21 May severe weather

A major severe weather event occurred across eastern NY and western New England on May 21st, as the forecast area had a cold front near the northern NY-northern New England and Canadian border. A weak surface wave was expected to move along the boundary, as the cold front would slowly move southward during the late afternoon and early evening. The timing of the weak surface wave and the amount of instability were in question in the morning.  This event qualified as W/NW flow aloft event too.

The air mass became increasingly hot and humid in the warm sector with temps rising into the mid 80's to lower 90's and surface dewpoints in the mid to u60s. The air mass continue to destabilize especially from the Mohawk Valley...Capital Region and Berkshires south and east. SPC dropped the "See Text" with the 1300 UTC update...but then increased the DAY 1 graphic to a "Slight Risk" for most of the ALY forecast area at the 1700 UTC update.

The air mass continue to destabilized in the early pm with SBCAPES in the 2000-3500 J/kg range from the Capital Region south and east according to the 1700 UTC SPC Meso-analysis page. There was much less instability over the northern zones. The abundance of instability was not in a region of strong deep layer shear /generally less 25 kts/. The better 0-6 km bulk shear was north of the Mohawk River Valley /30-35 kts/. The best shear and instability and shear were out of phase...HOWEVER...a remnant MCV approaching from SE Ontario...and a synoptic boundary nearby were good focusing mechanism for convection. There was concern that a widespread severe weather event would not occur due to mid level lapse rates only 6.5 C/km...and a mid-level cap in the 1200 UTC KALB sounding at 500-550 hPA. The thinking in the early afternoon was for a isolated or minor severe weather event with 5 or less events. The HIRES WRF hinted at better convection towards 00Z. All said, isolated-scattered convection started to fire over western and central NY in the around noontime.

At 2 pm...a Severe Thunderstorm Watch was issued until 9 pm for the forecast area. SPC was going to leave the northern zones out, but the concern from the ALY staff was for the next short-wave to get into those areas after dark with perhaps more strong to severe thunderstorms. The areal coverage for severe weather was still in question but the surface trough/wave approaching and the remnant MCV looked to move along the axis of best deep shear, and tap into the abundance of instability. The Maglenta Severe Weather Index was run with the following values:

CAPE = 2000 J/kg

Max Sounding Winds = 40 kts

EHI = 0.5

Storm Speed = 20 kts

SRH = 125

Result = Major Event

The forecast discussion at 200 pm mentioned a minor to major event was likely with the abundance of instability in place over the forecast area and the approaching disturbances. The thermodynamic environment was very volatile even though the dynamics looked a bit weak. The abundance of instability and the steepening lapse rates appealed towards pulse thunderstorm or multi-cell clusters that could produce large hail or damaging winds.

Strong to severe convection quickly fired from the Capital Region south and east in the mid afternoon. Several impressive large hail producing thunderstorms developed in the Capital Region and points to the south. Some of the cells exhibited hail monster characteristics with elevated cores of > 50 dBZ's way above the -20C height and in some cases exceeding 40 kft AGL ! A meso-analysis showed with the MSLP field and the wind directions from the ASOS's in east-central NY that a meso-low/weak sfc wave was moving across the region with the first batch of warnings.

The next short-wave trough impacted the northern zones into the Capital Region with another round of strong to severe convection between 00Z to 06Z. Some of the rainfall totals ranged from 1-3 inches from the Capital Region northeast into southern VT. This heavy rainfall actually caused the Walloomsac River in Bennington to reach the minor flood stage briefly overnight. The Hoosic River in Williamstown got close to flood, but there was a data outage. 22 SVR polygons were issued and 2 TOR's. 16 SVR polygons verified.

segments from SPC MCD's

Summary...thunderstorms should continue increasing in intensity...primarily focused along/s of a quasi-stationary front...across portions of western and central NY and into southern New England this afternoon. The primary threats will be damaging wind gusts and large hail. A weather watch may be needed if convective trends continue to increase.

Discussion...subjective surface analysis from 16z places a quasi-stationary front from near roc-alb. A few storms have recently initiated along this front...likely in response to weak low-level convergence...weakening convective inhibition...and modest midlevel moistening/ascent as noted on water vapor. Modified 12z soundings from both Buffalo and Albany suggest ample instability for the development of strong to severe thunderstorms...with mixed layer CAPE values likely exceeding 2000 j/kg...given the presence of moderately steep midlevel lapse rates and rich low-level moisture. Additionally...both sounding and recent VWP data show nearly 30 kts of midlevel flow and modestly veering winds with height...supportive for organized thunderstorms. Coverage of thunderstorms is somewhat uncertain given relatively weak forcing for ascent...but with the potential for large hail and damaging wind gusts...a weather watch may be required should trends continue to increase.

Discussion...scattered vigorous storm development is underway near/south of a frontal zone extending across the lower great lakes region into New England. In the presence of thermodynamic profiles characterized by fairly steep mid-level lapse rates and large cape...shear beneath 30 kt west northwesterly mid-level flow may support organized storm development...perhaps at least brief supercell structures. With time...consolidation of cold pools generated by activity may occur...resulting in one or more east southeastward progressing small storm clusters with the risk for potentially damaging wind gusts...in addition to severe hail.

Discussion...radar trends show pulsing isolated thunderstorms across much of western and central NY...along a quasi-stationary front draped w-e from approximately 10 n Buffalo to 10 southwest Worcester as of 20z. More organized/sustained convection has been noted across eastern NY into western MA...east of the Hudson valley...where a storm split occurred within the past couple hrs. Given strong heating s of the boundary/temperatures approaching 90 f/ maintaining a very unstable environment...convection appears likely to persist and maintain intensity into the early evening. However...coverage should continue to generally remain isolated/semi-discrete...owing to weak forcing for ascent and convergence along the boundary.

Discussion...as of 00z...regional radar imagery showed a loosely organized mesoscale convective system from Clinton and Essex counties to Madison and Chenango counties in NY with an eastward system motion of 35-40 kt. While the northern extent of this mesoscale convective system will move into a progressively cooler and more stable air mass in place over northern VT...the southern portion will maintain an inflow of at least modest instability /see 00z Albany sounding/...suggesting a continued damaging wind and hail threat as it moves into the Hudson valley. Over western and central parts of NY...isolated occurrences of marginally severe hail will remain possible over the next hour or two with storms developing along the outflow boundary trailing the mesoscale convective system.

In conjunction with the highest reflectivity core surpassing the -20C level, when looking at the dual pol products, the first indication that a cell may have hail beginning to form is by looking at the CC. The CC will begin to lower into the 0.9-0.97 range first, coincident with a spike in KDP (if not blocked out by CC <0.90) of around 2-3 deg/km. ZDR values at this time are still high, generally greater than 2 dB, which would signal big drops. Use all tilts and FSI to go up through the updraft to see how far these high ZDR values extend. This is a rough estimate for the strength of the updraft and how much liquid water is being pumped into the midlevels of the storm in the form of the ZDR column. Although CC initially tails off to values that would support possible hail, ZDR values remain high. This may be due to the ZDR column artifact and a delay in hail production as big water droplets are lofted into the updraft/meso. When the ZDR values drop below 2 dB and even less than 1 dB, I know that hail is being formed in the updraft and this is verified by the CC values continuing to drop along with an even bigger increase in KDP up to 3-5+ deg/km. When the ZDR drops off in conjunction with low CC and a high reflectivity core that extends well to -20C this can sometimes allow at least 5-15 minute lead time between warning issuance and time of first report, because the ZDR dropping off signifies hail growth, however there is a delay as the hail must grow big enough in size to escape the updraft and fall to the ground. The higher the ZDR column, the better lead time since a higher ZDR column is correlated to a stronger updraft that can recycle hail longer, especially if a meso is present.


Above:  SPC analysis of 925 hPa at 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  SPC analysis of 850 hPa at 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  SPC analysis of 700 hPa at 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  SPC analysis of 500 hPa at 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  SPC analysis of 300 hPa at 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  SPC analysis of 250 hPa at 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  Soundings from Albany, NY (KALB) for 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  Soundings from Buffalo, NY (KBUF) for 1200 UTC 21 May (left) and 0000 UTC 22 May (right).

Above:  Sounding from Upton, NY (KOKX) for 1200 UTC 21 May.

Above:  Loops of SPC mesoanalyses of (left to right) surface-based lapse rates, midlevel lapse rates, mixed-layer CAPE, surface-based CAPE/CIN and 0-3 Km storm-relative helicity.

Above:  Loops of SPC mesoanalyses of (left to right) supercell composite parameter, 0-1 Km storm-relative helicity, 0-6 Km bulk shear, 0-3 KM effective helicity and effective shear.

Above:  Loop of surface maps from HPC.

Above:  Radar loop from KENX.

Above:   Radar rainfall estimates from KENX Dual-Pol (left) and legacy algorithm (right).

Above:  Severe weather reports from the SPC.

Above:  Hail pictures from Canaan, CT (left) and Valatie, NY (right).