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.

 

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Upstate New York Supercell Through Dual-Pol Radar

A monster supercell on May 22, 2014 produced giant, destructive hail and an EF-3 tornado just west of Albany, New York. In general, the event was poorly forecast.

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The supercell formed outside of the “slight risk” area outlined by the Storm Prediction Center 1630 UTC forecast. Additionally the tornado probability was <2% and not contoured in the SPC forecast. The morning Area Forecast Discussion from Albany summed up the severe weather threat in the following way:

THEREFORE...WE FEEL THE THREAT OF SEVERE THUNDERSTORMS IS VERY LOW 
(BUT NOT ZERO PERCENT). THE BETTER THREAT APPEARS TO BE LARGE HAIL 
BUT AGAIN THAT IS A VERY LOW THREAT.

I don’t think anyone (myself included) expected a long-lived supercell producing a long path of significant severe weather. The storm gave us an opportunity to look at how this supercell looked with dual polarization radar – a relatively new tool in our severe weather arsenal.

The storm started out with a classic large hail signature in Fulton County, NY to the north of the New York State Thruway. The above video from the town of Broadalbin shows baseball size hail falling from the storm. While low level rotation was unimpressive the mid level mesocyclone was cranking.

Broadalbin2

From a dual pol perspective this was a classic large hail case. A large area of high reflectivity (near 70dbz in places) with ZDR near 0 and pockets of low CC. Because hail tends to tumble as it falls it appears spherical to the radar – hence a differential reflectivity near 0. Not shown is KDP which is fairly low over Broadalbin – only about 1º/km showing that liquid water was not a large contributor to the high reflectivity – the hail was! In addition, a three body scatter spike (note very low CC downraidial of hail core) was present from the lowest elevation slice (near 2,000 ft AGL) all the way up to 20,000 feet! Storm top divergence was also approximately 100 knots which is impressive for a northeastern US supercell.

amsterdam2

The supercell went on to produce large hail in the town of Amsterdam, NY as well. The largest hail fell in an area with somewhat low reflectivity (55dbz over Amsterdam as opposed to 65-70dbz farther east). This is another example of why dual pol variables are better at detecting large hail than using reflectivity alone!

Picca and Ryzhkov (2012) showed giant hail detection is possible by locating areas of wet hail growth above the freezing level, and in particular, in the hail growth zone near -15C. They cite CC <0.9 and ZDR <0 as clues. This event showed that signal over Amsterdam when giant hail was indeed falling. You can see very large Z near Amsterdam at 9500ft coincident with sub-zero ZDR and CC <0.9. This signature is also present up into the hail growth zone above the -10C isotherm.

wethailgrowth1

Rapid tornadogenesis took place following the large hail reports north of the Thruway. At 1928 UTC there is some sign of a reflectivity appendage at the lowest tilt. In addition the mid level mesocyclone is beginning to descend (not present at 0.5º or 0.9º tilts but is higher up at 1.3º). This was a tough event to warn for – as there weren’t many clues until the tornado was touching down.

By 1933 UTC tornadogenesis has occurred with a 56 knot gate-to-gate delta-V at 900 ft AGL. The tornado warning came out at 1939 UTC. By 1942 UTC a weak tornado debris signature is present and by 1951 UTC a clear tornado debris signature is present with very low CC, high Z, and near zero ZDR.

tds1

tds2

Given the strength of the tornado (EF-1 to EF-3), the proximity to the radar site, and the path of the tornado (through heavily wooded areas) I’m very surprised the TDS was not more dramatic. The “slam dunk” TDS only lasted 1 volume scan and only extended up to about 3500 ft AGL. Not particularly impressive.

One possible explanation for the relatively unimpressive debris signature was that there was a tremendous amount of rain and hail that was wrapped into the rear flank downdraft (KDP and Z quite high and ZDR near zero) that could have skewed the correlation coefficient higher than you’d typically expect with hydrometeors beings the dominant signal?

One other interesting feature is the presence of a ZDR arc shortly after tornadogenesis. The ZDR arc did not precede the tornado which frequently happens (no benefit for the radar operator) and was somewhat unusual because it appeared so rapidly.

zdrarc2

You can see the ZDR arc here at 1947 UTC along the reflectivity gradient of the storm’s forward flank. ZDR is quite high in places (nearly 7db) and there is clear separation between high ZDR and high KDP. This is an example of hydrometeor size sorting in a highly sheared environment. The high ZDR is indicative of large drops and the smaller drops are getting pushed farther into the storm’s forward flank where KDP shows high water content. The ZDR arc appeared as the tornado reached its maximum intensity (EF-3) in Duanesburg.

Courtesy: WNYT-TV

Courtesy: WNYT-TV

The sudden appearance of a ZDR arc might indicate that the storm was moving into a more strongly sheared environment where hydometeor size sorting occurred quickly. Not surprisingly, this is where the tornado was the strongest. At the time of tornadogenesis there is a region of high ZDR displaced well east of the low level mesocyclone but it is coincident with high KDP indicating little separation from size-sorting. Prior to and during tornadogenesis there was no real signal from a ZDR arc to help forecasters with this storm.

 

zdr4

 

The EF-3 tornado provided a unique opportunity to view the dual pol characteristics of this northeastern US supercell. Unfortunately, the storm’s rapid tornadogenesis was preceded with few, if any, clues on radar. When the tornado was on the ground the debris signal was surprisingly muted (many far weaker and farther from the radar tornadoes have produced much more dramatic TDS around here) but did eventually give forecasters confirmation of a tornado in the absence of any reports in real-time.

The Giant 1969 Memorial Day Weekend Storm

Until yesterday if you asked me about the year 1969 the only things that would probably come into my head were Hurricane Camille, the moon landing, and Woodstock. Am I missing anything else?

Well it turns out I am! Yesterday I was going back through some of my geeky weather archive sites to figure out how often New London County has received significant (>2″) hail. As it turns out there have been only 2 cases of significant hail in New London County since 1955 – one in 1969 and another in 1995 during the super hail storm. The 1969 report from May 29th (the day before Memorial Day) is of baseball size hail in Groton. You don’t see that everyday! Here’s how Storm Data described the 1969 thunderstorm in Connecticut.

Screen shot 2013-09-04 at 10.19.32 AM

The reports from Rhode Island and Massachusetts are equally as impressive.

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Screen shot 2013-09-04 at 10.20.56 AM

Radish size hail in Pawtucket – I love it! The storms obviously were quite intense given the dramatic descriptions in Storm Data. The weather pattern that day was favorable for severe thunderstorms – and one of those rare days where southeastern Connecticut was in the running for a violent evening storm.

Windsor Locks reported a sustained wind of 37 mph from the northeast during the storm with about 0.50″ of rain. Little rain fell in New Haven or Bridgeport – it would appear the storm missed them to the east.

In some of the cooperative weather reports from the Memorial Day morning you can get a general idea of where the storm(s) tracked.

  • “Trees and limbs” down in Brooklyn with a power outage and 0.56″ of rain
  • 0.23″ rain and thunder reported at the Cockaponset Ranger Station
  • 0.44″ of a “hard rain” along with thunder and hail in Colchester
  • 0.86″ of rain with thunder and hail in Groton
  • 0.86″ of rain at Pachaug State Forest

The morning weather balloon launch from JFK Airport in Queens showed very steep mid level lapse rates. The 500-700mb lapse rate was an impressive 7.9 C/KM! The weather map for 5/29/1969 showed a deep closed low over the Canadian Maritimes with a fast northwesterly flow through the mid troposphere. The JFK weather balloon data shows a northerly wind at 700mb of 46 knots.

comphour.71.234.167.147.246.8.14.7

There was also a fairly impressive backdoor cold front bisecting New England. While winds were generally out of the west and northwest in Connecticut during the day the winds were due east at Logan. The high temperature at Logan was only 72F while it was 86F at Worcester and 92F in New Haven, Bridgeport, and Bradley. Eastern New England can really suck in the spring!

The air was also quite sticky over Connecticut. By mid afternoon Bridgeport had a dew point of 68F with 92F on the thermometer. The combination of the moist and hot boundary layer and steep mid level lapse rates likely set up a very unstable atmosphere over western New England. While this is only conjecture (since I don’t have the data) one can envision severe storm initiation near the backdoor front as a shortwave raced south on the strong northwesterly flow aloft to get things going. The strong mid level flow also cranked up fairly impressive deep layer shear – a requisite for organized severe convection.

comphour.71.234.167.147.246.8.15.19

As it turns out this kind of setup is not unusual for severe weather in Connecticut. We’ve seen cases before where an elevated mixed layer (characterized by steep >8.0 C/KM lapse rates) have advected eastward along the US/Canadian border and have helped spawn severe weather in New England with deep northerly/northwesterly flow.

The 1995 hail storm comes to mind as one example of this. The storm occurred near a backdoor cold front. This storm was able to drop base ball size hail (for 20 minutes!!!!) in Deep River. The northerly/offshore flow can help keep the shoreline unstable and hold the marine influence at bay. You can see a similar setup here with a deep low over the Canadian Maritimes and northwest flow across New England.

comphour.71.234.167.147.246.8.16.35

It also is somewhat similar to the May 26, 2010 overnight severe weather event that managed to drop hail to ping pong ball size in Bridgeport in the middle of the night.

The offshore wind component is necessary for big severe storms in southeastern Connecticut. This is especially true around Memorial Day when water temperatures are still quite chilly.

I love looking back through old weather data to learn about storms I have never heard of and also how meteorologists and others described the weather back then. The “radish” size hail description in Pawtucket, RI was a new one for me. I may have to snag that one for myself next time we’re doing storm coverage. A Twitter follower wrote this to me last night after I tweeted about the radish size hail report. He nailed it – and made me laugh too.

Screen shot 2013-09-04 at 11.05.48 AM

One other interesting note from the May 1969 storm data report that caught my eye. On May 3rd around 3:30 Grosvenordale had one freaky storm…

Screen shot 2013-09-04 at 11.03.46 AMI’ll take their word for it.

New London County Hailstorm

Hail (and a screw driver) from Mark in Preston

Hail (and a screw driver) from Mark in Preston

When you think of severe weather in Connecticut you probably don’t think about New London County. The southeastern corner of the state is a pit for snow and 99 times out of 100 is where thunderstorms go to die. With the exception of hurricanes – weather weenies in New London County are used to let down after let down after let down.

Today, however, brought a severe storm bonanza to New London County. 3 separate storms brought severe hail to New London County including a report (and picture to go along with it!) of significant hail (hen egg size) in Ledyard.

2" diameter hail in Ledyard

2″ diameter hail in Ledyard from Josh

The severe weather threat today was a bit nebulous but still well forecast. A cold front had bisected the state by midday with a narrow corridor of marginal/moderate instability juxtaposed with fairly strong deep layer shear. Here’s the 18z RAP sounding for Groton around the time of the largest hail reports.

RAP BUFKIT analysis at 17z

RAP BUFKIT analysis at 17z

Dew points over 70F were underneath steep mid level lapse rates – on the order of 7.2 C/KM between 500 and 700mb. Impressive! While MLCAPE values were only around 1000 j/kg a fair amount of that CAPE was located in the hail growth zone (-10 to -30C) and 0-6km shear values were near 40 knots.

It shouldn’t come as a surprise that many of these storms exhibited mid level rotation and the mesocyclones were able to sustain some pretty impressive hail stones. I’ll also throw this out there that the models today were consistently underdone with progged CAPE values today. For example, the 12z OKX sounding had 2500 j/kg of CAPE which was far higher than model forecasts or even 12z model analyses. It’s conceivable that the 1000 j/kg of MLCAPE analyzed at 18z on the RAP may have also been underdone compared to reality.

Handfulls of quarter size hail in Groton

Handfulls of quarter size hail in Groton

The storm that dropped the largest hail pulsed up shortly before 2 p.m. over Montville and the Thames River just a (hail)stone’s throw from Mohegan Sun. The 1754 UTC volume scan from OKX shows an impressive hail core with 65 dbz echoes up to the -20c isotherm (over 20,000 ft AGL) and 72 dbz around 10,000 ft AGL.

1754 volume scan KOKX 88d

1754 volume scan KOKX 88d

While meteorological echoes (part anvil being blown northeast by 70 knot southwesterlies near the Equilibirum level and part other junk) masked the hail spike on the OKX radar but the radar out of Taunton showed an impressive (spatially and vertically) three body scatter spike.

KBOX 88d correlation coeffecient - note blue shading southwest of storm showing hail spike

KBOX 88d correlation coeffecient – note blue shading southwest of storm showing hail spike

Shortly after the storm pulsed over Montville the core of the storm dropped and so did golf ball and hen egg size hail in the far northwest corner of Ledyard near the Thames River and along and west of Avery Hill Road just south of Route 2A. Dual Polarization products showed the hail core descending with a clear signal of hail in Ledyard by 1758 UTC (at 4500 ft AGL).

ledyardhailannotate

1758 UTC KOKX volume scan. Clockwise from top left – 0.5 degree base reflectivity, correlation coefficient, specific differential phase, differential reflectivity

Within an area of high reflectivity (Z between 50 and 60 dbz) you can see a noticeably depressed area of correlation coefficient – in some cases near 0.90. There are also areas of ZDR near zero or even subzero which shows that hail is dominating the signal (hail tumbles as it falls so appears spherical to dual pol radar which leads to a differential reflectivity value near 0).

One thing that’s somewhat interesting is that the southeastern part of the storm has very high KDP values – nearly 4 deg/km while the northwest part of the storm was much lower. While the hail signal was present throughout this region the spike in KDP over the center of Ledyard may indicate a lot of water coated sub-severe hail while the lower KDP over the Thames River and northwest Ledyard in the hail core was where the larger/non-water coated hail was falling. Indeed, this matches up with the reports we received of hail near (or even over) 2″ on the Thames River in far northwest Ledyard with dime to quarter size hail in the center of town.

Golf ball size hail in Ledyard melted to this size about 10 minutes after it fell. Thanks Tom for the picture!

Golf ball size hail in Ledyard melted to this size about 10 minutes after it fell. Thanks Tom for the picture!

I went back and searched through SPC’s storm database and then double checked some of the events in NCDC’s storm data. It appears that the 2″ hail from Ledyard is the largest hail event reported in New London County since 2″ hail was reported in Old Lyme during the 1995 super hailstorm. Incidentally, the largest hail report I can find in New London County was from Groton in May 1969 where baseball size hail was reported. 2.75″ hail (baseball size) only shows up 4 times in the Connecticut SPC storm event database since 1955 and 3 of the 4 were from 1995 and all have been along or east of the Connecticut River.

Connecticut Hail and Dual Pol

Since dual polarization capability was installed on the 3 WSR-88ds that cover Connecticut we really haven’t had much large hail to play with. There was one notable exception on July 1, 2012 that I blogged about here.

The supercell that produced the hail dropped significant hail (2″+ in diameter) in a few locations including Cheshire, Clinton and Westbrook. There were many reports of golf ball size hail in Bethlehem, Watertown, Oakville,  Wolcott, Wallingford, and Essex.

The first reports of golf ball size hail came in around 3 p.m. around Watertown. Dual pol products from OKX certainly show the presence of hail with an area of depressed correlation coefficient (generally from 0.8 to 0.95) over Watertown. One thing that’s interesting is that the largest hail was likely falling on the western flank of the highest reflectivity. Notice that ZDR is quite high to the east (in some cases over 4 or 5 dB which would indicate melting hail stones). On the Watertown/Bethlehem border ZDR is much lower – near and even sub zero in some places – this is where the largest hail was falling. As hail falls it tends to tumble so the hail appears to be spherical to the radar – hence a ZDR value near 0 dB.

watertown1

Note throughout the hail core KDP values are quite high indicating there’s a large amount of liquid water mixed with the hail. The highest values of KDP are where you’d expect them – to the east where the hail is melting. The depressed area of CC becomes more impressive as the storm enters Wolcott with CC falling around 0.85 in area.

Wolcott

 

ZDR remains noisy but there is an area around 1 to 1.5 dB coincident with the depressed CC.

As the storm moves southeast toward the shoreline it actually pulsed up over Madison and Killingworth as it merged with another storm that had formed on its outflow boundary. One thing here that’s pretty cool is the BWER over North Madison that you can see on a cross section.

madisonbwer

 

On dual pol you can see a noisy, but still present, hail signature on CC and ZDR over Killingworth. Note, much like before over Watertown, the lowest ZDR values are to the west (also notice lower KDP here) while the higher ZDR values can be found in an area with high KDP showing melting hail on the eastern flank.

Killingworth

 

As I mentioned in the original post last year one of the more impressive things about this supercell was the monstrous storm top divergence. Nearly 130 knots over Killingworth and Watertown shows a super impressive updraft.  Any time I see storm top divergence over 100 knots in Connecticut I pay close attention to it – 130 knots is extremely impressive.

StormTopDiv

More recently, last week’s supercell that dropped hail in Salisbury and Canaan dropped 2″ diameter hail in Columbia County New York around Ghent, Kinderhook, and Valatie.

GhentHail

 

There was a classic dual pol signature here with ZDR near 0 coincident with a large area of depressed CC. This example is a bit more “classic” than last year’s hail in Connecticut. This stuff is just so neat!