it’s apparent that the whole system over Eastern New England was an MCV, it seems that the small vortex over CT was a separate vortex within the larger MCV. I looked up microscale meteorology and it’s certainly above the microscale. Perhaps it was a vortex between the storm scale and mesoscale because it did evolve from a single cell. Sort of like this storm http://www.spc.noaa.gov/coolimg/nc_storm/index.html

    • Hey Chris, yeah I actually thought of the North Carolina “tornadocane” but there’s a few differences. One is that the NC storm was much smaller. It also evolved from a supercell wheras this one did not. I do think you’re right that this was a smaller vortex within a larger one – you could see the larger one earlier in the day over the Delmarva. Since this small MCV was at least 40km wide it definitely would classify as “mesoscale”.

      • Thanks for the reply! Yeah I guess that makes sense. Definitely one of the weirder storms we’ve gotten this summer, though the fact that this was from a closed low reminds us fall’s around the corner

      • When I said closed low I meant attached to the jet stream. Closed isn’t the right term.

  2. Pingback: New England’s “Longest” Tornado | Way Too Much Weather

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