Memorial Day Weekend Hail

Courtesy: Keri Dallas / Woodstock

Courtesy: Keri Dallas / Woodstock

The May 25 hail storm in eastern Connecticut was an odd cluster of storms. The only severe reports in southern New England were of large hail with no reports of damaging wind.

Synoptically, forcing was marginal at best. The main forcing for convection appeared to be differential heating boundaries (storms fired first in Berkshire foothills and Worcester Hills) and some enhancement through sea breeze fronts and outflow boundaries. While there was modest instability present (MLCAPE around 500 j/kg) vertical shear between 30 and 40 knots was able to produce transient supercell structures. Very low wet bulb zero heights – around 7,500 ft AGL, also increased the hail threat.

One such transient supercell produced large hail near the Mass Pike southwest of Worcester and in Thompson and Woodstock, CT. On radar, 2 large three body scatter spikes (TBSS) were present downradial of 2 hail cores along with a side lobe (an artifact of the WSR-88d occasionally seen).

woodstockhail

 

As the storm moved south dozens of dime to quarter size hail reports were received across Windham and New London Counties. The mesocyclone, while never particularly strong, extended in the vertical through the hail growth zone (-10ºC to -20ºC). In addition, the height of the 60dbz level occasionally reached -20ºC when the storm was at its strongest.

Although the was one report of golf ball size hail (that report may be on the high side) most of the hail reports remained around 1″ in diameter. There is little evidence of giant wet hail growth (CC <0.9 in the hail growth zone) and in addition the typical proxys for updraft strength of radar (mesocyclone intensity, storm top divergence) weren’t exactly off the charts.

That said, the storm did produce a classic large hail (>1″ diameter) signal for most of its lifetime in eastern Connecticut.

griswoldhail

I’ve outlined the area here where Z>50 dbz and you can see a large chunk of that where the reflectivity is quite high (i.e. >60 dbz in white) is coincident with low ZDR (near or below zero) in addition to relatively low CC (<0.95). This, combined with the very large TBSS, is a good indication for severe hail falling. Many reports of quarter-size or even a bit larger came in under this echo from Plainfield and Griswold including this great video from my buddy Craig.

 

Advertisements

Hail Spikes!

One of the easiest ways to detect large hail on doppler radar is with the presence of a hail spike. The storm that produced large hail in East Hampton today showed several hail spikes at the time large hail was pelting down. Here’s a 3.1 degree base reflectivity scan from OKX at 20:41Z showing the hail size. The purple is DBZ >65. The hail spike is the light blue that extends downradial of the hail core.

Hail spikes, also known as Three Body Scatter Spikes, occur when the radar beam is deflected off a large object (hail in this case) down to the earth and then back to the hail stone and then finally back to the radar. Because the route of energy takes extra time the radar thinks there is something occuring further away from the radar than is actually the case. The narrow blue line extending northeast of the thunderstorm is the hail spike in this case.

Here’s a 2d cross section on a radial extending out from the OKX radar site. You can see the hail core suspended in the updraft with a hail spike extended on a radial on the radar beam downradial of the hail core. Awesome stuff!

While the largest “official” hail report we have is 0.88″ in East Hampton there was undoubtedly larger hail somewhere in town – possibly ping pong ball size!

Some Nasty Evening Storms Possible

No tornadoes tonight but some nasty storms in the Berkshires and near Albany are on their way south with strong instability acting as their fuel. Another elevated mixed layer overhead, coupled with high heat and moderate humidity near the surface, has our instability cranked.

The storms are capable of producing hail to golf balls (already been reported in New York) and 60 mph wind gusts. Lots of lightning too. One of the storms in New York near Coxsackie is producing large enough hail that the radar picked up a “hail spike”.

KENX 0.9 Degree Base Reflectivity

Hail spikes are also known as Three Body Scatter Spikes and occur when the radar beam is deflected off a large object (hail in this case) down to the earth and then back to the hail stone and to the radar. Because the route of energy takes extra time the radar thinks there is something occuring further away from the radar than is actually the case. The narrow blue line extending southeast of the thunderstorm is the hail spike in this case.